Obstetric Anesthesia PDF

51 Obstetric Anesthesia BRIAN J. KASSON Parturients who request or require obstetric anesthesia services repre- Plasma volume increases 40% to 50% whereas red blood cell (RBC) sent a unique challenge to anesthesia providers. Techniques for provid- volume increases by only 20%.13 As a resu...

51 Obstetric Anesthesia BRIAN J. KASSON Parturients who request or require obstetric anesthesia services repre- Plasma volume increases 40% to 50% whereas red blood cell (RBC) sent a unique challenge to anesthesia providers. Techniques for provid- volume increases by only 20%.13 As a result, a relative or dilutional ing pain relief during labor and delivery are continuously being refined anemia is commonly seen as the plasma volume increases to a greater in an effort to provide the best care possible to this uniquely vulnerable extent relative to the actual RBC volume. The increased plasma volume patient population. A thorough understanding of the anatomic and is likely caused by circulating levels of progesterone and estrogen result- physiologic changes that occur during pregnancy and the anesthetic ing in enhanced renin-angiotensin-aldosterone activity.14 RBC volume considerations associated with such changes are necessary for a safe and increases as a result of elevated erythropoietin levels seen after the effective anesthetic course. eighth week of gestation.15 Normal blood loss for vaginal delivery is less than 500 mL, and for an uncomplicated cesarean section, the blood Anatomic and Physiologic Changes During loss is 800 to 1000 mL. Normal blood losses at delivery are generally Normal Pregnancy well tolerated in the healthy parturient as a result of these compensatory The physiologic changes seen in pregnancy occur as a result of increased mechanisms. In labor each contraction moves 300 to 500 mL of blood metabolic demands and hormonal and anatomic changes. These from the contracting uterus to the central circulation.16 Pregnant changes begin early in pregnancy and continue into the postpartum women have greater baroreflex-mediated changes in HR at term than period. These marked changes have significant implications for the at 6 to 8 weeks postpartum.17 In the presence of adequate neuraxial anesthesia provider. analgesia and little sympathetic stimulation, there is often a correspond- ing decrease in maternal HR during uterine contractions due to the Cardiovascular Changes transiently increased preload. Cardiovascular changes begin as early as the fourth week of pregnancy Systemic vascular resistance (SVR) decreases as much as 21% by the and then continue into the postpartum period.1 The heart rate (HR) end of a term pregnancy, owing in large part to decreased resistance in is increased by 20% to 30% at term. This increase begins in the first the uteroplacental, pulmonary, renal, and cutaneous vascular beds.18,19 trimester and peaks by 32 weeks of gestation.2 Normal HR variability At term gestation 10% of the cardiac output perfuses the low resistance does not appear to be changed until late in pregnancy, when tachyar- intervillous space of the uterus. Baseline central sympathetic outflow is rhythmias are more common.3 Cardiac output increases by approxi- twice as high in normal, term pregnant women as in nonpregnant mately 40% over nonpregnant values.4 This increase in cardiac output women.20 The venous capacitance system loses tone, allowing pooling begins in the fifth week of pregnancy and results from an increase in of the larger blood volume. This decrease in SVR results in minimal stroke volume (SV) (20%–50%) and, to a lesser extent, HR.5 Cardiac overall systolic blood pressure change during normal pregnancy, despite output increases consistently throughout pregnancy.6 Some studies pre- the increased blood volume.21 A decrease in diastolic blood pressure of viously indicated cardiac output decreased in the third trimester, but up to 15 mm Hg may occur, resulting in a decrease in mean pressure. these results were likely due to aortocaval compression from studying subjects in the supine position.7 At term, approximately 10% of the Aortocaval Compression cardiac output perfuses the gravid uterus.8,9 When a woman is in labor, In the early 1950s, a syndrome involving supine hypotension was cardiac output increases during uterine contractions as a result of auto- identified in term or near-term pregnant women.22,23 This syndrome is transfusion from the contracting uterus to the central circulation.10 caused by compression of the vena cava and aorta by the gravid uterus, Immediately after delivery, cardiac output increases as much as 80% which restricts venous return to the heart when the parturient lies in above prelabor values. This occurs because of the autotransfusion of the supine position. Compression can be more severe when the blood into central circulation from the contracting uterus and increased abdomen is tense or when the uterus is larger than normal, as in poly- venous return from aortocaval decompression. As a result, patients with hydramnios or multiple gestation pregnancies. This decreased venous preexisting cardiac anomalies are at an increased risk for decompensa- return results in a significant reduction in SV and, ultimately, cardiac tion in the immediate postpartum period. Cardiac output gradually output. The resultant hypotension can be severe enough to cause loss returns to baseline within 14 days as HR and SV normalize.11 of consciousness in some women. Maximal decreases in blood pressure During pregnancy, the diaphragm rises, shifting the heart up and may require up to 10 minutes to develop; however, some women to the left, making the cardiac silhouette appear enlarged on x-ray experience the decrease almost immediately. The normal physiologic examination. The ventricular walls thicken and end-diastolic volume responses to aortocaval compression are tachycardia and vasoconstric- increases. A physical examination of the pregnant patient may appear tion of the lower extremities. Despite this attempted compensation, to elicit abnormal findings. A benign grade 1 or 2 systolic murmur or uterine blood flow and therefore fetal oxygenation are reduced.24 Fig. a third heart sound may be heard on auscultation.12 These findings 51.1 depicts changes in aortocaval compression with changes in would not be unusual; however, if the systolic murmur is greater than position. grade 3 or accompanied by chest pain or syncope, further evaluation In addition to compressing the vena cava, the gravid uterus may is necessary. Diastolic murmurs and cardiac enlargement are considered compress the abdominal aorta. For this reason, supine hypotensive pathologic. Normal pregnancy may also result in signs of cardiac abnor- syndrome is more correctly referred to as aortocaval compression. When mality such as exercise intolerance, shortness of breath, and edema. the abdominal aorta is compressed, upper-body blood pressure remains Total blood volume increases 25% to 40% throughout pregnancy, relatively normal, whereas blood pressure distal to the site of aortic in part to prepare for the normal blood loss associated with delivery. compression (uterus and lower extremities) may be significantly reduced. 1064  C H A PT E R 51 Obstetric Anesthesia 1065 Supine Left uterine displacement View from View from above above Vena cava Aorta Vena cava Aorta View from View from above above Aorta Vena cava Vena cava Aorta FIGURE 51.1 Effects of left uterine displacement on the diameter of the abdominal aorta and vena cava. Compression of the aorta and vena cava can usually be relieved by less than 150 × 109/L) in normal pregnancy has been shown to be shifting the uterus to the left (left uterine displacement) or by lying on 11.6% 29 and is not associated with increased morbidity or mortality. the side.25 Left uterine displacement can be accomplished by rotation The pathogenesis is not well understood but may involve factors such of the operating room table 15 degrees to the left or by placing a as hemodilution and/or accelerated platelet clearance.30,31 The white 15-cm-high wedge under the parturient’s right hip and back, as shown blood cell count rises in pregnancy. In the third trimester, the mean is in Fig. 51.1. Most anesthetists have been shown to underestimate the 10,500/mm3, and in labor it may increase to 20,000 to 30,000/mm3.32 angle of tilt they provide, so care should be taken to ensure adequate left uterine displacement.26 In women with an exceptionally large Respiratory Changes uterus, greater displacement may be necessary to be effective. The respiratory changes that accompany normal pregnancy are of par- There has been much debate regarding the optimal position for ticular importance to the anesthesia provider. Capillary engorgement performing neuraxial anesthesia for labor analgesia and cesarean deliv- in the upper airway results in a narrowed glottic opening and edema ery. Many clinicians feel that the lateral position is optimal to facilitate in the nasal and oral pharynx, larynx, and trachea. These changes carry both maternal and fetal hemodynamics. Researchers have found that over into labor, as has been shown by the Mallampati score changing cardiac index is improved in both the left and right lateral positions as labor progresses.33 Airway tissues are susceptible to damage and when compared to the sitting flexed position in healthy pregnant bleeding during placement of airway adjuncts. For this reason, nasal women. However, when umbilical Dopplers were monitored in the intubation in the parturient should generally be avoided. A 6.5–to same group as a surrogate for healthy fetal blood flow, no differences 7.0-mm cuffed oral endotracheal tube is recommended when intuba- were identified.27 Although positioning for neuraxial anesthesia may tion is necessary. Obese patients