Discontinuing Cardiopulmonary Bypass

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Chapter 25 Discontinuing Cardiopulmonary Bypass

Jack S. Shanewise, MD, Roberta Hines, MD, Joel A. Kaplan, MD

Cardiopulmonary bypass (CPB) has been used since the 1950s to facilitate surgery on the heart and great vessels, and even with the increased interest in off-pump coronary artery bypass grafting (CABG) CPB remains a critical part of most cardiac operations. Managing patients with CPB remains one of the defining characteristics of cardiac surgery and cardiac anesthesiology. Discontinuing CPB is a necessary part of every operation involving extracorporeal circulation. Through this process, the support of the circulation by the bypass pump and oxygenator is transferred back to the patient’s heart and lungs. In this chapter we review important considerations involved with discontinuing CPB and present an approach to managing this critical component of a cardiac operation, which may be routine and easy or extremely complex and difficult. The key to success in discontinuing CPB is proper preparation. The period during and immediately after weaning from CPB is usually very busy for the anesthesiologist, and having to do things that could have been accomplished earlier in the operation is not helpful. The preparations for bringing a patient off CPB may be organized into several parts: general preparations, preparing the lungs, preparing the heart, and final preparations.

GENERAL PREPARATIONS

Temperature

Because at least moderate hypothermia is used during CPB in most cardiac surgery cases, it is important that the patient is sufficiently rewarmed before attempting to wean from CPB (Table 25-1). Initiation of rewarming is a good time to consider whether additional drugs need to be given to keep the patient anesthetized. Anesthetic vaporizers need to be off for 10 to 20 minutes before coming off CPB to clear the agent from the patient if so desired. Monitoring the temperature of a highly perfused tissue such as the nasopharynx is useful to help prevent overheating the brain during rewarming, but these temperatures may rise more rapidly than others, such as bladder, rectum, or axilla temperatures, leading 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 getting enough heat into the patient to prevent significant dropoff after CPB.¹ After CPB, there is a tendency for the patient to lose heat, and measures to keep the patient warm such as fluid warmers, a circuit heater-humidifier, and forced-air warmers should be set up and turned on before weaning from CPB. 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.

Table 25-1 General Preparations for Discontinuing Cardiopulmonary Bypass

Temperature	Laboratory Results
Adequately rewarm before weaning from CPB	Correct metabolic acidosis
Avoid overheating the brain	Optimize hematocrit
Start measures to keep patient warm after CPB	Normalize K⁺
Use fluid warmer, forced air warmer	Consider giving Mg²⁺ or checking Mg²⁺ level
Warm operating room	Check Ca²⁺ level and correct deficiencies

Laboratory Results

Arterial blood gas analysis should be obtained before weaning from CPB and any abnormalities corrected. Severe metabolic acidosis depresses the myocardium and should be treated with sodium bicarbonate or tromethamine (Tham). The optimal hematocrit for weaning from CPB is controversial and probably varies from patient to patient.² It makes sense that sicker patients with lower cardiovascular reserve may benefit from a higher hematocrit, but the risks and adverse consequences of transfusion need to be considered as well. Suffice it to say that the hematocrit should be measured and optimized before weaning from CPB. The serum potassium level should be measured before weaning from CPB and may be high due to 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, calcium chloride, or insulin, but the levels usually decrease quickly after cardioplegia has been stopped. Low serum potassium levels should probably be corrected before coming off CPB, especially if arrhythmias are present. Administration of magnesium to patients on CPB decreases postoperative arrhythmias and may improve cardiac function, and many centers routinely give all CPB patients magnesium sulfate. Theoretical disadvantages include aggravation of vasodilation and inhibition of platelet function.^3,⁴ If magnesium is not given routinely, the level should be checked before weaning from CPB and deficiencies corrected. The ionized calcium level should be measured, and significant deficiencies corrected before discontinuing CPB. Many centers give all patients a bolus of calcium chloride just before coming off CPB because it transiently increases contractility and systemic vascular resistance. However, it has been argued that this practice is to be avoided because calcium may interfere with catecholamine action and aggravate reperfusion injury.

