Chapter 16

Clinical Monitoring I

Cardiovascular System

Monitoring of anatomic and physiologic variables during an anesthetic procedure is vital to patient safety and meeting established standards of care.¹ Many different monitors are commonly used to assist in the delivery of an anesthetic, and clinicians assimilate the data provided to make appropriate clinical judgments. Consequently, the application of critical thinking skills, thorough physical assessment, vigilance, and the appropriate selection and application of monitors are key requirements in the process of anesthesia monitoring.

Fundamental monitoring/assessment techniques include inspection, auscultation, and palpation. They provide essential objective and subjective data not available from advanced monitoring modalities and can alert the anesthetist to occult problems in select patients. Inspection of the patient can provide information regarding the adequacy of oxygen delivery and carbon dioxide elimination, fluid requirements, and positioning and alignment of body structures. Auscultation is used to verify correct placement of airway devices such as the endotracheal tube and laryngeal mask airway, to assess arterial blood pressure, and to continually monitor heart sounds and air exchange through the pulmonary system. Palpation can aid in assessing the quality of the pulse and degree of skeletal muscle relaxation, as well as locating major vascular structures when placing central venous lines or performing regional anesthesia techniques.

Critical thinking skills are cardinal prerequisites for successful monitoring of a patient’s anesthetic. In addition, it is well known that errors in anesthesia care are minimized when clinicians remain alert and vigilant. This chapter reviews the more commonly used noninvasive and invasive cardiovascular monitors in anesthesia practice.

ECG Monitoring

The continuous monitoring of the cardiovascular status via the electrocardiogram (ECG) is a requirement for any patient receiving an anesthetic. This includes assessment of heart rate, rhythm, and in particular for some patients, ST segments. Computerized real-time ST-segment analysis continues to be incorporated in operating rooms (ORs), intensive care units (ICUs), and postanesthesia care units (PACUs) across the country. Many factors support this trend, including the development of practice guidelines by professional societies that advocate such monitoring techniques in select patient populations ² and the demographics of the general surgical population. Approximately one third of patients scheduled for noncardiac surgery have risk factors for coronary artery disease (CAD), and postoperative myocardial infarction is three times as frequent in patients with ischemia.³ Research has shown prolonged stress-induced ischemia (i.e., ST-segment depression) to be the major cause for cardiac morbidity (myocardial infarction) after major vascular surgery.⁴ The overall incidence of perioperative ischemia in patients with CAD scheduled for cardiac or noncardiac surgery ranges from 20% to 80%.^5,6

Because of its low cost, noninvasiveness, widespread availability, and designation as a standard of care for monitoring of all anesthetized patients,¹ the ECG remains a common and required diagnostic tool in the operating room. Compared with Holter monitors, ST-segment trending monitors have on average a sensitivity of 74% and specificity of 73% in detecting myocardial ischemia.⁷ When used in high-risk cardiac patients to guide early treatment, they may reduce morbidity.⁴

Current recommendations for ST-segment deviation thresholds account for the influence of gender, ECG lead, age, and race on position of the ST segment.8–11 In particular, two chest leads (V₂ and V₃) have been shown to exhibit the greatest shift of the ST junction and as such must be accounted for in applying diagnostic criteria for myocardial injury. Otherwise the proportion of false positives would increase.

The degree of elevation (or depression) is relative to an isoelectric line, which is commonly referenced as the PR segment. The PR segment extends from the end of the P wave to the start of ventricular depolarization (e.g., appearance of a Q wave) (Figure 16-1). The ST junction is defined as the point at which the QRS complex ends and the ST segment begins. It is also synonymous with the J point. In the clinical setting some biomedical engineers have eliminated the J point in their computerized ST-segment analysis software algorithms (Figure 16-2). Recommended threshold values for ST-segment changes are listed in Table 16-1.⁸ When these threshold values are met, an imbalance between oxygen supply and demand may exist (e.g., myocardial injury).

