Weaning from Cardiopulmonary Bypass (CPB)

Overview

Pre-Weaning Checklist: The "CVP" Mnemonic

C - Temperature ("Cold")

• Patient temperature must be 36-37°C before weaning
• Nasopharyngeal and venous blood temperatures should never exceed 37°C - hyperthermia increases risk of postoperative neurologic complications

C - Conduction (Rate and Rhythm)

• Target heart rate: 80-100 bpm
• Bradycardia: treated with epicardial pacing wires and/or beta-adrenergic chronotropic drugs
• Tachycardia (>120 bpm): treat the underlying cause (anemia, hypovolemia, light anesthesia, chronotropic drugs)
• Third-degree AV block: requires pacing, ideally AV sequential pacing
• Sinus rhythm is preferable, especially in patients with poor LV compliance who depend on the atrial kick for adequate filling
• SVT: direct cardioversion; pharmacologic options include amiodarone, esmolol, verapamil, or adenosine

C - Cardiac Output

• Assessed before and during weaning
• Inotropic support initiated if cardiac output is inadequate
• Common inotropes: dobutamine, milrinone, epinephrine (see below)

C - Cells (Hematocrit)

• Optimal hematocrit (Hct) is checked
• Anemia may require transfusion before weaning

C - Calcium

• Ionized calcium levels are verified and corrected
• Hypocalcemia impairs myocardial contractility

C - Coagulation

• Labs reviewed; coagulation factors assessed as part of post-bypass management

V - Ventilation, Visualization, Vaporizer, Volume

Ventilation

• Lungs must be re-inflated and ventilation resumed before coming off bypass
• Atelectasis is corrected by manual lung recruitment

Visualization (TEE)

• TEE probe is used to:
  • Confirm de-airing of cardiac chambers (LV and aortic root)
  • Assess LV and RV function
  • Evaluate filling volumes
  • Detect residual surgical problems (valve regurgitation, wall motion abnormalities)

Vaporizer

• Volatile anesthetic from the oxygenator vaporizer is turned off when preparing to wean

Volume Expanders

• Blood remaining in the extracorporeal circuit (ECC) is used to transfuse volume back to the patient as venous drainage is reduced

P - Pressure, Pressors, Pacer, Potassium, Protamine

Pressure

• Systemic blood pressure is confirmed and supported as needed
• MAP targets are maintained with vasopressors if vasodilatory tone is excessive

Pressors / Inotropes

Pacer

• Epicardial pacing wires are placed routinely
• Pacing activated if bradycardia or AV block is present

Potassium

• Hyperkalemia from cardioplegia is corrected before weaning
• Hypokalemia is also arrhythmogenic and must be corrected

Protamine

• Administered after all cannulas are removed, when the patient is hemodynamically stable
• Reverses heparin anticoagulation
• Dose: 1 to 1.3 mg protamine per 100 units heparin in circulation
• Must be given slowly over 5-10 minutes to reduce risk of hypotension
• Critical communication: The anesthesiologist must clearly notify the perfusionist before giving protamine - any blood containing protamine that returns to the ECC via pump sucker will cause clotting and render the circuit unusable for emergency reinstitution of CPB
• After administration, ACT should return to baseline

The Weaning Process (Step-by-Step)

1. Confirm all pre-weaning criteria are met (CVP checklist above)
2. Resume ventilation - re-expand atelectatic lungs
3. Confirm cardiac electrical activity - support with pacemaker if needed
4. The perfusionist gradually reduces venous drainage to the ECC, allowing more blood to fill the heart
5. Simultaneously, blood from the ECC reservoir is transfused back to the patient to restore cardiac filling volumes
6. As the heart takes over cardiac output, the arterial pump flow is progressively decreased
7. The anesthesiologist ensures inotropic and/or volume requirements are met in real time
8. Once the patient's cardiac output is adequate, the pump is terminated
9. The patient is supported post-bypass with volume replacement from remaining ECC blood + vasopressors/inotropes as needed
10. Once hemodynamically stable, cannulas are removed and protamine is administered

PA Catheter Considerations During Weaning

• PA catheter-induced hemorrhage is most likely to occur during CPB weaning
• The catheter should be withdrawn 3-5 cm before CPB begins to avoid wedging in a pulmonary artery as lungs deflate
• If hemoptysis occurs during weaning: do not rapidly reverse anticoagulation (risks fatal asphyxiation)
• Instead: reinstitute full CPB to ensure oxygenation, then perform bronchoscopy to identify the bleeding site

Difficult Weaning / Failed Weaning

• Intra-aortic balloon pump (IABP) - reduces LV afterload, augments diastolic coronary perfusion
• Ventricular assist device (VAD) - mechanical circulatory support for severe LV/RV failure
• ECMO - for refractory cardiopulmonary failure

Algorithm Summary (Fig. 50.10)

1. ACT - adequate anticoagulation confirmed
2. Hct - acceptable anemia threshold
3. K+ - potassium corrected
4. HR - rate 80-100 bpm, pacing if needed
5. Temperature - 36-37°C
6. MAP - adequate perfusion pressure
7. LV function - assessed by TEE (CI target)
8. Inotrope/vasopressor adjustment based on CI and SVR