Chapter 5. Cardiovascular Monitoring

• The central venous pressure catheter’s tip should not be allowed to migrate into the heart chambers.
• Although the pulmonary artery catheter can be used to guide goal-directed hemodynamic therapy to ensure organ perfusion in shock states, other less invasive methods to determine hemodynamic performance are available, including transpulmonary thermodilution cardiac output measurements and pulse contour analyses of the arterial pressure waveform.
• Relative contraindications to pulmonary artery catheterization include left bundle-branch block (because of the concern about complete heart block) and conditions associated with greatly increased risk of arrhythmias, such as Wolff-Parkinson-White syndrome.
• Pulmonary artery pressure should be continuously monitored to detect an overwedged position indicative of catheter migration.
• Accurate measurements of cardiac output depend on rapid and smooth injection, precisely known injectant temperature and volume, correct entry of the calibration factors for the specific type of pulmonary artery catheter into the cardiac output computer, and avoidance of measurements during electrocautery.

Vigilant perioperative monitoring of the cardiovascular system is one of the primary duties of anesthesia providers. This chapter focuses on the specific monitoring devices and techniques used by anesthesiologists to monitor cardiac function and circulation in healthy and nonhealthy patients alike.

The rhythmic contraction of the left ventricle, ejecting blood into the vascular system, results in pulsatile arterial pressures. The peak pressure generated during systolic contraction (in the absence of aortic valve stenosis) approximates the systolic arterial blood pressure (SBP); the lowest arterial pressure during diastolic relaxation is the diastolic blood pressure (DBP). Pulse pressure is the difference between the systolic and diastolic pressures. The time-weighted average of arterial pressures during a pulse cycle is the mean arterial pressure (MAP). MAP can be estimated by application of the following formula:

Arterial blood pressure is greatly affected by where the pressure is measured. As a pulse moves peripherally through the arterial tree, wave reflection distorts the pressure waveform, leading to an exaggeration of systolic and pulse pressures (Figure 5-1). For example, radial artery systolic pressure is usually greater than aortic systolic pressure because of its more distal location. In contrast, radial artery systolic pressures often underestimate more “central” pressures following hypothermic cardiopulmonary bypass because of changes in hand vascular resistance. Vasodilating drugs may accentuate this discrepancy. The level of the sampling site relative to the heart affects the measurement of blood pressure because of the effect of gravity (Figure 5-2). In patients with severe peripheral vascular disease, there may be a significant difference in blood pressure measurements among the extremities. The higher value should be used in these patients.

Because noninvasive (palpation, Doppler, auscultation, oscillometry, plethysmography) and invasive (arterial cannulation) methods of blood pressure determination differ greatly, they are discussed separately.

Noninvasive Arterial Blood Pressure Monitoring

Indications

The use of any anesthetic, no matter how “trivial,” is an indication for arterial blood pressure measurement. The techniques and frequency of pressure determination depend on the patient’s condition and the type of surgical procedure. An oscillometric blood pressure measurement every 3-5 min is adequate in most cases.

Contraindications

Although some method of blood pressure measurement is mandatory, techniques that rely on a blood pressure cuff are best avoided in extremities with vascular abnormalities (eg, dialysis shunts) or with intravenous lines. Rarely, it may prove impossible to monitor blood pressure in cases (eg, burns) in which there may be no accessible site from which the blood pressure can be safely recorded.

Techniques & Complications

Palpation

SBP can be determined by (1) locating a palpable peripheral pulse, (2) inflating a blood pressure cuff proximal to the pulse until flow is occluded, (3) releasing cuff pressure by 2 or 3 mm Hg per heartbeat, and (4) measuring the cuff pressure at which pulsations are again palpable. This method tends to underestimate systolic pressure, however, because of the insensitivity of touch and the delay between flow under the cuff and distal pulsations. Palpation does not provide a diastolic pressure or MAP. The equipment required is simple and inexpensive.

Doppler Probe

When a Doppler probe is substituted for the anesthesiologist’s finger, arterial blood pressure measurement becomes sensitive enough to be useful in obese patients, pediatric patients, and patients in shock (Figure 5-3). The Doppler effect is the shift in the frequency of sound waves when their source moves relative to the observer. For example, the pitch of a train’s whistle increases as a train approaches and decreases as it departs. Similarly, the reflection of sound waves off of a moving object causes a frequency shift. A Doppler probe transmits an ultrasonic signal that is reflected by underlying tissue. As red blood cells move through an artery, a Doppler frequency shift will be detected by the probe. The difference between transmitted and received frequency causes the characteristic swishing sound, which indicates blood flow. Because air reflects ultrasound, a coupling gel (but not corrosive electrode jelly) is applied between the probe and the skin. Positioning the probe directly above an artery is crucial, since the beam must pass through the vessel wall. Interference from probe movement or electrocautery is an annoying distraction. Note that only systolic pressures can be reliably determined with the Doppler technique.

A variation of Doppler technology uses a piezoelectric crystal to detect lateral arterial wall movement to the intermittent opening and closing of vessels between systolic and diastolic pressure. This instrument thus detects both systolic and diastolic pressures. The Doppler effect is routinely employed by perioperative echocardiographers to discern both the directionality and velocity of both blood flow within the heart and the movement of the heart’s muscle tissue (tissue Doppler).

Auscultation

Inflation of a blood pressure cuff to a pressure between systolic and diastolic pressures will partially collapse an underlying artery, producing turbulent flow and the characteristic Korotkoff sounds. These sounds are audible through a stethoscope placed under—or just distal to—the distal third of the blood pressure cuff. The clinician measures pressure with an aneroid or mercury manometer.

Occasionally, Korotkoff sounds cannot be heard through part of the range from systolic to diastolic pressure. This auscultatory gap is most common in hypertensive patients and can lead to an inaccurate diastolic pressure measurement. Korotkoff sounds are often difficult to auscultate during episodes of hypotension or marked peripheral vasoconstriction. In these situations, the subsonic frequencies associated with the sounds can be detected by a microphone and amplified to indicate systolic and diastolic pressures. Motion artifact and electrocautery interference limit the usefulness of this method.

Oscillometry

Arterial pulsations cause oscillations in cuff pressure. These oscillations are small if the cuff is inflated above systolic pressure. When the cuff pressure decreases to systolic pressure, the pulsations are transmitted to the entire cuff, and the oscillations markedly increase. Maximal oscillation occurs at the MAP, after which oscillations decrease. Because some oscillations are present above and below arterial blood pressure, a mercury or aneroid manometer provides an inaccurate and unreliable measurement. Automated blood pressure monitors electronically measure the pressures at which the oscillation amplitudes change (Figure 5-4). A microprocessor derives systolic, mean, and diastolic pressures using an algorithm. Machines that require identical consecutive pulse waves for measurement confirmation may be unreliable during arrhythmias (eg, atrial fibrillation). Oscillometric monitors should not be used on patients on cardiopulmonary bypass. Nonetheless, the speed, accuracy, and versatility of oscillometric devices have greatly improved, and they have become the preferred noninvasive blood pressure monitors in the United States and worldwide.