TABLE 16-1

Threshold Values for ST-Segment Deviation

mV, Millivolt;

^∗right ventricular ECG leads;

^†posterior chest leads.

From Galen SW, et al. AHA/ACCF/HRS Expert Consensus Document: AHA/ACCF/HRS Recommendations for the standardization and interpretation of the electrocardiogram. Part VI: Acute ischemia/infarction: A Scientific Statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society. Endorsed by the International Society for Computerized Electrocardiology. J Am Coll Cardiol. 2009;53:1003-1011.

FIGURE 16-1 A single cardiac cycle (ST snippet) and enlarged section of the snippet providing greater details. The PR segment is measured from the end of the P wave to the beginning of the QRS complex. A, Depicts a depressed ST-segment that is upsloping. The junction between the S wave and ST segment defines the ST pointB, The PR segment is extended out via a horizontal line. This serves as an isoelectric reference (no deviation of the ECG stylus upward or downward) to determine the degree of ST-segment shift. The distance from the extended PR segment to the ST point demonstrates that the ST segment is 3.7 mm depressed (i.e., J point depression of 3.7 mm). (Reprinted with permission from Kossick MA. EKG Interpretation: Simple, Thorough, Practical. 3rd ed. Park Ridge, Ill: AANA Publishing. In press.)

FIGURE 16-2 Two ST snippets illustrating two different techniques to calculate ST-segment depression values. ST snippet (A) measures ST-segment deviation 80 ms (2 mm) from the J point. ST snippet (B) measures ST-segment depression at the J point. The most recent American Heart Association/American College of Cardiology Foundation/Heart Rhythm Society recommendations for standardizing and interpreting the electrocardiogram advocate measuring ST-segment changes at the J point (i.e., B). (Reprinted with permission from Kossick MA. EKG Interpretation: Simple, Thorough, Practical. 3rd ed. Park Ridge, Ill: AANA Publishing. In press.)

It should be appreciated when critiquing the literature that researchers may use the J point in combination with the ST point as a means to determine the degree of ST-segment deviation. Examples would include assessing the extent of ST-segment shift by measuring 60 ms (1.5 mm) or 80 ms (2 mm) from the J point. Figure 16-2 contrasts these two means of calculating ST-segment deviation. Measuring the degree of ST-segment depression or elevation at the J point is the author’s preferred method to most accurately assess ST-segment deviation values.

For anesthesia providers who use ST-segment analysis software that incorporates a J point in combination with an ST point, caution should be exercised in blindly accepting numeric ST-segment deviation values. Shortened ST segments are predictably associated with tachyarrhythmias, which can result in T waves encroaching on ST segments. Should this occur, the use of a J + 80-ms or even J + 60-ms distance could lead to an ST point intersecting a T wave instead of the ST segment. In this circumstance, the computer-derived ST-segment deviation value would reflect a false significant shift in the ST segment, suggesting myocardial injury (false positive) or masking a significant ST-segment depression (false negative) (Figure 16-3).

FIGURE 16-3 Two cardiac cycles from lead III and V₄ illustrating a shortened ST segment. Use of a J + 80 ms value to measure ST-segment deviation in each lead results in the ST point intersecting the T wave, thus producing inaccurate ST-segment deviation values. For lead III the misplaced ST point produces a false positive and for lead V₄ a false negative. (Reprinted with permission from Kossick MA. EKG Interpretation: Simple, Thorough, Practical. 3rd ed. Park Ridge, Ill: AANA Publishing. In press.)

