Weaning from cardiopulmonary bypass

INTRODUCTION — Weaning from cardiopulmonary bypass (CPB) is the transition from the complete mechanical circulatory and respiratory support provided by CPB to the unassisted function of the patient's native heart and lungs. It is a gradual, multistep, interdisciplinary effort involving the surgeon(s), anesthesiologist(s), perfusionist(s), and operating room nurse(s). This topic will discuss the process of weaning from CPB after completion of the cardiac surgical procedure.

Intraoperative problems are commonly encountered in the immediate postbypass period; these are discussed separately. (See "Intraoperative problems after cardiopulmonary bypass".)

The physiology of CPB as well as preparations, initiation, and management of the patient during CPB are discussed separately. (See "Management of cardiopulmonary bypass".)

PREPARATION FOR WEANING — A standardized weaning protocol and/or checklist is employed at most institutions during the initial attempt to wean from the mechanical support of CPB [1,2].

Anesthesiologist's preparation

Checklist — Checklists ensure that critical tasks are completed, correctable abnormalities have been addressed prior to the weaning attempt, and important information has been transmitted among team members [3-6]. One example is the mnemonic "WAAARRRRMM" (table 1):

●Warm (temperature) – Generally, rewarming should be accomplished slowly to a target nasopharyngeal temperature of 37.0°C (but not higher), and a bladder (core) temperature of approximately 35.5°C prior to weaning. The target nasopharyngeal temperature should not be exceeded as this may result in cerebral injury [7-9]. (See "Management of cardiopulmonary bypass", section on 'Temperature'.)

●Anesthesia – Plan for postbypass anesthesia. Either a volatile anesthetic administered via a vaporizer attached to the CPB circuit or a total intravenous anesthesia (TIVA) is typically used to maintain anesthesia during CPB [10-13] (see "Management of cardiopulmonary bypass", section on 'Maintenance of anesthesia and neuromuscular blockade'). Subsequently, during weaning from CPB, either volatile anesthetic administration is reinstituted via the anesthesia machine, or the TIVA technique is continued. Additional monitoring such as bispectral index (BIS) may be useful to guide titration of inhaled and/or intravenous (IV) anesthetics during weaning and the postbypass period [10,14].

●Adjunct drugs – Prepare adjunct drugs that are likely to be used during weaning from CPB, such as antiarrhythmic agents, inotropic drugs, and vasoactive infusions, should be prepared and immediately available to avoid distractions during weaning. Protamine is an exception, and should only be prepared just prior to administration to avoid unintentional premature neutralization of heparin. Adjunct drugs may also include inhaled agents such as inhaled nitric oxide (iNO) or epoprostenol for pulmonary vasodilation. These agents may require time to prepare for delivery and may be initiated by a member of the respiratory care team.

●Air – Air in the cardiac chambers is removed by the surgeon (see 'Surgeon's preparation' below), aided by transesophageal echocardiography (TEE) monitoring and close communication with the anesthesiologist to assess adequacy of air removal.

Air may accumulate in the aortic root, left ventricle (LV), and left atrium (LA) during surgery, particularly if the left heart chambers have been opened, and may embolize to the systemic circulation after removal of the aortic cross-clamp (movie 1A-D). When the lungs are reinflated and mechanical ventilation is reestablished, pulmonary venous blood is displaced into the LA, along with any air bubbles trapped in the pulmonary veins. Embolization to the coronary arteries may cause myocardial ischemia, arrhythmias, and ventricular dysfunction, while embolization to the cerebral circulation may cause neurologic dysfunction.

The goal of de-airing procedures is to reduce cerebral and systemic air embolization as much as possible. The anesthesiologist typically places the patient into the Trendelenburg position during the de-airing process, since this may reduce the quantity of air entering the cerebral circulation.