Arterial Tonometry

Arterial tonometry measures beat-to-beat arterial blood pressure by sensing the pressure required to partially flatten a superficial artery that is supported by a bony structure (eg, radial artery). A tonometer consisting of several independent pressure transducers is applied to the skin overlying the artery (Figure 5-5). The contact stress between the transducer directly over the artery and the skin reflects intraluminal pressure. Continuous pulse recordings produce a tracing very similar to an invasive arterial blood pressure waveform. Limitations to this technology include sensitivity to movement artifact and the need for frequent calibration.

Clinical Considerations

Adequate oxygen delivery to vital organs must be maintained during anesthesia. Unfortunately, instruments to monitor specific organ perfusion and oxygenation are complex, expensive, and often unreliable, and, for that reason, an adequate arterial blood pressure is assumed to predict adequate organ blood flow. However, flow also depends on vascular resistance:

Even if the pressure is high, when the resistance is also high, flow can be low. Thus, arterial blood pressure should be viewed as an indicator—but not a measure—of organ perfusion.

The accuracy of any method of blood pressure measurement that involves a blood pressure cuff depends on proper cuff size (Figure 5-6). The cuff’s bladder should extend at least halfway around the extremity, and the width of the cuff should be 20% to 50% greater than the diameter of the extremity.

Automated blood pressure monitors, using one or a combination of the methods described above, are frequently used in anesthesiology. A self-contained air pump inflates the cuff at set intervals. Incorrect or too frequent use of these automated devices has resulted in nerve palsies and extensive extravasation of intravenously administered fluids. In case of equipment failure, an alternative method of blood pressure determination must be immediately available.

Invasive Arterial Blood Pressure Monitoring

Indications

Indications for invasive arterial blood pressure monitoring by catheterization of an artery include induced current or anticipated hypotension or wide blood pressure deviations, end-organ disease necessitating precise beat-to-beat blood pressure regulation, and the need for multiple arterial blood gas measurements.

Contraindications

If possible, catheterization should be avoided in smaller end arteries with inadequate collateral blood flow or in extremities where there is a suspicion of preexisting vascular insufficiency.

Selection of Artery for Cannulation

Several arteries are available for percutaneous catheterization.

1. The radial artery is commonly cannulated because of its superficial location and substantial collateral flow (in most patients the ulnar artery is larger than the radial and there are connections between the two via the palmar arches). Five percent of patients have incomplete palmar arches and lack adequate collateral blood flow. Allen’s test is a simple, but not reliable, method for assessing the safety of radial artery cannulation. In this test, the patient exsanguinates his or her hand by making a fist. While the operator occludes the radial and ulnar arteries with fingertip pressure, the patient relaxes the blanched hand. Collateral flow through the palmar arterial arch is confirmed by flushing of the thumb within 5 sec after pressure on the ulnar artery is released. Delayed return of normal color (5-10 s) indicates an equivocal test or insufficient collateral circulation (>10 s). The Allen’s test is of such questionable utility that many practitioners routinely avoid it. Alternatively, blood flow distal to the radial artery occlusion can be detected by palpation, Doppler probe, plethysmography, or pulse oximetry. Unlike Allen’s test, these methods of determining the adequacy of collateral circulation do not require patient cooperation.

2. Ulnar artery catheterization is usually more difficult than radial catheterization because of the ulnar artery’s deeper and more tortuous course. Because of the risk of compromising blood flow to the hand, ulnar catheterization would not normally be considered if the ipsilateral radial artery has been punctured but unsuccessfully cannulated.

3. The brachial artery is large and easily identifiable in the antecubital fossa. Its proximity to the aorta provides less waveform distortion. However, being near the elbow predisposes brachial artery catheters to kinking.

4. The femoral artery is prone to atheroma formation and pseudoaneurysm, but often provides an excellent access. The femoral site has been associated with an increased incidence of infectious complications and arterial thrombosis. Aseptic necrosis of the head of the femur is a rare, but tragic, complication of femoral artery cannulation in children.

5. The dorsalis pedis and posterior tibial arteries are some distance from the aorta and therefore have the most distorted waveforms.

6. The axillary artery is surrounded by the axillary plexus, and nerve damage can result from a hematoma or traumatic cannulation. Air or thrombi can quickly gain access to the cerebral circulation during vigorous retrograde flushing of axillary artery catheters.

Technique of Radial Artery Cannulation

One technique of radial artery cannulation is illustrated in Figure 5-7. Supination and extension of the wrist provide optimal exposure of the radial artery. The pressure-tubing-transducer system should be nearby and already flushed with saline to ensure easy and quick connection after cannulation. The radial pulse is palpated, and the artery’s course is determined by lightly pressing the tips of the index and middle fingers of the anesthesiologist’s nondominant hand over the area of maximal impulse or by use of ultrasound. Using aseptic technique, 1% lidocaine is infiltrated in the skin of awake patients, directly above the artery, with a small gauge needle. A larger gauge needle can then be used as a skin punch, facilitating entry of an 18-, 20-, or 22-gauge catheter over a needle through the skin at a 45° angle, directing it toward the point of palpation. Upon blood flashback, a guidewire may be advanced through the catheter into the artery and the catheter advanced over the guidewire. Alternatively, the needle is lowered to a 30° angle and advanced another 1-2 mm to make certain that the tip of the catheter is well into the vessel lumen. The catheter is advanced off the needle into the arterial lumen, after which the needle is withdrawn. Applying firm pressure over the artery proximal to the catheter tip, with the middle and ring fingertips, prevents blood from spurting from the catheter while the tubing is connected. Waterproof tape or suture can be used to hold the catheter in place.

Complications

Complications of intraarterial monitoring include hematoma, bleeding (particularly with catheter tubing disconnections), vasospasm, arterial thrombosis, embolization of air bubbles or thrombi, pseudoaneurysm formation, necrosis of skin overlying the catheter, nerve damage, infection, necrosis of extremities or digits, and unintentional intraarterial drug injection. Factors associated with an increased rate of complications include prolonged cannulation, hyperlipidemia, repeated insertion attempts, female gender, extracorporeal circulation, the use of larger catheters in smaller vessels, and the use of vasopressors. The risks are minimized when the ratio of catheter to artery size is small, saline is continuously infused through the catheter at a rate of 2-3 mL/hr, flushing of the catheter is limited, and meticulous attention is paid to aseptic technique. Adequacy of perfusion can be continually monitored during radial artery cannulation by placing a pulse oximeter on an ipsilateral finger.

Clinical Considerations

Because intraarterial cannulation allows continuous beat-to-beat blood pressure measurement, it is considered the optimal blood pressure monitoring technique. The quality of the transduced waveform, however, depends on the dynamic characteristics of the catheter-tubing-transducer system (Figure 5-8). False readings can lead to inappropriate therapeutic interventions.