with enlarged breasts may benefit from influence maternal hemodynamic variables, there were no differences the use of a short-handled laryngoscope. in healthy fetal blood flow indices among positions, suggesting that Term pregnancy is accompanied by an increase in oxygen (O2) these changes are not clinically significant. consumption by up to 33% at rest and 100% or more during the second stage of labor. Minute ventilation at term is increased by 50%. Hematologic Changes This is primarily due to a 40% increase in tidal volume, whereas the In general, the parturient is said to be in a hypercoagulable state. respiratory rate remains unchanged or increases by only 10%.34 By 12 Because of the parturient’s hypercoagulable state, concentration of weeks’ gestation the normal arterial partial pressure of carbon dioxide factors VII through IX and fibrinogen are increased, whereas fibrino- (PaCO2) decreases to approximately 30 to 32 mm Hg and remains in lytic activity is decreased.28 In the nonpregnant state, fibrinogen levels this range throughout pregnancy.35 Metabolic alkalosis is rarely seen average from 200 to 400 mg/dL. Late in pregnancy, fibrinogen levels because there is a compensatory decrease in the serum bicarbonate from are normally at least 400 mg/dL and may be as high as 650 mg/dL. 26 to 22 mEq/L. The normal arterial partial pressure of O2 is greater These increased levels place the parturient at risk for thromboembolic than 100 mm Hg. events, which remain one of the leading causes of maternal mortality. The functional residual capacity (FRC), expiratory reserve volume, The platelet count remains stable or is decreased slightly in the third and residual volume are decreased primarily as a result of upward pres- trimester. The prevalence of maternal thrombocytopenia (platelet count sure on the diaphragm, with results functionally similar to restrictive 1066 U N I T V Intraoperative Management lung disease. The FRC plays an important role in preserving O2 satura- moderate amount of clear fluids in uncomplicated laboring patients. tion during periods of hypoventilation or apnea. The decrease in FRC Uncomplicated patients undergoing elective cesarean delivery may have (20%) combined with the increase in O2 consumption in pregnancy modest amounts of clear liquids up to 2 hours prior to induction of commonly results in rapid arterial desaturation in the apneic pregnant anesthesia. The same guidelines state that patients with additional risk patient.36 Morbid obesity, labor, and sepsis exaggerate this effect. For factors for aspiration (e.g., morbid obesity, diabetes, difficult airway), this reason, preoxygenation with 100% O2 prior to induction of general or patients at increased risk for operative delivery (e.g., nonreassuring anesthesia (GA) is important. Closing capacity (CC) does not change, fetal heart rate [FHR] pattern) may have further restrictions of oral which results in a decreased FRC/CC ratio often leading to small- intake.46 Administration of nonparticulate antacids, H2 receptor antag- airway closure before the tidal volume has been exhaled. This mecha- onists (to increase gastric pH), and metoclopramide (to promote gastric nism may explain the reduction in O2 saturations seen in parturients emptying, reduce nausea and vomiting, and increase lower esophageal during natural sleep. When compared with nonpregnant controls, sphincter tone) may be beneficial prior to cesarean section and should whose average oxygen saturation during sleep was 98.5%, healthy near- be considered. The use of these drugs is advocated when a general term pregnant women averaged only 95.2%, with temporary desatura- anesthetic becomes necessary. When a general anesthetic is used, a rapid tions below 90% not being uncommon.37 The increased cardiac output sequence induction with cricoid pressure using a cuffed endotracheal and a shift to the right in the oxyhemoglobin dissociation curve help tube is necessary from the twentieth week of gestation extending to the to maximize oxygen delivery. immediate postpartum period. Minute ventilation can increase up to 300% during contractions and cause maternal PaCO2 to drop below 15 mm Hg. This alkalemia Hepatic Changes can cause hypoventilation between contractions, resulting in hypox- During pregnancy, levels of aspartate aminotransferase, alanine amino- emia. Some evidence indicates that hyperventilation may cause a transferase, lactate dehydrogenase, and alkaline phosphatase increase to decrease in uterine blood flow. However, in animal studies, this finding the upper limits of nonpregnant normal levels. Serum albumin con- has usually been associated with stressful events such as intubation and centration decreases and may result in increased free fractions of highly invasive procedures. In pregnant, laboring human volunteers, hyper- protein-bound drugs. Serum cholinesterase activity decreases by 30% ventilation to a PaCO2 of 20 mm Hg does not appear to harm the fetus. or more during the first or second trimester. Although activity recovers Specifically, the fetus does not develop hypoxia or acidosis as deter- slightly by term, it remains reduced compared with prepregnant values. mined by analysis of a scalp blood sample. However, in the presence Despite these decreases in cholinesterase activity, clinically relevant of a complicated labor with preexisting fetal compromise, the effects prolongation of the duration of action of drugs that depend on cho- could be detrimental.38 linesterase for elimination, such as succinylcholine and remifentanil, is uncommon in women with genotypically normal cholinesterase Nervous System Changes enzymes. Beginning in the first trimester pregnant women have an increased sensitivity to local and general anesthetics.39 The exact mechanism Renal Changes remains unclear, but animal studies have demonstrated a variable reduc- During pregnancy, increased cardiac output leads to increased renal tion in the minimum alveolar concentration (MAC) of inhalation plasma flow and increased glomerular filtration rate (GFR). Creatinine agents in rabbits chronically exposed to progesterone.40 In pregnant clearance rises to 140 to 160 mL/min. As a result, the level of blood rats, the effects of endorphins on pain thresholds are also demonstrated, urea nitrogen decreases to approximately 8 mg/dL and creatinine which may also be an important factor.41 Additionally, research and decreases to approximately 0.5 mg/dL. Urinary excretion of glucose is clinical experience have demonstrated an increase in the sensitivity of common in the absence of disease and is attributable to increased GFR nerves to local anesthetics during pregnancy.42 Mechanical changes and reduced renal absorption. The urinary excretion of protein in within the epidural space may also play a role in the increased block normal pregnancy is slightly elevated toward the upper limits of normal. height seen in pregnancy.43 Epidural veins become engorged as a result The gravid uterus may occasionally produce mechanical obstruction of of increased intraabdominal pressure and cause a decrease in the volume a ureter. A summary of the physiologic changes in pregnancy at term of both the epidural and subarachnoid spaces. appears in Box 51.1 and Table 51.1. Gastrointestinal Changes The parturient is at increased risk for regurgitation and aspiration of TA B L E 5 1.1 gastric contents because of anatomic and physiologic changes associated with pregnancy. A significant number of pregnant women, and women Summary of the Physiologic Changes in immediately postpartum, have gastric volumes in excess of 25 mL and Pregnancy at Term gastric pH below 2.5.44 Ultrasound has demonstrated solid food in the stomach of almost two-thirds of women in whom neuraxial analgesia Parameter Change Amount had been instituted and in the stomach of more than 40% of laboring women who had not eaten in 12 to 24 hours.45 Increased levels of Heart rate ↑ 20%–30% Stroke volume ↑ 20%–50% gastrin during pregnancy result in greater gastric volume and lower pH. Cardiac output ↑ 40% Upward displacement of the stomach by the gravid uterus may result Systemic vascular resistance ↓ 20% in mechanical obstruction to outflow through the pylorus, delayed Total blood volume ↑ 25%–40% gastric emptying, and increased intragastric pressure. Elevated levels of Plasma volume ↑ 40%–50% progesterone, a smooth-muscle relaxant, also decrease gastric motility Red blood cell volume ↑ 20% and cause a reduction in lower-esophageal sphincter tone. This explains Coagulation factors ↑↑ the heartburn frequently experienced by pregnant women. Gastro- Platelets No change or ↓ intestinal changes do not completely normalize until several weeks Minute ventilation ↑ 50% postpartum. Tidal volume ↑ 40% The onset of labor is accompanied by a further reduction in the rate Respiratory rate ↑ of gastric emptying in part due to pain and the use of parenteral Functional residual capacity ↓ 20% opioids. Practice guidelines (2007) allow for the ingestion of a  C H A PT E R 51 Obstetric Anesthesia 1067 Umbilical vein BOX 51.1 Umbilical arteries Umbilical cord Embryonic veins and artery Key Points Regarding Physiologic Changes in Pregnancy Chorion (embryonic part Cardiac output increases mostly because of an increase in stroke volume and, to of placenta) a lesser extent, an increase in heart rate. Blood volume is markedly increased and prepares the parturient for the blood Villus loss associated with delivery. Placenta Intervillous space Plasma volume is increased to a greater extent than red blood cell volume, (embryonic part resulting in a dilutional anemia. of placenta) Minute ventilation increases 45%, and this is due mostly to an increase in tidal volume. Decidua basalis Oxygen consumption is markedly increased; carbon dioxide production is similarly (maternal part Uterine