PREPARING THE LUNGS

As the patient is weaned from CPB and the patient’s heart starts to support the circulation, the lungs again become the site of gas exchange, delivering oxygen and eliminating carbon dioxide. Before weaning from CPB, the lung function must be restored (Table 25-2). The lungs are reinflated by hand gently and gradually, with sighs using up to 30 cmH₂O pressure, and then mechanically ventilated with 100% oxygen. Care should be taken not to allow the left lung to injure an in situ internal mammary artery graft as the lung is reinflated. The compliance of the lungs can be judged by their feel with hand ventilation, with stiff lungs suggesting more difficulty with oxygenation or ventilation after CPB. If visible, both lungs should be inspected for residual atelectasis, and they should be rising and falling with each breath. Ventilation alarms and monitors should be activated. If prolonged expiration or wheezing is detected, bronchodilators should be given. The surgeon should inspect both pleural spaces for pneumothorax, which should be treated with chest tubes. Any fluid present in the pleural spaces should be removed before attempting to wean the patient from CPB.

Table 25-2 Preparing the Lungs for Discontinuing Cardiopulmonary Bypass

1. Suction trachea and endotracheal tube, if needed.

2. Inflate lungs gently by hand.

3. Ventilate with 100% oxygen.

4. Treat bronchospasm with bronchodilators.

5. Check for pneumothorax and pleural fluid.

6. Consider need for positive end-expiratory pressure, intensive care unit ventilator, and nitric oxide.

PREPARING THE HEART

Preparing the heart to resume its function pumping blood involves optimizing the five hemodynamic parameters that can be controlled: rhythm, rate, contractility, afterload, and preload (Table 25-3).

Table 25-3 Preparing the Heart for Discontinuing Cardiopulmonary Bypass

Parameter	Preparation
Rhythm	Normal sinus rhythm is ideal. Defibrillate if necessary when temperature > 30°C. Consider antiarrhythmic drugs if ventricular fibrillation persists more than a few minutes. Try synchronized cardioversion for atrial fibrillation or flutter. Look at the heart to diagnose atrial rhythm. Try atrial pacing if AV conduction exists. Try AV pacing for heart block.

Heart rate

Rate should be between 75 and 95 beats per minute in most cases.

Treat slow rates with electrical pacing.

Treat underlying causes of fast heart rates.

Heart rate may decrease as the heart fills.

Control fast supraventricular rates with drugs and then pace as needed.

Always have pacing available during heart surgery.

Contractility

Inotropic support is more likely needed with depressed cardiac function before CPB, advanced age, long bypass or clamp time, poor preservation, or incomplete revascularization.

Look for the vigorous “snap” of a heartbeat with good contractility.

If depressed contractility is likely, begin inotropic drugs before weaning from CPB.

Severely impaired function may require mechanical support.

Afterload

Systemic vascular resistance is a major component of afterload.

Keep MAP between 60 and 80 mmHg at full CPB flow.

Consider a vasoconstrictor if the MAP is low and a vasodilator if the MAP is high.

Preload

End-diastolic volume is the best measure of preload and can be seen with TEE.

Filling pressures provide a less direct measure of preload.

Consider baseline filling pressures.

Assess RV volume and function with direct inspection.

Assess LV volume and function with TEE.

Cardiac distention may cause MR and TR.

AV = atrioventricular; CPB = cardiopulmonary bypass; LV = left ventricular; MAP = mean arterial pressure; MR = mitral regurgitation; RV = right ventricular; TEE = transesophageal echocardiography; TR, tricuspid regurgitation.

Rhythm

There must be an organized, effective, and stable cardiac rhythm before attempting to wean from CPB. This 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 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 or amiodarone 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, can often be converted to normal sinus rhythm with synchronized cardioversion, especially if antiarrhythmic drugs are administered. It is often helpful to look directly at the heart when there is any question about the cardiac rhythm. Atrial contraction, flutter, and fibrillation are easily seen on CPB. Ventricular arrhythmias should be treated by correcting underlying causes such as potassium or magnesium deficits and, if necessary, with 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 when there is heart block, which is frequently present for 30 to 60 minutes as the myocardium recovers after 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.