Regarding the significance of the various forms of ST-segment depression, it is important to recall that a horizontal or downsloping depressed ST segment has greater specificity (fewer false positives) than an upsloping depressed ST segment. Adding upsloping ST-segment changes to myocardial injury diagnostic criteria does improve overall sensitivity but at a sacrifice to specificity and positive predictive value.12,13

Setting the ST-Segment Parameters

Most manufacturers of computerized ST-segment analysis monitors have sophisticated algorithms that allow fairly consistent and accurate placement of ST measurement points. Nevertheless, these parameters should be periodically assessed these parameters and changed as needed; responding to false trends secondary to incorrectly placed ST measurement points could lead to iatrogenic injury. In fact, manufacturers have incorporated software that permits healthcare providers to override the monitor’s placement of ST measurement points. A common technique for setting ST measurement points involves adjustment of two (Iso point and ST point) or three (Iso point, J point, and ST point) variables. Manipulation of a keypad or touchscreen device on the ECG monitor permits the operator to scroll each of these “points” along a horizontal axis. The “points” are depicted as vertical lines that intersect various components of a single cardiac cycle (Figure 16-2). Figure 16-4 illustrates the consequences when real-time ST-segment analysis software incorrectly places the Iso point on the apex of the P wave. The application of an ST-segment deviation algorithm can reduce the occurrence of such mishaps and improve overall management of patients at risk for ischemic changes (Figure 16-5).

FIGURE 16-4 Iso point incorrectly placed on top of the P wave by an electrocardiogram (ECG) monitor, producing an exaggerated ST-segment deviation value. (Reprinted with permission from Kossick MA. EKG Interpretation: Simple, Thorough, Practical. 3rd ed. Park Ridge, Ill: AANA Publishing. In press.)

FIGURE 16-5 An ST-segment deviation assessment and treatment algorithm. (Reprinted with permission from Kossick MA. EKG Interpretation: Simple, Thorough, Practical. 3rd ed. Park Ridge, Ill: AANA Publishing. In press.)

Other significant variables to account for when monitoring patients at risk for ischemic events include ECG electrode placement, ECG lead selection, gain setting, and frequency bandwidth. Each of these is briefly reviewed here.

Electrocardiograph Electrode Placement

It is fairly common to see ECG electrodes placed incorrectly on a patient in an attempt to “move an operating room schedule along.” Many times with a physical status (PS) I or II patient, accurate ECG electrode placement is not a critical issue. However, in patients with risk factors for CAD, such inattentiveness can lead to iatrogenic injury by producing deviated ST segments, inverted T waves, or pathologic Q waves that can be viewed as “real” problems. Proper placement of the limb lead and chest lead electrodes is described in Table 16-2. For emphasis, the precordial leads should be placed via palpation of the costae, not by gross visual estimation of an intercostal space (Figure 16-6). Understandably, some surgical procedures do not permit the use of optimal ECG lead selection and placement; ECG electrode(s) can interfere with skin preparation and surgical incision. Under these circumstances, a less than optimal ECG lead placement is acceptable and rationale should be documented.

TABLE 16-2

Proper Placement of Electrocardiographic Electrodes for Monitoring Chest Leads and Limb Leads via the Mason-Likar Lead Position

Lead Name	Placement
RA	Over the outer right clavicle^∗
LA	Over the outer left clavicle^∗
LL	Near the left iliac crest or midway between the costal margin and left iliac crest, anterior axillary line^∗
RL	At any convenient location on the body (e.g., upper right shoulder)
V₁	Fourth intercostal space right of the sternal border
V₂	Fourth intercostal space left of the sternal border
V₃	Equal distance between V₂ and V₄
V₄	Midclavicular line at the fifth intercostal space
V₅	Horizontal to V₄ on the anterior axillary line or if difficult to identify (anterior axillary line), then midway between V₄ and V₆
V₆	Horizontal to V₅ on the midaxillary line
V₇	Horizontal to V₆ on the posterior axillary line
V₈	Horizontal to V₇ below the left scapula
V₉	Horizontal to V₈ at the left paravertebral border
V₃R	Placed right side of chest wall in mirror image to chest lead V₃
V₄R	Placed right side of chest wall in mirror image to chest lead V₄

LA, Left arm electrocardiographic (ECG) electrode; LL, left leg ECG electrode; RA, right arm ECG electrode; RL, right leg ECG electrode;

^∗Mason-Likar ECG electrode placement.

