Chestnut Chapter 2- AI Lesson 1

• Marked anatomical and physiological shifts occur in pregnancy that allow adaptation to the developing fetus and its metabolic needs.

Body Weight and Composition

• The average weight gain during pregnancy is about 17% of pre-pregnancy weight, around 12 kg.
• This increase stems from uterus growth and its contents:
  • Uterus is 1 kg
  • Amniotic fluid is 1 kg
  • Fetus and placenta are 4 kg
  • Blood volume and interstitial fluid increase by 1 kg each.
  • New fat and protein deposition accounts for approximately 4 kg.
• The Institute of Medicine's weight gain recommendations are structured based on pre-pregnancy body mass index (BMI),
• In nonobese individuals, expect a 1-2 kg weight gain in the first trimester, with an additional 5-6 kg increase in each of the last two trimesters.
• Obese individuals receive recommendations for less weight gain.
• Excessive weight gain during pregnancy can increase the risk of long-term increases in BMI.

Cardiovascular Changes

Physical Examination and Cardiac Studies

• Pregnancy induces an enlargement of the heart as a result of increased blood volume, stretching, and stronger contractions.
• The upward shift of the diaphragm due to the growing uterus contributes to notable alterations in both physical exams and cardiac evaluations.
• Heart sound variations include a more pronounced first heart sound, along with a clearer split in the mitral and tricuspid elements.
• A mild systolic ejection murmur is often detected alongside the left edge of the sternum. This murmur is considered benign
• Diaphragm elevation displaces the heart forward and to the left.
• The point of maximum cardiac impulse is shifted higher to the fourth intercostal area and left to at least the midclavicular line.
• Echocardiography shows left ventricular hypertrophy at 12 weeks’ gestation followed by 23% increase in LV mass from the first to third trimester and 50% increase in mass at term.
• Mitral, tricuspid, and pulmonic valves increase in diameter with 94% of term pregnant women exhibiting tricuspid and pulmonic regurgitation, and 27% exhibiting mitral regurgitation.
• The electrocardiogram undergoes changes, notably during the third trimester.
• Heart rate rises progressively through the first and second trimesters, while the PR interval as well as the uncorrected QT interval shorten.

Central Hemodynamics

• Measurements of central hemodynamic values during pregnancy should occur while the patient is in a resting position with left uterine displacement to minimize vena caval compression
• Cardiac output starts rising by the fifth week of gestation, reaching levels 35% to 40% higher than the baseline by the end of the first trimester.
• Cardiac output keeps rising throughout the second trimester, peaking at roughly 50% above pre-pregnancy levels.
• Cardiac output usually remains stable during the third trimester.
• The heart rate increases by 15% to 25% above normal by the end of the first trimester, remaining fairly consistent for rest of the pregnancy.
• Stroke volume climbs around 20% within the first trimester and escalates by 25% to 30% above its initial value through the second trimester.
• Estrogen levels correlate with increased stroke volume
• Stroke volume index goes down throughout pregnancy, while cardiac index stays slightly elevated above pre-pregnancy levels.
• Left ventricular end-diastolic volume increases during pregnancy, whereas end-systolic volume remains unchanged. Elevated velocity of left ventricular circumferential fiber shortening indicates increased myocardial contractility.
• Mild diastolic dysfunction may be seen in the third trimester compared with earlier in pregnancy.
• Uterine blood flow increases from a baseline value of 50 mL/min to 700-900 mL/min.
• Skin blood flow is approximately three to four times the nonpregnant level, resulting in higher skin temperature.
• Renal plasma flow is increased by 80% at 16 to 26 weeks’ gestation but is only 50% above the prepregnancy baseline at term.

Blood Pressure and Systemic Vascular Resistance

• Blood pressure measurements are influenced by positioning, gestational age, and parity.
• Brachial sphygmomanometry yields the highest measurements in the supine position and the lowest measurements in the lateral position, especially with the cuff on the upper arm.
• Blood pressure generally rises with maternal aging, and for a given age, nulliparous women typically register higher mean blood pressure than parous women.
• Systolic, diastolic, and mean blood pressure experience decreases during mid-pregnancy, returning to baseline near term.
• Diastolic blood pressure sees a more pronounced decrease than systolic blood pressure, usually around 20%.
• Systemic vascular resistance mirrors blood pressure changes, decreasing in early gestation, reaching its lowest point (a 35% decline) at 20 weeks’ gestation, then rising toward prepregnancy baseline during late gestation.

