Cardiovascular Monitoring PDF

36 Cardiovascular Monitoring BECKY SCHROEDER, JONATHAN MARK, and ATILIO BARBEITO KEY POINTS □  onitoring of the electrocardiogram (ECG) provides continuous monitoring of heart rate, iden- M...

36 Cardiovascular Monitoring BECKY SCHROEDER, JONATHAN MARK, and ATILIO BARBEITO KEY POINTS □  onitoring of the electrocardiogram (ECG) provides continuous monitoring of heart rate, iden- M tification of arrhythmias and conduction abnormalities, and detection of myocardial ischemia. □ Accurate and reliable ECG monitoring requires attention to lead placement and selection, choice of filter, and gain that will influence the displayed ECG tracing. □ An anterolateral precordial lead (V3, V4, or V5) should be selected for the most sensitive detec- tion of myocardial ischemia. □ Demand-mediated subendocardial ischemia resulting in ST-segment depression is the most commonly observed form of perioperative ischemia. ST-segment depression is most commonly observed in an anterolateral precordial lead regardless of the coronary territory responsible. □ Supply-mediated transmural ischemia resulting in ST-segment elevation is uncommonly ob- served intraoperatively except during cardiac operations. In contrast to ST-segment depression, ST-segment elevation is indicative of the myocardial territory and coronary artery involved. □ Most automated noninvasive arterial blood pressure measuring devices use an oscillometric measurement technique and rarely cause complications. Caution should be exercised in pa- tients who cannot complain of arm pain, those with irregular rhythms that force repeated cuff inflation, and those receiving anticoagulant therapy. □ The Allen test for palmar arch collateral arterial flow is not a reliable method to predict com- plications from radial artery cannulation. Despite the absence of anatomic collateral flow at the elbow, brachial artery catheterization for perioperative blood pressure monitoring is a safe alternative to radial or femoral arterial catheterization. □ The accuracy of a directly recorded arterial pressure waveform is determined by the natural fre- quency and damping coefficient of the pressure monitoring system. Optimal dynamic response of the system will be achieved when the natural frequency is high, thereby allowing accurate pressure recording across a wide range of damping coefficients. □ The preferred position for alignment (or “leveling”) of external pressure transducers for measuring arterial or central venous pressure (CVP), which eliminates confounding hydrostatic pressure artifacts, lies approximately 5 cm posterior to the sternomanubrial junction. The more conventional location for the reference level used for hemodynamic monitoring including central venous and pulmonary artery pressures, is the mid-thoracic level, which corresponds most closely to the mid-left atrial posi- tion and is located halfway between the anterior sternum and the bed surface in the supine patient. □ Because of wave reflection and other physical phenomena, the arterial blood pressure recorded from peripheral sites has a wider pulse pressure than when measured more centrally. □ Dynamic measures of cardiac preload, such as stroke volume and pulse pressure variation, are better predictors of intravascular volume responsiveness than static indicators, such as CVP and pulmonary capillary wedge pressure. □ Selecting the best site, catheter, and method for safe and effective central venous cannulation requires that the physician consider the purpose of catheterization, the patient’s underlying medical condition, the intended operation, and the skill and experience of the physician per- forming the procedure. Right internal jugular vein cannulation is preferred due to its consistent, predictable anatomic location and its relative ease of access intraoperatively. □ Mechanical complications from central venous catheters can be decreased by the use of ultra- sound vessel localization, venous pressure measurement before large catheter insertion, and radiographic confirmation that the catheter tip lies outside the pericardium and parallel to the walls of the superior vena cava. □ CVP is the result of a complex and diverse interplay among many different physiologic vari- ables, the main ones being venous return and cardiac function. No simple relationship exists between CVP and circulating blood volume. Despite this, important pathophysiologic informa- tion can be obtained by careful assessment of the CVP waveform morphology. □ Catheter misuse and data misinterpretation are among the most common complications of central venous and pulmonary artery catheters. □ Pulmonary artery wedge pressure is a delayed and damped reflection of left atrial pressure. The wedge pressure provides a close estimate for pulmonary capillary pressure in many cases, but it may underestimate capillary pressure when postcapillary pulmonary vascular resistance is increased, as in patients with sepsis. 1145 Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 1146 SECTION III Anesthesia Management □  se of central venous, pulmonary artery diastolic, or pulmonary artery wedge pressures as U estimates of left ventricular preload is subject to many confounding factors, including changes in diastolic ventricular compliance and juxtacardiac pressure. □ Pulmonary artery catheter monitoring has not been shown to improve patient outcomes. Reasons cited for these results include misinterpretation of catheter-derived data and failure of hemodynamic therapies that are guided by specific hemodynamic indices. □ Thermodilution cardiac output monitoring, the most widely used clinical technique, is sub- ject to measurement errors introduced by rapid intravenous fluid administration, intracardiac shunts, and tricuspid valve regurgitation. □ Mixed venous hemoglobin oxygen saturation is a measure of the adequacy of cardiac output relative to body oxygen requirements. This measurement is also dependent on the arterial hemoglobin oxygen saturation and hemoglobin concentration. Introduction to Cardiovascular or arterial blood pressure monitor. Considering both in monitoring and clinical evaluation improves accuracy and Monitoring: Focused Physical reduces measurement errors and false alarms.3 Examination Electronic devices currently provide the vast majority of Electrocardiography Monitoring information used in monitoring a patient’s cardiovascu- lar status. However, the physician’s senses, augmented by The value and importance of intraoperative monitoring clinical context, continue to provide global insight into the of the ECG is evidenced by its requirement as a basic cir- patient’s condition and remain critical in evaluating, and culatory monitoring standard by the American Society of interpreting data derived from other sources.1 Manual pal- Anesthesiologists (ASA)4: “Every patient receiving anes- pation of an arterial pulse will differentiate true asystole thesia shall have the ECG continuously displayed from from monitoring artifact more efficiently than troubleshoot- the beginning of anesthesia until preparing to leave the ing any monitor. Regardless of the manner used, though, it anesthetizing location.” Similar recommendations have is important to understand the strengths and limitations of been recently updated by the American Heart Association monitoring techniques. (AHA) to guide indications, duration, and implementation of ECG monitoring in hospitalized patients outside of the operating room.5 Heart Rate and Pulse Rate The three primary reasons for ECG monitoring are contin- Monitoring uous monitoring of heart rate, identification of arrhythmias and conduction abnormalities, and detection of myocar- The ability to estimate the heart rate quickly with a “finger dial ischemia. Furthermore, with many patients coming to on the pulse” is as important as this expression is common surgery with pacemakers or implantable cardiac defibrilla- despite near-universal use of electronic devices for continu- tors in place, the ECG monitor enables the anesthesiologist ous monitoring. The electrocardiogram (ECG) is the most to follow the proper function of these devices during the common heart rate monitoring method used in the oper- perioperative period. (Perioperative management of these ating room, even though any device measuring the period devices is described in Chapter 38.) In order for bedside ECG of the cardiac cycle will suffice. Accurate detection of the monitoring to be accurate and effective, the clinician must R wave and measurement of the interval from the peak of attend to proper lead placement and selection, filter mode, one QRS complex to the peak of the next on an ECG (R-R and gain adjustment. interval) serve as the basis from which digitally displayed values are derived and periodically updated (e.g., at 5- to ELECTROCARDIOGRAM LEAD PLACEMENT AND 15-second intervals) (Fig. 36.1).2 The distinction between heart rate and pulse rate lies SELECTION in the difference between electrical depolarization with Standard Lead Systems systolic contraction of the heart (heart rate) and a detect- Current operating room and intensive care monitoring sys- able peripheral arterial pulsation (pulse rate). Pulse deficit tems have five leads that allow monitoring of the standard describes the extent to which the pulse rate is less than the limb leads (I, II, III), the augmented limb leads (aVR, aVL, heart rate and may arise in conditions such as atrial fibril- aVF), and a single precordial lead (V1, V2, V3, V4, V5, or lation in which stroke volume is periodically compromised V6). Typically, two of these 12 standard leads are simulta- by a very short R-R interval to such an extent that no arte- neously displayed on the bedside monitor. Historically, the rial pulse is detectable for that systolic ejection. Electrical- augmented limb leads and precordial leads were described mechanical dissociation and pulseless electrical activity are as unipolar, whereas the standard limb leads were described extreme examples of pulse deficit in which cardiac contrac- as bipolar. A recent scientific statement from the AHA and tion is completely unable to generate a palpable peripheral others6 discourages this distinction because in effect, all the pulse. The heart rate is reported from the ECG trace and lead configurations are effectively bipolar in their recording the pulse rate from the pulse oximeter plethysmograph of surface electrical potentials. Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  36 Cardiovascular Monitoring 1147 HR 49 1 sec 135 90 45 0 Fig. 36.1 Digital heart rate (HR) displays may fail to warn of dangerous bradyarrhythmias. Direct observation of the electrocardiogram (ECG) and the arterial blood pressure traces reveals complete heart block and a 4-second period of asystole, whereas the digital display reports an HR of 49 beats/ min. Note that the ECG filter (arrow) corrects the baseline drift so that the trace remains on the recording screen. (From Mark JB. Atlas of Cardiovascular Monitoring. New York: Churchill Livingstone; 1998.) artifacts in the limb leads caused by movement.8 Although the limb lead QRS complexes are slightly different in ampli- tude and axis, and the precordial leads may vary slightly RA LA from the standard 12-lead ECG recording, studies have shown that the ST-segment measurements during exercise stress testing were generally similar when the Mason-Likar 12-lead ECG system is used, as compared with the standard 12-lead ECG.8-11 Consequently, torso-positioned limb leads have become standard for monitoring in the operating room and ICU because of convenience and the potential to obviate motion artifact. In some cases, surgical incisions, patient positioning, or RL LL other procedural aspects may mandate adjusting these limb lead electrode locations. However, for reliable recording of standard limb leads, the electrodes must be outside the car- diac borders, in the transverse plane above and below the heart, and in the sagittal plane to the left and right of the heart (Fig. 36.3). In practice, limb lead placement closer to the heart may lead to unintended distortion of the ECG tracings.10 Placement of the precordial lead (brown) electrode requires more attention than the limb leads, since this lead is often misplaced and its position is critical for the reliable and Fig. 36.2 Standard ECG limb lead placement for patient monitor- ing. LA, Left arm; LL, left leg; RA, right arm; RL, right leg. (From Mark JB. sensitive detection of myocardial ischemia. The V5 precor- Atlas of Cardiovascular Monitoring. New York: Churchill Livingstone;1998.) dial lead is the one most commonly chosen for monitoring patients at risk for myocardial ischemia, since historically Based on AHA guidelines, ECG monitoring leads have a it has been shown to be the most sensitive single lead for standard color-coding system: right arm (white), left arm detecting ischemia during exercise stress testing12,13 and (black), right leg (green), left leg (red), and precordial lead during anesthesia.14,15 Importantly, the V5 precordial lead (brown). Of note, this color scheme is not the same used can be monitored during cardiac operations without inter- internationally or the one recommended by the Interna- fering with the surgical prep and median sternotomy inci- tional Electrotechnical Commission. Unlike recording a sion. For patients undergoing other operations, particularly standard 12-lead ECG, where the limb leads are attached to high-risk patients undergoing vascular procedures, leads electrodes placed on the wrists and ankles or arms and legs, V4 or V3 may be chosen because there is good evidence that when monitoring patients in the operating room or inten- these leads are even more sensitive for detecting prolonged sive care unit (ICU), the limb leads are typically placed on postoperative myocardial ischemia.16 the torso, with the right and left arm leads placed just below Standard locations for the six precordial lead electrodes the clavicles and the leg leads placed above the hips (Fig. are shown in Fig. 36.4. Given that there is considerable 36.2). Of note, placement of the right leg lead (green lead) evidence that precordial lead placement is inaccurate can be anywhere on the body because it is a ground elec- even during diagnostic 12-lead ECG recordings,6 it is likely trode, and its location will not alter the display of any of the that this is also commonplace during ECG monitoring in selected standard leads.7 the operating room and ICU. Accurate lead placement is Placement of the limb electrodes on the torso has been facilitated by locating the manubrial-sternal junction, its utilized since 1966, when Mason and Likar introduced immediately inferior rib interspace (the second), and then a variation on positioning the standard limb electrodes of palpating down to identify the fourth and fifth interspaces the 12-lead ECG during exercise stress testing to minimize for accurate precordial lead location. Note that lead V4 lies Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 1148 SECTION III Anesthesia Management Lead I Lead II Lead III 1 sec A 1 mV B RA LA Position B (LL) Position A (LL) Fig. 36.3 Importance of ECG limb lead placement. ECG recording of leads I, II, and III from position A, with the left leg (LL) lead in the standard loca- tion below the heart near the left iliac crest compared with position B, with the LL lead placed over the precordium (near the V5 position). Standard ECG limb lead positioning should be outside the cardiac borders, as shown here in position A. If the LL lead electrode is placed inappropriately in position B, the recording of leads II and III will be modified. Note, however, that the recording of lead I is not affected by the misplaced LL lead, since lead I measures the difference between left arm (LA) and right arm (RA) electrodes. (From Mark JB. Atlas of Cardiovascular Monitoring. New York: Churchill Livingstone;1998.) LA RA V1 V6 V5 V2 V4 V3 RL LL Fig. 36.4 Proper anatomic location of the six standard precordial ECG leads. Precise positioning of the lateral precordial leads is ensured by iden- tifying the fifth intercostal space in the