From Krucoff MW, et al. Simultaneous ST-segment measurements using standard and monitoring-compatible torso limb lead placements at rest and during coronary occlusion. Am J Cardiol. 1994; 74(10):997-1001; Paul K, et al. AHA/ACC/HRS scientific statement: Recommendations for the standardization and interpretation of the electrocardiogram. Part I: The electrocardiogram and its technology: A Scientific Statement from the American Heart Association Electrocardiography and Arrhythmias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society. Endorsed by the International Society for Computerized Electrocardiology. Heart Rhythm. 2007;4:394-412.

FIGURE 16-6 Precordial electrodes V₂, V₃, V₄ positioned across the ventrolateral aspect of the thorax. V₂ placement: 4th intercostal space (ICS) left of the sternum; V₃: equal distance between V₂ and V₄.; V₄: left midclavicular line at the 5th ICS. (Reprinted with permission from Kossick MA. EKG Interpretation: Simple, Thorough, Practical. 3rd ed. Park Ridge, Ill: AANA Publishing. In press.)

Electrocardiographic Lead Selection

The decision regarding which ECG leads to monitor during the course of an anesthetic can be extremely important relative to the medical history of the patient. Improper selection can result in unrecognized myocardial ischemia, injury, or infarction. Research has validated that use of a single ECG lead for ischemic monitoring in patients with documented CAD is inadequate; monitoring with multiple leads enhances patient safety. In patients at risk for ischemic events, this author recommends the maximum number of ECG leads be displayed (e.g., 3, 7, 12 [derived 12-lead]) during the perioperative period to enhance continuous and comprehensive assessment of ST-segment and T-wave changes (Figures 16-7 and 16-8). Which lead(s) is/are best in detecting significant ST-segment changes remains somewhat controversial. First and foremost, if a preoperative 12-lead ECG has been done, “fingerprinting” of this tracing should serve as the primary guide for lead selection during the perioperative period. If the baseline 12-lead shows significant primary ST-segment changes in limb leads III and aVF, then this lead set should be prioritized for continuous display in the operating room. The ECG monitoring system software will dictate what lead display options can be configured. For example, with Philips software and a five-cable ECG lead system, a derived 12-lead (EASI) can be continuously displayed (see Figure 16-8). With other manufacturers and a five-cable ECG lead system, a true V lead (e.g., V₃), a modified chest lead V₅ (e.g., central subclavicular 5 [CS₅]), and a bipolar limb lead (e.g., lead II) can be configured for ECG monitoring (Figure 16-9).

FIGURE 16-7 Monitoring in three electrocardiogram (ECG) leads during anesthetic administration captured significant ST-segment elevation. The greatest change in ST segments occurred in limb lead III, followed by limb lead II. Noteworthy was the failure of lead V₅ to demonstrate any appreciable change in the ST segment. Postoperatively a cardiology consultation resulted in a diagnosis of Prinzmetal angina.

FIGURE 16-8 Derived 12-lead electrocardiogram (ECG [EASI]) recorded just prior to anesthetic induction in the operating room. The ECG monitor can be configured to continuously display all 12 leads for comprehensive assessment of ST segments and rhythm changes.

FIGURE 16-9 Preoperative electrocardiogram (ECG) configured with a five-cable ECG system to continuously display limb lead II (right arm [RA] negative and left leg [LL] positive), true V₃ chest lead (C), and modified chest lead 5 (MCL₅, i.e., central subclavicular 5 [CS₅]). V₃ electrode placement (C): equal distance between V₂ and V₄. Lead selector switch set to display lead I causes the RA electrode to become negative and the left arm (LA) electrode (placed in the V₅ position) to become positive (CS₅). (Reprinted with permission from Kossick MA. EKG Interpretation: Simple, Thorough, Practical. 3rd ed. Park Ridge, Ill: AANA Publishing. In press.)