A complex waveform, such as an arterial pulse wave, can be expressed as a summation of simple harmonic waves (according to the Fourier theorem). For accurate measurement of pressure, the catheter-tubing-transducer system must be capable of responding adequately to the highest frequency of the arterial waveform (Figure 5-9). Stated another way, the natural frequency of the measuring system must exceed the natural frequency of the arterial pulse (approximately 16-24 Hz).

Most transducers have frequencies of several hundred Hz (>200 Hz for disposable transducers). The addition of tubing, stopcocks, and air in the line all decrease the frequency of the system. If the frequency response is too low, the system will be overdamped and will not faithfully reproduce the arterial waveform, underestimating the systolic pressure. Underdamping is also a serious problem, leading to overshoot and a falsely high SBP.

Catheter-tubing-transducer systems must also prevent hyperresonance, an artifact caused by reverberation of pressure waves within the system. A damping coefficient (β) of 0.6-0.7 is optimal. The natural frequency and damping coefficient can be determined by examining tracing oscillations after a high-pressure flush (Figure 5-10).

System dynamics are improved by minimizing tubing length, eliminating unnecessary stopcocks, removing air bubbles, and using low-compliance tubing. Although smaller diameter catheters lower natural frequency, they improve underdampened systems and are less apt to result in vascular complications. If a large catheter totally occludes an artery, reflected waves can distort pressure measurements.

Pressure transducers have evolved from bulky, reusable instruments to miniaturized, disposable devices. Transducers contain a diaphragm that is distorted by an arterial pressure wave. The mechanical energy of a pressure wave is converted into an electric signal. Most transducers are resistance types that are based on the strain gauge principle: stretching a wire or silicone crystal changes its electrical resistance. The sensing elements are arranged as a “Wheatstone bridge” circuit so that the voltage output is proportionate to the pressure applied to the diaphragm (Figure 5-11).

Transducer accuracy depends on correct calibration and zeroing procedures. A stopcock at the level of the desired point of measurement—usually the midaxillary line—is opened, and the zero trigger on the monitor is activated. If the patient’s position is altered by raising or lowering the operating table, the transducer must either be moved in tandem or zeroed to the new level of the midaxillary line. In a seated patient, the arterial pressure in the brain differs significantly from left ventricular pressure. In this circumstance, cerebral pressure is determined by setting the transducer to zero at the level of the ear, which approximates the circle of Willis. The transducer’s zero should be checked regularly, as some transducer measurements can “drift” over time.

External calibration of a transducer compares the transducer’s reading with a manometer, but modern transducers rarely require external calibration.

Digital readouts of systolic and diastolic pressures are a running average of the highest and lowest measurements within a certain time interval. Because motion or cautery artifacts can result in some very misleading numbers, the arterial waveform should always be monitored. The shape of the arterial wave provides clues to several hemodynamic variables. The rate of upstroke indicates contractility, the rate of downstroke indicates peripheral vascular resistance, and exaggerated variations in size during the respiratory cycle suggest hypovolemia. MAP is calculated by integrating the area under the pressure curve.

Intraarterial catheters also provide access for intermittent arterial blood gas sampling and analysis. The development of fiberoptic sensors that can be inserted through a 20-gauge arterial catheter enables continuous blood gas monitoring. Unfortunately, these sensors are quite expensive and are often inaccurate, so they are rarely used. Analysis of the arterial pressure waveform allows for estimation of cardiac output (CO) and other hemodynamic parameters. These devices are discussed in the section on CO monitoring.

Indications & Contraindications

All patients should have intraoperative monitoring of their electrocardiogram (ECG). There are no contraindications.

Techniques & Complications

Lead selection determines the diagnostic sensitivity of the ECG. ECG leads are positioned on the chest and extremities to provide different perspectives of the electrical potentials generated by the heart. At the end of diastole, the atria contract, which provides the atrial contribution to CO, generating the “P” wave. Following atrial contraction, the ventricle is loaded awaiting systole. The QRS complex begins the electrical activity of systole following the 120-200 msec atrioventricular (AV) nodal delay. Depolarization of the ventricle proceeds from the AV node through the interventricular system via the His-Purkinje fibers. The normal QRS lasts approximately 120 msec, which can be prolonged in patients with cardiomyopathies and heart failure. The T wave represents repolarization as the heart prepares to contract again. Prolongation of the QT interval secondary to electrolyte imbalances or drug effects can potentially lead to life-threatening arrhythmias (les torsade de pointes).

The electrical axis of lead II is approximately 60° from the right arm to the left leg, which is parallel to the electrical axis of the atria, resulting in the largest P-wave voltages of any surface lead. This orientation enhances the diagnosis of arrhythmias and the detection of inferior wall ischemia. Lead V5 lies over the fifth intercostal space at the anterior axillary line; this position is a good compromise for detecting anterior and lateral wall ischemia. A true V5 lead is possible only on operating room ECGs with at least five lead wires, but a modified V5 can be monitored by rearranging the standard three-limb lead placement (Figure 5-12). Ideally, because each lead provides unique information, leads II and V5 should be monitored simultaneously. If only a single-channel machine is available, the preferred lead for monitoring depends on the location of any prior infarction or ischemia. Esophageal leads are even better than lead II for arrhythmia diagnosis, but have not yet gained general acceptance in the operating room.

Electrodes are placed on the patient’s body to monitor the ECG (Figure 5-13). Conductive gel lowers the skin’s electrical resistance, which can be further decreased by cleansing the site with alcohol. Needle electrodes are used only if the disks are unsuitable (eg, with an extensively burned patient).

Clinical Considerations

The ECG is a recording of the electrical potentials generated by myocardial cells. Its routine use allows arrhythmias, myocardial ischemia, conduction abnormalities, pacemaker malfunction, and electrolyte disturbances to be detected (Figure 5-14). Because of the small voltage potentials being measured, artifacts remain a major problem. Patient or lead-wire movement, use of electrocautery, 60-cycle interference from nearby alternating current devices, and faulty electrodes can simulate arrhythmias. Monitoring filters incorporated into the amplifier to reduce “motion” artifacts will lead to distortion of the ST segment and may impede the diagnosis of ischemia. Digital readouts of the heart rate (HR) may be misleading because of monitor misinterpretation of artifacts or large T waves—often seen in pediatric patients—as QRS complexes.

Depending on equipment availability, a preinduction rhythm strip can be printed or frozen on the monitor’s screen to compare with intraoperative tracings. To interpret ST-segment changes properly, the ECG must be standardized so that a 1-mV signal results in a deflection of 10 mm on a standard strip monitor. Newer units continuously analyze ST segments for early detection of myocardial ischemia. Automated ST-segment analysis increases the sensitivity of ischemia detection, does not require additional physician skill or vigilance, and may help diagnose intraoperative myocardial ischemia.