●Rhythm – Reestablish an adequate perfusing heart rhythm before weaning. Normal sinus rhythm is ideal, but temporary epicardial pacing is employed if sinus rhythm cannot be achieved or to treat bradycardia. When atrioventricular (AV) conduction is normal, atrial (A) pacing is used. Otherwise, AV pacing is preferable to ventricular (V) pacing so that AV synchrony and optimal ventricular preloading are maintained. (See 'Maintenance of optimal pacemaker function' below.)

●Rate – A heart rate between 80 to 90 beats/minute (bpm) is optimal to maximize cardiac output without sacrificing coronary diastolic perfusion time [15,16].

●Resistance – Vasodilation with hypotension in patients with a normal or elevated cardiac output (pump flow rate) indicates low systemic vascular resistance (SVR) and a need for vasopressors (table 2). The use of vasopressors on CPB indicated that such agents will likely be needed during the weaning process. (See "Intraoperative problems after cardiopulmonary bypass", section on 'Vasoplegia'.)

●Respiration – Restore adequate ventilation and oxygenation by native lungs by resuming positive pressure mechanical lung ventilation. Respiratory problems that become evident with reestablishment of ventilation (eg, bronchospasm or high airway pressure) should be addressed prior to weaning from bypass.

●Metabolism/labs – Correct hemoglobin to a level appropriate for the patient, typically >7 to 8 g/dL [16,17]. Decisions to transfuse depend on reservoir volume and red blood cell (RBC) mass (see 'Perfusionist's preparation' below). Check for electrolyte (eg, potassium, calcium) or acid–base abnormalities with core laboratory or point-of-care testing, and treat as needed. Ionized calcium levels should be 1.09 to 1.3 mmol/L [18,19], and potassium levels should be 4 to 5.5 mmol/L to allow normal myocardial function [1]. An international survey revealed that the majority of programs (71 percent) use calcium during the weaning process, with calcium chloride used most frequently by 66 percent of the centers surveyed [18].

●Monitoring – Ensure that all monitors are functioning. The volume on the pulse oximeter should be audible. Pressure transducers are adjusted to ensure accurate "zero" and reference level. Additional monitors, such as a femoral arterial line may be needed; if so, preparation prior to weaning may reduce distraction during the actual weaning process. Note that central to peripheral arterial pressure gradients may be present (figure 1).

Surgeon's preparation — The surgeon's responsibilities in preparation for weaning include:

●Removal of intracardiac catheters that are no longer necessary (eg, coronary sinus catheter for retrograde cardioplegia, left ventricular vent) and repair of catheter entry sites.

●Removal of intracardiac air remaining within the left-sided chambers of the heart. The surgeon achieves air removal largely by "venting" through the suction applied to the anterograde cardioplegia line in the ascending aorta (termed the "aortic root vent") or other vent catheters in the left side of the heart. Also, the surgeon often "massages" the heart to force air out of the left cardiac chambers. The de-airing process is facilitated by TEE monitoring by the anesthesiologist.

●Placement of epicardial pacing wires directly into the right atrial (RA) and right ventricular (RV) myocardium for A, V, or AV pacing, as needed. (See 'Maintenance of optimal pacemaker function' below.)

●Final visual inspection of the surgical repair, with attention to hemostasis.

Perfusionist's preparation — The perfusionist's tasks must be accomplished prior to the first attempt to wean from bypass. These include:

●Ensuring adequate rewarming while avoiding hyperthermia [7]. (See "Management of cardiopulmonary bypass", section on 'Temperature'.)

●Treatment of anemia (ie, Hgb levels <7 to 8 g/dL) with hemoconcentration if the venous reservoir volume is high, which avoids loss of coagulation factors, platelets, and proteins associated with the use of cell scavenging technology after bypass. Other options include adding cell saver (autologous) blood if available, or administering allogeneic blood to the venous reservoir if necessary [17,20].

●Discontinuing unnecessary venting of blood from the surgical field, as well as vacuum assistance for venous drainage.