mid-clavicular line for lead V4, moving laterally to the anterior axillary line for V5, and further laterally to the mid- axillary line for V6. LA, Left arm; LL, left leg; RA, right arm; RL, right leg. in the fifth interspace at the mid-clavicular line, and leads detection of ischemia, it is important that the precordial V5 and V6 are located directly lateral to V4 in the anterior lead not be placed haphazardly or too laterally. In some and mid-axillary lines, respectively. cases (e.g., patients undergoing left thoracotomy), none of Given anatomic considerations and the strong evidence these precordial lead placement sites are possible. In this that the mid-precordial leads (V3, V4, V5) are best for instance, it is reasonable to choose another standard lead Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  36 Cardiovascular Monitoring 1149 position, such as V1, but it is important that the clinician recognize that ECG monitoring for ischemia will not be as sensitive. RA RA I I Alternative Lead Systems Electrocardiographic evidence for right ventricular (RV) LA LA ischemia and infarction is best identified by right-sided pre- cordial leads, particularly lead V4R, a mirror image of the V4 cs5 CM5 lead, positioned in the fourth intercostal space at the mid- clavicular line. Although never chosen for routine ischemia monitoring, this lead has proven valuable for detecting RV ischemia and infarction, and might be chosen to monitor RA patients at risk for inferior left ventricular (LV) ischemia, which is commonly accompanied by RV involvement.6,17 I Three-lead ECG monitoring systems, while less com- LA I LA mon today than five-lead systems, remain available par- RA ticularly in non-operating room procedural settings and for patient transport. A three-lead system is also the standard CB5 CC5 ECG monitoring system incorporated in external defibril- lator devices. The three leads (right arm, left arm, left leg) can be placed in similar locations to their five-lead system III LA counterparts, and this allows reliable monitoring of heart LL rate, detection of R waves for synchronized direct current cardioversion, and monitoring for potentially life-threat- ening arrhythmias such as ventricular fibrillation. These systems are more limited than five-lead systems (or stan- MCL1 dard 12-lead ECG recordings) for diagnosing more complex Fig. 36.5 When only a three-electrode lead set is available, modified arrhythmias and detection of myocardial ischemia. Modifi- bipolar limb leads may be recorded as surrogates for standard precordial cations of these standard limb leads allow better sensitivity leads. Alternatives to precordial lead V5 are recorded by selecting lead I for ischemia monitoring by placing the positive (left arm) on the bedside monitor and placing the positive exploring left arm (LA) electrode in the standard precordial V5 position and select- electrode in the V5 position. The nomenclature describing these leads ing an appropriate limb lead (usually lead I) to create this derives from the location of the positive exploring electrode located in the V5 position and the negative right arm (RA) electrode position being modified lead recording. These three-lead variations useful located as follows: (CS5) central subclavicular, (CM5) central manubrial, for ischemia monitoring are created by placing the right (CB5) central back (shown overlying the right scapula), and (CC5) central arm lead in the following locations: CS5 (central subclavicu- chest. In contrast, note that the (MCL1) lead (modified central lead 1) is lar), CM5 (central manubrial), CB5 (central back), and CC5 recorded by selecting lead III on the bedside monitor and placing the positive exploring left leg (LL) electrode in the V1 position with the nega- (central chest) (Fig. 36.5). Each of these lead recordings will tive LA lead in a modified position beneath the left clavicle. (From Mark be a modification of the standard precordial V5 lead, and JB. Atlas of Cardiovascular Monitoring. New York: Churchill Livingstone;1998.) owing to differences in R-wave amplitude and ST-segment morphology, may lead to over- or underestimation of ST- artifacts and improving signal quality. As the name sug- segment changes.18 gests, a bandpass filter allows signal frequencies within a Another modified three-lead system is a surrogate for certain range to pass or be displayed, while attenuating or precordial lead V1. This lead (MCL1) is recorded by placing functionally eliminating signal frequencies both at a low the left arm lead in standard position, the left leg lead at the range and a high range. V1 position, and selecting lead III (see Fig. 36.5). This modi- Low-frequency artifact is typically caused by respiration fied lead system is useful for monitoring in the ICU or other or patient movement that causes the ECG tracing to wan- circumstances where detection of P-wave morphology and der above and below the baseline (Fig. 36.7). Often this will arrhythmias is of paramount importance. appear as the ECG tracing being cut off or only partially dis- Following cardiac surgical procedures, recording the played on the bedside monitor channel. Therefore, low-fre- ECG signal from an epicardial atrial pacing wire may also quency filters (also called high-pass filters) are used in ECG be useful for detecting P waves that may not be as evident monitoring. The heart rate forms a rough lower bound for from skin surface ECG recordings (Fig. 36.6). This alterna- the frequency content of the ECG signal and is measured in tive lead recording is usually performed in the ICU using a Hertz (Hz, cycles per second). Since heart rates slower than 12-lead ECG system and attaching one of the atrial pacing 40 beats per minute (bpm) (0.67 Hz) are uncommon, tra- wires to a precordial lead wire. ditional low-frequency analog filters are used to cut off sig- nals at frequencies below 0.5 Hz. However, such filters may introduce considerable distortion into the ECG, particularly ELECTROCARDIOGRAM FILTER SELECTION with respect to the level of the ST segment, resulting from The ECG signal is subject to artifacts (noise) both in the phase nonlinearities that occur in areas of the ECG signal low-frequency and high-frequency ranges. Consequently, where frequency content and wave amplitude abruptly all ECG monitors use bandpass filters to narrow the signal change, as occurs where the end of the QRS complex meets bandwidth, preserving the signal of interest while reducing the ST segment. The 1975 AHA recommendations included Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 1150 SECTION III Anesthesia Management 1sec 1mV II aVF Atrial Fig. 36.6 Simultaneous recording of surface ECG leads II and aVF and an atrial epicardial lead (atrial) recorded from a pacing wire attached to the surface of the right atrium. Onset of atrial electrical activity is denoted by the P wave in the surface leads and marked by the dashed vertical line. Note that the amplitude of the atrial electrical signal is greatest in the atrial lead recording. In addition, the sixth beat is a ventricular premature beat, and the resulting retrograde atrial depolarization is clearly evident in the atrial lead (arrow) but not seen as easily in standard surface ECG leads II or aVF. (From Mark JB. Atlas of Cardiovascular Monitoring. New York: Churchill Livingstone;1998.) Fig. 36.7 ECG recording of lead II (top) and lead V5 (bottom) at standard speed (25 mm/s) and gain (10 mm/mV) with a diagnostic bandpass filter. The respiratory artifact is evident from the varying or wandering baseline most noticeable in lead II. This artifact will be eliminated by using the monitor bandpass filter. a 0.05-Hz low-frequency cutoff for diagnostic ECGs. This maintain diagnostic accuracy during visual inspection of an recommendation preserves the fidelity of repolarization, but ECG, although it has long been recognized that higher-fre- baseline drift can still be a problem. Current modern digital quency components of the QRS complex may have clinical filtering provides more sophisticated methods for a higher significance in