Monitoring of the Heart and Vascular System

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Chapter 9 Monitoring of the Heart and Vascular System

HEMODYNAMIC MONITORING

For patients with severe cardiovascular disease and those undergoing surgery associated with rapid hemodynamic changes, adequate hemodynamic monitoring should be available at all times. With the ability to measure and record almost all vital physiologic parameters, the development of acute hemodynamic changes may be observed and corrective action may be taken in an attempt to correct adverse hemodynamics and improve outcome. Although outcome changes are difficult to prove, it is a reasonable assumption that appropriate hemodynamic monitoring should reduce the incidence of major cardiovascular complications. This is based on the presumption that the data obtained from these monitors are interpreted correctly and that therapeutic decisions are implemented in a timely fashion.

Many devices are available to monitor the cardiovascular system. These devices range from those that are completely noninvasive, such as the blood pressure (BP) cuff and ECG, to those that are extremely invasive, such as the pulmonary artery (PA) catheter. To make the best use of invasive monitors, the potential benefits to be gained from the information must outweigh the potential complications. In many critically ill patients, the benefit obtained does seem to outweigh the risks, which explains the widespread use of invasive monitoring. Transesophageal echocardiography (TEE), a minimally invasive technology, has gained in popularity as an alternative to the PA catheter and is considered a standard-of-care monitoring device in the perioperative management for certain procedures, such as mitral valvuloplasty or surgery for congenital heart defects.

Standard monitoring for cardiac surgical patients includes BP, ECG, central venous pressure (CVP), urine output, temperature, capnometry, pulse oximetry, and intermittent arterial blood gas analysis (Box 9-1). The next tier of monitoring includes PA catheters, left atrial pressure (LAP) catheters, thermodilution cardiac output (CO) measurements, TEE, and indices of tissue oxygen transport (Box 9-2). All of these measurements and their derivatives can be obtained and recorded. The interpretation of these complex data, however, requires an astute clinician who is aware of the patient’s overall condition and the limitations of the monitors.1

ARTERIAL PRESSURE MONITORING

Blood pressure monitoring is the most commonly used method of assessing the cardiovascular system. The magnitude of the BP is directly related to the CO and the systemic vascular resistance (SVR). This is analogous to Ohm’s law of electricity (voltage = current × resistance), in which BP is analogous to voltage, CO to flow, and SVR to resistance. An increase in the BP may reflect an increase in CO or SVR, or both. Although BP is one of the easiest cardiovascular variables to measure, it gives only indirect information about the patient’s cardiovascular status.

Mean arterial pressure (MAP) is probably the most useful parameter to measure in assessing organ perfusion, except for the heart, in which the diastolic BP is the most important. MAP is measured directly by integrating the arterial waveform tracing over time, or using the formula:

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where SBP is systolic blood pressure and DBP is diastolic blood pressure. The pulse pressure is the difference between SBP and DBP.

Anesthesia for cardiac surgery is frequently complicated by rapid and sudden lability of the BP because of several factors, including direct compression of the heart, impaired venous return due to retraction and cannulation of the venae cavae and aorta, arrhythmias from mechanical stimulation of the heart, and manipulations that may impair right ventricular (RV) outflow and pulmonary venous return. Sudden losses of significant amounts of blood may induce hypovolemia at almost any time. The cardiac surgical population also includes many patients with labile hypertension and atherosclerotic heart disease. A safe and reliable method of measuring acute changes in the BP is required during cardiac surgery with cardiopulmonary bypass (CPB).

Continuous BP monitoring with noninvasive devices have not proven to be suitable for cardiac surgery. Intra-arterial monitoring provides a continuous, beat-to-beat indication of the arterial pressure and waveform, and having an indwelling arterial catheter enables frequent sampling of arterial blood for laboratory analyses. Direct intra-arterial monitoring remains the gold standard for cardiac surgical procedures.

Arterial Cannulation Sites

Factors that influence the site of arterial cannulation include the location of surgery, the possible compromise of arterial flow due to patient positioning or surgical manipulations, and any history of ischemia of or prior surgery on the limb to be cannulated. Another factor that may influence the cannulation site is the presence of a proximal arterial cutdown. The proximal cutdown may cause damped waveforms or falsely low BP readings due to stenosis or vascular thrombosis.