In patients without a preoperative 12-lead or those who have a baseline 12-lead that is unremarkable, the literature suggests leads V₃, V₄, V₅, limb lead III, and aVF (in this order of preference) be selected for continuous monitoring for ST-segment elevation or depression.14–18 Lead II is recommended for assessment of narrow QRS complex rhythms, particularly if the P wave is significant for diagnostic criteria (e.g., atrial flutter, atrial fibrillation, junctional rhythms).

The 1988 recommendation for V₅ and limb lead II as preferred ECG leads ¹⁹ in patients where ST-segment monitoring is desired has been mitigated by other researchers, critical care task forces, and major publications.^{2,15,17,20,21} In 2002 Landesberg and colleagues studied 185 consecutive patients undergoing vascular surgery who were monitored by continuous 12-lead ST-trend analysis during the perioperative period and up to 72 hours postoperatively. Chest lead V₃ was found to detect ischemia earliest and most frequently (86.8%). Lead V₄ was the second most diagnostic lead (78.9%), and V₅ was third (65.8%). With those patients sustaining a myocardial infarction, V₄ was the most sensitive lead (83.3%), with V₃ and V₅ being the second most sensitive (75%).¹⁷ In this study, myocardial infarction was diagnosed if cardiac troponin I levels were greater than 3.1 ng/mL and were accompanied by symptoms of ischemia or the presence of ECG criteria (i.e., ST-segment elevation, ST-segment depression, or large Q waves). Of interest was the observation that 97% of ischemic events were expressed as ST-segment depression—not elevation—and ST shifts were considered significant if their duration of change exceeded 10 minutes. As reported elsewhere in the literature, monitoring in multiple leads was advocated as a means to improve sensitivity for detecting ST-segment changes.^4,17–19

Given this information, it is prudent for anesthesia providers to monitor and assess multiple ECG leads in the operating room. In the absence of an ST-segment fingerprint, this author advocates the following ECG lead combinations (for ST-segment elevation or depression) in patients with documented or identified significant risk factors for ischemic heart disease:

1. For a five-cable ECG recording system, the three-lead set of V₃, MCL₅, and aVF (the use of MCL₅ precludes the use of limb lead III) or V₃ combined with limb lead III and aVF.

2. For a three-cable ECG recording system, a two-lead set comprising MCL₅ combined with limb lead aVF.

Extended monitoring capabilities help to optimize detection of regionalized myocardial ischemia. Many times this entails nothing more than changing the lead selector switch to another ECG lead (e.g., III changed to aVF) or displaying a multilead ECG when indicated or continuously during an anesthetic (Figure 16-10). The latter produces an ECG recording of all six limb leads and a single chest lead, permitting the anesthetist to more comprehensively assess ECG data, including arrhythmias, the mean QRS axis (limb leads I and aVF), T-wave morphology, ST-segment changes, QT intervals, and the presence of right bundle branch block.

FIGURE 16-10 Multilead electrocardiogram (ECG) displayed in the operating room. ECG leads displayed include limb leads I, II, III, aVR, aVL, aVF, and true chest lead V₃. This configuration permits a more comprehensive view of cardiac anatomy. (Reprinted with permission from Kossick MA. EKG Interpretation: Simple, Thorough, Practical. 3rd ed. Park Ridge, Ill: AANA Publishing. In press.)

With the introduction into clinical practice of the derived 12-lead system (EASI), nurses and physicians now have a convenient method with which to globally assess the overall well-being of the myocardium (see Figure 16-8). The 12-lead is derived from modified vectorcardiographic leads and requires the use of a five-cable ECG lead system.²² To monitor with this system, the five ECG electrodes are placed in the following locations: LA electrode over the manubrium; chest (V) electrode over the lower body of the sternum; LL electrode, left midaxillary, horizontal to the chest electrode; RA electrode, right midaxillary, also horizontal to the chest electrode; and RL electrode in any convenient location. Current and past research suggests the derived 12-lead is comparable (but not equivalent) to the standard 12-lead for multiple cardiac diagnosis in adults and children (e.g., ST-segment changes, myocardial infarction, wide QRS-complex tachycardia, QT-interval measurements).^23–25 It is possible that patients at substantial risk for CAD would benefit from global ischemic monitoring via a derived 12-lead. This software option also eliminates any need to consider “preferred” ECG leads because all six limb leads and six chest leads can be viewed during an anesthetic.