The weaning checklist — The use of a checklist to assure completion of tasks and to improve team performance as well as patient safety is recommended according to published guidelines [21]. Studies have shown that checklists in the perioperative period reduce communication failures and surgical complications, and increase detection of potential safety hazards [22,23]. (See "Patient safety in the operating room", section on 'Approaches to risk reduction'.)

THE WEANING PROCESS — Successful weaning from CPB is defined as adequate oxygenation, ventilation, circulation, and organ perfusion without the aid of ongoing extracorporeal circulation [15].

Steps in the weaning process — After preparations for weaning have been completed, the process of weaning proceeds in a controlled manner. All team members must be present in the operating room and focused on their roles.

●We agree with professional society recommendations for using a checklist prior to initiating weaning, including the European Association for Cardio-Thoracic Surgery (EACTS), the European Association of Cardiothoracic Anesthesiology (EACTA), and the European Board of Cardiovascular Perfusion (EBCP) [1]. Routine review of critical factors in the weaning process via a checklist enhances compliance with weaning measures and reduces task omission during simulated separation from bypass [24]. An example of a pre-weaning checklist includes:

•Confirmation of adequate rewarming

•Blood gas, hemoglobin, and electrolytes reviewed

•Ventilation initiated

•Vacuum assist venous drainage is turned off

•Shunts are closed

•Aortic root vent is on or off depending upon procedure

•Vasopressor control returned to anesthesiologist

•Management of anesthetic returned to anesthesia team and volatile agent on bypass machine turned off

●The surgeon and anesthesiologist direct the perfusionist to gradually decrease the amount of blood diverted to the CPB circuit, which allows some blood to remain in the right atrium (RA). This process is evident on the monitor because pulsatility is noted on the central venous pressure (CVP) tracing (and pulmonary artery pressure [PAP] tracing, if available), followed by the arterial blood pressure (BP) +tracing (waveform 1).

●The perfusionist continues to infuse blood through the aortic cannula, thereby restoring the circulating blood volume as venous drainage is further reduced. Once the left ventricle (LV) is adequately filled and is ejecting a sufficient stroke volume (as evidenced by the pulse pressure on the arterial pressure waveform), the perfusionist stops diverting blood from the RA to the CPB circuit, clamps the venous drainage line completely, and communicates that the patient is now "off bypass" (ie, weaned from bypass).

●The anesthesiologist and surgeon continuously assess adequacy of ventricular filling (preload) by observing the LV and right ventricle (RV) with transesophageal echocardiography (TEE) [1]. The RV is also assessed by direct visualization in the open chest. The CVP and/or PAP are monitored; target values are often selected to approximate those recorded before CPB, rather than absolute values.

●When adequate circulation is established as indicated by the hemodynamic variables noted above, the surgeon reinspects suture lines and other sites for excessive bleeding. Both the surgeon and anesthesiologist judge the integrity of the surgical repair by assessing pertinent TEE images and monitoring hemodynamic data before proceeding with protamine administration and vascular decannulation. (See 'Venous decannulation and initial return of pump blood' below and 'Reversal of anticoagulation with protamine' below and 'Aortic decannulation' below.)

Hemodynamic targets during weaning — Hemodynamic goals during the weaning process are targeted to achieve stable physiologic parameters and adequate organ perfusion. Typical goals include:

●Heart rate 80 to 90 beats/minute (bpm) to optimize ventricular filling and cardiac index (CI). Pacing of either the RA, RV, or both may be necessary.

●Mean arterial pressure (MAP) 60 to 90 mmHg to maintain perfusion to the myocardium and other vital organs. Higher pressures are avoided while the aortic cannula is still in place.

●Adequate filling of the ventricles (preload) to maintain adequate CI (or adequate end-diastolic volume in both the LV and RV observed on TEE). Although CVP may be monitored, the optimal target to ensure adequate filling depends on the patient's baseline CVP.

