32 Discontinuing Cardiopulmonary Bypass
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, 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 (see Chapters 28 to 31). 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. This chapter reviews important considerations involved with discontinuing CPB and presents 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 usually is 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 32-1).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 20 to 30 minutes before coming off CPB to clear the agent from the patient if so desired.2 However, currently, the patient is frequently removed from CPB with low doses of the inhalation agent still on to continue the anesthetic preconditioning of the heart (see Chapter 9). 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 increase more rapidly than others, such as bladder, rectum, or axilla temperatures, leading to inadequate rewarming and temperature drop-off after CPB as the heat continues to distribute throughout the body.3 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 drop-off after CPB4 (see Chapters 28 and 29). 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 32-1 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 |
CPB, cardiopulmonary bypass.
Laboratory Results
An arterial blood gas should be measured before weaning from CPB and any abnormalities corrected. Severe metabolic acidosis depresses the myocardium and should be treated with NaHCO3 or THAM.5–8 The optimal hematocrit for weaning from CPB is controversial and probably varies from patient to patient.9,10 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 need to be considered as well. Suffice it to say that the hematocrit should be measured and optimized before weaning from CPB (see Chapters 30 and 31). Serum potassium 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 NaHCO3, CaCl2, or insulin, but the levels usually decrease quickly after cardioplegia has been stopped. Low serum potassium levels probably should be corrected before coming off CPB, especially if arrhythmias are present. Administration of magnesium (Mg2+) to patients on CPB decreases postoperative arrhythmias and may improve cardiac function, and many centers routinely give all CPB patients magnesium sulfate.11 Theoretic disadvantages include aggravation of vasodilation and inhibition of platelet function.12 If Mg2+ 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 (SVR).13 However, it has been argued that this practice is to be avoided because Ca2+ may interfere with catecholamine action and aggravate reperfusion injury.14
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 32-2). The trachea should be suctioned and, if necessary, lavaged with saline to clear secretions. If the abdomen appears to be distended, the stomach should be suctioned so that gastric distention does not impair ventilation after CPB. The lungs are reinflated by hand gently and gradually, with sighs using up to 30 cm H2O 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 opening the pleural space. Any fluid present in the pleural spaces should be removed before attempting to wean the patient from CPB. In its most severe form, pulmonary dysfunction after CPB may require positive end-expiratory pressure, an intensive care unit–type ventilator, or nitric oxide (see Chapters 33, 35, and 37). If needed, this equipment should be obtained before attempting to wean the patient from CPB.
TABLE 32-2 Preparing the Lungs for Discontinuing Cardiopulmonary Bypass
| Suction trachea and endotracheal tube. |
| Inflate lungs gently by hand. |
| Ventilate with 100% oxygen. |
| Treat bronchospasm with bronchodilators. |
| Check for pneumothorax and pleural fluid. |
| Consider need for positive end-expiratory pressure, intensive care unit ventilator, and nitric oxide. |
Preparing the heart
Preparing the heart to resume its function of pumping blood involves optimizing the five hemodynamic parameters that can be controlled: rhythm, rate, contractility, afterload, and preload (Table 32-3).
TABLE 32-3 Preparing the Heart for Discontinuing Cardiopulmonary Bypass
| Hemodynamic Parameters | 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/min 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 immediately 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 mm Hg 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.15–17 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.18,19 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 there is any question 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 potassium or magnesium deficits and, if necessary, with antiarrhythmic drugs such as amiodarone.20 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 frequently is 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 system21,22 (see Table 32-3).
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 before surgery or before CPB), advanced age, long CPB time, long aortic cross clamp time, inadequate myocardial preservation, and incomplete revascularization.23 A heart with good contractility often has a vigorous snap with contraction that can be seen while on CPB, in contrast with 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 and resting the heart for 10 to 20 minutes while inotropic therapy with a catecholamine or phosphodiesterase (PDE) 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 (see Pharmacologic Management of Ventricular Dysfunction section later in this chapter; see also Chapters 27 and 34).
Afterload
Afterload is the tension developed within the ventricular muscle during contraction. An important component of afterload in patients is the SVR (see Chapters 5, 14, and 34).24 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 usually is within a reasonable range when the arterial pressure is between 60 and 80 mm Hg 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
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 left atrial pressure (LAP), pulmonary artery occlusion pressure, and pulmonary artery diastolic pressure, but there may be a poor relation between end-diastolic pressure and volume during cardiac surgery25,26 (see Chapters 5, 14, and 34). 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 ventricle27–29 (see Chapters 11 to 13). 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 at hand. 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, rezeroing 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 32-4).