In contrast to a derived 12-lead or five-cable ECG electrode system, a three-cable system offers challenges to anesthesia providers concerning potential errors with ECG lead configuration. The literature documents that healthcare providers consistently struggle with modified chest lead configuration—even those who routinely monitor the ECG.²⁶ Modified chest leads offer an alternative to true chest leads when only a three-cable ECG recording system is available. Recently introduced into clinical practice is the modified chest lead MAC_1(L) (modified augmented chest lead V₁). This modified chest lead is configured using limb lead aVL and has been shown to have a diagnostic accuracy similar to true chest lead V₁. The internal validity of this finding was based on His-bundle recordings, used as the gold standard for distinguishing between premature ventricular ectopy and premature aberrantly conducted beats.²⁷ The simplicity of this unique ECG lead has the potential to reduce modified chest lead configuration errors. Research would need be done to substantiate this theoretical advantage (e.g., ease of configuration of MAC_1(L) versus modified chest lead V₁ [MCL₁]). Figure 16-11 illustrates the ECG configuration of MAC_1(L), as well as the similarities in morphologic characteristics of single cardiac cycles recorded in V₁ and MAC_1(L).

FIGURE 16-11 Modified augmented chest lead V₁ (MAC_{1 [L]}). It is configured by (1) using limb lead aVL, which causes the left arm (LA) electrode to become positive, and (2) placing the LA electrode in the V₁ position. The remaining ECG electrodes are placed in their normal positions. The two single cardiac cycles shown above illustrate great similarity in cardiac cycle morphology (rSR₁) between V₁ and MAC_1(L). (Modified and reprinted with permission from Kossick MA. Evaluation of a New Modified Chest Lead in Diagnosing Wide Complex Beats of Unknown Origin. Dissertation. Memphis, Tenn: The University of Tennessee Health Science Center; 2003:2, 9.)

In summary, practitioners who limit ECG monitoring and assessment to a single lead or pair of leads in patients with documented or recognized risk factors for ischemic heart disease are potentially compromising patient safety by not using (when available) multiple-ECG-lead display configuration options.² In such patients, the continuous display of three-ECG leads, a multilead ECG (six limb leads and one true chest lead), or a derived 12-lead could be of clinical benefit. The literature substantiates myocardial ischemia (T-wave and/or ST-segment changes) can be regionalized and completely missed when viewing two or fewer ECG leads. Similar concerns exists when three-cable ECG lead systems are used in place of available five-cable ECG lead systems. The latter permits the viewing of a true chest lead, which is always preferable over a modified chest. Unarguably, critical assessment of all available patient data will help anesthetists exercise better judgment during an anesthetic and potentially improve anesthetic outcome.

Gain Setting and Frequency Bandwidth

Two other potential problems with continuous ST-segment monitoring relate to the amplitude at which the ECG monitor has been set and whether filtering of the electrical signal is excessive. When accurate visual assessment of ST segments is a priority during an anesthetic, the gain of the ECG monitor should be set at standardization (i.e., a 1-mV signal delivered by the ECG monitor produces a 10-mm calibration pulse). This gain setting fixes the ratio of the ST-segment and QRS-complex size so that a 1-mm ST-segment change is accurately assessed (e.g., potential myocardial injury). Failure to recognize the use of other gain settings can lead to overdiagnosis or underdiagnosis of myocardial injuries (ST-segment changes). Figure 16-12 illustrates how changes in gain settings and incorrect ECG electrode and/or lead wire placement can confound ST-segment assessment.^28–30