●A CI of 2.0 to 2.4 L/min per m², indicating adequate organ perfusion (assuming adequate oxygen carrying capacity of the blood).

Although these goals may be achieved, unstable physiologic parameters and inadequate organ perfusion may subsequently occur in the immediate postbypass period due to inadequate preload (most commonly resulting from bleeding), inadequate vascular tone (ie, vasodilation), or myocardial dysfunction. (See "Intraoperative problems after cardiopulmonary bypass", section on 'Cardiovascular problems'.)

THE IMMEDIATE POSTBYPASS PERIOD — Goals during the immediate postbypass period are to reverse anticoagulation, achieve hemostasis, reinfuse residual pump blood, maintain an optimal hemodynamic state, complete the surgical procedure, and prepare for patient transport to the intensive care unit (ICU) (table 3).

Venous decannulation and initial return of pump blood — After complete separation from CPB, with no venous return to the pump (or arterial outflow from the pump), the surgeon removes the venous cannula and closes the right atrial (RA) cannulation site or caval cannulation sites. Blood remaining in the venous tubing is returned to the venous reservoir for transfusion back into the patient by the perfusionist. This is typically accomplished by administering small aliquots of 100 to 200 mL through the aortic cannula as directed by the anesthesiologist and/or surgeon to augment cardiac index (CI) as needed.

When ventricular filling is adequate and there is no anticipated need for returning to CPB, the surgeon removes the aortic cannula and repairs this cannulation site. Any residual blood remaining in the extracorporeal circuit reservoir is returned to the patient via a peripheral intravenous (IV) or central venous catheter (CVC), as described below.

Reversal of anticoagulation with protamine — After weaning from CPB, protamine is administered to reverse systemic anticoagulation with heparin. Techniques for protamine administration, dosing considerations, and recognition, treatment, and prevention of adverse reactions to protamine are discussed separately. (See "Protamine: Administration and management of adverse reactions during cardiovascular procedures", section on 'Protamine administration after cardiopulmonary bypass'.)

Aortic decannulation — Exact timing of initiation of protamine administration (before or after aortic decannulation) varies according to institutional and surgical preferences. However, the aortic cannula should be removed before heparin neutralization is complete, so that clot does not form on the aortic cannula tip, which might lead to arterial thromboembolism (picture 1). (See "Protamine: Administration and management of adverse reactions during cardiovascular procedures", section on 'Timing of administration'.)

Systolic BP is controlled (typically <100 mmHg) during aortic decannulation to reduce risks of bleeding, aortic dissection, or other vascular injury [25]. Methods to reduce BP include decreasing preload or afterload, or, rarely, altering temporary pacemaker settings. For example, preload may be decreased with nitroglycerin administration, afterload may be decreased by administering anesthetic agents with vasodilatory properties (eg, small bolus doses of propofol 10 to 50 mg or increased concentration of an inhalation agent), or vasodilator agents such as nicardipine, clevidipine, or nitroprusside. Also, temporary pacemaker settings may be temporarily adjusted (eg, a change from atrial or atrioventricular [AV] sequential pacing to ventricular pacing alone to reduce left ventricular [LV] filling and BP with loss of AV synchrony [atrial kick]).

The anesthesiologist closely controls systolic BP until the surgeon closes the decannulation site with aortic purse string sutures.

Return of residual pump blood — Typically, 500 to 1500 mL of residual heparinized whole blood remains in the venous reservoir and tubing of the CPB pump after removal of the venous and aortic cannulae. This whole blood has not been further processed; therefore, it contains all of the clotting factors and platelets that are helpful for restoring hemostasis after reinfusion. However, residual pump blood also contains heparin. There are several methods to safely reinfuse this pump blood into the patient, and to neutralize any residual heparin (if necessary) [1]:

●Residual pump blood may be hemoconcentrated by the perfusionist using the CPB circuit and then infused into the patient over a brief period (typically 10 to 15 minutes) via a primed IV tubing leading from the circuit that is connected by the anesthesiologist into an existing peripheral IV catheter or CVC port. Such direct reinfusion allows rapid delivery of the blood, including platelets and clotting factors. However, to avoid heparin rebound, residual heparin in the residual pump blood must be neutralized with administration of protamine timed to coincide with completion of reinfusion. If it is necessary to reinfuse blood slowly (eg, to avoid hypervolemia), an additional small dose of protamine (eg, 25 to 50 mg) is typically administered after the infusion is complete because protamine has a short half-life and will have been rapidly cleared after the initial dose. (See "Protamine: Administration and management of adverse reactions during cardiovascular procedures", section on 'Initial protamine dosing strategies' and "Protamine: Administration and management of adverse reactions during cardiovascular procedures", section on 'Avoiding heparin rebound'.)

●An alternative method is collection of residual pump blood into a separate blood transfusion bag, thereby allowing reinfusion later in the postbypass period. If this method is selected, residual heparin contained in the pump blood is neutralized by administering an additional small dose of protamine (eg, 25 to 50 mg) or a protamine infusion at approximately 25 mg/hour over two to four postoperative hours [26,27]. (See "Protamine: Administration and management of adverse reactions during cardiovascular procedures", section on 'Avoiding heparin rebound'.)

●Another method used by many centers is processing residual reservoir blood and any additional blood that is subsequently suctioned from the surgical field with a blood salvage device such as a "cell saver." Since salvaged blood is concentrated and washed by cell saver devices, there is minimal residual heparin concentration. Thus, additional protamine administration is not necessary. However, platelets and other clotting factors are removed during cell saver processing. (See "Surgical blood conservation: Intraoperative blood salvage", section on 'Technology and procedural steps'.)

Maintenance of optimal pacemaker function — Temporary epicardial pacing is usually employed after cardiac surgery with CPB via atrial (A) and ventricular (V) pacing wires placed by the surgeon before weaning from CPB. (See 'Preparation for weaning' above.)

Temporary epicardial A pacing is preferred to treat sinus bradycardia because this option provides an atrial kick and anterograde conduction through the His Bundle, resulting in coordinated ventricular contraction. However, since the AV delay may be prolonged after cardiac surgery, optimal pacing is achieved in some patients with AV sequential pacing to improve mechanical coupling between atria and ventricles [28].

The following general principles for temporary pacemaker management apply to patients during weaning and the immediate postbypass period:

●Prior to separation from CPB, a regular rhythm with a rate of 80 to 90 beats per minute allows adequate diastolic filling time to optimize CI [28].

●For bradycardia, AOO, DOO, or VOO pacing is used to allow asynchronous pacing that will not be interrupted by oversensing of the surgical electrocautery. In these modes, the atrium, ventricle, or both are paced as necessary, but the pacemaker has no sensing response and there is a higher risk of R on T phenomena. (See "Modes of cardiac pacing: Nomenclature and selection", section on 'AOO, VOO, or DOO mode'.)

●When the patient's intrinsic heart rate (HR) is adequate (ie, >75), a synchronous pacemaker mode (DDD, AAI) may be employed. Such synchronous modes avoid the potential for pacing competition and R on T phenomena. (See "Modes of cardiac pacing: Nomenclature and selection", section on 'DDD or DDDR pacing' and "Modes of cardiac pacing: Nomenclature and selection", section on 'AAI or AAIR pacing'.)

However, surgical electrocautery causes electromechanical interference (EMI), with resultant atrial and ventricular oversensing and inhibition of pacing function. Also, the radiofrequency identification wand used to detect retained surgical sponges near the end of the procedure may cause EMI [29]. Thus, before applying a DDD or AAI pacemaker setting, the anesthesiologist must confirm that the patient has an underlying perfusing rhythm and adequate HR. This is done by pausing the pacemaker output, checking the ECG, and confirming appropriate ventricular ejection by inspecting the arterial BP tracing on the monitor. (See "Perioperative management of patients with a pacemaker or implantable cardioverter-defibrillator", section on 'Electromagnetic interference'.)