patients with various forms of heart disease. cutoff for low-frequency filtration without those phase According to current AHA recommendations, to measure distortions observed with analog filtering. Thus, to reduce routine duration and amplitudes accurately in adults, ado- artifactual distortion of the ST segment, current AHA rec- lescents, and children, an upper frequency cutoff of at least ommendations suggest the low-frequency cutoff should be 150 Hz is required, and an upper frequency cutoff of 250 Hz either 0.05 Hz for monitors with analog filters or 0.67 Hz or is more appropriate for infants.6 below for monitors and ECG recording devices with linear Current ECG monitors allow the clinician a choice among digital filters with zero phase distortion.6 several filtering modes or bandwidths. The actual filter fre- High-frequency ECG artifacts are typically caused by quencies tend to vary among manufacturers, but in general muscle fasciculations, tremors, and most importantly, the there are three different filters that may be selected, termed ever present 60 cycle (Hz) electromagnetic interference from diagnostic mode, monitoring mode, and filter mode. The other electrical equipment in the monitoring environment. diagnostic mode typically has a bandpass of 0.05 to 150 These artifacts can be eliminated with high-frequency fil- Hz, and this filter should always be selected for the most ters (also termed low-pass filters), but like low-frequency fil- undistorted and accurate display of the ST segment and the ters, the high-frequency filters may distort the ECG signal in identification of pacing spikes. The monitor mode typically undesirable ways. The higher frequencies in the ECG signal has a bandpass of 0.5 to 40 Hz, and while both low-fre- include features such as rapid upstroke velocity (QRS com- quency (respiratory drift) and high-frequency (60 Hz) noise plex), peak amplitude (R wave), and waves of short dura- is reduced or attenuated, the ST segments are often distorted tion. Most importantly, pacing spikes, which by definition and typically show an exaggerated deviation that is artifac- are high frequency and low amplitude, are often eliminated tually introduced by the filter (Figs. 36.8A and B).19 The by high-frequency filters and make bedside identification of filter mode bandpass is 0.5 to 20 Hz and may incorporate pacemaker function impossible. A high-frequency cutoff of a notch filter aimed at further attenuating and eliminating 100 Hz was considered adequate by the AHA in 1975 to 60 Hz interference from nearby electrical equipment. Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  36 Cardiovascular Monitoring 1151 1 sec 1 sec 1 mV 1 mV Diagnostic 0.05 -130 Hz Monitor Monitor 0.5 - 40 Hz Diagnostic Filter 0.5 - 25 Hz A B Fig. 36.8 Effects of filter selection on the ST segment. Application of the monitor mode filter (8A, top panel) produces artifactual J-point depression and upsloping ST-segment depression (box). These abnormalities are not seen when the diagnostic mode filter (8A, bottom panel) is used to record the ECG. Filter selection also effects ECG electrical interference originating from the wall power source (8B). Compared to the diagnostic mode filter (bandpass 0.05-130 Hz), the narrower bandpass of the monitor mode (0.5-40 Hz) reduces this high-frequency artifact, and the filter mode (0.5-25 Hz) that incorporates an additional notch filter at 60 Hz eliminates this electrical artifact entirely. (From Mark JB. Atlas of Cardiovascular Monitoring. New York: Churchill Livingstone;1998.) II 1 mV Mason-Likar 0.05–150 Hz Non-Paced V 1 mV Fig. 36.9 ECG gain is indicated by a 1 mV rectangular calibration signal on a paper recording or by a 1 mV vertical marker at the edge of the bedside monitor ECG waveform. In this example, the ECG is shown at standard gain, 10 mm/mV. ELECTROCARDIOGRAM GAIN SELECTION when the monitor detection of heart rate is inaccurate. For example, one might reduce the ECG signal gain when the In addition to lead and filter selection, bedside monitors monitor is inappropriately counting a tall T wave as an R also allow adjustment and selection of the ECG signal gain. wave and displaying an artifactual heart rate that is twice A standard ECG is recorded at a gain of 10 mm/mV and the true rate. Conversely, one would increase the ECG gain is indicated by a 1 mV rectangular calibration signal on a when there are very small R waves and the monitor cannot paper recording (Fig. 36.9) or by a 1 mV vertical marker record and display the heart rate. at the edge of the bedside monitor ECG waveform. Bed- Gain adjustment is important because all features of side monitors may be set to an autogain mode, where the the ECG are equally amplified or reduced when the gain available display space is filled by the ECG tracing and the is changed from the 10 mm/mV standard. As a conse- corresponding increase or decrease from standard gain is quence, ST-segment deviations can be obscured when indicated by the vertical marker. Alternatively, the clini- gain is reduced and thereby impair clinical observation of cian may adjust the ECG signal gain manually. This is done important ST-segment changes. Alternatively, if the gain is Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 1152 SECTION III Anesthesia Management 1 sec 1 mV 1 mV 1 mV 0.5 mV Fig. 36.10 Effect of gain adjustment on the magnitude of ST-segment shift. The magnitude of ST-segment depression and R-wave amplitude vary in direct proportion to overall signal gain denoted by the vertical calibration signals adjacent to the traces (10 mm/mV, upper left panel; 5 mm/mV, upper right panel; 20 mm/mV, lower left panel; 20 mm/0.5 mV or 40 mm/mV, lower right panel). Note that at four times standard gain (lower right panel), the ST depression increases to 6 mm but R-wave amplitude only increases to 21 mm because the R-wave peak is “cut off” at the upper limits of the recording display. (From Mark JB. Atlas of Cardiovascular Monitoring. New York: Churchill Livingstone;1998.) 1 mV Mason-Likar 0.05–150 Hz Paced 1 mV Fig. 36.11 Leads II (top) and V5 (bottom) are recorded during atrial pacing (86 beats/min) at standard speed (25 mm/ms) and gain (10 mm/mV). Atrial pacing spikes are small and difficult to discern, particularly in lead V5. Prior to the 10th beat, the Pacing Mode is selected and the monitor marks the atrial pacing spikes clearly on the remaining five beats. increased, ST deviations will also be amplified proportion- function easier. When this monitoring mode is selected, ally. While common clinical communication of ST-segment clinicians should recognize that the displayed ECG trac- shifts is always described in terms of millimeters (mm) of ing shows regular markers, often in a different color than depression or elevation, interpretation of bedside monitor the ECG tracing, that indicate and highlight presence of a ECG tracings must always be tempered by consideration of pacemaker stimulus output. These are not amplified pace- the display gain (Fig. 36.10). maker signals, but rather monitor-generated markers of their detection. While very helpful to the clinician at the bedside, pacemaker mode monitoring may not reliably ELECTROCARDIOGRAM PACING MODE detect pacemaker spikes in all patients. In other instances, While older ECG monitors recorded analog (continuous) the intrinsic features of the pacemaker stimulus in differ- signals, current devices convert the analog ECG signal to ent leads might allow their detection in some leads but not digital form by sampling the signal at very high rates up to others (Fig. 36.11). 