FIGURE 16-12 Series of electrocardiographic (ECG) recordings illustrating how the gain setting and incorrect ECG electrode placement can lead to misinterpretation of ST-segment changes. In strip A, the gain setting (arrow) on the ECG monitor has been set at half standardization (1 mV = 5 mm); it grossly gives the appearance of a minor ST-segment change. When concurrent strip B is compared with strip A, it becomes apparent that use of smaller gain settings can mask ST-segment deviation. Therefore if a 0.5-mm ST-segment deviation in ECG strip A were to occur, it would equate with a 1-mm change. A similar error in assessment of ST-segment changes can occur secondary to misplaced ECG electrodes. ECG strip recording C has all limb lead electrodes properly placed and displays an ST-segment elevation of approximately 0.75 mm. In contrast, ECG rhythm strip D mistakenly has the left leg electrode placed in the second intercostal space, midclavicular line, and therefore is not representative of a lead II. The end result is an ST segment that is falsely elevated (approximately 1.5 mm), suggesting inferior transmural myocardial injury. (Reprinted with permission from Kossick MA. EKG Interpretation: Simple, Thorough, Practical. 2nd ed. Park Ridge, Ill: AANA Publishing; 1999:26-27.)

The filtering capacity of the ECG monitor is yet another potential source of artifact. Research has demonstrated that filtering out the low end of the frequency bandwidth (e.g., 0.05 to 0.5 to produce a new bandwidth range of 0.5 to 40 Hz) of the monitor’s electrical signal can lead to distortion of the ST segment (elevation or depression) and T wave (inverted).31,32 For this reason in many (but not all) cases, the diagnostic mode of an ECG monitor should be used when ST-segment analysis is a priority during an anesthetic.

Clearly, the sensitivity and specificity of computerized real-time ST-segment analysis software is dependent on the ability of the anesthetist to critically analyze the large number of factors that influence ST-segment values. Attentiveness to such variables as the patient’s physical status, ECG lead placement and selection, verification of proper placement of the Iso point, ST point, type of electronic filtering used by the ECG monitor, and gain setting may affect anesthetic outcome in patients at risk for myocardial ischemia or injury.

Central Venous and Arterial Hemodynamic Measurements

Central venous and pulmonary artery catheters (PAC) are not commonly used in the general surgical population. In fact, since the introduction of the PAC in 1970, the frequency of its use as a monitoring tool for significant surgical procedures has significantly diminished. Even during cardiac or large invasive vascular surgical procedures, many surgeons and anesthesia providers have opted for less invasive means to assess hemodynamic measurements (e.g., Flo Trac sensor). Part of the rationale for this change in practice relates to insufficient evidence demonstrating clinical benefit from use of the PAC.33,34 The literature is clear in regard to numerous challenges healthcare providers face in accurately interpreting data derived from PACs and central venous lines³⁵; cardinal concepts that relate to critical assessment of hemodynamic data are reviewed in this section of the clinical monitoring chapter.

Practice guidelines for PAC use have been recommended and established by various professional societies.³⁶ It is also recognized that the competency of healthcare providers to manage central venous and arterial hemodynamic parameters can vary significantly. In one study, attending physicians from the departments of medicine, surgery, and anesthesia were unable to demonstrate the basic skill of correctly determining the pulmonary artery occlusive pressure [PAOP] from a clear tracing and applying PAC data for proper patient treatment.³⁷ Research with anesthesiologists who specialize in cardiovascular anesthesia care also demonstrated cognitive deficits with the use of the PAC (e.g., 39% of cardiovascular anesthesiologists could not correctly interpret a PAOP waveform).³⁸ Results from these two studies suggest that the understanding of PAC data among healthcare providers is extremely variable, and misinterpretation of PAC data may result in increased morbidity and mortality. It is likely that similar deficiencies in the application and interpretation of PAC data exists for advanced practice nurses (including Certified Registered Nurse Anesthetists [CRNAs]), knowing failing scores were noted on competency tests used to assess other areas of critical care.²⁶ Such research findings have caused several groups to develop guidelines for the indications of a PAC, along with competency requirements for interpretation of data.^36,39