●The temporary epicardial pacemaker should be set to a synchronous (sensing) mode for all patients before transport to the intensive care unit. Inappropriate asynchronous pacing may cause competition between the pacemaker and the patient's intrinsic rhythm, or more importantly, a rare but life threatening "R-on-T" phenomenon that results in ventricular fibrillation.

Achievement of hemostasis — After decannulation and placement of pacemaker wires, the surgeon inspects all surgical sites for bleeding or compromise of the surgical repair. Hemostasis must be achieved prior to sternal closure.

MANAGEMENT OF FAILURE TO WEAN FROM CARDIOPULMONARY BYPASS — The term "failure to wean" from CPB is applied to patients with low cardiac output syndrome (LCOS) who are unable to maintain oxygenation, adequate cardiac output (CO), and end-organ perfusion when withdrawal from extracorporeal circulation support is attempted [30]. In one large study, failure to wean from CPB on the first attempt or a requirement for mechanical assist devices to achieve weaning occurred in 14.5 percent of 6120 patients [31]. Factors associated with failure to wean included older age, left ventricular dysfunction, previous cardiac surgery, presence of mitral valve regurgitation, coagulopathy, and longer duration of CPB.

Appropriate planning and communication are necessary during management of postbypass low CO syndrome (ie, cardiogenic shock). The anesthesiologist prepares additional vasoactive drugs and performs ongoing assessment of the need for mechanical circulatory support such as an intraaortic balloon pump (IABP), ventricular assist device (VAD), or extracorporeal membrane oxygenation (ECMO) [30,31].

Return to CPB after a failed weaning attempt — The causes of unsuccessful weaning from CPB are usually multifactorial, as one problem compounds others. For example, it may not be possible to sustain satisfactory blood pressure (BP) and myocardial perfusion in a hypotensive patient when adequate preload cannot be maintained due to excessive bleeding.

Urgent reinstitution of CPB may become necessary if BP, CO, and organ perfusion are inadequate [1].

Return to CPB involves several steps that must occur in rapid sequence, including:

●The perfusionist prepares the CPB machine and circuit. In general, these CPB components should remain primed (ie, filled with the patient's blood) until it is certain that the patient is stable and will not need to return to CPB.

●If protamine has been administered, the anesthesiologist must re-administer heparin before reinstitution of CPB. As with initial systemic heparinization, a dose of 300 to 400 units/kg is given; alternatively, the heparin dose may be determined by point-of-care testing (ie, HepCon) if available. It may be necessary to administer additional heparin to offset the effect of any residual protamine. (See "Blood management and anticoagulation for cardiopulmonary bypass", section on 'Heparin administration and monitoring' and "Protamine: Administration and management of adverse reactions during cardiovascular procedures", section on 'Redosing protamine after a reaction'.)

●Once CPB has been reinstituted, the surgeon corrects any underlying technical problems. Often, surgical issues are addressed with the heart still beating. In some cases (eg, bleeding from an incision site or inadequate flow through a graft), replacement of the aortic cross-clamp and a repeated dose of cardioplegia are necessary to achieve electromechanical silence to facilitate adequate surgical repair.

●Temporary mechanical circulatory support may be employed in cases of refractory myocardial or pulmonary dysfunction that precludes weaning from CPB [30]. Selection of a circulatory assist device (eg, IABP, VAD, ECMO) depends on individual patient hemodynamic factors, surgical preferences, and institutional resources. Further details are available in a separate topic review. (See "Intraoperative problems after cardiopulmonary bypass", section on 'Cardiogenic shock'.)

●If a specific surgical problem is not identified, the patient is assessed for other cardiac and noncardiac problems before further attempts to wean from CPB. (See "Intraoperative problems after cardiopulmonary bypass".)