Commonly accepted criteria for diagnosing myocardial ischemia require that the ECG be recorded in “diagnostic mode” and include a flat or downsloping ST-segment depression exceeding 1 mm, 80 msec after the J point (the end of the QRS complex), particularly in conjunction with T-wave inversion. ST-segment elevation with peaked T waves can also represent ischemia. Wolff-Parkinson-White syndrome, bundle-branch blocks, extrinsic pacemaker capture, and digoxin therapy may preclude the use of ST-segment information. The audible beep associated with each QRS complex should be loud enough to detect rate and rhythm changes when the anesthesiologist’s visual attention is directed elsewhere. Some ECGs are capable of storing aberrant QRS complexes for further analysis, and some can even interpret and diagnose arrhythmias. The interference caused by electrocautery units, however, has limited the usefulness of automated arrhythmia analysis in the operating room.

Indications

Central venous catheterization is indicated for monitoring central venous pressure (CVP), administration of fluid to treat hypovolemia and shock, infusion of caustic drugs and total parenteral nutrition, aspiration of air emboli, insertion of transcutaneous pacing leads, and gaining venous access in patients with poor peripheral veins. With specialized catheters, central venous catheterization can be used for continuous monitoring of central venous oxygen saturation.

Contraindications

Relative contraindications include tumors, clots, or tricuspid valve vegetations that could be dislodged or embolized during cannulation. Other contraindications relate to the cannulation site. For example, subclavian vein cannulation is relatively contraindicated in patients who are receiving anticoagulants (due to the inability to provide direct compression in the event of an accidental arterial puncture). Some clinicians avoid central venous cannulation on the side of a previous carotid endarterectomy due to concerns about the possibility of unintentional carotid artery puncture. The presence of other central catheters or pacemaker leads may reduce the number of sites available for central line placement.

Techniques & Complications

Central venous cannulation involves introducing a catheter into a vein so that the catheter’s tip lies with the venous system within the thorax. Generally, the optimal location of the catheter tip is just superior to or at the junction of the superior vena cava and the right atrium. When the catheter tip is located within the thorax, inspiration will increase or decrease CVP, depending on whether ventilation is controlled or spontaneous. Measurement of CVP is made with a water column (cm H2O), or, preferably, an electronic transducer (mm Hg). The pressure should be measured during end expiration.

Various sites can be used for cannulation (Figure 5-15 and Table 5-1). All cannulation sites have an increased risk of line-related infections the longer the catheter remains in place. Compared with other sites, the subclavian vein is associated with a greater risk of pneumothorax during insertion, but a reduced risk of other complications during prolonged cannulations (eg, in critically ill patients). The right internal jugular vein provides a combination of accessibility and safety. Left-sided internal jugular vein catheterization has an increased risk of pleural effusion and chylothorax. The external jugular veins can also be used as entry sites, but due to the acute angle at which they join the great veins of the chest, are associated with a slightly increased likelihood of failure to gain access to the central circulation than the internal jugular veins. Femoral veins can also be cannulated, but are associated with an increased risk of line-related sepsis. There are at least three cannulation techniques: a catheter over a needle (similar to peripheral catheterization), a catheter through a needle (requiring a large-bore needle stick), and a catheter over a guidewire (Seldinger’s technique; Figure 5-16). The overwhelming majority of central lines are placed using Seldinger’s technique.

The following scenario describes the placement of an internal jugular venous line. The patient is placed in the Trendelenburg position to decrease the risk of air embolism and to distend the internal jugular (or subclavian) vein. Venous catheterization requires full aseptic technique, including scrub, sterile gloves, gown, mask, hat, bactericidal skin preparation (alcohol-based solutions are preferred), and sterile drapes. The two heads of the sternocleidomastoid muscle and the clavicle form the three sides of a triangle (Figure 5-16A). A 25-gauge needle is used to infiltrate the apex of the triangle with local anesthetic. The internal jugular vein can be located using ultrasound, and we strongly recommend that it be used whenever possible (Figure 5-17). Alternatively, it may be located by advancing the 25-gauge needle—or a 23-gauge needle in heavier patients—along the medial border of the lateral head of the sternocleidomastoid, toward the ipsilateral nipple, at an angle of 30° to the skin. Aspiration of venous blood confirms the vein’s location. It is essential that the vein (and not the artery) be cannulated. Cannulation of the carotid artery can lead to hematoma, stroke, airway compromise, and possibly death. An 18-gauge thin-wall needle or an 18-gauge catheter over needle is advanced along the same path as the locator needle (Figure 5-16B), and, with the latter apparatus, the needle is removed from the catheter once the catheter has been advanced into the vein. When free blood flow is achieved, a J wire with a 3-mm radius curvature is introduced after confirmation of vein puncture (Figure 5-16C). The needle (or catheter) is removed, and a dilator is advanced over the wire. The catheter is prepared for insertion by flushing all ports with saline, and all distal ports are “capped” or clamped, except the one through which the wire must pass. Next, the dilator is removed, and the final catheter is advanced over the wire (Figure 5-16D). The guidewire is removed, with a thumb placed over the catheter hub to prevent aspiration of air until the intravenous catheter tubing is connected to it. The catheter is then secured, and a sterile dressing is applied. Correct location is confirmed with a chest radiograph.

The possibility of placement of the vein dilator or catheter into the carotid artery can be decreased by transducing the vessel’s pressure waveform from the introducer needle (or catheter, if a catheter over needle has been used) before passing the wire (most simply accomplished by using a sterile intravenous extension tubing as a manometer). Alternatively, one may compare the blood’s color or Pao2 with an arterial sample. Blood color and pulsatility can be misleading or inconclusive, and more than one confirmation method should be used. In cases where transesophageal echocardiography (TEE) is used, the guide wire can be seen in the right atrium, confirming venous entry (Figure 5-18).

The risks of central venous cannulation include line infection, blood stream infection, air or thrombus embolism, arrhythmias (indicating that the catheter tip is in the right atrium or ventricle), hematoma, pneumothorax, hemothorax, hydrothorax, chylothorax, cardiac perforation, cardiac tamponade, trauma to nearby nerves and arteries, and thrombosis.

Clinical Considerations

Normal cardiac function requires adequate ventricular filling by venous blood. CVP approximates right atrial pressure. Ventricular volumes are related to pressures through compliance. Highly compliant ventricles accommodate volume with minimal changes in pressure. Noncompliant systems have larger swings in pressure with less volume changes. Consequently, an individual CVP measurement will reveal only limited information about ventricular volumes and filling. Although a very low CVP may indicate a volume-depleted patient, a moderate to high pressure reading may reflect either volume overload or poor ventricular compliance. Changes associated with volume loading coupled with other measures of hemodynamic performance (eg, blood pressure, HR, urine output) may be a better indicator of the patient’s volume responsiveness. CVP measurements should always be considered within the context of the patient’s overall clinical perspective.

The shape of the central venous waveform corresponds to the events of cardiac contraction (Figure 5-19): a waves from atrial contraction are absent in atrial fibrillation and are exaggerated in junctional rhythms (“cannon” a waves); c waves are due to tricuspid valve elevation during early ventricular contraction; v waves reflect venous return against a closed tricuspid valve; and the x and y descents are probably caused by the downward displacement of the tricuspid valve during systole and tricuspid valve opening during diastole.