TABLE 32-4 Final Preparations for Discontinuing Cardiopulmonary Bypass
| Anesthesiologist’s Preparations | Surgeon’s Preparations |
|---|---|
| Level operating table. | Remove macroscopic collections of air from the heart. |
| Rezero transducers. | Control major sites of bleeding. |
| Activate monitors. | Ensure CABG is lying nicely without kinks. |
| Check drug infusions. | Turn off or remove cardiac vents. |
| Have resuscitation drugs and fluid volume at hand. | Take clamps off the heart and great vessels. |
| Reestablish TEE/PAC monitoring. | Loosen tourniquets around caval cannulas. |
CABG, coronary artery bypass graft; PAC, pulmonary artery catheter; TEE, transesophageal echocardiography.
The surgeon must confirm that he or she has completed the necessary preparations in the surgical field before discontinuing CPB. Macroscopic collections of air in the heart should be evacuated before starting to wean from CPB. These are detected most easily with TEE, which also can be helpful in monitoring and directing the deairing process.30 Major sites of bleeding should be controlled, cardiac vent suction should be off, all clamps on the heart and great vessels should be removed, coronary artery bypass grafts should be checked for kinks and bleeding, and tourniquets around the caval cannulas should be loosened or removed before starting to wean a patient from CPB.
Routine weaning from cardiopulmonary bypass
The actual process of weaning from CPB begins with partially occluding the venous return cannula with a clamp (Figure 32-1). This may be done in the field by the surgeon or at the pump by the perfusionist. This causes blood to flow into the right ventricle. As the right ventricle fills and begins to pump blood through the lungs, the left heart will begin to fill. When this occurs, the left ventricle will begin to eject and the arterial waveform will become pulsatile. Next, the perfusionist will gradually decrease the pump flow rate. As more of the venous return goes through the heart and less to the pump reservoir, it becomes necessary to gradually decrease the pump flow to avoid emptying the pump reservoir. One approach to weaning from CPB is to bring the filling pressure being monitored (e.g., central venous pressure, pulmonary artery occlusion pressure, LAP) to a specific, predetermined level somewhat lower than may be necessary and then assess the hemodynamics. Volume (preload) of the heart also may be judged by direct observation of its size or with TEE. Further filling is done in small increments (50 to 100 mL) while closely monitoring the preload until the hemodynamics appear satisfactory as judged by the arterial pressure, the appearance of the heart, and the trend of the Svo2. It typically is easy to see the right-heart volume and function directly in the surgical field and the left heart with TEE, and combining the two observations is a useful approach for weaning from CPB. Overfilling and distention of the heart should be avoided because it may stretch the myofibrils beyond the most efficient length and dilate the annuli of the mitral and tricuspid valves, rendering them incompetent, which easily is detected with TEE. If the patient has two venous cannulas, the smaller of the two may be removed when the pump flow is half of the full flow rate to improve movement of blood from the great veins into the right atrium. When the pump flow has been decreased to 1 L/min or less in an adult and the hemodynamics are satisfactory, the venous cannula may be completely clamped and the pump flow turned off. At this point, the patient is “off bypass” (Figure 32-2).
After discontinuing CPB, the anticoagulation by heparin is reversed with protamine. Depending on institutional preference, protamine may be administered before or after removal of the arterial cannula. Giving it before removal allows for continued transfusion from the pump and easier return to CPB if there is a severe protamine reaction (see Chapter 31). Giving protamine after removal of the arterial cannula may decrease the risk for thrombus formation and systemic embolization. After the infusion of protamine is started, pump suction return to the reservoir should be stopped to keep protamine out of the pump circuit in case subsequent return to CPB becomes necessary. Protamine should be given slowly through a peripheral intravenous catheter over 5 to 15 minutes while watching for systemic hypotension and pulmonary hypertension, which may indicate that an untoward (allergic) reaction to protamine is occurring.31–33 Technically flawed coronary artery bypass grafts may thrombose after protamine administration, causing acute ischemia mimicking a protamine reaction.
When transfusion of the pump reservoir blood is completed, a thorough assessment of the patient’s condition should be made before removing the arterial cannula, because after this is done, returning to CPB becomes much more difficult. The cardiac rhythm should be stable. Cardiac function is assessed by evaluating pressures, cardiac output, and TEE. Hemodynamics should be satisfactory and stable. Adequate oxygenation and ventilation should be confirmed by arterial blood gas or pulse oximetry and capnography. Bleeding from the heart should be at a manageable level before removal of the arterial cannula. The perfusionist should not have to transfuse significant amounts of blood through the arterial cannula before removing it, because it may be difficult to keep up with the blood loss through intravenous infusions alone. Bleeding sites behind the heart may have to be repaired on CPB if the patient cannot tolerate lifting the heart to expose the problem area. At the time of arterial decannulation, the systolic pressure should be between 85 and 100 mm Hg to minimize the risk for dissection or tearing of the aorta.34 The head of the bed may be raised, or small boluses of a short-acting vasodilator (e.g., nitroglycerin, nitroprusside) may be given to lower the systemic blood pressure as necessary. Tight control of the arterial blood pressure may be needed for a few minutes until the cannulation site is secure.