The radial artery is the most commonly used artery for continuous BP monitoring because it is easy to cannulate with a short (20-gauge) catheter. It is readily accessible during surgery, and the collateral circulation is usually adequate and easy to check. It is advisable to assess the adequacy of the collateral circulation and the absence of proximal obstructions before cannulating the radial artery for monitoring purposes.

The ulnar artery provides most blood flow to the hand in about 90% of patients. The radial and ulnar arteries are connected by a palmar arch, which provides collateral flow to the hand in the event of radial artery occlusion. It has been shown that if there is adequate ulnar collateral flow, circulatory perfusion pressure to the fingers is adequate after radial arterial catheterization. Many clinicians routinely perform Allen’s test before radial artery cannulation to assess the adequacy of collateral circulation to the hand.

Allen’s test is performed by compressing the radial and ulnar arteries and by exercising the hand until it is pale. The ulnar artery is then released (with the hand open loosely), and the time until the hand regains its normal color is noted. With a normal collateral circulation, the color returns to the hand in about 5 seconds. If, however, the hand takes longer than 15 seconds to return to its normal color, cannulation of the radial artery on that side is controversial. The hand may remain pale if the fingers are hyperextended or widely spread apart, even in the presence of a normal collateral circulation. Variations on Allen’s test include using a Doppler probe or pulse oximeter to document collateral flow. If Allen’s test demonstrates that the hand depends on the radial artery for adequate filling, and other cannulation sites are not available, the ulnar artery may be selected.2

Chest wall retractors, such as the Favaloro retractor, may impede radial arterial pressure monitoring in cardiothoracic procedures in some patients. The arm on the affected side may have diminished perfusion during extreme retraction of the chest wall. If the left internal mammary artery is used during myocardial revascularization, the right radial artery could be monitored to avoid this problem. Alternatively, a noninvasive BP cuff on the right side could be used to confirm the accuracy of the radial artery tracing during extreme chest wall retraction.

Monitoring of the radial artery distal to a brachial arterial cutdown site is not recommended. Acute thrombosis or residual stenosis of the brachial artery will lead to falsely low radial arterial pressure readings. Other considerations related to the choice of a radial arterial monitoring site include prior surgery of the hand, selection of the nondominant hand, and the preference of the surgeon, the anesthesiologist, or both.

The brachial artery lies medial to the bicipital tendon in the antecubital fossa, in close proximity to the median nerve. Brachial artery pressure tracings resemble those in the femoral artery, with less systolic augmentation than radial artery tracings. Brachial arterial pressures were found to more accurately reflect central aortic pressures than radial arterial pressures before and after CPB. The complications from percutaneous brachial artery catheter monitoring are lower than those after brachial artery cutdown for cardiac catheterization.3 A few series of perioperative brachial arterial monitoring have documented the relative safety of this technique.

The femoral artery may be cannulated for monitoring purposes but is usually reserved for situations in which other sites are unable to be cannulated or it is specifically indicated (e.g., descending thoracic aortic aneurysm surgery for distal pressure monitoring). Peripheral artery cannulation for hemodynamic monitoring, including 3899 femoral artery cannulations, has been studied. Temporary occlusion was found in 10 patients (1.45%), whereas serious ischemic complications requiring extremity amputation were reported in 3 patients (0.18%). Other complications were pseudoaneurysm formation in 6 patients (0.3%), sepsis in 13 patients (0.44%), local infection (0.78%), bleeding (1.58%), and hematoma (6.1%). The femoral artery for hemodynamic monitoring purposes was as safe as radial artery cannulation.

In patients undergoing thoracic aortic surgery, distal aortic perfusion (using partial CPB, left-heart bypass, or a heparinized shunt) may be performed during aortic cross-clamping to preserve spinal cord and visceral organ blood flow. In these situations, it is useful to measure the distal aortic pressure at the femoral artery (or, dorsalis pedis or posterior tibial artery) to optimize the distal perfusion pressure. In repairs of aortic coarctation, simultaneous femoral and radial arterial monitoring may help determine the adequacy of the surgical repair by documenting the pressure gradient after the repair. It is necessary to consult with the surgeon before cannulating the femoral vessels because these vessels may be used for extracorporeal perfusion or placement of an intra-aortic balloon pump during the surgical procedure.