Physiology and Morphology of Hemodynamic Waveforms

Essential to accurate interpretation of hemodynamic data derived from central venous lines is a solid foundation in what constitutes “normal” distances, pressures, and waveform morphology for central venous pressure (CVP), right ventricular (RV), PA, and PAOP recordings. Table 16-3 illustrates the approximate distances for reaching the junction of the venae cavae and the right atrium (RA) from various distal anatomic sites. Table 16-4 lists the anticipated distances for reaching various cardiac and pulmonary structures from the right internal jugular vein. Advancement of a catheter 10 cm beyond these distances without the production of a characteristic waveform could indicate coiling of the central line. If this problem arises with a PAC, the balloon should be deflated and the catheter withdrawn. If any resistance is met during withdrawal, a chest radiograph should be taken to rule out knotting or entanglement with the chordae tendineae.

TABLE 16-3

Distance to the Junction of the Venae Cavae and Right Atrium from Various Distal Anatomic Sites

TABLE 16-4

Distance from the Right Internal Jugular Vein to Distal Cardiac and Pulmonary Structures

Right Atrial Pressure Waveform

Familiarity with the anticipated distances of relevant hemodynamic anatomy, normal intracardiac pressures, pulmonary pressures (Table 16-5), and waveform morphology facilitates accurate interpretation of PAC data and placement of central lines. For example, under normal circumstances, a CVP tracing will generate mean RA pressures in the range of 1 to 10 mmHg. The fidelity of the transducing system determines whether discernible a, c, and v waves will be displayed once the distal tip of a central line lies just above the junction of the venae cavae and the RA (Figure 16-13). The a wave is produced by contraction of the RA, the c wave by closure of the tricuspid valve, and the v wave by passive filling of the RA (which encompasses a portion of RV systole). The reason the a wave is commonly larger than the c wave is based on the position of the catheter relative to the physiologic event responsible for the pressure change. In essence, RA systole and the subsequent increase in atrial pressure is detected by a catheter positioned just above (or inappropriately within) the RA, whereas RV systole (a more distal physiologic event relative to the position of a CVP catheter) indirectly increases RA pressure by closure of the tricuspid valve.

TABLE 16-5

Normal Intracardiac and Pulmonary Pressures

LVEDP, Left ventricular end-diastolic pressure; MLAP, mean left atrial pressure; MPAP, mean pulmonary artery pressure; MRAP, mean right atrial pressure; PA, pulmonary artery; PAOP, pulmonary artery occlusive pressure; RV, right ventricular; S/D, systolic/diastolic.

^∗Values are systolic pressure/diastolic pressure.

FIGURE 16-13 Positive and negative waveforms of a central venous pressure (CVP) tracing. The third cardiac cycle in this figure does not produce a c wave.

Right Ventricular Pressure Waveform

Further advancement of a PAC (approximately 10 cm) produces dramatic changes in the morphology of the hemodynamic waveform. As shown in Figure 16-14, a brisk upstroke (isovolumetric contraction and rapid ejection [RV systole]) and steep downslope (reduced ejection and isovolumetric relaxation [RV systole and diastole]) are viewed on an oscilloscope when a PAC is advanced through the right intraventricular cavity. A PAC with the distal balloon inflated should remain in the RV for as short a time as possible to reduce the incidence of ventricular ectopy, or the development of a conduction defect such as bundle branch block. Because it is undesirable to leave the tip of a central line in the RV, pressures generated during RV systole and RV diastole are assessed indirectly via the CVP port of a PAC and distal tip of the PAC. The former is used to estimate RV end-diastolic pressure (EDP) and the latter RV systolic pressure via the PA systolic recording. Thus RVEDP is used to estimate RVED volume (RVEDV), which approximates RV preload (and less accurately left ventricular [LV] preload).