Discontinuing Cardiopulmonary Bypass

Temperature

Because at least moderate hypothermia is used during CPB in most cardiac surgical cases, it is important that the patient is sufficiently rewarmed before attempts are made to wean the patient from CPB ( Table 28.1 ). Initiation of rewarming is a good time to consider whether additional drugs must be given to keep the patient anesthetized and to prevent shivering. Monitoring the temperature of a highly perfused tissue such as the nasopharynx is useful to help prevent overheating of the brain during rewarming. Cerebral hyperthermia may lead to neurologic injury and postoperative cognitive dysfunction. The central nervous system receives a greater proportion of warm blood, thus resulting in a more rapid increase in temperature compared with other sites such as the bladder, rectum, or axilla. This situation may lead to inadequate rewarming and temperature dropoff after CPB as the heat continues to distribute throughout the body. Different institutions have various protocols for rewarming, but the important point is to warm gradually, avoiding hyperthermia of the central nervous system while providing enough heat to the patient to prevent significant dropoff after CPB. After CPB, the tendency is for the patient to lose heat, and measures to keep the patient warm (eg, fluid warmers, a circuit heater-humidifier, and forced-air warmers) should be set up and turned on before weaning from CPB is begun. The temperature of the operating room may need to be increased as well; this is probably an effective measure to keep a patient warm after CPB, but it may make the scrubbed and gowned personnel uncomfortable.

Laboratory Results

Arterial blood gases should be measured before the patient is weaned from CPB, and any abnormalities should be corrected. Severe metabolic acidosis depresses the myocardium and necessitates correction before separation from bypass. The optimal hematocrit for weaning from CPB is controversial and probably varies from patient to patient. It makes sense that sicker patients with lower cardiovascular reserve may benefit from a higher hematocrit (optimal is considered to be 30%), but the risks and adverse consequences of transfusion must be considered as well. The hematocrit should be measured and optimized before the patient is weaned from CPB. The serum potassium (K ⁺ ) level should be measured before weaning from CPB and may be high because of cardioplegia or low, especially in patients receiving loop diuretics. Hyperkalemia may make establishing an effective cardiac rhythm difficult and can be treated with sodium bicarbonate (NaHCO ₃ ), calcium chloride (CaCl ₂ ), or insulin, but the levels usually decrease quickly after cardioplegia has been stopped. Low serum K ⁺ levels should be corrected before CPB is discontinued, especially if arrhythmias are present. Administration of magnesium (Mg ²⁺ ) to patients on CPB decreases postoperative arrhythmias and may improve cardiac function, and many centers routinely give all CPB-treated patients magnesium sulfate. Theoretic disadvantages include aggravation of vasodilation and inhibition of platelet function. If Mg ²⁺ is not given routinely, the level should be checked before weaning from CPB, and deficiencies should be corrected. The ionized calcium (Ca ²⁺ ) level should be measured, and significant deficiencies should be corrected before discontinuing CPB. Many centers give all patients a bolus of CaCl ₂ just before coming off CPB because it transiently increases contractility and systemic vascular resistance (SVR). However, investigators have argued that this practice is to be avoided because Ca ²⁺ may interfere with catecholamine action and aggravate reperfusion injury.

Management of Intracardiac Air

During the bypass period, the heart is empty, cooled, and usually electrically silent to minimize consumption of adenosine triphosphate (ATP). Air is often introduced into the heart during the operation and can eventually cause deleterious effects during separation from CPB and in the postoperative period. TEE can be helpful in identifying and locating air in the heart and assisting in de-airing before CPB is discontinued. On TEE, air is often seen as echo-dense or bright foci floating to the highest point within the chamber.