Insertion Techniques

Direct Cannulation

Proper technique is helpful in obtaining a high degree of success in arterial catheterization. The wrist should be placed in a dorsiflexed position on an armboard and immobilized in a supinated position. It is helpful to draw the course of the artery for 1 inch and to be comfortably seated. Doppler devices and ultrasonic vessel finders may also be of value. Local anesthetic is injected intradermally over the artery, and a small skin nick may be made to allow passage of the catheter-over-needle assembly into the subcutaneous tissue without crimping secondary to penetration of the unit through the skin. A 20-gauge or smaller, 3- to 5-cm, nontapered Teflon catheter over needle is used, without a syringe attached, to make the puncture. If a syringe is used, the plunger should be removed to allow free flow of blood to detect when the artery has been punctured. The angle between the needle and the skin should be shallow (30 degrees or less), and the needle should be advanced parallel to the course of the artery. When the artery is entered, the angle between the needle and skin is reduced to 10 degrees, the needle is advanced another 1 to 2 mm to ensure that the tip of the catheter also lies within the lumen of the vessel, and the outer catheter is then threaded off the needle while watching that blood continues to flow out of the needle hub (Fig. 9-1). After insertion of the catheter, the wrist should be taken out of the dorsiflexed position, because continued extreme dorsiflexion can lead to median nerve damage by stretching of the nerve over the wrist. An armboard may still be used to prevent the wrist from flexing, which causes kinking of the catheter and damping of the arterial waveform.

CENTRAL VENOUS PRESSURE MONITORING

Central venous pressure catheters are used to measure the filling pressure of the right ventricle, give an estimate of the intravascular volume status, and assess RV function. For accurate pressure measurement, the distal end of the catheter must lie within one of the large intrathoracic veins or the right atrium. Although water manometers have been used in the past, an electronic system is preferred because it allows the observation of the right atrial (RA) waveform, which provides additional information. In any pressure monitoring system, it is necessary to have a reproducible landmark (e.g., the midaxillary line) as a zero reference. This is especially important in monitoring venous pressures, because small changes in the height of the zero reference point produce proportionately larger errors compared with arterial pressure monitoring.

The normal CVP waveform consists of three upward deflections (A, C, and V waves) and two downward deflections (X and Y descents) (Fig. 9-2). The A wave is produced by right atrial contraction and occurs just after the P wave on the ECG. The C wave occurs because of the isovolumic ventricular contraction forcing the tricuspid valve to bulge upward into the right atrium. The pressure within the right atrium then decreases as the tricuspid valve is pulled away from the atrium during RV ejection, forming the X descent. RA filling continues during late ventricular systole, forming the V wave. The Y descent occurs when the tricuspid valve opens and blood from the right atrium empties rapidly into the right ventricle during early diastole.

The CVP is a useful monitor if the factors affecting it are recognized and its limitations are understood. The CVP reflects the patient’s blood volume, venous tone, and RV performance. Following serial measurements (trends) is more useful than individual numbers. The response of the CVP to a volume infusion is a useful test.

The CVP does not give a direct indication of left-heart filling pressure, but it may be used as an estimate of left-sided pressures in patients with good LV function. A good correlation has been shown between the CVP and left-sided filling pressures during a change in volume status in patients with coronary artery disease and left ventricular ejection fraction (LVEF) greater than 0.4.

Internal Jugular Vein

Cannulation of the internal jugular vein (IJV) was first described by English and coworkers in 1969. Its popularity among anesthesiologists has steadily increased since that time. Advantages of this technique include the high success rate as a result of the relatively predictable relationship of the anatomic structures; a short, straight course to the right atrium that almost always assures RA or superior vena cava (SVC) localization of the catheter tip; easy access from the head of the operating room table; and fewer complications than with subclavian vein catheterization. The IJV is located under the medial border of the lateral head of the sternocleidomastoid (SCM) muscle (Fig. 9-3). The carotid artery is usually deep and medial to the IJV. The right IJV is preferred, because this vein takes the straightest course into the SVC, the right cupola of the lung may be lower than the left, and the thoracic duct is on the left side.