The pulmonary artery (PA) catheter (or Swan-Ganz catheter) was introduced into routine practice in operating rooms and intensive care units in the 1970s. It quickly became common for patients undergoing major surgery to be managed with PA catheterization. The catheter provides measurements of both CO and PA occlusion pressures and was used to guide hemodynamic therapy, especially when patients became unstable. Determination of the PA occlusion or wedge pressure permitted (in the absence of mitral stenosis) an estimation of the left ventricular end-diastolic pressure (LVEDP), and, depending upon ventricular compliance, an estimate of ventricular volume. Through its ability to perform measurements of CO, the patient’s stroke volume (SV) was also determined.

CO = SV × HR

SV = CO/HR

Blood pressure = CO × systemic vascular resistance (SVR)

Consequently, hemodynamic monitoring with the PA catheter attempted to discern why a patient was unstable so that therapy could be directed at the underlying problem.

If the SVR is diminished, such as in states of vasodilatory shock (sepsis), the SV may increase. Conversely, a reduction in SV may be secondary to poor cardiac performance or hypovolemia. Determination of the “wedge” or pulmonary capillary occlusion pressure (PCOP) by inflating the catheter balloon estimates the LVEDP. A decreased SV in the setting of a low PCOP/ LVEDP indicates hypovolemia and the need for volume administration. A “full” heart, reflected by a high PCOP/ LVEDP and low SV, indicates the need for a positive inotropic drug. Conversely, a normal or increased SV in the setting of hypotension could be treated with the administration of vasoconstrictor drugs to restore SVR in a vasodilated patient.

Although patients can present concurrently with hypovolemia, sepsis, and heart failure, this basic treatment approach and the use of the PA catheter to guide therapy became more or less synonymous with perioperative intensive care and cardiac anesthesia. However, numerous large observational studies have shown that patients managed with PA catheters had worse outcomes than similar patients who were managed without PA catheters. Other studies seem to indicate that although PA catheter-guided patient management may do no harm, it offers no specific benefits.

Despite numerous reports of its questionable utility and the increasing number of alternative methods to determine hemodynamic parameters, the PA catheter is still employed perioperatively more often in the United States than elsewhere. Although echocardiography can readily inform hemodynamic decision-making through imaging the heart to determine if it is full, compressed, contracting, or empty echocardiography requires a trained individual to obtain and interpret the images. Alternative less invasive hemodynamic monitors have gained acceptance in Europe and may expand in the United States, further decreasing the use of PA catheters.

Until other alternatives are available, PA catheterization should be considered whenever cardiac index, preload, volume status, or the degree of mixed venous blood oxygenation need to be known. These measurements might prove particularly important in surgical patients at high risk for hemodynamic instability (eg, those who recently sustained myocardial infarction) or during surgical procedures associated with an increased incidence of hemodynamic complications (eg, thoracic aortic aneurysm repair).

Contraindications

Techniques & Complications

Although various PA catheters are available, the most popular design integrates five lumens into a 7.5 FR catheter, 110-cm long, with a polyvinylchloride body (Figure 5-20). The lumens house the following: wiring to connect the thermistor near the catheter tip to a thermodilution CO computer; an air channel for inflation of the balloon; a proximal port 30 cm from the tip for infusions, CO injections, and measurements of right atrial pressures; a ventricular port at 20 cm for infusion of drugs; and a distal port for aspiration of mixed venous blood samples and measurements of PA pressure.

Insertion of a PA catheter requires central venous access, which can be accomplished using Seldinger’s technique, described above. Instead of a central venous catheter, a dilator and sheath are threaded over the guidewire. The sheath’s lumen accommodates the PA catheter after removal of the dilator and guidewire (Figure 5-21).

Prior to insertion, the PA catheter is checked by inflating and deflating its balloon and irrigating all three intravascular lumens with saline. The distal port is connected to a transducer that is zeroed to the patient’s midaxillary line.

The PA catheter is advanced through the introducer and into the internal jugular vein. At approximately 15 cm, the distal tip should enter the right atrium, and a central venous tracing that varies with respiration confirms an intrathoracic position. The balloon is then inflated with air according to the manufacturer’s recommendations (usually 1.5 mL) to protect the endocardium from the catheter tip and to allow the right ventricle’s CO to direct the catheter forward. The balloon is always deflated during withdrawal. During the catheter’s advancement, the ECG should be monitored for arrhythmias. Transient ectopy from irritation of the right ventricle by the balloon and catheter tip is common and rarely requires treatment. A sudden increase in the systolic pressure on the distal tracing indicates a right ventricular location of the catheter tip (Figure 5-22). Entry into the pulmonary artery normally occurs by 35-45 cm and is heralded by a sudden increase in diastolic pressure.

To prevent catheter knotting, the balloon should be deflated and the catheter withdrawn if pressure changes do not occur at the expected distances. In particularly difficult cases (low CO, pulmonary hypertension, or congenital heart anomalies), flotation of the catheter may be enhanced by having the patient inhale deeply; positioning the patient in a head-up, right lateral tilt position; injecting iced saline through the proximal lumen to stiffen the catheter (which also increases the risk of perforation); or administering a small dose of an inotropic agent to increase CO. Occasionally, the insertion may require fluoroscopy or TEE for guidance.

After attaining a PA position, minimal PA catheter advancement results in a pulmonary artery occlusion pressure (PAOP) waveform. The PA tracing should reappear when the balloon is deflated. Wedging before maximal balloon inflation signals an overwedged position, and the catheter should be slightly withdrawn (with the balloon down, of course). Because PA rupture may cause mortality and can occur because of balloon overinflation, the frequency of wedge readings should be minimized.

Correct catheter position can be confirmed by a chest radiograph.

The numerous complications of PA catheterization include all complications associated with central venous cannulation plus bacteremia, endocarditis, thrombogenesis, pulmonary infarction, PA rupture, and hemorrhage (particularly in patients taking anticoagulants, elderly or female patients, or patients with pulmonary hypertension), catheter knotting, arrhythmias, conduction abnormalities, and pulmonary valvular damage (Table 5-2). Even trace hemoptysis should not be ignored, as it may herald PA rupture. If the latter is suspected, prompt placement of a double-lumen tracheal tube may maintain adequate oxygenation by the unaffected lung. The risk of complications increases with the duration of catheterization, which usually should not exceed 72 hr.

Clinical Considerations

The introduction of PA catheters into the operating room revolutionized the intraoperative management of critically ill patients. PA catheters allow more precise estimation of left ventricular preload than either CVP or physical examination, as well as the sampling of mixed venous blood. Catheters with self-contained thermistors (discussed later in this chapter) can be used to measure CO, from which a multitude of hemodynamic values can be derived (Table 5-3). Some catheter designs incorporate electrodes that allow intracavitary ECG recording and pacing. Optional fiberoptic bundles allow continuous measurement of the oxygen saturation of mixed venous blood.

