The Allen’s test is used to detect the presence of adequate collateral ulnar arterial circulation. The radial and ulnar arteries are occluded by the examiner’s fingers, and the patient is then asked to make a tight fist repeatedly until the palmar skin is blanched (to empty the blood from the hand). If the patient is under anesthesia, the blood in the hand may be drained by a third person squeezing the hand. The hand is held above the heart level to help venous drainage. Then, the fist is opened slowly and put down to the heart level. Only the ulnar compression is released, and the flush of the palmar skin is watched. The Allen’s test is:

A modified Allen’s test may be done with a Doppler detector or a pulse oximeter. The results of Allen’s test are abnormal in approximately 3% of young healthy individuals. However, Slogoff et al. studied the complications following radial artery cannulation in 1,699 cardiovascular surgical patients. They concluded that in the absence of peripheral vascular disease, Allen’s test is not a predictor of ischemia of the hand during or after radial artery cannulation, and that radial artery cannulation is a low-risk, high-benefit monitoring technique that deserves wide clinical use. However, in the current litigious setting and in the face of some evidence to the contrary, it is probably prudent to continue to perform Allen’s test and document the results. Some authorities still consider the use of Allen’s test as the standard of care.

In many cardiac surgery centers, temperature is measured from two or more sites in order to estimate the accurate core temperature and achieve even distribution of body heat during cooling and rewarming by extracorporeal circulation (ECC). Esophageal, nasopharyngeal, bladder, or tympanic sites measure core temperature; the axillary and rectal sites measure peripheral temperature. Urinary bladder temperature monitoring through a urinary catheter is quite convenient and popular and reflects a body temperature between core and periphery. During cooling and rewarming by ECC, the core temperature changes rapidly, whereas the peripheral temperature changes slowly. The opposite changes are witnessed during surface cooling or warming.

Temperatures are measured during cooling on ECC to ensure that the organs most susceptible to potential hypoperfusion, such as the brain, actually receive the protective effect of the desired hypothermia. In this regard, nasopharyngeal or tympanic membrane temperatures are the monitoring sites, although they, at times, overestimate or underestimate the actual brain temperature. During rewarming, cerebral hyperthermia should be avoided (by maintaining a gradient of 5°C between the arterial blood and brain, with the brain having the lower temperature) and should be corrected immediately because hyperthermia increases the cerebral oxygen consumption and may exacerbate postoperative neuropsychological dysfunction.

There are three main differences in the pressure tracings, as shown in Figure 5.4.

Diastolic pressure is higher in PA than in RV because the closed pulmonary valve maintains a diastolic gradient between the PA and the RV.

The indication to monitor PA pressure includes the following:

For four decades following classic observation on the effects of coronary occlusion on myocardial contraction, it was believed that transient severe ischemia caused either irreversible cardiac injury—that is, infarction—or prompt recovery. However, in the 1970s, it became clear that after a brief episode of severe ischemia, prolonged myocardial dysfunction with gradual return of contractile activity occurred, a condition termed myocardial stunning. Alternatively, severe chronic ischemia can result in diminished contractile performance manifested by chronic RWMA (hibernation).

Acute myocardial ischemia affects systolic and diastolic cardiac function. Diastolic dysfunction usually manifests before alterations in systolic function. The immediate impact on ventricular compliance is related to the etiology of the ischemic event. Decreased myocardial oxygen supply is initially accompanied by an increase in compliance. In contrast, increased myocardial oxygen demand is associated with an immediate loss of ventricular compliance (e.g., the ventricle becomes stiffer). Therefore, the ventricle requires a higher filling pressure (end-diastolic pressure) to maintain a given stroke volume. A cascade of events occurs, which may include wall motion abnormalities, dysrhythmias, and conduction block. An 80% reduction of coronary blood flow causes akinesis (<10% of radial systolic thickening), whereas a 95% decrease causes dyskinesis (thinning, instead of thickening, during systole). If the ischemia becomes severe, the continuously elevated LVEDP will lead to pulmonary edema (Fig. 5.5).

Although a number of sensitive techniques are available for detection of ischemia, such as magnetic resonance spectroscopy, radiolabeled lactate determinations, or direct measurement of end-diastolic pressure, but all are impractical intraoperatively. The most popularly recognized sign of ischemia is ECG ST-segment changes. The ECG criteria for ischemia are horizontal or down-sloping ST-segment depression at least 0.1 mV at 0.06 second from J-point, up-sloping ST-segment depression at least 0.2 mV at 0.08 second from J-point, and ST-segment elevation at least 0.15 mV (1 mV = 10 mm). Other ECG signs of ischemia include inverted T waves and a new onset of arrhythmias or conduction abnormalities.