15,000 times per second.6 This oversampling of the ECG signal was originally introduced to allow identification and ELECTROCARDIOGRAM DISPLAYS AND recording of pacemaker stimulus outputs (pacing spikes), RECORDINGS which are generally shorter than 0.5 ms, but current sys- tems do not always detect these small-amplitude high- Monitoring the ECG during anesthesia and in the ICU frequency signals reliably. As a result, most bedside ECG includes both observation of the bedside monitor display and monitors now include a pacing mode selection, which periodic recording of the ECG “rhythm strip” for documen- when activated, employs an algorithm to detect and high- tation or more careful analysis. The most common bedside light these pacing spikes, making detection of pacemaker recording system provides a 2-inch strip that records two Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  36 Cardiovascular Monitoring 1153 HR(V) 146 PR 58 1 sec ECG 135 ART 0 HR(V) 58 PR 58 ECG 135 ART 0 Fig. 36.12 Electrosurgical unit interference with heart rate measurement from the ECG. (Top panel) The ECG signal from lead V is distorted by the electrosurgical unit. As a result, the digital value for heart rate (HR) is erroneous (146 beats/min), although the pulse rate (PR) is measured accurately by the monitor from the arterial blood pressure waveform (58 beats/min). (Lower panel) Correct (and identical) digital values for both HR and PR are displayed by the monitor. ART, Arterial blood pressure. (From Mark JB. Atlas of Cardiovascular Monitoring. New York: Churchill Livingstone;1998.) ECG leads (see Fig. 36.11), but other recording systems may ELECTROCARDIOGRAM ARTIFACTS provide larger formats for multi-lead or multi-waveform recording. Monitoring systems may also have “full disclo- As is the case for all bedside monitoring,20 artifacts com- sure” capability whereby monitored ECG (and other) wave- monly distort the monitored ECG tracing and must be identi- forms are stored for up to 24 hours and can be retrieved and fied to prevent misinterpretation or inappropriate treatment. printed for review. This last system is particularly useful for In the operating room, the most common cause of ECG arti- the retrospective identification, interpretation, and docu- fact is the electrosurgical unit (ESU). Since some of the fre- mentation of arrhythmias or other cardiovascular changes quencies generated by the ESU fall within the QRS frequency that may have escaped detection in real time by clinicians range, and the amplitude of these signals can be very high (1 at the bedside. kV or 1 million times the typical QRS amplitude of 1 mV),21 The ECG tracing on the bedside monitor can be displayed even the best current advanced filtering techniques can- at varying sweep speeds, but most typically is displayed at not eliminate this artifact that often totally masks the ECG 25 mm/s, the same speed as a standard 12-lead ECG trac- signal (Fig. 36.12). This not only precludes identification ing. As described previously, the ECG gain can be adjusted of any ECG waveform features, but it can also prevent ECG and the displayed gain should be indicated by a vertical monitoring of heart rate. The almost universal availability of marker overlying the ECG tracing, the standard being 10 additional simultaneously displayed waveforms, such as the mm/mV. pulse oximetry plethysmogram or an arterial blood pressure Paper recordings of the ECG should have standard millime- waveform, allow safe patient monitoring during these brief ter grids, such that with a standard sweep speed of 25 mm/s, periods of ESU deployment (Fig. 36.12). each 1 mm is a 40 ms interval, and a darker line appearing Other common ECG artifacts have been identified, and every 5 mm indicates a 200 ms interval (see Fig. 36.11). As include common sources such as the 60 Hz interference noted, the recorded gain is indicated by a 1 mV rectangular from other medical devices near the patient (see Fig. 36.8B) calibration signal, the standard being 10 mm/mV. and less common sources such as the cardiopulmonary Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 1154 SECTION III Anesthesia Management BOX 36.1 Equipment or Component- V Related Electrocardiographic Artifacts □ Monitor/components □ Manufacturing problems (50/60-Hz filter) 0.1 □ Defective monitor insulation □ Orthopedic shaver R □ Intraoperative MRI □ Sinus endoscope □ Pressure-controlled irrigation pumps ISO ST □ Flexible bronchoscopes T □ ESWL □ Digital urine output-core temperature monitors □ Intravenous fluid warmer/warming sets □ Cardiopulmonary bypass machine □ Ventilator—HFOV □ Electrostimulators □ Spinal cord, peripheral nerve, thalamic, vagal nerve, transcu- Fig. 36.13 Enlarged display of ECG lead V, showing the isoelectric (ISO) taneous nerve, other and ST-segment (ST) measurement points during continuous computer- □ Evoked potential monitoring units aided ST-segment monitoring. The ECG R and T waves are also identified. □ Hemodialysis machines This V lead ECG complex is shown at standard gain (10 mm/mV) in the □ Cellular telephones upper right corner of the panel. The computer measures and displays 0.1 mm (0.01 mV) of ST-segment elevation in this lead. (From Mark JB. Atlas of ESWL, Extracorporeal shock-wave lithotripsy; HFOV, high-frequency oscil- Cardiovascular Monitoring. New York: Churchill Livingstone;1998.) latory ventilation; MRI, magnetic resonance imaging. Adapted from Patel SI, Souter MJ. Equipment-related electrocardio- 4 15 min graphic artifacts: causes, characteristics, consequences, and correc- ST Lead V5 tion. Anesthesiology. 2008;108(1):138–148. 0 bypass machine.22 Patel and Souter provide a comprehen- sive list of reported sources of ECG artifact (Box 36.1).23 -4 ELECTROCARDIOGRAM MONITORING FOR MYOCARDIAL ISCHEMIA 100 The ST segment, representing myocardial repolarization, is the ECG component most sensitive to acute myocardial isch- HR emia. ST elevation, with or without tall positive (hyperacute) T waves, indicates transmural ischemia and is most often the result of acute coronary artery occlusion either by coro- 0 nary thrombosis or vasospasm (Prinzmetal-variant angina). Reciprocal ST-segment depression may appear in the contra- 200 lateral leads. Ischemia confined to the subendocardial area is usually denoted by ST-segment depression. Subendocardial, MAP ST-depression–type ischemia typically occurs during epi- sodes of symptomatic or asymptomatic (silent) stable angina pectoris, and is characteristic of ischemia occurring during exercise, tachycardia, or pharmacologic stress testing in 0 patients with significant but stable coronary artery disease. Fig. 36.14 One-hour trend displays for computer-aided continuous ST-segment monitoring in lead V5, heart rate (HR, beats/min), and mean Automated Real-Time ST-Segment Monitoring arterial pressure (MAP, mm Hg). A 15-minute episode of ST-segment depression (arrows) occurs without significant accompanying changes Real-time ST-segment analysis first appeared in cardiac in HR or MAP. (From Mark JB. Atlas of Cardiovascular Monitoring. New York: monitoring in the mid-1980s and is currently standard in Churchill Livingstone;1998.) most ECG monitors. On some monitors, the ST-segment analysis is set up to turn on automatically, but while ST- the addition of computerized ST-segment ischemia moni- segment analysis is commonplace in the operating room, it toring improves patient outcomes after surgery. is underutilized in other monitoring settings. A recent study Computerized ST-segment analysis is achieved by the moni- has shown that even among coronary care units, less than tor measuring the ST segment at 60 or 80 ms after the J point 50% routinely use ST-segment monitoring for the detec- (termed as J+60 or J+80 ms) and comparing it with the iso- tion of myocardial ischemia in patients admitted with acute electric point measured during the PR interval (Fig. 36.13). coronary syndromes.24 Chief among the reasons for the One millimeter of ST-segment deviation is equivalent to a 0.1 underuse of ST-segment analysis are the frequent number mV difference. The changes in ST-segment level over time of false alarms and the lack of education on how to use the in each lead can be displayed as ST-segment trends, just like technology. In addition, no evidence exists as to whether trend displays of other hemodynamic variables (Fig. 36.14). Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  36 Cardiovascular Monitoring 1155 4 ST Lead II 0 15 min -4 R ISO ST T Fig. 36.15 Erroneous identification of the isoelectric (ISO) point measurement during computer-aided continuous ST-segment monitoring. A 1-hour trend recording of ST-segment deviations in lead II, displayed as millimeters of ST displacement, shows approximately 15 minutes of ST-segment depression, which reaches 3 mm in magnitude during this episode (top panel). The enlarged display of ECG lead II showing the ISO and ST-segment (ST) measurement points during the episode of ST-segment depression reveals that the ISO point is identified inappropriately at the peak of the P wave, thereby producing artifactual ST-segment depression (bottom panel). (From Mark JB. Atlas of Cardiovascular Monitoring. New York: Churchill Livingstone;1998.) Simultaneously, V5 S-wave amplitude and absolute ST- When the computer misidentifies the appropriate isoelectric segment deviation were reduced. These investigators or ST monitoring point, the clinician can manually adjust the concluded that inclusion of an R-wave gain factor might J-point or the ST-segment measurement point (Fig. 36.15). improve perioperative ECG ischemia monitoring. One major advantage of continuous ST-segment moni- 2. Many patients have preexisting ECG abnormalities that toring is that the electrodes stay in place and do not vary as confound interpretation of ST-segment changes. Early they may with serial 12-lead ECG recordings. However, for repolarization (a normal variant), intraventricular con- improved diagnostic accuracy of ST-segment monitoring, duction delays, LV hypertrophy, digitalis, pericarditis, the following points should be recognized:    and other conditions may cause baseline ST-segment 1.  Changes in body position may cause ST-segment abnormalities. In these conditions, standard ECG crite- changes and lead to false ST-segment alarms. However, ria for diagnosing myocardial ischemia are less specific. changes in the QRS complex almost always accompany 3. Most cardiac monitors with ST-segment monitoring these positional ST-segment changes and therefore can software provide displays of ST-segment trends in a sin- be easily distinguished from true ST-segment devia- gle lead or the sum of absolute ST-segment deviations tions (Fig. 36.16). Changes in position of the heart in from multiple leads (Fig. 36.18). Although such graphic the mediastinum have also shown to affect the ST seg- trends are convenient for the quick identification of ment. Mark and associates observed that placement of a potential ischemic events, analysis of the ECG waveform sternal retractor during cardiac surgery was associated on the monitor screen or by recording the ECG tracing is with a reduction in V5 R-wave amplitude (Fig. 36.17).25 of paramount importance for verification. Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 1156 SECTION III Anesthesia Management 1 sec 1 sec 1 mV V5 1 mV Supine I II V5 Standing 0.1 -0.6 -0.8 I II V5 0.1 0.4 0.1 (R) Side Down Fig. 36.18 Computer-aided continuous ST-segment monitoring. A baseline recording of lead V5 shows isoelectric ST segments (top panel). Shortly after induction of anesthesia, the ST-segment monitoring display shows the three monitored leads I, II, and V5 and the absolute amount of ST-segment elevation (0.1 mm in lead I) or depression (−0.6 mm in lead II, −0.8 mm in lead V5) in each lead (middle panel). A trend line is (L) Side Down displayed on the right side of the panel and demonstrates that the sum of ST-segment deviations in these three monitored leads has increased and reached a plateau over the previous few minutes. Another ST-seg- ment display recorded 5 minutes later shows resolution of these subtle Fig. 36.16 Effect of changing body position on the ECG in a patient with ST changes (bottom panel). Note that the appearance of lead V5 in this preexisting ST-segment depression. Lead CC5, a surrogate for lead V5, is last display closely resembles the baseline recording and that the trend recorded with the patient in four different positions: supine, standing, right line has returned to pre-induction baseline level. (From Mark JB. Atlas of (R) side down, and left (L) side down. The magnitude of ST-segment depres- Cardiovascular Monitoring. New York: Churchill Livingstone;1998.) sion changes in direct proportion to the R-wave amplitude. (From Mark JB. Atlas of Cardiovascular Monitoring. New York: Churchill Livingstone;1998.) 1 Sec 1 sec 1 mV 1 mV Fig. 36.19 Subendocardial ischemia produces ST-segment depression. Fig. 36.17 Effect of surgical retraction on the ECG. Baseline lead As heart rate increases progressively from 63 beats/min (top panel) V5 recording shows 2 mm ST-segment depression and 27 mm R-wave to 75 beats/min (middle panel) and finally to 86 beats/min (bottom, amplitude (top panel). Placement of a sternal retractor during cardiac panel) in this patient with left main coronary artery disease, the ST seg- surgery displaces the precordial lead electrode relative to the heart, ment becomes more depressed and more downsloping, owing to an resulting in a marked reduction in R-wave amplitude to 10 mm and a increase in myocardial oxygen demand. (From Mark JB. Atlas of Cardiovas- proportional reduction in the magnitude of ST-segment displacement cular Monitoring. New York: Churchill Livingstone;1998.) (bottom panel). (From Mark JB. Atlas of Cardiovascular Monitoring. New York: Churchill Livingstone;1998.) testing and with acute subendocardial ischemia, the electri- cal forces responsible for the ST segment are deviated toward Electrocardiogram Criteria for Acute Myocardial the inner layer of the heart, causing ST-segment depression Ischemia or demand-mediated ischemia (Fig. 36.19). With acute trans- The ECG criteria most accepted for detecting myocardial isch- mural epicardial ischemia, the electrical forces in the ischemic emia during continuous ECG monitoring are those established area are deviated toward the outer layer of the heart, caus- and validated during exercise stress testing.26 During stress ing ST-segment elevation or supply-mediated ischemia in Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  36 Cardiovascular Monitoring 1157 1 sec responsible for the ischemic episode. Because the major- ity of modern patient-monitoring systems do not simul- taneously monitor all 12 ECG leads, selecting which chest leads to monitor is of great importance, particu- 1 mV larly in noncardiac surgery. During exercise stress test- ing, investigators have identified leads V4 and V5 as the most sensitive leads to detect exercise-induced ischemia (90%-100% sensitivity).28 London and colleagues stud- ied high-risk patients undergoing noncardiac surgery and showed that the greatest sensitivity for ischemia was obtained with lead V5 (75%), followed by lead V4 (61%).15 Combining leads V4 and V5 increased the sensitivity to 90%, whereas with the standard lead II and V5 combina- tion, the sensitivity was only 80%. They also suggested that if three leads (II, V4, and V5) could be simultane- ously examined, the sensitivity would increase to 98%. More recently, Landesberg and associates monitored continuous 12-lead ST-segment changes greater than 0.2 mV from baseline in a single lead or more than 0.1 mV in two contiguous leads at J+60 ms, lasting longer than 10 minutes in patients undergoing major vascular surgery.16 They showed that leads V3 and V4 were more Fig. 36.20 Transmural ischemia produces ST-segment elevation. sensitive than V5 in detecting perioperative ischemia Occlusion of a patent saphenous vein graft during repeat coronary (87%, 79%, and 66%, respectively). As a result of these artery bypass surgery causes an abrupt reduction of coronary blood and other investigations, it appears most appropriate to supply and results in progressive ST-segment elevation (middle and monitor lead V3, V4, or V5 for optimal detection of peri- bottom panels). (From Mark JB. Atlas of Cardiovascular Monitoring. New operative ST-segment depression, choosing the specific York: Churchill Livingstone;1998.) lead location based on whether the lead placement might interfere with the surgical prep and procedure. the overlying leads (Fig. 36.20). Note that most commonly, intraoperative or perioperative