Rate

In most situations for adult patients, the heart rate (HR) should be between 75 and 95 beats per minute for weaning from CPB. Lower rates may theoretically be desirable for hearts with residual ischemia or incomplete revascularization. Higher HRs may be needed for hearts with limited stroke volume, 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 deal with, 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 should always be immediately available during cardiac surgery. Supraventricular tachycardias should be electrically cardioverted if possible, but drugs such as β-antagonists or calcium channel antagonists may be needed to control the ventricular rate if they persist, most typically occurring in patients with chronic atrial fibrillation. If drug therapy lowers the rate too much, pacing may be used.

Contractility

The contractile state of the myocardium should be considered before attempting to wean from CPB. The likelihood of decreased contractility requiring inotropic support after CPB is greater with preexisting ventricular impairment (e.g., low ejection fraction, high left ventricular end-diastolic pressure [LVEDP] preoperatively or before CPB), advanced age, long CPB time, long aortic cross-clamp time, inadequate myocardial preservation, and incomplete revascularization. A heart with good contractility often has a vigorous snap with contraction that can be seen while on CPB, in contrast to the weak contractions of a heart with impaired contractility, but it may be difficult to assess global ventricular function while the heart is empty and on CPB. If significant depression of contractility is likely, inotropic support can be started before attempting to wean the patient from CPB. If depressed myocardial contractility becomes evident during weaning, the safest approach is to prevent cardiac distention by resuming CPB, resting the heart for 10 to 20 minutes while inotropic therapy with a catecholamine or phosphodiesterase inhibitor drug is started. Extreme depression of contractile function of the myocardium may require mechanical support with an intra-aortic balloon pump (IABP) or ventricular assist device (VAD).

Afterload

An important component of afterload in patients is the systemic vascular resistance (SVR). While on CPB at full flow, usually about 2.2 L/min/m², mean arterial pressure (MAP) is directly related to SVR and indicates whether the SVR is appropriate, too high, or too low. Low SVR after CPB can cause inadequate systemic arterial perfusion pressure, and high SVR can significantly impair cardiac performance, especially in patients with poor ventricular function. SVR is usually within a reasonable range when the arterial pressure is between 60 and 80 mmHg at full pump flow. If below that range, infusion of a vasopressor may be needed to increase SVR before attempting to wean from CPB. If the MAP is high while on CPB, vasodilator therapy may be needed.

Preload

In the intact heart, the best measure of preload is end-diastolic volume. Less direct clinical measures of preload include left atrial pressure (LAP), pulmonary artery occlusion pressure (PAOP), and pulmonary artery diastolic pressure, but there may be a poor relationship between end-diastolic pressure and volume during cardiac surgery. Transesophageal echocardiography (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.⁵ The process of weaning a patient from CPB involves increasing the preload (i.e., 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 particular 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.

FINAL PREPARATIONS

The final preparations before discontinuing CPB include leveling the operating table, re-zeroing the pressure transducers, ensuring the proper function of all monitoring devices, confirming that the patient is receiving only intended drug infusions, ensuring the immediate availability of resuscitation drugs and appropriate fluid volume, and verifying that the lungs are being ventilated with 100% oxygen (Table 25-4).

Table 25-4 Final Preparations for Discontinuing Cardiopulmonary Bypass

Anesthesiologist’s Preparations	Surgeon’s Preparations
Level operating table	Remove macroscopic collections of air from the heart
Re-zero transducers	Control major sites of bleeding
Activate monitors	CABG lying nicely without kinks
Check drug infusions	Cardiac vents off or removed
Have resuscitation drugs and fluid volume on hand	Clamps off the heart and great vessels
Reestablish TEE/PA catheter monitoring	Tourniquets around caval cannulas loose