Starling demonstrated the relationship between left ventricular function and left ventricular end-diastolic muscle fiber length, which is usually proportionate to end-diastolic volume. If compliance is not abnormally decreased (eg, by myocardial ischemia, overload, ventricular hypertrophy, or pericardial tamponade), LVEDP should reflect fiber length. In the presence of a normal mitral valve, left atrial pressure approaches left ventricular pressure during diastolic filling. The left atrium connects with the right side of the heart through the pulmonary vasculature. The distal lumen of a correctly wedged PA catheter is isolated from right-sided pressures by balloon inflation. Its distal opening is exposed only to capillary pressure, which—in the absence of high airway pressures or pulmonary vascular disease—equals left atrial pressure. In fact, aspiration through the distal port during balloon inflation samples arterialized blood. PAOP is an indirect measure of LVEDP which depending upon ventricular compliance approximates left ventricular end diastolic volume.

Whereas central venous pressure may reflect right ventricular function, a PA catheter may be indicated if either ventricle is markedly depressed, causing disassociation of right- and left-sided hemodynamics. CVP is poorly predictive of pulmonary capillary pressures, especially in patients with abnormal left-ventricular function. Even the PAOP does not always predict LVEDP. The relationship between left ventricular end-diastolic volume (actual preload) and PAOP (estimated preload) can become unreliable during conditions associated with changing left atrial or ventricular compliance, mitral valve function, or pulmonary vein resistance. These conditions are common immediately following major cardiac or vascular surgery and in critically ill patients who are on inotropic agents or in septic shock.

Ultimately, the information provided by the PA catheter is like that from any perioperative monitor dependent upon its correct interpretation by the patient’s care givers. In this context, the PA catheter is a tool to assist in goal-directed perioperative therapy. Given the increasing number of less invasive methods now available to obtain similar information, we suspect that PA catheterization will become mostly of historic interest.

Indications

CO measurement to permit calculation of the SV is one of the primary reasons for PA catheterization. Currently, there are a number of alternative, less invasive methods to estimate ventricular function to assist in goal-directed therapy.

Techniques & Complications

Thermodilution

The injection of a quantity (2.5, 5, or 10 mL) of fluid that is below body temperature (usually room temperature or iced) into the right atrium changes the temperature of blood in contact with the thermistor at the tip of the PA catheter. The degree of change is inversely proportionate to CO: Temperature change is minimal if there is a high blood flow, whereas temperature change is greater when flow is reduced. After injection, one can plot the temperature as a function of time to produce a thermodilution curve (Figure 5-23). CO is determined by a computer program that integrates the area under the curve.

A modification of the thermodilution technique allows continuous CO measurement with a special catheter and monitor system. The catheter contains a thermal filament that introduces small pulses of heat into the blood proximal to the pulmonic valve and a thermistor that measures changes in PA blood temperature. A computer in the monitor determines CO by cross-correlating the amount of heat input with the changes in blood temperature.

Transpulmonary thermodilution (PiCCO® system) relies upon the same principles of thermodilution, but does not require PA catheterization. A central line and a thermistor-equipped arterial catheter (usually placed in the femoral artery) are necessary to perform transpulmonary thermodilution. Thermal measurements from radial artery catheters have been found to be invalid. Transpulmonary thermodilution measurements involve injection of cold indicator into the superior vena cava via a central line (Figure 5-24). A thermistor notes the change in temperature in the arterial system following the cold indicator’s transit through the heart and lungs and estimates the CO.

Transpulmonary thermodilution also permits the calculation of both the global-end diastolic volume (GEDV) and the extravascular lung water (EVLW). Through mathematical analysis and extrapolation of the thermodilution curve, it is possible for the transpulmonary thermodilution computer to calculate both the mean transit time of the indicator and its exponential decay time (Figure 5-25). The intrathoracic thermal volume (ITTV) is the product of the CO and the mean transit time (MTT). The ITTV includes the pulmonary blood volume (PBV), EVLW, and the blood contained within the heart. The pulmonary thermal volume (PTV) includes both the EVLW and the PBV and is obtained by multiplying the CO by the exponential decay time (EDT). Subtracting the PTV from the ITTV gives the GEDV (Figure 5-26). The GEDV is a hypothetical volume that assumes that all of the heart’s chambers are simultaneously full in diastole. With a normal index between 640 and 800 mL/m2, the GEDV can assist in determining volume status. An extra vascular lung water index of less than 10 mL/kg is normative. The EVLW is the ITTV minus the intrathoracic blood volume (ITBV). The ITBV = GEDV × 1.25 (Figure 5-26).

Thus EVLW = ITTV - ITBV. An increased EVLW can be indicative of fluid overload. Thus, through mathematical analysis of the transpulmonary thermodilution curve, it is possible to obtain volumetric indices to guide fluid replacement therapy. Moreover, the PiCCO® system calculates SV variation and pulse pressure variation through pulse contour analysis, both of which can be used to determine fluid responsiveness. Both SV and pulse pressure are decreased during positive pressure ventilation. The greater the variations over the course of positive pressure inspiration and expiration, the more likely the patient is to improve hemodynamic measures following volume administration.

Dye Dilution

If indocyanine green dye (or another indicator such as lithium) is injected through a central venous catheter, its appearance in the systemic arterial circulation can be measured by analyzing arterial samples with an appropriate detector (eg, a densitometer for indocyanine green). The area under the resulting dye indicator curve is related to CO. By analyzing arterial blood pressure and integrating it with CO, systems that use lithium (LiDCOTM) also calculate beat-to-beat SV. In the LiDCOTM system, a small bolus of lithium chloride is injected into the circulation. A lithium-sensitive electrode in an arterial catheter measures the decay in lithium concentration over time. Integrating the concentration over time graph permits the machine to calculate the CO. The LiDCOTM device, like the PiCCO® thermodilution device, employs pulse contour analysis of the arterial wave form to provide ongoing beat-to-beat determinations of CO and other calculated parameters. Lithium dilution determinations can be made in patients who have only peripheral venous access. Lithium should not be administered to patients in the first trimester of pregnancy. The dye dilution technique, however, introduces the problems of indicator recirculation, arterial blood sampling, and background tracer buildup, potentially limiting the use of such approaches perioperatively. Nondepolarizing neuromuscular blockers may affect the lithium sensor.

Pulse Contour Devices

Pulse contour devices use the arterial pressure tracing to estimate the CO and other dynamic parameters, such as pulse pressure and SV variation with mechanical ventilation. These indices are used to help determine if hypotension is likely to respond to fluid therapy.

Pulse contour devices rely upon algorithms that measure the area of the systolic portion of the arterial pressure trace from end diastole to the end of ventricular ejection. The devices then incorporate a calibration factor for the patient’s vascular compliance, which is dynamic and not static. Some pulse contour devices rely first on transpulmonary thermodilution or lithium thermodilution to calibrate the machine for subsequent pulse contour measurements. The FloTrac (Edwards Life Sciences) does not require calibration with another measure and relies upon a statistical analysis of its algorithm to account for changes in vascular compliance occurring as a consequence of changed vascular tone.