ischemia is demand-mediated subendocardial ischemia, which is manifest as ST-segment Blood Pressure Monitoring depression.27 Although the anterolateral precordial leads are most sensitive for detecting these changes, they are non- Like heart rate, blood pressure has long been a fundamen- localizing as to the coronary distribution responsible. In tal cardiovascular vital sign included in the mandated contrast, supply-mediated transmural ischemia, while much standards for basic anesthetic monitoring.29 Measuring less common perioperatively, causes ST-segment elevation in blood pressure is primarily performed with either indirect leads overlying the involved coronary bed and thus are able cuff devices or direct arterial cannulation with pressure to localize the responsible coronary territory (e.g., ST eleva- transduction. These techniques measure different physical tions in inferior leads II, III, aVF suggest occlusion of the right signals and differ in their degree of invasiveness, but both or posterior descending coronary artery). are subject to numerous confounding factors that often With demand-mediated ischemia, as heart rate increases, result in significant discrepancies among simultaneous J-point depression and upsloping ST-segment depression measurements.30 occurs. As the severity of ischemia progresses, the ST seg- ment typically becomes horizontal (flattens) and the extent INDIRECT MEASUREMENT OF ARTERIAL BLOOD of ST-segment depression may increase and the ST seg- ment may become downsloping (see Fig. 36.19). Standard PRESSURE criteria for stress-induced ischemia are 1 mm (0.1 mV) or Manual Intermittent Techniques more of horizontal or downsloping ST-segment depression Most indirect methods of blood pressure measurement uti- measured 60 to 80 ms after the J point. As noted earlier, lize a sphygmomanometer, first described by Riva-Rocci in patients with preexisting ST-segment abnormalities make 1896.31 The systolic pressure was identified using an inflat- ST-segment interpretation more difficult. able elastic cuff around the arm and a mercury manometer During the perioperative period, ECG monitoring to measure cuff pressure, while the radial arterial pulse most commonly identifies stress-induced, ST-depression, was palpated as the cuff pressure was increased or rapidly demand-mediated ischemia, or other causes of supply- decreased. The technique was later modified to detect both demand imbalance such as prolonged or severe hypo- systolic and diastolic pressure with description of ausculta- tension. Such ECG changes do not provide information tory method of blood pressure measurement by Korotkoff about the location of the ischemic myocardial area. In in 1905.32 Korotkoff sounds are a complex series of audible contrast, ST-segment elevation indicating transmural or frequencies produced by turbulent flow beyond the par- supply-mediated ischemia, observed particularly during tially occluding cuff. Put simply, the systolic pressure is cardiac surgery, provides useful information about the associated with first sound heard (beginning of turbulent myocardial segment and coronary perfusion territory flow through the vessel) and diastolic pressure at the point Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 1158 SECTION III Anesthesia Management when the sounds disappear (when vessel flow becomes lam- inar).33 Mean blood pressure cannot be measured using this BOX 36.2 Complications of Noninvasive technique. Blood Pressure (NIBP) Measurement A fundamental principle of the auscultatory method is its Pain reliance on blood flow to generate Korotkoff sounds. Physi- Petechiae and ecchymoses ologic conditions that interfere with sound detection (e.g., Limb edema severe edema, obesity, abnormal compliance of overlying Venous stasis and thrombophlebitis tissue) or blood flow (shock, intense vasoconstriction) will Peripheral neuropathy frustrate manual blood pressure measurement.33-37 Fur- Compartment syndrome thermore, the cuff must also be snugly fitted, with a blad- der that measures 40% of arm circumference and 80% of length of the upper arm, and centered over the artery. A cuff that is too large will often yield acceptable results when result in significant error in blood pressure measurement, used for manual measurements, but a small cuff will usually especially in systolic and diastolic estimation, although yield falsely high readings.36 averaging three measurements seems to minimize the clini- cal impact.49,50 Automated Intermittent Techniques While the upper arm is the most common cuff location, Automated noninvasive blood pressure (NIBP) devices are various factors may force choice of an alternative site. In the most commonly used means of measuring blood pres- obese patients, there is little agreement between any alter- sure in the operating room. Small oscillations in pressure nate location and invasively measured pressures while amplitude are measured in an air-filled cuff that slowly ankle, calf, and thigh cuffs have never been validated at deflates from a pressure well in excess of that needed to col- all.51 Interestingly, the forearm may be a preferable site to lapse the underlying artery. The point of maximal oscillation upper arm in obese patients, although such cuffs display a marks the mean arterial blood pressure (MAP), with systolic reversed pattern of bias; overestimation of the systolic and and diastolic being calculated by various proprietary algo- underestimation of the diastolic pressure.48 rithms specific to individual device manufacturers.33,35,36 It is important to remember the auscultatory method In general, a cuff that is too large will underestimate the measures the systolic and diastolic pressures while oscil- blood pressure while a small cuff will overestimate.36,38 Of lometric devices measure the mean and calculate the the three possible measurements, systolic has the poorest systolic and diastolic, albeit in different and non-inter- agreement with invasive blood pressure values.38,39 changeable ways. Furthermore, directly-measured arte- Although automated NIBP measurements have gener- rial pressure measurements utilize another technique ally been shown to approximate directly measured arterial altogether. In some authors’ opinions, “current protocols pressures, there are also important shortcomings to keep for validating blood pressure monitors give no guarantee in mind.36,38 For reasons involving the ethics of validation of accuracy in clinical practice.”40,52 Expecting them to against invasive measurements, standards for device per- yield identical values is unrealistic, especially in complex formance established by the Association for the Advance- and unstable clinical situations. The sources of error vary ment of Medical Instrumentation (AAMI) and the British significantly for each of these measurement techniques Hypertension Society are defined by auscultation.40 New and should closely guide evaluation and therapeutic inter- devices must demonstrate average differences ≤ ± 5 mm vention, especially when there is discrepancy between the Hg and standard deviations ≤ 8 mm Hg, which means that measured values or between measurements and clinical deviations of up to 20 mm Hg are still considered “accept- conditions. able performance.”41 Clinical studies comparing NIBP with direct arterial pres- Complications of Noninvasive Blood Pressure sure measurements reflect the problematic nature of NIBP Measurement monitoring. Direct comparisons of oscillometric devices to Although automated NIBP measurement is generally safe, invasive monitoring have shown that mean blood pres- there have been reports of rare but severe complications sure measurements generally show the greatest degree of (Box 36.2).53 Compartment syndrome is possible after pro- agreement with invasive blood pressure readings while longed periods of frequent cycling and is most likely related systolic measurements are the most divergent.39,42-44 to trauma or impaired distal limb perfusion. Caution should Agreement is especially problematic in critically ill or older be exercised