RWMA, detected with two-dimensional echocardiography, have been shown to be the earliest and most sensitive sign of myocardial ischemia. RWMA appear with just 25% decrease in coronary blood flow in the absence of ECG changes, whereas ECG signs of ischemia are apparent with a 50% decrease in coronary blood flow. During exercise, patients with CAD develop RWMA after 30 seconds, whereas ECG changes do not occur until after 90 seconds. RWMA was four times more sensitive than ECG ST-segment change in detecting intraoperative ischemia. Moreover, ST-segment changes cannot be analyzed if the patient has conduction disturbances, such as bundle branch block or ventricular-paced rhythms. Worsening wall motion after CABG surgery should be considered a prognostic indicator of adverse cardiovascular outcome.

Measurement of PAOP has been suggested as an early and sensitive indicator of ischemia to be used when the ECG is not diagnostic. Although acute increases in PAOP or development of V waves may reflect ischemia, the absence of a change in PAOP does not ensure the absence of ischemia. Häggmark et al. reported that the sensitivity, specificity, and predictive value (positive and negative) of PAOP abnormalities for ischemia ranged between 40% and 60%. Leung et al. found that 61% of TEE wall motion abnormalities in CABG patients occurred without significant changes in HR, systolic arterial pressure, or PA pressure. Only 10% of episodes were accompanied by 5 mm Hg or greater changes in PA pressure. These studies question the value of PA catheterization and monitoring for detection of intraoperative ischemia.

Multiple-lead ECG monitoring provides the best clinically available method of detecting perioperative ischemia. Based primarily on results obtained from exercise treadmill testing, 96% of ischemic events can be detected by combining ECG leads II and V₅; therefore, leads II and V₅ are the optimal leads for intraoperative myocardial ischemia. However, London et al. found that the standard combination of leads II and V₅ was only 80% sensitive, whereas combining leads V₄ and V₅ increased sensitivity to 90% in patients with known or suspected CAD undergoing noncardiac surgery with general anesthesia. The sensitivity increased to 96% by combining leads II, V₄, and V₅. If only one lead can be displayed, V₅ should be used because lead V₅ has the greatest sensitivity: 75% intraoperatively and 89% during exercise treadmill testing. See also Chapter 14, section C.3.

TEE is a well-established technique to visualize cardiac anatomy and function. Echocardiography is based on fundamental ultrasonic principles. Ultrasound is defined as sound above the upper threshold of human hearing (20,000 Hz). The ultrasound waves (1 to 7 MHz) are created by applying an alternating electric current of 500 to 1,500 pulses per second on a piezoelectric crystal. A short burst (pulse) of high-frequency, low-intensity sound is then emitted and directed through the human body to detect boundaries between structures of different acoustic impedance. The ultrasound wave is partially reflected at the boundary of media with different acoustic impedance, a property that is primarily determined by the slight difference in density between different tissues. The transmission of ultrasound through the heart, with detection of the returning echoes detailing the position and movement of cardiac structures, is termed cardiac ultrasound or echocardiography. Two-dimensional ultrasound reconstructs cardiac anatomy in shades of gray, from the time delay and decreased intensity of the reflected ultrasound. The ultrasound beam sweeps in an arc to give a panoramic view of the heart that results in cross-sectional images that are anatomically recognizable. With M-mode ultrasound, the ultrasonic beam is aimed in one direction only and therefore depicts only one dimension of the target structure in an image that does not resemble cardiac structure. See also Chapter 7, section A.6.

Doppler echocardiography blood flow is measured by applying Doppler frequency shift analysis to echoes reflected by a moving target (red blood cells or cardiac tissue). According to the Doppler principle, the frequency of the reflected ultrasound is different from the frequency of the emitted ultrasound. This change in frequency is proportional to the speed of the moving target. Color-coded Doppler flow imaging (color flow Doppler) simultaneously presents real-time images of intracardiac blood flow and moving cardiac structures in two dimensions. Color flow Doppler permits the evaluation of valvular function and intracardiac shunts.