wall of placenta) increased. Pregnant women have an increased sensitivity to local anesthetics and a decreased minimum alveolar concentration (MAC) for all general anesthetics. Platelet count remains stable or decreases slightly; coagulation factors and Maternal veins and fibrinogen are increased, resulting in a hypercoagulable state in pregnancy. arteries Aortocaval compression results in profound hypotension and can be relieved by FIGURE 51.2 Cross section of the uteroplacental interface and the maternal left uterine displacement. and fetal blood supply. All pregnant women are at increased risk of aspiration because of the anatomic and physiologic changes to the gastrointestinal system and should be considered to have a full stomach after week 12 of gestation. Pregnancy and labor are associated with significant airway changes that can despite somewhat reduced blood pressure; this may be a function of result in a difficult intubation. This highlights the importance of a comprehensive altered uterine blood flow, altered fetal O2 requirements, or both. airway evaluation prior to general anesthesia. Placental Transfer and Fetal Effects of Drugs Placental transfer of free (non–protein-bound) drug is dependent on the magnitude of the concentration gradient, the molecular weight, lipid solubility, and state of ionization. Drugs with molecular weights Uterine Blood Flow greater than 1000 Daltons (Da) cross the placenta poorly, whereas The uterus undergoes tremendous changes during pregnancy. The drugs with weights less than 500 Da cross easily. Most drugs that are uterus enlarges, and its blood flow increases to meet both uterine and administered to the parturient are relatively small compounds and are fetal metabolic demands. Uterine blood flow is supplied by two uterine thus able to cross to the placenta readily. However, size is only one of arteries that are thought to be maximally dilated throughout pregnancy. the determinants of permeability. Placental blood flow on the uterine side is supplied via the maternal Transfer of drugs from the maternal circulation to the fetal unit is arcuate, radial, and spiral arteries. The spiral arteries expel blood into determined primarily by diffusion. Factors that favor diffusion include the intervillous space. The maternal venous sinuses receive blood from low molecular weight, high lipid solubility, low degree of ionization, the intervillous space and return it to the general circulation. Uterine and low protein binding. Cell membranes consist primarily of phos- blood flow at term increases to a maximum of 800 mL/min, account- pholipids such that a drug’s degree of lipid solubility favors its passage ing for approximately 10% of maternal cardiac output. Of this, approx- through a cell membrane. Highly lipid-soluble drugs such as fentanyl imately 150 mL/min supplies nutritive flow to the myometrium, and cross readily. On the other hand, ionized drugs are polar and water 100 mL/min flows to the decidua (the lining of the uterus); the remain- soluble, which inhibits diffusion through lipophilic cell membranes. der flows to the intervillous space. Local anesthetics are examples of variably ionized basic compounds, The fetus sends O2-poor blood to the placenta via two umbilical whereby the degree of ionization is dependent on the ambient pH; the arteries. These vessels perfuse capillary networks within placental villi more alkaline the pH, the greater the degree of nonionization. It is the that protrude into the pool of maternal blood. Placental villi are small, nonionized portion that crosses phospholipid membranes readily. Non- fingerlike projections, the purpose of which is to maximize the placental depolarizing muscle relaxants are an example of large, ionized drugs surface area in contact with maternal blood. Each villus contains a that are not affected by ambient pH because of the quaternary groups capillary network that exchanges respiratory gases, nutrients, and wastes in their structure and are inhibited from crossing the placenta. with maternal blood (Fig. 51.2). Both O2 and CO2 diffuse through When a drug enters the fetal circulation, a variety of factors mini- placental tissue readily and are considered to be “perfusion limited,” mize the effects on the fetus. First among them is dilution. Before meaning their transfer to the fetus is limited only by the perfusion of reaching the fetus, a drug is diluted in intervillous blood, absorbed by the placenta, not by the rate of diffusion of the gases. Therefore, the placenta, further diluted in placental blood, and then circulated decreases in maternal uterine artery blood flow or increases in placental to the fetus. Once in the fetus, the drug is distributed within the vascular resistance decrease fetal oxygenation. fetal intravascular volume and redistributed to fetal tissues. Umbilical Autoregulation of intervillous blood flow does not seem to occur. venous blood from the placenta must first pass through the liver on Thus the uteroplacental perfusion is solely dependent on maternal the way to the fetal heart. The resultant increase in maternal hepatic blood pressure. The spiral arteries, however, do constrict in response to enzyme activity likely reduces serum drug levels before entering the α-agonists (e.g., phenylephrine). However, human studies indicate general fetal circulation. Other factors limit the effects of maternally phenylephrine does not appear to be harmful to the fetus when used administered drugs on a fetus. Approximately one-fifth of the fetal at clinically relevant doses.47 Patients with preeclampsia can develop cardiac output returns directly to the placenta because of shunt flow placental insufficiency at systolic pressures greater than 100 mm Hg. through the foramen ovale and ductus arteriosus. This shunted blood Unlike patients who receive neuraxial anesthesia, patients receiving does not circulate, and any drug it contains does not have a systemic inhalation anesthesia seem to maintain adequate placental blood flow, fetal effect. 1068 U N I T V Intraoperative Management The acid-base status of the fetus may affect the accumulation of a range from the presence or absence of social support to physical factors drug. A fetus who has become acidotic will alter the degree of ionization such as fetal presentation. Labor and vaginal delivery can be described of a drug, potentially resulting in “ion trapping” leading to accumula- as a process in which three essential components must work in concert. tion. For a complete discussion of fetal ion trapping see Chapter 10. First, the fetus must be properly positioned and of an appropriate size to ensure passage through the bony pelvis, dilating cervix, and vagina. Labor and Delivery Second, the uterus must contract regularly and effectively. Third, the pelvic outlet itself must be configured in a way that the fetus can pass. Pain in Labor and Delivery A flaw in any of these three elements can result in a difficult, painful, First-stage labor pain is primarily the result of cervical distension, and prolonged labor. A few women are able to tolerate labor and deliv- stretching of the lower uterine segment, and possibly, myometrial ische- ery without significant discomfort; others experience pain in excess of mia. The resultant nonspecific nociceptor visceral stimulation is carried their ability to cope. When women choose labor analgesia, it can make to the central nervous system (CNS) by afferent unmyelinated C fibers the birth process more enjoyable and provide the opportunity to better that enter the cord at the T10, T11, T12, and L1 segments (Table 51.2 control their bodies and environment, thereby allowing them to main- and Fig. 51.3). Thus, the pain of first-stage labor is frequently described tain personal dignity. as nonlocalized aching or cramping. Second-stage labor begins when cervical dilation is complete and the presenting part descends into the Stages of Labor and Delivery pelvis. Compression and stretching of the pelvic musculature and Labor is defined as progressive dilation of the cervix in response to perineum produce pain that is mediated by somatic afferent fibers uterine contractions. Although sporadic and irregular third-trimester carried via the pudendal nerves that enter the spinal cord at the S2, S3, contractions are common, labor is not said to begin until those contrac- and S4 levels. A neuraxial anesthetic that extends from T10 to the sacral tions are regular and result in a change to the cervix. Labor is generally nerve roots can be effective in relieving the pain of both first- and divided into three stages. First-stage cervical changes consist of efface- second-stage labor. ment and dilation. The first stage is further divided into at least the The experience of pain is a highly individualized phenomenon, and latent and active phases. The latent phase is of variable duration and is pain tolerance varies widely among patients. Many factors outside a defined as the period between the onset of labor and the point at which parturient’s control are also related to the perception of pain; these the cervix begins to rapidly change. Active phase usually begins at 2 to 3 cm dilation and is the period during which the cervix undergoes its maximum rate of dilation. TABL E 51.2 The second stage begins at full cervical dilation (10 cm) and ends with delivery of the fetus. The third stage encompasses the delivery of Pain Pathways During Labor the placenta. The length of time it takes to progress through these stages Area Innervation Comments is dependent on parity, effective uterine contractions, the size and type of pelvis, and fetal presentation. The Friedman curve is used to track Uterus and cervix T10 to L1–L2 Pain impulses carried by visceral the normal progress of labor (Fig. 51.4). Cervical dilation normally afferent type C fibers progresses 1 to 1.2 cm/hr during the active phase of labor. When labor Perineum S2-S4 Pain impulses carried by somatic nerve progress no longer follows the normal pattern, it is considered a “dys- fibers; pudendal nerves functional labor,” and may require the use of oxytocin to augment contractions. T9 Intrapartum Fetal Evaluation T10 Intrapartum FHR monitoring is now used routinely in the United T11 States. Although continuous or intermittent FHR monitoring is not a T12 specific predictor of fetal well-being, it is the most readily available method for the assessment of fetal condition. Intermittent monitoring can be performed using a manual stethoscope known as a fetoscope. Uterine L1 wall L2 10 3 4 8 Cervical dilation (cm) 5 6 Accel. S1 phase 4 Phase of max. slope Decel. S2, S3, 2 phase Sec. S4 stage Latent phase Active phase Pudendal 0 nerve Cervix 0 2 4 6 8 10 12 14 Vagina Time (hr) FIGURE 51.4 Curve representing average dilation for nulliparous patients in FIGURE 51.3 Lower thoracic, lumbar, and sacral regions of the spine showing labor. accel., Acceleration; decal., deceleration; sec., second. (Redrawn with spinal cord, nerve roots, and sensory innervation to the uterus, cervix, and modifications from Miller RD, et al. Miller’s Anesthesia. 