in cases of peripheral neuropathy, arterial or patients.38,42-44 venous insufficiency, severe coagulopathies, or recent use Oscillometric NIBP values tend to underestimate MAP of thrombolytic therapy. values during periods of hypertension and overestimate during hypotension, potentially biasing clinical decisions in Automated Continuous Techniques unstable patients.45 And they often underestimate the sys- Continuous methods for NIBP monitoring are being tolic while overestimating the diastolic measurements.46 developed with various degrees of success. The most cur- A well-fitted arm cuff does appear to be helpful in identi- rent version is based on the volume clamp technique and fying hypotension in unstable patients and differentiating involves photoplethysmography and closed loop continu- responses to therapy in such situations, but below a MAP ous control of a pressure cuff around a finger. This creates of 65 mm Hg, it is not useful for titration of therapy, and a a stable arterial pressure waveform via quantification of more frequent interval of measurement is probably required an infrared beam applied distal to the finger cuff. Many to be considered reliable.38,42,47,48 Dysrhythmia may also of these require initial calibration with a standard NIBP Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved.  36 Cardiovascular Monitoring 1159 radial artery harvest have reported no significant decrease BOX 36.3 Indications for Arterial relative to the contralateral hand in either the early or late Cannulation postoperative periods.73-78 Continuous, real-time blood pressure monitoring Before radial artery cannulation, some clinicians assess Anticipated pharmacologic or mechanical cardiovascular manipu- collateral blood flow to the hand by performing a modified lation Allen test, originally described in 1929 to assess arterial Repeated blood sampling stenosis in patients suffering from thromboangiitis obliter- Failure of indirect arterial blood pressure measurement ans.79 The radial and ulnar arteries are both compressed Supplementary diagnostic information from the arterial waveform while the patient makes a tight fist to exsanguinate the palm and then slowly reopens it. As occlusion of the ulnar artery is released, the color of the open palm is observed. Normally, cuff, and all are significantly affected by changes in vascu- the color will return to the palm within several seconds; lar tone and perfusion in the finger, as well as movement, severely reduced collateral flow is present when the palm vascular disease, and a host of other factors.54 While none remains pale for more than 6 to 10 seconds. Unfortunately, of these devices technically meet AAMI standards when the predictive value of this test is poor. There are numerous compared with invasive pressures, clinical studies have reports of ischemic sequelae in the face of a normal Allen shown their correlation to be reasonable in a variety of test, and conversely, reports of uncomplicated radial cathe- operative cases.35,55-58 ter use and even harvest for bypass grafting in the presence Other devices use technologies related to pulse transit of an abnormal result.72,73,80,81 In recent years, the radial time or arterial tonometry.59,60 All of these techniques, artery has become more popular for coronary catheteriza- however, have limitations, including need for calibration, tion and stenting access, even in individuals with abnormal sensitivity to motion artifact, and limited applicability in Allen tests.82 Overall, the diagnostic accuracy of the modi- critically ill patients.36,61,62 It remains unclear whether any fied Allen test with a 5-second threshold is only 80% with noninvasive technique will reduce the need for direct arte- 76% sensitivity and 82% specificity. It appears that the rial pressure monitoring during anesthesia or critical care, test is unable to provide a cutoff point below which perfu- but with continued development and technical refinement, sion can be deemed vulnerable.83 Use of pulse oximetry, they remain promising. plethysmography, or Doppler ultrasound as adjuncts does not seem to improve its accuracy. Oximetry detects blood DIRECT MEASUREMENT OF ARTERIAL BLOOD flows at extremely low flows, leading to poor specificity, while there are no established ultrasound criteria by which PRESSURE to evaluate radial or ulnar blood flow.73,84,85 In general, it Arterial cannulation with continuous pressure transduc- seems that while a normal modified Allen test may be useful tion remains the accepted reference standard for blood pres- in identifying patients unacceptable for radial artery use for sure monitoring despite its risk, cost, and need for technical bypass graft or coronary angiography, there is no evidence expertise for placement and management (Box 36.3). Its that it predicts clinical outcomes following cannulation for superiority over noninvasive techniques for early detec- blood pressure monitoring.73 tion of interoperative hypotension was confirmed by The Techniques for radial artery cannulation have been Australian Incident Monitoring Study of 1993.42,63 More unchanged for decades with the notable exception of the use recently, though, the use of waveform analysis in physi- of ultrasound in guiding catheter placement. Evidence sup- ologic monitoring has become more popular. This was ini- ports its use, especially as a rescue method following a failed tially proposed more than a half century ago by Eather and attempt.86 Although evidence in the critical care setting sug- associates, who advocated monitoring of “arterial pressure gests that ultrasound techniques improve first-pass cannula- and pressure pulse contours” in anesthetized patients.64 tion success rates, it is not clear that this translates to improved Arterial pressure waveform characteristics used in current clinical outcomes, nor that the impact on time required to per- clinical practice include the dicrotic notch as a trigger for form the procedure or other factors warrants routine intra- intra-aortic balloon counterpulsation as well as respira- operative use.87-90 Ultrasound guidance in catheterization of tory-induced variation in an array of directly-measured other vessels other than the radial artery or in special popula- and derived pressure measurements to indicate preload tions, such as those on mechanical support without pulsatile reserve and volume responsiveness.65-68 Rates of intraop- flow, is more likely to be of substantial benefit.91 erative direct arterial blood pressure measurement vary significantly across clinical environments even for similar Alternative Arterial Pressure Monitoring Sites procedures.69 If the radial arteries are unsuitable or unavailable, there are multiple alternatives. The ulnar artery has been used safely Percutaneous Radial Artery Cannulation even following failed attempts to access the ipsilateral radial The radial artery is the most common site for invasive artery.72,92 Similarly, the brachial artery, while lacking col- blood pressure monitoring because it is technically easy lateral branches to protect the hand, has a long track record to cannulate and complications are rare.70,71 Slogoff et al. of safe use. Several investigators have reported large series described 1700 cardiovascular surgical patients who of brachial artery catheters in patients undergoing cardiac underwent radial artery cannulation without ischemic surgery with very few vascular, neurologic, or thrombotic complications despite evidence of arterial occlusion after sequelae.93,94 The axillary artery has the advantages of decannulation in more than 25% of patients.72 Further- patient comfort and mobility, and complications appear more, most investigations of hand perfusion following to be similar in incidence to those for radial and femoral Downloaded for Vicente Gonzalez ([email protected]) at Florida International University from ClinicalKey.com by Elsevier on June 02, 2024. For personal use only. No other uses without permission. Copyright ©2024. Elsevier Inc. All rights reserved. 1160 SECTION III Anesthesia Management