The clinical applications of intraoperative echocardiography are as follows:

Midazolam 1 to 2 mg is given as soon as the standard monitors are applied and prior to the arterial line is inserted (under local anesthesia). This is important in alleviating anxiety, which can exacerbate myocardial ischemia. A smooth induction is essential to prevent hypotension, hypertension, and tachycardia. Different techniques may be used to achieve a smooth induction. For patients with good left ventricular function, anesthesia is induced with fentanyl, 2 to 5 µg per kg, and propofol, 1 to 2 mg per kg. The patient is ventilated by mask with 100% oxygen. After administration of a nondepolarizing muscle relaxant (vecuronium, rocuronium), the trachea is intubated. The choice of nondepolarizing muscle relaxant varies extremely from center to among practitioners. Succinylcholine is used less frequently in cardiac surgery. If the patient has a history of hypertension or the initial systolic BP is over 150 mm Hg, lidocaine or a β-blocker (or a mixed α– and β-blocker) is administered. For patients with poor left ventricular function, potent inhalation agents such as isoflurane, sevoflurane, and desflurane should be used cautiously during induction of anesthesia because of their dose-dependent reduction in myocardial contractility and systemic arterial resistance. In lieu of the increasing incidence of delirium in the elderly postoperatively, benzodiazepines should be avoided in patients older than 70 years. Alternatively, etomidate 0.2 mg per kg may be administered for induction. However, it is more important to maintain stable hemodynamics than the choice of induction anesthetic agents. See also
Chapter 11, section C.6 for other measures to prevent tachycardia and hypertension at the time of intubation.

Again, smooth anesthesia is essential to achieve a balance between myocardial oxygen demand and supply. Different agents and techniques may be used to accomplish the same goal. A combination of fentanyl (or another synthetic opioid) and isoflurane, sevoflurane, or desflurane (or propofol) is a popular choice. After the patient is intubated, a mixture of 60% air and 40% oxygen is administered to lessen the possibility of oxygen toxicity. Clearly, this ratio is subject to underlying pulmonary disease and ongoing intraoperative clinical conditions.

The depth of anesthesia must be titrated to meet the requirements of the varying intensities of surgical stimulation. Skin incision and sternal splitting are very painful. However, the strongest stimulation is usually from sternal retraction with the self-retaining retractor. Fentanyl, around 5 µg (0.1 mL) per kg (or another synthetic opioid), is given right before the skin incision, before sternotomy, and intermittently throughout surgery as needed to maintain adequate analgesia. Very high doses of fentanyl or sufentanil have been successfully used for cardiac anesthesia. There have been significant changes in practice over the past decade, whereby much lower doses of narcotic are administered to facilitate early extubation. Nowadays, maintenance of anesthetic is based on administration of volatile agents and smaller opioid dosages as was the practice in the 1980s and 1990s. Vasoactive agents are also used to regulate the hemodynamics (HR, systemic arterial pressure, and filling pressures).

Both inhalation, IV, and combined agents have been used successfully. Both have advantages and disadvantages. Understanding the cardiovascular effects of each anesthetic agent and careful titration of each drug will improve the balance between myocardial oxygen demand and supply. Early detection and appropriate control of the major determinants of myocardial oxygen consumption (BP, HR, PAOP) are mandatory if myocardial ischemia is to be avoided. Another consideration is the desirability of early extubation. High-dose narcotic and large doses of sedatives and muscle relaxants should be avoided if early extubation is planned.

Studies point toward a beneficial effect of volatile agents in preconditioning the myocardium against an ischemic injury. De Hert et al. reviewed various animal studies that demonstrate exposure of myocardium to volatile anesthetics before ischemia prevents ischemia-reperfusion injury, resulting in better recovery and cardiac function after ischemia. Although in two metaanalyses, Symons and Myles and Yu and Beattie reported that anesthetic choice did not affect incidence of perioperative morbidity and mortality, another two suggest that there is a benefit. In one of these, Landoni et al. performed a meta-analysis isolating studies comparing either desflurane or sevoflurane to total intravenous anesthesia (TIVA) and found that desflurane and sevoflurane significantly decreased the rate of MI and death in patients undergoing cardiac surgery as well as other end points of decreased cardiac biomarker release, MI, and postoperative use of inotropic drugs. Currently, there is significant variability in anesthetic technique during cardiac surgery and no consensus on which technique is superior for cardioprotection.