8th ed. Philadelphia: vagina. Elsevier; 2015:2335.)  C H A PT E R 51 Obstetric Anesthesia 1069 Continuous FHR monitoring is accomplished using either external As stated, uterine arteries are maximally dilated in pregnancy, which Doppler ultrasonography or an internal fetal electrocardiography elec- results in the inability of the uterus to autoregulate blood flow. Decreases trode. The internal method requires ruptured amniotic membranes and in maternal blood pressure and uterine artery blood flow ultimately a partially dilated cervix through which the electrode can be inserted. compromise placental blood flow, resulting in fetal hypoxia and acido- The continuous FHR is recorded in a two-channel format with the sis, which can be manifested as FHR changes. The normal FHR ranges FHR displayed directly above a graphic representation of the uterine from 110 to 160 beats per minute (bpm). An immature fetus has a contractions. In this way, it is possible to relate FHR changes to uterine higher HR. Tachycardia is defined as a HR greater than 160 bpm in a activity. The most commonly used scaling is a paper speed of 3 cm/ term fetus. It can result from fetal asphyxia, fetal arrhythmias, maternal min on the horizontal axis and 30 beats/min per centimeter of paper fever, chorioamnionitis, and maternally administered drugs such as for the FHR (vertical axis).48 In this manner each sequential vertical terbutaline and atropine. Ephedrine can increase both FHR and vari- line is 10 seconds apart and every sixth vertical line (bold) represents ability if given to the parturient in large enough doses.53 Fetal brady- a period of 1 minute. cardia is present when the FHR is less than 110 bpm. Bradycardia can Uterine contractions are monitored either externally via a tocody- be caused by maternally administered drugs, hypoxia (fetal or mater- namometer or internally with an intrauterine pressure catheter attached nal), or fetal head or cord compression. to a transducer placed between the fetus and uterine wall. Data obtained from external contraction monitoring is limited to temporal elements Variability such as contraction duration and interval, and cannot be used to esti- Fetal heart rate variability is thought to be the single best indicator of mate contraction strength. External monitoring is subject to a great fetal wellbeing because it likely indicates adequate fetal oxygen reserve. deal of artifact from maternal movement and errors in transducer place- Variability is the result of interaction between the fetal sympathetic and ment. Internal contraction monitoring is more reliable and allows for parasympathetic autonomic nervous systems. Baseline variability is precise determination of intrauterine pressure and, hence, contraction defined as fluctuations in the FHR of two cycles per minute or greater strength. Its use is also predicated on ruptured amniotic membranes that are irregular in amplitude and frequency.52 It is quantified by and a partially dilated cervix. Internal pressure monitoring is com- measuring the amplitude from peak to trough in beats per minute. monly seen with high-risk parturients or those in whom labor augmen- Variability is described as absent, minimal (less than 5 bpm), moderate tation with oxytocin is being used. Most commercially available (6–25 bpm), and marked (greater than 25 bpm). In general, baseline intrauterine pressure catheter systems also allow for the infusion of FHR variability increases with advancing gestational age. Variability fluids into the amniotic space (amnioinfusion), which is thought to represents an intact CNS and normal cardiac function. Hypoxia causes reduce the risk of fetal aspiration of meconium-stained amniotic fluid.49 fetal CNS depression, which results in decreased variability. Other Fetal pulse oximetry has been introduced but is largely investiga- causes of decreased variability include fetal sleep, acidosis, anencephaly, tional at this time. A wide range in normal values has been reported, drugs (CNS depressants or autonomic agents), and defects of the fetal and use of oximetry has not been shown to decrease overall cesarean cardiac conduction system. For example, administration of opioids to section rates, making its ultimate benefit questionable.50 the mother, can decreases FHR variability for up to 30 minutes.54 Information about the baseline status of a fetus should be obtained Maternal magnesium sulfate administration may also attenuate FHR during the preanesthetic assessment. The FHR can also reveal informa- variability.55 Variability refers to the baseline FHR and, as such, does tion about the fetal response to anesthetic interventions. This informa- not include accelerations or decelerations. Beat-to-beat variability can tion is useful before neuraxial anesthetic placement, for assisted or be accurately assessed only by direct FHR monitoring with a fetal scalp operative delivery, and for nonobstetric surgery during pregnancy. electrode. Examples of absent and moderate FHR variability are shown Current American Association of Nurse Anesthetists (AANA) practice in Fig. 51.5. guidelines state that the Certified Registered Nurse Anesthetist (CRNA) shall be aware of the fetal status prior to each anesthetic intervention Accelerations and Decelerations and that the fetal status be monitored and documented in the patient’s An acceleration is an abrupt increase in the FHR above baseline. Accel- record.51 erations occur in response to fetal movement and indicate adequate oxygenation. A FHR pattern is said to be reactive when there are two Changes in Fetal Heart Rate or more accelerations in a 20-minute period. Decelerations are classi- The individual components of the FHR pattern do not occur alone and fied as early, variable, or late. changes generally evolve over time. Therefore, a full description of a Early Decelerations. Early decelerations occur in concert with FHR tracing requires a qualitative and quantitative description of base- uterine contractions. The deceleration begins when the contraction line rate, variability, presence of accelerations or decelerations, and begins, and it returns to baseline when the contraction ends. An early trends over time.52 In the presence of a normally functioning uteropla- deceleration occurs with each uterine contraction and has the same cental unit, fetal oxygenation is limited primarily by uterine blood flow. appearance from one contraction to the next. Compression of the fetal 240 210 180 150 120 90 60 30 100 75 12 50 8 25 4 A 0 0 FIGURE 51.5 A, Normal intrapartum fetal heart rate (FHR) tracing. The infant had Apgar scores of 8 and 8 at 1 and 5 minutes, respectively. 1070 U N I T V Intraoperative Management 240 210 180 150 120 90 60 30 100 12 75 50 8 25 4 B 0 0 240 210 180 150 120 90 60 30 100 12 75 50 8 25 4 C 0 0 240 210 180 150 120 90 60 30 100 75 12 50 8 25 4 D 0 0 240 210 180 150 120 90 60 30 100 75 12 50 8 25 4 E 0 0 FIGURE 51.5, cont’d B, Absence of variability in a FHR tracing. Placental abruption was noted at cesarean delivery. The infant had an umbilical arterial blood pH of 6.75 and Apgar scores of 1 and 4, respectively. C, Early FHR decelerations. After a normal spontaneous vaginal delivery, the infant had Apgar scores of 8 and 8, respectively. D, Late FHR decelerations. The amniotic fluid surrounding this fetus was meconium stained. Despite the late FHR decelerations, the variability remained acceptable. The infant was delivered by cesarean delivery and had an umbilical venous blood pH of 7.30. Apgar scores were 9 and 9, respectively. E, Variable FHR decelerations. A tight nuchal cord was noted at low-forceps vaginal delivery. The infant had Apgar scores of 6 and 9, respectively. Numerical scales: Left upper panel margin, FHR in beats per minute; left lower panel margin, uterine pressure in mm Hg; right lower panel margin, uterine pressure in kilopascal (kPa) (From Chestnut DH, et al. Chestnut’s Obstetric Anesthesia, 5th ed. St. Louis: Elsevier; 2014:153.)  