The time to begin looking with TEE for intracardiac air on CPB is usually after all the chambers and the aorta are closed and the aortic cross-clamp is removed. It is essential to identify macroscopic accumulations of air within the left side of the heart to minimize systemic emboli. With the patient in the supine position, air often is visualized in the left atrium along the interatrial septum, left atrial (LA) appendage, and near the entry points of the pulmonary veins. In the left ventricle and aortic root, air often accumulates along the apical portion of the interventricular septum and right coronary sinus of Valsalva. As the heart ejects, close inspection of the left ventricular (LV) outflow tract (LVOT) and aortic root at this image plane may facilitate visualization of air emboli, mandating aggressive aspiration of the aortic root vent.

Although a correlation with the amount of intracardiac air seen with TEE and neurologic outcome has not been shown, one of the major concerns with systemic air emboli after CPB is the potential for cerebral injury. It is reasonable to proceed with the assumption that the less air pumped into the systemic circulation during and after CPB, the better. Another adverse consequence is the passage of air into the coronary circulation that leads to myocardial ischemia. In the supine patient, the right coronary artery takes off from the highest point of the aortic root, and intracoronary air is most commonly manifested by dramatic inferior ST-segment elevation and acute right-sided heart dysfunction. Saphenous vein grafts typically are anastomosed to the anterior aspect of the ascending aorta and are susceptible to air emboli as well. If this occurs while the patient is still on CPB or before decannulation, it is a simple matter to go back on CPB and wait a few minutes until the air clears from the coronary circulation, the ST segments normalize, and ventricular function improves before trying to wean the patient from CPB again. If, however, coronary air embolization occurs after decannulation, the hemodynamic status can quickly deteriorate to cardiac arrest. Smaller air emboli can be moved through the coronary vessels by acutely increasing the blood pressure with a vasopressor while dilating the coronary arteries with nitroglycerin (NTG). Perhaps the worst-case scenario is when a macroscopic air bubble in the left side of the heart is shaken loose while moving the patient from the operating table at the end of the case; acute right-sided heart failure (HF) and circulatory collapse may occur either then or while the patient is being transported to the intensive care unit.

Numerous maneuvers may be used to de-air the chambers. They may include shaking the vented heart on partial CPB to jar loose any pockets of air, elevating and aspirating LV air directly from the apex, applying positive pressure to the lungs to squeeze air out of the pulmonary veins, and tipping the table from side to side to help the passage of bubbles through the heart to the ascending aorta where they are released through a vent. Additional air may appear in the left side of the heart during weaning from CPB as increasing flow through the pulmonary veins flushes air out from the lungs into the left atrium. Passage of air from the left atrium to the left ventricle may be facilitated with the head and right-side-down position, as well as from the left ventricle to the ascending aorta with the head and right-side up. It may be impossible to evacuate every last trace of air from the left side of the heart before discontinuing CPB, especially tiny bubbles trapped in the trabeculae of the left ventricle; it therefore becomes a matter of judgment and experience to know when enough is enough. The persistence of a macroscopic air-fluid level in the left side of the heart visible with TEE, however, suggests that more de-airing probably is needed before closing the vent in the ascending aorta and weaning from CPB. After adequate de-airing, preparing the heart to resume its function of pumping blood involves optimizing the determinants of cardiac output (CO). The five hemodynamic parameters that can be controlled are rate, rhythm, preload, contractility, and afterload ( Table 28.2 ).

Heart Rate

Establishing an effective heart rate (HR) is a critical prerequisite and major determinant of CO. In most situations for adult patients, the HR should be between 75 and 95 beats/minute for weaning from CPB. It may be prudent to establish electrical pacing early in the weaning process to ensure a means to control the HR precisely. Lower rates theoretically may be desirable for hearts with residual ischemia or incomplete revascularization. Higher HRs may be needed for hearts with limited stroke volume (SV) such as after ventricular aneurysmectomy. Slow HRs are best treated with electrical pacing, but β-agonist or vagolytic drugs also may be used to increase the HR. Tachycardia before weaning from CPB is more worrisome and difficult to manage, and treatable causes such as inadequate anesthesia, hypercarbia, and ischemia should be identified and corrected. The HR often decreases as the heart is filled in the weaning process, and electrical pacing always should be immediately available during cardiac operations to treat sudden bradycardias. Supraventricular tachycardias should be electrically cardioverted if possible, but drugs such as β-antagonists or Ca ²⁺ channel antagonists may be needed to control the ventricular rate if these arrhythmias persist, most typically in patients with chronic atrial fibrillation. If drug therapy decreases the HR too much, pacing may be used.