Pharmacologic management of ventricular dysfunction
Severe ventricular dysfunction, specifically the low cardiac output syndrome (LCOS), occurring after CPB and cardiac surgery differs from chronic congestive heart failure (CHF) (Box 32-1). Patients emerging from CPB have hemodilution, moderate hypocalcemia, hypomagnesemia, and altered potassium levels. Depending on temperature and depth of anesthesia, these individuals may demonstrate low, normal, or high SVR. Increasing age, female sex, decreased LVEF, and increased duration of CPB are associated with a greater likelihood that inotropic support will be needed after CABG surgery23 (Table 32-5).
Contractile dysfunction during or after cardiac surgery can result from preexisting impairment in contractility or be a new-onset condition. Abnormal contraction, especially in the setting of coronary artery disease (CAD), usually is caused by myocardial injury resulting in ischemia or infarction. The magnitude of contractile dysfunction corresponds to the extent and duration of injury. Brief periods of myocardial oxygen deprivation (< 10 minutes) produce regional contractile dysfunction, which can be rapidly reversed by reperfusion. Extension of the ischemia to 15 to 20 minutes also is associated with restoration of cardiac function with reperfusion; however, this process is very slow and can take hours to days. This condition of postischemic reversible myocardial dysfunction in the presence of normal flow is referred to as myocardial stunning.35–37 Irreversible cell injury will occur with longer periods of ischemia, producing a myocardial infarction characterized by release of intracellular enzymes, disruption of cell membranes, influx of calcium, persistent contractile dysfunction, and eventual cellular swelling and necrosis.38
In addition to the previously described factors, right ventricular (RV) dysfunction and failure are potential sources of morbidity and mortality after cardiac surgery. Numerous factors may predispose patients to the development of perioperative RV dysfunction, including CAD, RV hypertrophy, previous cardiac surgery, and operative considerations such as inadequate revascularization or hypothermic protection. Technical and operative difficulties are associated with various cardiac surgical procedures (e.g., right ventriculotomy), RV trauma, rhythm and conduction abnormalities, injury to the right ventricle during cessation of CPB, or protamine reaction (see Chapter 31).
The following discussion provides an overview of the pharmacologic approach to management of perioperative ventricular dysfunction in the setting of cardiac surgery. Management goals are described in Table 32-6.39 These are extensions of the routine preparations made for discontinuing CPB shown in Table 32-3.
| Variable | Physiologic Management |
|---|---|
| Heart rate and rhythm | Maintain normal sinus rhythm, avoid tachycardia; for tachycardia or bradycardia, consider pacing or chronotropic agents (atropine, isoproterenol, epinephrine), correct acid-base, electrolytes, and review of current medications |
| Preload | Reduce increased preload with diuretics or venodilators (nitroglycerin or sodium nitroprusside); monitor CVP, PCWP, and SV; obtain echocardiogram to rule out ischemia, valvular lesions, tamponade, and intracardiac shunts; consider using inotropes, IABP, or both |
| Afterload | Avoid increased afterload (increased wall tension), use vasodilators (sodium nitroprusside); avoid hypotension; maintain coronary perfusion pressure; consider IABP, inotropes devoid of α1-adrenergic effects (dobutamine or milrinone), or both IABP and inotropes |
| Contractility | Assess hemodynamics, rule out ischemia/infarction, assess rate/rhythm, preload, and afterload, use inotropes; if uncertain, obtain echocardiogram to assess cardiac function; consider combination therapy with inotropes and vasodilators and/or assist devices (IABP/LVAD/RVAD) |
| Oxygen delivery | Increase Fio2 and CO; check ABGs and chest radiograph; mechanical ventilation if indicated; correct acid-base disturbances |
ABG, arterial blood gas; CO, cardiac output; CVP, central venous pressure; Fio2, inspired oxygen concentration; IABP, intraaortic balloon pump; LVAD, left ventricular assist device; PCWP, pulmonary capillary wedge pressure; RVAD, right ventricular assist device; SV, stroke volume.
Sympathomimetic Amines
Sympathomimetic drugs (i.e., catecholamines) are pharmacologic agents capable of providing inotropic and vasoactive effects (Box 32-2). Catecholamines exert positive inotropic action by stimulation of the β1 receptor. The predominant hemodynamic effect of a specific catecholamine depends on the degree to which the various α, β, and dopaminergic receptors are stimulated (Tables 32-7 and 32-8).
The physiologic effect of an adrenergic agent is determined by the sum of its actions on α, β, and dopaminergic receptors. The effectiveness of any adrenergic agent will be influenced by the availability and responsiveness of adrenergic receptors. Chronically increased levels of plasma catecholamines (e.g., chronic CHF and long CPB time) cause downregulation of the number and sensitivity of β receptors.40 Maintenance of normal acid-base status, normothermia, and electrolytes also improve the responsiveness to adrenergic-receptor stimulation.