C H A PT E R 51 Obstetric Anesthesia 1071 head resulting in vagal stimulation is thought to be the cause of early of a compromised fetus in utero. These interventions include changing decelerations. Early decelerations have the following characteristics: maternal position, rapid infusion of intravenous (IV) fluids, discon- Occur with each uterine contraction tinuing oxytocin, IV pressor support in the presence of maternal hypo- Start and end with the contraction tension, the use of tocolytic agents and maternal oxygen administration Gradually decrease in rate and then end in a return to baseline to correct maternal hypoxia.59 Are uniform in appearance Are associated with a mild decrease in FHR (20 bpm or less) Analgesia for Labor and Vaginal Delivery Are accompanied by a loss in beat-to-beat variability during the deceleration Intravenous Analgesia in the Parturient Variable Decelerations. Variable decelerations are the most fre- Although less effective than neuraxial analgesia, systemic medications quent FHR changes related to labor and are described as a sudden can be used for labor pain relief when neuraxial analgesia is unavail- decrease in FHR that occurs irrespective of uterine contractions.56 Vari- able, refused, or contraindicated. However, the use of systemic medi- able decelerations are abrupt in both onset and recovery and vary in cations presents several disadvantages. The pain relief they afford is occurrence, onset, depth, duration, and appearance. Beat-to-beat vari- often inadequate, and both fetal and maternal respiratory depression, ability is normally present during the decelerations. Variable decelera- nausea, vomiting, and decreased lower esophageal sphincter tone may tions are thought to be caused by a baroreflex-mediated response to result. Controlled, randomized trials investigating neuraxial versus IV umbilical cord compression and are often considered to be a non- analgesia in labor are difficult to design and execute due to high ominous sign for neonatal outcome. Alternatively, if the fetus is com- protocol failure rates. In a systematic review of studies involving over promised, then the recovery phase of the deceleration may be delayed. 9600 women that compared neuraxial analgesia to opiates, neuraxial Variable decelerations have the following characteristics: techniques offered better pain relief and a reduced risk of fetal aci- Vary in appearance, duration, depth, and shape dosis. On the other hand, neuraxial analgesia was associated with a Demonstrate abrupt onset and recovery longer second stage labor and an increased incidence of assisted vaginal Maintain beat-to-beat variability with the deceleration delivery.60 Late Decelerations. Late decelerations begin late in the contrac- Because opioids are lipid soluble and relatively small (less than tion and represent uteroplacental insufficiency. The lowest point of the 500 Da), they rapidly cross the placenta to gain access to the fetal deceleration occurs after the peak of the contraction. Like early decel- circulation and may result in fetal depression. A neonate’s blood-brain erations, they are smooth in both onset and recovery. Late decelerations barrier is less developed and its ability to metabolize narcotics is less are normally repetitive. Beat-to-beat variability may or may not be mature when compared to an adult. Opioids can be administered by present during the deceleration, depending on the degree of fetal IV bolus or patient-controlled analgesia (PCA). The advantage of PCA hypoxia and myocardial depression. Late decelerations are nonreassur- is it allows the parturient greater control over drug dosing and has ing and should be investigated for potential causes.57 Fig. 51.5 shows resulted in greater patient satisfaction. However, in one study PCA was a FHT with late decelerations and minimal variability. compared with nurse-administered boluses, and there was no difference Late decelerations have the following characteristics: in pain scores or maternal and fetal side effects.61 The use of parenteral Occur with each uterine contraction narcotics in early labor has declined as the practice of obstetric anes- Low point of the deceleration after the peak of the contraction thesia has evolved to include those patients in early labor as acceptable Gradually decrease in rate and end in a return to baseline candidates for neuraxial analgesia. Are uniform in appearance Vary in depth according to the strength of the uterine contraction Meperidine May or may not be accompanied by beat-to-beat variability Meperidine is administered as a parenteral opioid for labor analgesia. A typical dose is 50 to 100 mg intramuscular (IM) and can be repeated Categories for the Evaluation of Fetal Heart Rate every 4 hours. Meperidine crosses the placenta easily and has been The American College of Obstetrics and Gynecologists recommends a recovered from the fetus within 2 minutes of IV administration. The three-tiered system to evaluate FHR tracings. Category I FHT are maternal half-life of meperidine is 2.5–3 hours. Like all opioids, it can considered normal and include normal baseline HR and moderate cause neonatal respiratory depression, although less so than morphine. variability with absent variable and late decelerations. Category I trac- Because of differences in pH and protein binding, the level of meperi- ings predict normal acid-base balance. Category II FHT include all dine in the fetus is likely to be higher than the maternal blood level. tracings that are not classified as category I or III. Category II tracings Normeperidine is an active metabolite of meperidine with an elimina- do not predict abnormal acid-base status and warrant continued obser- tion half-life of 30 hours. Normeperidine remains in the neonate for vation. Category III tracings include those with fetal bradycardia or several days after delivery and may lead to depression of neonatal absent variability accompanied by recurrent late or variable decelera- behavioral assessment scores. Both meperidine and normeperidine can tions. Category III tracings predict abnormal acid-base status and be antagonized by naloxone. require prompt intervention.58 The overall analgesic effect of meperidine in labor is marginal. In a random trial of 102 women acetaminophen 1000 mg IV was found to Anesthetic Considerations in the Presence of provide similar analgesia when compared to IV meperidine 50 mg. The Nonreassuring FHT incidence of maternal side effects was 64% in the meperidine group If the FHR tracing recorded during the preanesthesia assessment sug- and 0 in the acetaminophen group.62 gests hypoxia, caution should be used in making the decision to proceed with neuraxial analgesia. Careful consideration must be given to the Fentanyl severity of the fetal compromise and to the possibility that it may be The pharmacologic profile of fentanyl, that is, its high potency and worsened by anesthetic intervention. However, the obstetrical team short duration of action, make it a reasonable choice for labor. It is may request a neuraxial anesthetic for a patient with a nonreassuring highly lipid soluble and protein bound and can be detected in fetal FHT in anticipation of a possible urgent and unplanned operative circulation after 1 minute of IV administration. Depressant effects, vaginal delivery or cesarean section. including a reduction in beat-to-beat variability, have been seen.63 Intrauterine resuscitation describes interventions that can be used Fentanyl can be administered IM, IV or most commonly by PCA. by the obstetrical staff and anesthesia provider to improve the condition Dosages used for labor analgesia range from 25 to 100 mcg IV in 1072 U N I T V Intraoperative Management hourly increments64 or PCA bolus 25–50 mcg with a lockout interval hallucinations limit the effectiveness of ketamine for routine use in of 3 to 6 minutes and a 4-hour limit of 1–1.5 mg.65 obstetrics. Uterine arterial blood flow does not decrease after ketamine administration. Ketamine is lipid soluble and crosses the placenta easily. Morphine Neonatal depression has not been demonstrated after maternal ket- Morphine has been used in early labor to provide analgesia and sedation amine doses of up to 1 mg/kg. Doses larger than 1 mg/kg may result but is no longer widely accepted due to high rates of maternal seda- in neonatal respiratory depression, muscular hypertonicity, and lower tion, neonatal depression, and an undesirable prolonged duration.66 Apgar scores.77 Morphine crosses the placenta easily and, in animal studies, crosses the fetal blood-brain barrier more readily than in adults.67 Nonpharmacologic Methods A multitude of nonpharmacologic techniques are currently in use but, Butorphanol and Nalbuphine as Wong has stated, the effectiveness of these techniques generally lack Butorphanol and nalbuphine are opioid agonist-antagonists that are rigorous scientific study and conclusions regarding their efficacy cannot associated with a “ceiling effect” whereby incrementally higher doses be made.78 Nonpharmacologic alternatives include hydrotherapy, hyp- do not result in increasing respiratory depression. They may also result notherapy, massage, movement, and positioning. Although maternal in less nausea and vomiting than pure opioids. Agonist-antagonists satisfaction is without a doubt influenced by the degree of pain endured, have typically been used in first-stage latent-phase labor to provide it is influenced to a greater extent by whether the actual birth event sedation and a period of maternal rest. A typical butorphanol dose for met the mother’s personal expectations.79 The role of an obstetric anes- labor is 1 to 2 mg IV or IM. Butorphanol is five times as potent as thesia provider is to support laboring women such that they can make morphine and has a half-life of 3 hours. Unlike morphine, butorphanol informed choices that meet their individual expectations, while at the increases pulmonary artery pressure and myocardial work.64 Butorpha- same time ensuring the safety of both the mother and infant.80 nol has no active metabolites. The nalbuphine dose in labor is 5 to 10 mg IV, IM, or subcutaneous (SC) and is equivalent to morphine Neuraxial Analgesia for Labor and Vaginal Delivery 10 mg. Nalbuphine results in a greater reduction in FHR variability Neuraxial analgesia is currently the best method of pain relief for labor compared with meperidine.68 