Aortocaval Compression

• The degree to which the aorta and inferior vena cava are compressed by the uterus depends on body positioning and gestational age. 
• At term, partial vena caval compression occurs when the woman is in the lateral position while in the supine position, significant and sometimes complete compression of the inferior vena cava is evident.
• In the supine position, the aorta can undergo compression as well.
• Blood flow in the upper extremities is normal, whereas uterine blood flow decreases by 20% and lower extremity blood flow decreases by 50%.
• Some term pregnant women exhibit an increase in brachial artery blood pressure when they assume the supine position due to higher systemic vascular resistance from compression of the aorta.
• The supine hypotension syndrome consists of up to 15% of women at term experience bradycardia and a substantial decrease in blood pressure when supine.

Hemodynamic Changes during Labor and the Puerperium

• Cardiac output increases during labor by 10% in the early first stage, 25% in the late first stage, and 40% in the second stage
• Cardiac output may reach values as high as 75% above predelivery measurements and 150% above prepregnancy baseline during the immediate postpartum period.
• Uterine contractions displace 300 to 500 mL of blood.
• Cardiac output decreases to just below prelabor values at 24 hours postpartum and returns to prepregnancy levels between 12 and 24 weeks postpartum.
• Heart rate decreases rapidly after delivery, reaches prepregnancy levels by 2 weeks postpartum, and is slightly below the prepregnancy rate for months.

Respiratory System 

• Pregnancy has a relatively minor impact on lung function.

Anatomy

• The thorax experiences structural changes due to mechanical and hormonal influences.
• Relaxin allows the ligamentous attachments to the lower ribs to relax, widening the subcostal angle from around 69 to 104 degrees.
• The chest wall's anteroposterior and transverse diameters increase by about 2 cm each, resulting in an overall increase of 5 to 7 cm.
• The vertical measurement of the chest cavity decreases by as much as 4 cm because of the diaphragm's elevated position.
• Capillary engorgement affects the larynx along with nasal and oropharyngeal mucous membranes.

Airflow Mechanics

• Inspiration is mainly attributable to movement of the diaphragm in pregnant women at term.
• Airway function changes minimally during pregnancy.
• There is no significant change in respiratory muscle strength during pregnancy despite the cephalad displacement of the diaphragm.
• Excursion of the diaphragm increases by 2 cm

Lung Volumes and Capacities

• Total lung capacity is slightly reduced, while the tidal volume increases by 45%.
• Early in pregnancy, a transient reduction in inspiratory reserve volume occurs.
• The inspiratory capacity increases by 15% during the third trimester.
• The functional residual capacity (FRC) drops by 400 to 700 mL to 80% of prepregnancy value at term.
• Assumption of the supine position causes the FRC to decrease further to 70% of the prepregnancy value, it can then be increased by 10% by placing the patient in a 30-degree head-up position.

Ventilation and Blood Gases

• Respiratory patterns remain relatively unchanged.
• Minute ventilation increases via an increase in tidal volume from 450 to 600 mL; this occurs primarily during the first 12 weeks of gestation.
• Alveolar ventilation increases by 30% to 50% above baseline.
• Dyspnea affects up to 75% of women and is attributable to increased respiratory drive, decreased Paco2, increased oxygen consumption.
• Exercise does not affect pregnancy-induced changes in ventilation or alveolar gas exchange.
• Hypoxic ventilatory response increases to twice the normal level. Pa02 increases, and Pac02 declines by 12 weeks but does not change during the remainder of the pregnancy.
• Women in the supine position have an increased alveolar-to-arterial gradient due to small airways.
• Exercise has no effect on pregnancy-induced changes in ventilation or alveolar gas exchange.

The Gastrointestinal System 

Anatomy, Barrier Pressure, and Gastroesophageal Reflux

• Upward displacement of the stomach toward the left along with a roughly 45-degree rotation to the right, can lead to reduction in tone of the lower esophageal high-pressure zone (LEHPZ).
• Approximately 30% to 50% of women experience gastroesophageal reflux disease (GERD) during pregnancy.