Esophageal Doppler

Esophageal Doppler relies upon the Doppler principle to measure the velocity of blood flow in the descending thoracic aorta. The Doppler principle is integral in perioperative echocardiography, as discussed below. The Doppler effect has been described previously in this chapter and is the result of the apparent change in sound frequency when the source of the sound wave and the observer of the sound wave are in relative motion. Blood in the aorta is in relative motion compared with the Doppler probe in the esophagus. As red blood cells travel, they reflect a frequency shift, depending upon both the direction and velocity of their movement. When blood flows toward the transducer, its reflected frequency is higher than that which was transmitted by the probe. When blood cells move away from the transducer, the frequency is lower than that which was initially sent by the probe. By using the Doppler equation, it is possible to determine the velocity of blood flow in the aorta. The equation is:

Velocity of blood blow = {frequency change/cosine of the angle of incidence between the Doppler beam and the blood flow} × {speed of sound in tissue/2 (source frequency)}

For Doppler to provide a reliable estimate of velocity, the angle of incidence should be as close to zero as possible, since the cosine of 0 is 1. As the angle approaches 90°, the Doppler measure is unreliable, as the cosine of 90° is 0.

The esophageal Doppler device calculates the velocity of flow in the aorta. As the velocities of the cells in the aorta travel at different speeds over the cardiac cycle, the machine obtains a measure of all of the velocities of the cells moving over time. Mathematically integrating the velocities represents the distance that the blood travels. Next, using normograms, the monitor approximates the area of the descending aorta. The monitor thus calculates both the distance the blood travels, as well as the area: area × length = volume.

Consequently, the SV of blood in the descending aorta is calculated. Knowing the HR allows calculation of that portion of the CO flowing through the descending thoracic aorta, which is approximately 70% of total CO. Correcting for this 30% allows the monitor to estimate the patient’s total CO. Esophageal Doppler is dependent upon many assumptions and nomograms, which may hinder its ability to accurately reflect CO in a variety of clinical situations.

Thoracic Bioimpedance

Changes in thoracic volume cause changes in thoracic resistance (bioimpedance) to low amplitude, high frequency currents. If thoracic changes in bioimpedance are measured following ventricular depolarization, SV can be continuously determined. This noninvasive technique requires six electrodes to inject microcurrents and to sense bioimpedance on both sides of the chest. Increasing fluid in the chest results in less electrical bioimpedance. Mathematical assumptions and correlations are then made to calculate CO from changes in bioimpedance. Disadvantages of thoracic bioimpedance include susceptibility to electrical interference and reliance upon correct electrode positioning. The accuracy of this technique is questionable in several groups of patients, including those with aortic valve disease, previous heart surgery, or acute changes in thoracic sympathetic nervous function (eg, those undergoing spinal anesthesia).

Fick Principle

The amount of oxygen consumed by an individual (Vo2) equals the difference between arterial and venous (a-v) oxygen content (C) (Cao2 and Cvo2) multiplied by CO. Therefore

Mixed venous and arterial oxygen content are easily determined if a PA catheter and an arterial line are in place. Oxygen consumption can also be calculated from the difference between the oxygen content in inspired and expired gas. Variations of the Fick principle are the basis of all indicator-dilution methods of determining CO.

Echocardiography

There are no more powerful tools to diagnose and assess cardiac function perioperatively than transthoracic echocardiography (TTE) and transesophageal echocardiography (TEE). Both approaches are increasingly used in the operative setting. In the operating rooms, limited access to the chest makes TEE an ideal option to visualize the heart. Both TTE and TEE can be employed preoperatively and postoperatively. TTE has the advantage of being completely noninvasive; however, acquiring the “windows” to view the heart can be difficult. Disposable TEE probes are now available that can remain in position in critically ill patients for days, during which intermittent TEE examinations can be performed.

Echocardiography can be employed by anesthesia staff in two ways, depending upon degrees of training and certification. Basic (or hemodynamic) TEE permits the anesthesiologist to discern the primary source of a patient’s hemodynamic instability. Whereas in past decades the PA flotation catheter would be used to determine why the patient might be hypotensive, the anesthetist performing hemodynamic TEE is attempting to determine if the heart is adequately filled, contracting appropriately, not externally compressed, and devoid of any grossly obvious structural defects. At all times, information obtained from hemodynamic TEE may be correlated with other information as to the patient’s general condition.

Anesthesiologists performing advanced TEE make therapeutic and surgical recommendations based upon their TEE interpretations. Various organizations and boards have been established worldwide to certify individuals in all levels of perioperative echocardiography. More importantly, individuals who perform echocardiography should be aware of the credentialing requirements of their respective institutions.

Echocardiography has many uses, including:

• Diagnosis of the source of hemodynamic instability, including myocardial ischemia, systolic and diastolic heart failure, valvular abnormalities, hypovolemia, and pericardial tamponade
• Estimation of hemodynamic parameters, such as SV, CO, and intracavitary pressures
• Diagnosis of structural diseases of the heart, such as valvular heart disease, shunts, aortic diseases
• Guiding surgical interventions, such as mitral valve repair.

Various echocardiographic modalities are employed perioperatively by anesthesiologists, including TTE, TEE, epiaortic and epicardiac ultrasound, and three-dimensional echocardiography. Some advantages and disadvantages of the modalities are as follows:

• TTE has the advantage of being noninvasive and essentially risk free. Limited scope TTE exams are now increasingly common in the intensive care unit (Figure 5-27).
• Unlike TTE, TEE is an invasive procedure with the potential for life-threatening complications (esophageal rupture and mediastinitis) (Figure 5-28). The close proximity of the esophagus to the left atrium eliminates the problem of obtaining “windows” to view the heart and permits great detail. TEE has been used frequently in the cardiac surgical operating room over the past decades. Its use to guide therapy in general cases has been limited by both the cost of the equipment and the learning necessary to correctly interpret the images. Both TTE and TEE generate two-dimensional images of the three-dimensional heart. Consequently, it is necessary to view the heart through many two-dimensional image planes and windows to mentally recreate the three-dimensional anatomy. The ability to interpret these images at the advanced certification level requires much training.
• Epiaortic and epicardiac ultrasound imaging techniques employ an echo probe wrapped in a sterile sheath and manipulated by thoracic surgeons intraoperatively to obtain views of the aorta and the heart. The air-filled trachea prevents TEE imaging of the ascending aorta. Because the aorta is manipulated during cardiac surgery, detection of atherosclerotic plaques permits the surgeon to potentially minimize the incidence of embolic stroke. Imaging of the heart with epicardial ultrasound permits intraoperative echocardiography when TEE is contraindicated because of esophageal or gastric pathology.
• Three-dimensional echocardiography (TTE and TEE) has become available in recent years (Figure 5-29). These techniques provide a three-dimensional view of the heart’s structure. In particular, three-dimensional images can better quantify the heart’s volumes and can generate a surgeon’s view of the mitral valve to aid in guiding valve repair.