Because these compounds possess signifi- and delivery. When properly placed and dosed, a neuraxial anesthetic cant antagonist properties, their use in opioid-dependent patients is the only modality available that can provide complete relief from should be avoided. labor pain with minimal depression of the parturient or fetus. Common neuraxial anesthetics include epidural, combined spinal-epidural Remifentanil (CSE), and spinal techniques. The American College of Obstetricians Remifentanil is an ultra–short-acting opioid receptor agonist. It is and Gynecologists (ACOG) has stated that, absent medical contrain- rapidly hydrolyzed by plasma and tissue esterases to an inactive metabo- dications, maternal request is sufficient reason to provide labor analge- lite. The context-sensitive half-life remains short (3.2 min) regardless sia.81 However, labor is a prerequisite for neuraxial analgesia and is of prolonged administration times.69 This unique pharmacokinetic defined as regular uterine contractions that result in cervical dilation profile makes it suitable for PCA use in labor. It crosses the placenta and effacement. readily but is similarly rapidly redistributed and metabolized by the Early initiation of neuraxial anesthesia may be considered for anes- fetus.70 Douma et al.71 compared remifentanil, fentanyl, and meperi- thetic or obstetrical complications.82 These patients include those with dine PCA’s in labor and found that remifentanil PCA was associated morbid obesity, severe scoliosis or known difficult airway. Obstetrical with the greatest reduction in pain scores; however, the relief was indications for early placement include multiple gestation pregnancies described as mild to moderate. The same group has found that remi- and severe preeclampsia. Early initiation allows for better cooperation fentanil PCA is less effective when compared with standard epidural with positioning during placement and adequate time to confirm labor analgesia.72 A PCA bolus of 0.25 mcg/kg with a lockout interval proper block function. If cesarean section becomes urgently necessary of 2 minutes and a background infusion of 0.025–0.05 mcg/kg per the indwelling epidural catheter can be used for anesthesia, reducing min is common.63,73,74 Use of remifentanyl during labor is a relatively the need for a general anesthetic. new technique that appears to have an acceptable safety profile. Absolute contraindications to neuraxial anesthesia include patient However, its use mandates careful monitoring due to the potential for refusal, inability to cooperate, uncorrected severe hypovolemia, uncor- adverse maternal events.75 rected coagulopathy or pharmacologic anticoagulation, elevated intra- cranial pressure secondary to a mass, or infection at the site of insertion. Ketamine There are many relative contraindications such as stable preexisting Ketamine is a derivative of phencyclidine and an N-methyl-D-aspartate CNS disease, chronic severe headaches or back pain, untreated bacte- (NMDA) receptor antagonist that produces a dissociative anesthetic remia, and severe stenotic valvular lesions. Patients with these and other effect. It produces profound analgesia at subhypnotic doses. Its use as preexisting conditions should undergo careful preanesthetic evaluation a labor analgesic is limited, however, by its short duration of action and and consultation that takes into consideration the risks and benefits of high incidence of amnesia. Low-dose ketamine preserves airway reflexes the proposed procedure. With recognition and proper optimization of while causing somatic analgesia, sedation and occasionally a dreamlike these preexisting conditions, patients can often safely receive neuraxial state. Satisfactory labor analgesia has been described by using a continu- anesthesia. ous infusion of ketamine at 0.2 mg/kg per hr.76 Small intermittent IV Historically, the platelet count at which a neuraxial anesthetic could doses of ketamine can be used to supplement a suboptimal neuraxial be safely administered has been considered to be greater than 100,000 anesthetic when replacement or adjustment is not possible. Ketamine × 109/L. However, contemporary practice surveys indicate that most doses of 0.2 to 0.5 mg/kg produce rapid analgesia with a dose- practitioners are comfortable providing a regional anesthetic with the dependent duration of action of approximately 5 to 15 minutes. platelet count as low as 80,000 × 109/L.83 Careful consideration of Ketamine has sympathomimetic properties that result in increased thrombocytopenic parturients should include a thorough history and HR, blood pressure and cardiac output which may be desirable in a physical examination for signs and symptoms of coagulopathy with an hypovolemic parturient. The hyperdynamic effects, however, may be emphasis on previously administered medications including herbal problematic in the parturient with hypertension or preeclampsia. It is supplements. A review of laboratory findings should include all prior the agent of choice for induction in the patient with acute asthma platelet counts (to establish any trends) as well as other relevant labora- requiring urgent cesarean section. The emergence delirium and tory findings.  C H A PT E R 51 Obstetric Anesthesia 1073 Patients receiving prenatal and/or intrapartum antithrombotic or A second confounding issue with regard to informed consent is thrombolytic therapy require special consideration. Normal pregnancy caring for the pregnant minor. Competence describes the legal author- is associated with a hypercoagulable state that, alone or in combination ity to make a decision and is recognized in the United States as occur- with certain pathologic conditions, can predispose parturients to ring at 18 years of age. However, various state laws make exceptions thromboembolic events. Conditions that may require anticoagulation (emancipation) for minors who are free of parental care, control, and therapy in the obstetric population include deep vein thrombosis, custody. This may occur when a minor is married, a member of the antiphospholipid antibody syndrome, factor V Leiden mutations, and military, or a high school graduate. Becoming pregnant does not neces- protein S and C deficiencies. The concern with neuraxial anesthetics is sarily constitute emancipation. With respect to obstetric anesthesia, it the potentially catastrophic complication of epidural hematoma. Epi- is best to include the legal guardian of a minor patient in the informed dural hematoma results from uncontrolled bleeding in the nondistend- consent process. Anesthesia providers must have a working knowledge able epidural space, which, if untreated, can cause ischemic injury to of various local, state, and federal guidelines, as well as their institu- the cord resulting in persistent neurologic dysfunction. Epidural hema- tions’ applicable policies and procedures. toma is a rare complication that is estimated to occur in less than 1 in Informed consent should include those risks that are reasonably 150,000 epidural and 1 in 220,000 spinal anesthetics.84 The American foreseeable, but it does not have to include every possible risk. Material Society of Regional Anesthesia and Pain Medicine has published con- risks are those that a reasonable person would want to be made aware sensus statements in an effort to guide practitioners in the management of before deciding on a recommended therapy.90 They include risks that of these complex patients.85 commonly occur, as well as those risks that are rare but may result in severe morbidity and mortality. There exists no distinct and inclusive Preprocedure Preparation list of potential risks and complications; however, several surveys of History and Physical. It is optimal to complete the history and practitioners demonstrate some consensus regarding what should be physical for every patient whose plan includes a neuraxial anesthetic disclosed.91,92 early in the labor process, often before the regional anesthetic is indi- Drugs and Equipment. It is the provider’s responsibility to ensure cated. An early history and physical allows for recognition of potential that the anesthesia equipment available in the obstetric area is consis- problems and provides time for additional work-up if necessary. Valu- tent with other anesthetizing locations in the facility.93 An oxygen able time can also be saved with a previously completed history and source, a positive pressure apparatus capable of delivering 100% physical if an anesthetic intervention becomes urgently necessary. oxygen, and an appropriate suction assembly must be present at every The history should include questions regarding pertinent systemic location where neuraxial anesthesia is provided. Emergency airway diseases, as well as the obstetric history and course of the current preg- supplies such as face masks, oral and nasal airways, laryngeal mask nancy. Particular attention should be directed toward problems with airways, laryngoscope handle/blades, endotracheal tubes and stylet, and previous pregnancies, deliveries, or anesthetics. A review of the patient’s an Eschmann intubating stylet need to be immediately available. medical record to include previous analgesic interventions, such as IV Carbon dioxide monitoring can be accomplished with a qualitative narcotics, should be performed. The physical examination should carbon dioxide detector. Emergency drugs include an induction drug include an inspection of the back, heart, and lungs, as well as an airway (for terminating a local anesthetic–induced seizure), succinylcholine, examination. It has been demonstrated that the airway examination ephedrine, epinephrine, phenylephrine, naloxone, atropine, calcium can change during labor.32 Therefore a repeat examination is indicated chloride (for treating magnesium sulfate