Rhythm

The patient must have an organized, effective, and stable cardiac rhythm before attempts are made to wean the patient from CPB. This rhythm can occur spontaneously after removal of the aortic cross-clamp, but the heart may resume electrical activity with ventricular fibrillation. If the blood temperature is greater than 30°C, the heart may be defibrillated with internal paddles applied directly to the heart by using 10 to 20 J. Defibrillation at lower temperatures may be unsuccessful because extreme hypothermia can cause ventricular fibrillation. If ventricular fibrillation persists or recurs repeatedly, antiarrhythmic drugs such as lidocaine, amiodarone, or Mg ²⁺ may be administered to help achieve a stable rhythm. It is not unusual for the rhythm to remain unstable for several minutes immediately after cross-clamp removal, but persistent or recurrent ventricular fibrillation should prompt concern about impaired coronary blood flow. Because it provides an atrial contribution to ventricular filling and a normal, synchronized contraction of the ventricles, normal sinus rhythm is the ideal cardiac rhythm for weaning from CPB. Atrial flutter or fibrillation, even if present before CPB, often can be converted to normal sinus rhythm with synchronized cardioversion, especially if antiarrhythmic drugs are administered. It often is helpful to look directly at the heart when any question exists about the cardiac rhythm. Atrial contraction, flutter, and fibrillation are easily seen on CPB when the heart is visible. Ventricular arrhythmias should be treated by correcting underlying causes such as K ⁺ or Mg ²⁺ deficits and, if necessary, by administering antiarrhythmic drugs such as amiodarone. If asystole or complete heart block occurs after cross-clamp removal, electrical pacing with temporary epicardial pacing wires may be needed to achieve an effective rhythm before weaning from CPB. If atrioventricular conduction is present, atrial pacing should be attempted because, as with normal sinus rhythm, it provides atrial augmentation to filling and synchronized ventricular contraction. Atrioventricular sequential pacing is used in patients with heart block, which may be temporarily present for 30 to 60 minutes as the myocardium recovers after cardioplegia and cross-clamp removal. Ventricular pacing remains the only option if no organized atrial rhythm is present, but this sacrifices the atrial “kick” to ventricular filling and the more efficient synchronized ventricular contraction of the normal conduction system (see Table 28.2 ).

Preload

Once control of the rate and rhythm is established, priming the heart with volume or preload is the next step. Preload is the amount of stretch on the myocardial muscle fibers just before contraction. In the intact heart, the best measure of preload is end-diastolic volume. Less direct clinical measures of preload include LA pressure (LAP), pulmonary artery occlusion pressure, and pulmonary artery diastolic pressure, but the relationship between end-diastolic pressure and volume during cardiac surgical procedures may be poor. TEE is a useful tool for weaning from CPB because it provides direct visualization of the end-diastolic volume and contractility of the left ventricle. TEE may also provide a means to calculate serial CO measurements during volume loading of the heart. In addition, diastolic filling indices (transmitral and pulmonary venous inflow) may assist in assessing fluid responsiveness and elevations in LA and LV filling pressures. The process of weaning a patient from CPB involves increasing the preload (ie, filling the heart from its empty state on CPB) until an appropriate end-diastolic volume is achieved. When preparing to discontinue CPB, some thought should be given to the appropriate range of preload for the individual patient. The filling pressures before CPB may indicate what they need to be after CPB; a heart with high filling pressures before CPB may require high filling pressures after CPB to achieve an adequate preload.