Delayed deterioration after cardiopulmonary bypass — The majority of patients wean from cardiopulmonary successfully and are transported to the intensive care unit (ICU) without life-threatening problems. However, certain problems may appear later (eg, left or right ventricular failure, bleeding, arrhythmias, graft compromise, air entry into the coronary arteries, or compression of the heart upon removal of the sternal retractor. Thus, continued vigilance is critical even after successful weaning and includes assessment of hemodynamic status, trends in doses of inotropic agents, and myocardial function (assess using transesophageal echocardiography [TEE]), as well as acid-base status, and lactate levels. Myocardial function can be assessed using TEE.

Furthermore, removal of the sternal retractor followed by chest closure is associated with hemodynamic changes including a reduction in cardiac index and stroke volume index [32]. The causes include a reduction in venous return due to compression of the heart by the sternum and a limitation on the ability of the ventricle to relax and accommodate returning blood. These changes may require medication adjustment, fluid administration, or reopening of the chest and reassessment. Patients who do not tolerate closure may be managed with delayed sternal closure. (See "Intraoperative problems after cardiopulmonary bypass", section on 'Inability to close the sternum'.)

Further discussion of management of problems after CPB can be found in a separate topic. (See "Intraoperative problems after cardiopulmonary bypass".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Management of cardiopulmonary bypass".)

SUMMARY AND RECOMMENDATIONS

●Preparation for weaning from cardiopulmonary bypass (CPB) – Weaning from CPB is a complex, multistep, interdisciplinary team effort. We suggest a checklist for preparations for weaning (Grade 2C) (table 1). This ensures that the patient is adequately rewarmed; blood gases, electrolytes, and hemoglobin are checked; and ventilation is reestablished. Transesophageal echocardiography (TEE) is employed before weaning to assess residual intracardiac air, during weaning to assess adequacy of left ventricular (LV) and right ventricular (RV) filling and function, and after weaning to perform a complete TEE assessment of the surgical repair. (See 'Preparation for weaning' above.)

●The weaning process – Hemodynamic goals during the weaning process are targeted to achieve stable physiologic parameters and adequate organ perfusion. Typical goals include heart rate 80 to 90 beats/minute (which may require pacing of the atrium, ventricle, or both), mean arterial pressure 60 to 90 mmHg, and adequate filling of the ventricles to achieve a cardiac index 2.0 to 2.4 L/min per m². (See 'The weaning process' above.)

●Management immediately after weaning from CPB – Key steps during the immediate postbypass period include venous and arterial decannulation, reversal of anticoagulation with protamine administration, reinfusion of pump blood, maintenance of optimal pacemaker function, and achievement of hemostasis (table 3). (See 'The immediate postbypass period' above.)

●Failure to wean from CPB – If urgent reinstitution of CPB becomes necessary, the perfusionist prepares the CPB machine and circuit, and the anesthesiologist re-administers the initial systemic heparin (typically 300 to 400 units/kg) if protamine has already been infused. Once CPB has been reinstituted, the surgeon corrects any underlying technical problems. The patient is assessed for other cardiac and noncardiac problems before further attempts to wean from CPB. In selected patients with refractory ventricular dysfunction resulting in persistently low cardiac output, mechanical circulatory support may be necessary (eg, intraaortic balloon pump [IABP], ventricular assist device [VAD], extracorporeal membrane oxygenator [ECMO]). (See 'Return to CPB after a failed weaning attempt' above.)

●Delayed deterioration after weaning from CPB – Patients who are successfully weaned from CPB may demonstrate instability or deterioration in the period after bypass. The cause may be delayed LV or RV failure, bleeding, arrhythmias, graft compromise, air entry into the coronary arteries, or compression of the heart upon removal of the sternal retractor. Continued vigilance is necessary throughout the surgical procedure. Some patients may benefit from delayed sternal closure to allow these factors to resolve. (See 'Delayed deterioration after cardiopulmonary bypass' above.)