overdose), and sodium bicar- immediately before administering anesthesia in labor regardless of prior bonate. The equipment should be checked prior to each anesthetic. examination results. As services are often provided at each bedside in a labor and delivery The maternal vital signs and fetal condition should be documented. unit, an “epidural” cart that contains routine supplies, as well as emer- This includes dilation, effacement, station, FHR, membrane status, and gency equipment and drugs, is preferred. A standard “crash cart” with variability. The obstetric history includes gestational age and the par- a defibrillator and a supply of intralipid 20% to treat intravascular turient’s gravidity (i.e., number of conceptions) and parity (i.e., number injection of local anesthetics should be readily available (see Chapter of live births). Obtaining a platelet count before the institution of 10 for a complete discussion of local anesthetic toxicity). Noninvasive neuraxial anesthesia in healthy patients with uncomplicated pregnan- automatic blood pressure monitoring and pulse oximetry are required cies has not been shown to reduce overall complication rates.86 However, in each labor and delivery room. In addition to proper drugs and in the presence of hypertensive disorders or coagulopathies with anes- equipment, a knowledgeable assistant is essential for the safe insertion thetic implications, the elective anesthetic should generally be delayed of a neuraxial anesthetic. An obstetric nurse who can assist with FHR until appropriate laboratory results (e.g., coagulation studies, platelet monitoring and positioning is desirable. IV access is required prior to count) are available. In the absence of comprehensive prenatal care, initiating a neuraxial anesthetic and should be maintained throughout obtaining baseline laboratory values may be indicated due to the pos- its duration. IV insertion sites should take into consideration the vigor- sibility of undiagnosed medical conditions. ous arm movement that can occur during second-stage labor. The dilute Informed Consent. Those who provide anesthesia for laboring concentrations of local anesthetic commonly used in labor analgesia women have an obligation to obtain informed consent prior to the today result in significantly less sympathetic blockade and hypotension procedure. The elements of informed consent include a description of compared with traditional high-dose blocks.94 As a result, the admin- the procedure, the risks, benefits, potential complications, and alterna- istration of a fixed volume of IV fluid is not required before neuraxial tives. However, there are several issues unique to obstetric anesthesia analgesia is initiated.86 that can complicate informed consent. The first involves capacity and whether a woman in great pain and distress has the ability to under- Local Anesthetics stand and reason.87 There is evidence to indicate that women retain the Bupivacaine. Bupivacaine is an amino-amide local anesthetic with ability to assimilate information and make good decisions despite the a relatively long duration of action and the ability to produce a dif- pain of labor. Jackson et al.88 found that most women in active labor ferential block whereby sensory fibers are blocked more readily than were able to voluntarily consent and that their ability to understand motor fibers. When compared with lidocaine, there is less tachyphy- was not affected by pain, premedication, or duration of labor. Affleck laxis during long-term administration. In lower doses, it has limited et al.89 determined that recall of risks by parturients is similar to the placental transfer, as well as minimal neonatal effects. These charac- recall of risks by other patients and does not appear to be affected by teristics have made it the most commonly used local anesthetic in parity or the reported level of pain. obstetric anesthesia. Bupivacaine is marketed as a racemic mixture of 1074 U N I T V Intraoperative Management the S− and R+ isomers. Refractory cardiac arrest has been associated ropivacaine and bupivacaine. It is possible that ropivacaine is less car- with high concentrations of bupivacaine inadvertently administered diotoxic than bupivacaine at high doses, but at the more dilute con- intravascularly. It is more difficult to resuscitate patients who experi- centrations clinically relevant to obstetric anesthesia, this difference is ence bupivacaine-induced cardiac arrest when compared with other less likely to be important.106 Synthetic opioids increase the potency local anesthetics. The exact cause remains unclear; however, evidence of amide local anesthetics. Significantly longer labor analgesia can be suggests that toxicity occurs primarily at sodium channels, and there achieved with ropivacaine–sufentanil and levobupivacaine–sufentanil, may be qualitative differences in cardiotoxicity caused by low- and and ropivacaine is associated with comparatively less motor blockade. high-potency local anesthetics.95 The epidural administration of 0.75% Labor duration after epidural analgesia has been found to be shorter bupivacaine has been prohibited for use in obstetrical anesthesia due when bupivacaine–sufentanil combinations are used and there is a low to its increased potential for toxicity. Clinical techniques to reduce the incidence of instrumental delivery.107 incidence of cardiac toxicity include fractional dosing (less than 5 mL), frequent aspiration for blood during injection, and use of a test dose. Neuraxial Anesthetic Placement The use of low concentrations for continuous epidural infusion and Strict aseptic technique must always be used during the insertion of low doses (less than 15 mg) for spinal anesthesia are not associated neuraxial anesthetics. This includes preprocedural hand hygiene, with cardiac toxicity.96 wearing of sterile gloves, cap and mask (new for each case), removal of Lidocaine. Lidocaine is an amino-amide local anesthetic that has rings and watches, and sterile draping of the back. A newly opened been used extensively in obstetric anesthesia for decades. It has a rapid single-use container of preparation solution should be used for skin onset and an intermediate duration of action. When given in the epi- surface disinfection. Chlorhexidine gluconate in an alcohol base should dural space, it produces a dense motor block when combined with be considered the antiseptic of choice. When compared with other epinephrine, which makes it well suited for epidural anesthesia for commonly used antiseptic solutions, it is effective against a wider array cesarean section. This dense motor block, however, makes it less desir- of pathogens, has a greater degree of potency, shows a more rapid onset able for labor analgesia. Lidocaine administered in the subarachnoid of action with an extended duration of effect, and has fewer and less space is potentially neurotoxic.97 In 1991 four cases of cauda equina severe localized skin reactions.108 In addition, the efficacy of chlorhexi- syndrome were reported after the use of lidocaine in small-diameter dine gluconate is maintained in the presence of protein-based organic catheters designed for continuous spinal anesthesia. In all four cases, compounds such as blood and other body fluids. A sterile occlusive there was initial evidence of a focal sensory block, and to achieve dressing should then be placed over the catheter insertion site. adequate analgesia, a dose of local anesthetic was given that was greater Baseline blood pressure and pulse oximetry should be obtained than that usually administered with a single-injection technique. The preprocedure then measured at regular intervals once neuraxial dosing authors postulated, and subsequent data substantiated, that the com- is begun. Typically, blood pressure is monitored every 2 minutes for 15 bination of maldistribution and a relatively high dose of local anesthetic minutes, then every 5 minutes for another 15 minutes, thereby enabling resulted in neuronal injury.98 Transient neurologic symptoms consisting the early detection and treatment of neuraxial anesthesia–induced primarily of lower extremity pain also have been attributed to 5% hypotension. It is preferable to remain in the anesthetizing location hyperbaric lidocaine99 (see Chapter 10). For this reason, obstetric anes- during this time to monitor the proper onset and progression of the thesia providers do not use lidocaine in the subarachnoid space. neuraxial block and to enable prompt recognition and treatment of 2-Chloroprocaine. Chloroprocaine is an amino-ester local anes- potentially catastrophic complications such as with total spinal anes- thetic that is rapidly metabolized by ester hydrolysis. As such, its thetic or local anesthetic systemic toxicity. potential to produce cardiac and CNS toxicity after inadvertent IV The sitting position is often easiest for the laboring parturient to administration is low. It has a rapid onset and brief duration of action. assume and maintain and normally offers the anesthetist the maximum Chloroprocaine is used primarily in obstetric anesthesia to rapidly interspace width. On the other hand, the use of the lateral position has produce a surgical block in the presence of a preexisting epidural been found to reduce the incidence of intravascular catheter place- in the case of an emergency cesarean section. However, the epidu- ment.109 The lateral position can make identification of the midline ral administration of chloroprocaine has been shown to reduce the more difficult due to skin shifting and requires meticulous attention to effectiveness of subsequently administered epidural morphine, pos- shoulder position. The upper shoulder naturally rotates anteriorly sibly by antagonism at the opioid receptors.100 Neurotoxicity has been resulting in a