The preferred middle approach to the right IJV is shown in Figure 9-4. With the patient supine or in Trendelenburg position and the head turned toward the contralateral side, the fingers of the left hand are used to palpate the two heads of the SCM muscle and the carotid pulse. These fingers then hold the skin stable over the underlying structures while local anesthetic is infiltrated into the skin and subcutaneous tissues. A 22-gauge “finder” needle is placed at the apex of the triangle formed by the two heads of the SCM muscle at a 45-degree angle to the skin and directed toward the ipsilateral nipple. If venous blood return is not obtained, the needle is withdrawn to the subcutaneous tissue and then passed in a more lateral or medial direction until the vein is located. This needle reduces the risk of consequences related to inadvertent carotid arterial puncture and tissue trauma if localization of the vein is difficult. When venous blood is aspirated through the “finder” needle, the syringe and needle are withdrawn, leaving a small trail of blood on the drape to indicate the direction of the vein. Alternatively, the needle and syringe can be fixated and used as an identifying needle. Then, a syringe attached to an 18-gauge intravenous catheter-over-needle is inserted in an identical fashion. When venous return is present, the whole assembly is lowered to prevent the needle from going through the posterior wall of the central vein and advanced an additional 1 to 2 mm until the tip of the catheter is within the lumen of the vein. The catheter is then threaded into the vein.

Once the catheter is advanced into the vein, the needle is removed, and an empty syringe is attached to the cannula to withdraw a sample of blood. To confirm that an artery has not been inadvertently cannulated, comparison of the color of the blood sample to an arterial sample drawn simultaneously is recommended. If this is inconclusive or there is no arterial catheter in place, the cannula may be attached to a transducer by sterile tubing to observe the pressure waveform. Another option is to attach the cannula to sterile tubing and allow blood to flow retrograde into the tubing. The tubing is then held upright as a venous manometer, and the height of the blood column is observed. If the catheter is in a vein, it will stop rising at a level consistent with the CVP and demonstrate respiratory variation. A guidewire is then passed through the 18-gauge catheter, and the catheter is exchanged over the wire for a CVP catheter. The use of more than one technique to confirm the venous location of the catheter may provide additional reassurance of correct placement before cannulation of the vein with a larger cannula.

ultrasonic guidance of internal jugular vein cannulation

Ultrasound has been increasingly used to define the anatomic variations of the IJV. A review and meta-analysis of randomized controlled trials looking at ultrasound-guided central venous cannulation found that real-time two-dimensional ultrasound for IJV cannulation had a significantly higher success rate overall and on the first attempt compared with the landmark method in adults.4 Most studies have demonstrated that two-dimensional ultrasonic guidance of IJV cannulation is helpful in locating the vein, permits more rapid cannulation, and decreases the incidence of arterial puncture.5 Circumstances in which ultrasonic guidance of IJV cannulation can be advantageous include patients with difficult neck anatomy (e.g., short neck, obesity), prior neck surgery, anticoagulated patients, and infants.

Ultrasound has provided more precise data regarding the structural relationship between the IJV and the carotid artery (Fig. 9-5). Troianos and associates found that in more than 54% of patients, more than 75% of the IJV overlies the carotid artery. Patients who were older than 60 years were more likely to have this type of anatomy.6 There was greater overlap of the IJV and the carotid artery when the head is rotated 80 degrees compared with head rotation of only 0 to 40 degrees. The data from 2 and 4 cm above the clavicle did not differ, and the percentage overlap was larger on the left side of the neck compared with the right. Excessive rotation of the head of the patient toward the contralateral side may distort the normal anatomy in a manner that increases the risk of inadvertent carotid artery puncture.7 Doppler ultrasonography has also been used to demonstrate that the Valsalva maneuver increases IJV cross-sectional area by approximately 25% and that the Trendelenburg position increases it by approximately 37%.

Indications

Central venous pressure monitoring is often performed to obtain an indication of intravascular volume status. The accuracy and reliability of CVP monitoring depend on many factors, including the functional status of the right and left ventricles, the presence of pulmonary disease, and ventilatory factors, such as positive end-expiratory pressure (PEEP). The CVP may reflect left-sided heart filling pressures, but only in patients with good LV function. Elderly patients have a high incidence of coronary artery and pulmonary disease, and the CVP is therefore less likely to accurately reflect left-sided filling pressures in this population. Perioperative indications for the insertion of a central venous catheter are listed in Box 9-4.

The CVP should be monitored in all patients during CPB. When the catheter tip is in the SVC, it indicates RA pressure and cerebral venous pressure. Significant increases in CVP can produce critical decreases in cerebral perfusion pressure. This is occasionally caused by a malpositioned SVC cannula during CPB and must be corrected immediately by the surgeon to avoid cerebral edema and poor cerebral perfusion.