Echocardiography employs ultrasound (sound at frequencies greater than normal hearing) from 2 to 10 MHz. A piezoelectrode in the probe transducer converts electrical energy delivered to the probe into ultrasound waves. These waves then travel through the tissues, encountering the blood, the heart, and other structures. Sound waves pass readily through tissues of similar acoustic impedance; however, when they encounter different tissues, they are scattered, refracted, or reflected back toward the ultrasound probe. The echo wave then interacts with the ultrasound probe, generating an electrical signal that can be reconstructed as an image. The machine knows the time delay between the transmitted and the reflected sound wave. By knowing the time delay, the location of the source of the reflected wave can be determined and the image generated. The TEE probe contains myriad crystals generating and processing waves, which then create the echo image. The TEE probe can generate images through multiple planes and can be physically manipulated in the stomach and esophagus, permitting visualization of heart structures (Figure 5-30). These views can be used to determine if the heart’s walls are being delivered adequate blood supply (Figure 5-31). In the healthy heart, the heart’s walls thicken and move inwardly with each beat. Wall motion abnormalities, in which the heart’s walls fail to thicken during systole or move in a dyskinetic fashion, can be associated with myocardial ischemia.

The Doppler effect is routinely used in echocardiographic examinations to determine the heart’s function. In the heart, both the blood flowing through the heart and the heart tissue move relative to the echo probe in the esophagus or on the chest wall. By using the Doppler effect, it is possible for echocardiographers to determine both the direction and the velocity of blood flow and tissue movement.

Blood flow in the heart follows the law of the conservation of mass. Therefore, the volume of blood that flows through one point (eg, the left ventricular outflow tract) must be the same volume that passes through the aortic valve. When the pathway through which the blood flows becomes narrowed (eg, aortic stenosis), the blood velocity must increase to permit the volume to pass. The increase in velocity as blood moves toward an esophageal echo probe is detected. The Bernoulli equation (pressure change = 4V2) allows echocardiographers to determine the pressure gradient between areas of different velocity, where v represents the area of maximal velocity (Figure 5-32). Using continuous wave Doppler, it is possible to determine the maximal velocity as blood accelerates through a pathologic heart structure. For example, a blood flow of 4 m/sec reflects a pressure gradient of 84 mm Hg between an area of slow flow (the left ventricular outflow tract) and a region of high flow (a stenotic aortic valve).

Likewise, the Bernoulli equation permits echocardiographers to estimate PA and other intracavitary pressures, if assumptions are made.

Assume P1 >> P2

Blood flow proceeds from an area of high pressure P1 to an area of low pressure P2.

The pressure gradient = 4V2, where V is the maximal velocity measured in meters per second.

Thus,

4V2 = P1 − P2

Thus, assuming that there is a jet of regurgitant blood flow from the left ventricle into the left atrium and that left ventricular systolic pressure (P1) is the same as systemic blood pressure (eg, no aortic stenosis), it is possible to calculate left atrial pressure (P2). In this manner, echocardiographers can estimate intracavitary pressures when there are pressure gradients, measurable flow velocities between areas of high and low pressure, and knowledge of either P1 or P2 (Figure 5-33).

The Doppler principle is also used by echocardiographers to identify areas of abnormal flow using color flow Doppler. Color flow Doppler creates a visual picture of the heart’s blood flow by assigning a color code to the velocities in the heart. Blood flow directed away from the echocardiographic transducer is color-coded blue, whereas that which is moving toward the probe is red. The higher the velocity of flow, the lighter the color hue (Figure 5-34). When the velocity of blood flow becomes greater than that which the machine can measure, flow toward the probe is misinterpreted as flow away from the probe, creating images of turbulent flow and “aliasing” of the image. Such changes in flow pattern are used by echocardiographers to identify areas of pathology.

Doppler can also be used to provide an estimate of SV and CO. Similar to esophageal Doppler probes previously described, TTE and TEE can be used to estimate CO. Assuming that the left ventricular outflow tract is a cylinder, it is possible to measure its diameter (Figure 5-35). Knowing this, it is possible to calculate the area through which blood flows using the following equation:

Area = πr2 = 0.785 × diameter2

Next, the time velocity integral is determined. A Doppler beam is aligned in parallel with the left ventricular outflow tract (Figure 5-36). The velocities passing through the left ventricular outflow tract are recorded, and the machine integrates the velocity/time curve to determine the distance the blood traveled.

Area × length = volume

In this instance, the SV is calculated:

SV × HR = CO

Lastly, Doppler can be used to examine the movement of the myocardial tissue. Tissue velocity is normally 8-15 cm/sec (much less than that of blood, which is 100 cm/s). Using the tissue Doppler function of the echo machine, it is possible to discern both the directionality and velocity of the heart’s movement. During diastolic filling, the lateral annulus myocardium will move toward a TEE probe. Reduced myocardial velocities (<8 cm/s) are associated with impaired diastolic function and higher left ventricular end-diastolic pressures.

Ultimately, echocardiography can provide comprehensive cardiovascular monitoring. Its routine use outside of the cardiac operating room has been hindered by both the costs of the equipment and the training required to correctly interpret the images. As equipment becomes more readily available, it is likely that anesthesia staff will perform an increasing number of echocardiographic examinations for hemodynamic monitoring perioperatively. When questions arise beyond those related to hemodynamic guidance, interpretation by an individual credentialed in advanced perioperative echocardiography is warranted.

Hemodynamic Monitoring and Management of a Complicated Patient

A 68-year-old male presents with a perforated colon secondary to diverticulitis. Vital signs are: heart rate, 120 beats/min; blood pressure, 80 mm Hg/55 mm Hg; respiratory rate, 28 breaths/min; and body temperature, 38°C. He is scheduled for emergency exploratory laparotomy. His past history includes placement of a drug-eluting stent in the left anterior descending artery two weeks earlier. His medications include metoprolol and clopidogrel.

What hemodynamic monitors should be employed?

This patient presents with multiple medical issues that could lead to perioperative hemodynamic instability. He has a history of coronary artery disease for which he has been given stents. His previous and current ECGs should be reviewed for signs of new ST- and T-wave changes, heralding ischemia. He is both tachycardic and febrile, and, consequently, may be concurrently ischemic, vasodilated, and hypovolemic. All of these conditions could complicate perioperative management.

Arterial cannulation and monitoring will provide beat-to-beat blood pressure determinations intraoperatively and will also provide for blood gas measurements in a patient likely to be acidotic and hemodynamically unstable. Central venous access is obtained to permit volume resuscitation and to provide a port for delivery of fluid for transpulmonary measurements of CO and SV variation. Alternatively, pulse contour analysis can be employed from an arterial trace to determine volume responsiveness, should the patient become hemodynamically unstable. Echocardiography can be used to determine ventricular function, filling pressures, and CO and to provide surveillance for the development of ischemia-induced wall motion abnormalities.

PA catheters can also be placed to measure CO and pulmonary capillary occlusion pressure.

The choice of hemodynamic monitors remains with the individual physician and the availability of various monitoring techniques. It is important to also consider monitors that will be available in the postoperative setting to insure continuation of goal-directed therapy.