concomitant rotation of the vertebral column that, if not reported after inadvertent spinal administration of large doses intended corrected, can result in anatomical distortion. The lateral position may for the epidural space but was likely a result of the low pH and/or the also help limit movement if the patient is having a difficult time with preservatives.101 positioning. Occasionally, obstetrical emergencies such as fetal head Ropivacaine. Ropivacaine is an amino-amide local anesthetic that entrapment, prolapsed umbilical cord, and footling breech presentation is produced as the pure levorotatory enantiomer and is structurally may preclude the use of the sitting position for neuraxial anesthesia similar to bupivacaine. It has a propyl group attached to the aromatic placement. ring as opposed to bupivacaine, which has a butyl group attached to It has long been observed that the failure rate associated with epi- the ring. Ropivacaine was developed largely to address the cardiac toxic- dural anesthetics is greater in the obese patient. At the same time, ity associated with bupivacaine and when equivalent masses of drug are neuraxial anesthetics are particularly desirable in obese parturients compared, appears to be less toxic. Both levobupivacaine and ropiva- because of their increased cesarean section rates and concerns surround- caine have a clinical profile similar to that of bupivacaine, and the ing potential airway difficulty. The distance between the skin and the minimal differences observed are mainly related to different anesthetic epidural space increases as the body mass index (BMI) increases, which potency, with bupivacaine > levobupivacaine > ropivacaine.102,103 When can make epidural insertion more difficult.110 However, morbid obesity, clinical dosing is adjusted for the decreased potency the toxicity advan- when considered alone, is not a predictor of difficult block placement. tage of ropivacaine is diminished.104 Although equipotent doses of In a study of 427 morbidly obese patients receiving neuraxial anesthet- ropivacaine have been shown to produce less motor block when com- ics, four variables were evaluated to determine whether they could pared with bupivacaine, it has not been shown to influence the mode predict difficult insertion. The authors found that only the ability to of delivery, the duration of labor, or neonatal outcome.105 palpate bony landmarks and the patient’s ability to flex her back were The potential cardiotoxicity of ropivacaine are limited to animal significant predictors of difficulty, whereas BMI and the practitioners models and in vitro preparations, and they have found either an advan- experience level were not predictive.111 Prelabor consultation for women tage for ropivacaine in regard to toxicity or no difference between with a BMI greater than 50 kg/m2 should be considered where an  C H A PT E R 51 Obstetric Anesthesia 1075 anesthesia plan can be made after a focused history and physical exam. to reliably increase HR in nonpregnant patients. The maternal HR Neuraxial anesthesia for the morbidly obese parturient should include should be monitored continuously using either pulse oximetry or elec- a thorough check of equipment for appropriate size, consideration of trocardiogram (ECG) during administration of the test dose to detect early placement and anticipation of difficulty with prolonged labor and these HR changes. In the laboring parturient, increases in HR are a the likelihood of cesarean delivery. Frequent evaluation of the block less specific indicator of intravascular injection because of the changes and communication with all caregivers is paramount.82 in HR that normally occur during uterine contractions. Even when a The use of ultrasound imaging as an aid in the placement of neur- test dose is carefully timed to be administered between contractions, axial anesthetics is a relatively new and growing technique. The place- it lacks both sensitivity and specificity. As a result, it is not univer- ment of neuraxial anesthetics relies primarily on palpation of underlying sally accepted for use in obstetric anesthesia. Furthermore, concern anatomy and visualization of surface landmarks and is therefore an exists that epinephrine may cause significant uterine artery constric- essentially blind technique. Preprocedure imaging and, eventually, real- tion in a small number of patients, resulting in a decrease in fetal O2 time ultrasound guidance, may ultimately improve clinical practice and delivery.121 the ability to teach these techniques.112 Ultrasound imaging is particu- Some practitioners believe that by using multiple-orifice catheters larly useful in determining the skin to epidural space depth, the loca- in combination with incrementally administered (every 3–5 min) small tion of the midline, and the precise interspace. Palpation alone, doses (less than 5 mL) of dilute (less than 0.125%) local anesthetic, an regardless of BMI, is a relatively ineffective method for determining epinephrine containing test dose is unjustified for labor analgesia. vertebral interspace, which is of particular importance when perform- Careful aspiration of the epidural catheter for blood or CSF alone, ing techniques that involve dural puncture.113 Morbidly obese patients without the administration of a test dose, may be effective in revealing or those with a history of difficult block placement may benefit from an intrathecal or intravascular catheter.122 If a parturient receiving a this technique.114 continuous epidural infusion of dilute local anesthetic remains com- The type of epidural catheter used influences the anesthetic. Spiral fortable, the catheter must, by default, be in the epidural space. wire-embedded flexible polyurethane catheters with a soft tip have been However, if the catheter was intravascular, the parturient would have shown to result in fewer paresthesias and intravascular placements when inadequate analgesia, and, if it were intrathecal, a significant motor compared with nylon catheters.115 Previous comparisons between mul- block would ensue.123 tiple- and single-orifice catheters focused on standard nylon (stiff) Observing for blood or CSF while aspirating from the epidural designs and found an advantage favoring multiple-orifice catheters. A catheter had a 0% false-positive rate and a 0.2% false-negative rate in study comparing both versions (single orifice and multiple orifice) of over 1000 women in whom 60 intravascular or subarachnoid catheter soft catheters has found no difference.116 Multiple-orifice closed-end placements were performed.124 However, a test dose with epinephrine catheters are less prone to obstruction and likely result in better spread is necessary when administering larger and more concentrated doses of local anesthetic in the epidural space.117 such as those given in an epidural for cesarean delivery.122 Ideally, the Historically, when larger doses of local anesthetic were used, a bolus administration of every epidural dose should be considered a “test dose” of IV fluids was given prior to placement of the regional analgesic in in which the catheter is carefully aspirated and incrementally injected. an effort to reduce the incidence and magnitude of hypotension. In summary, every precaution to ensure proper placement of the epi- However, when dilute concentrations of local anesthetics combined dural catheter should be employed including the use of minimum with lipid-soluble narcotics are used to initiate neuraxial analgesia for effective doses, careful aspiration, and incremental injection, coupled labor and are combined with efforts to reduce aortocaval compression, with the use of intravascular markers when large doses are used. Epi- the incidence of hypotension is approximately 5%.118 Zamora et al.119 nephrine remains the most widely used and studied marker, but its found that a 1-liter preload did not provide added protection against reliability is impaired in the face of β-blockade, advanced age, and hypotension but likely resulted in decreasing contraction frequency and active labor. As an alternative, the use of subtoxic doses of local anes- was associated with delays in providing analgesia. thetics themselves can produce subjective symptoms in unpremedicated patients. Fentanyl has also been confirmed to produce sedation in Test Doses pregnant women when used as an alternative. The use of ultrasound High cephalic spread of a neuraxial block remains one of the most observation of needle placement and injection may be useful. Constant common causes of anesthesia related maternal mortality in obstetrics.120 vigilance and suspicion are still needed along with a combination of as A test dose is intended to identify epidural catheters that are inadver- many of these safety steps as practical.125,126 tently inserted into either the subarachnoid space or an epidural vein. First, it is essential to aspirate for blood or cerebrospinal fluid (CSF) Anesthetic Effects on the Progress of Labor after placement of the epidural catheter and before every manually It is likely that women who request early labor analgesia have more administered dose. However, negative aspiration does not conclusively pain than women who do not, and pain is a marker for risk of cesarean indicate that the catheter is not in the subarachnoid or intravascular delivery.127 In a systematic review of 10 trials that enrolled over 2300 space. Therefore the epidural test dose is designed to reveal inadvertent patients in which the effects of epidural versus parenteral opioids in subarachnoid or intravascular injection of local anesthetic without pro- labor were examined, Halpern et al.128 found that epidural labor anal- ducing systemic toxicity or widespread subarachnoid block. The test gesia was not associated with increased rates of instrumented