Gastrointestinal Motility

• Esophageal peristalsis and intestinal transit are slowed during pregnancy

The Liver and Gallbladder

• Liver structure and blood flow remain unchanged, although it may be slightly displaced.
• Serum levels of bilirubin and certain liver enzymes go up, while total alkaline phosphatase activity increases twofold to fourfold.
• Biliary stasis and elevated cholesterol levels in bile raise the risk of gallbladder disease during pregnancy. Progesterone inhibits contractions of gastrointestinal smooth muscle.

The Kidneys

• Renal size, renal vascular, and interstitial volume expands.
• Hydronephrosis may occur, with dilation of the various parts of the collecting system. 
• GFR and renal plasma flow both increase in pregnancy.
• Increased creatinine clearance along with reduced blood concentrations of nitrogenous metabolites like blood urea nitrogen and serum creatinine.
• There is an elevation in glucose excretion Average 24-hour total protein and albumin excretion are 200 mg and 12 mg, respectively
• Proteinuria is associated with increased progression to preeclampsia.

Hematology 

Blood Volume

• Plasma volume expands starting at 6 weeks’ gestation & increases by almost 50% by 34 weeks.
• After 34 weeks, plasma volume stabilizes or slightly decreases.
• The red blood cell volume decreases during first 8 weeks and increases to a level 30% above pre-pregnancy.
• The increase in plasma volume outpaces the increases in red blood cell volume.
• Blood volume is positively correlated with the fetus, is higher in multiple gestations, and protects from hemorrhage at delivery
• Lower blood viscosity creates lower resistance to blood flow.

Plasma Proteins

• Plasma albumin concentration goes down.
• Maternal colloid osmotic pressure goes down during pregnancy.

Coagulation

• Enhanced platelet turnover, clotting and fibrinolysis.
• Pregnancy represents a state of accelerated but compensated intravascular coagulation.
• The concentrations of most coagulation factors typically increase.

The Immune System 

• The blood leukocyte count increases, reflecting an increase in the number of polymorphonuclear cells, with the appearance of immature granulocytic forms
• Despite elevated concentration, polymorphonuclear, leukocyte function is impaired during pregnancy.
• Levels of IGA, IGG, and IGM remain unchanged.

Nonplacental Endocrinology

Thyroid Function

• The thyroid gland increases significantly in size during pregnancy due to hyperplasia and greater vascularity
• There is an increase in total triiodothyronine and thyroxine concentrations during the first trimester The concentrations of free T3 and T4 do not change and the concentration of thyroid stays normal.

Glucose Metabolism

• The mean blood glucose concentration stays within the normal range during pregnancy, although concentrations can vary.
• The glucose demand of the fetus and the placenta leads to fasting hypoinsulinemia.
• Pregnant women are relatively insulin-resistant

Adrenal Cortical Function

• Concentrations of corticosteroid-binding globulin double, leading to increases of both plasma cortisol concentrations & of unbound, metabolically active cortisol

The Musculoskeletal System

• Back pain during pregnancy is common.
• Etiology is multifactorial: the enlarging uterus results in exaggerated lumbar lordosis alongside associated hormonal changes which leads to collagen remodeling.
• Backpain responds to exercises to strengthen the abdominal and back muscles. Scheduled rest periods with elevation of feet also helpful.
• Mobilility increases in the sacroiliac.

The Nervous System

Sleep

• Sleep disturbances from mechanical and hormonal factors include the following: influenced by progesterone and estrogen, complaints of insomnia + daytime sleepiness.
• Pregnancy associates w transients restless legs syndrome

Anesthetic Implications

Positioning

_Aortocaval compression, decreased BP, and impaired uteroplacental blood flow can be consequences of a supine position

• Supine positions should be avoided and uterus should be tilted if maternal blood pressure is unattainable.

Blood replacement

_The vasculature now does not need to provide for the intervillous space and therefore that quantity of blood need not be replaced

General Anesthesia

• Proportion of women with mallampati IV classification increases as pregnancy progresses
• Airway edema in exacerbated in patients, leads to difficult incubation
• Pregnant woman's airway is different and may cause complication

Neuraxial Analgesia and Anesthesia

• Neuraxial anesthesia increases the likelihood of hypotension via sympathetic block, but reduces pressure in vertebral column, and spinal CSF Volume
• Pregnant patients exhibit decreased need for anesthesia

Explore the significant physiological adaptations during pregnancy and labor, including changes in cardiac output, blood volume, and respiratory function. Understand the hormonal influences and mechanical effects that shape the maternal response to pregnancy.