Weaning From CPB
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Direct answer
Weaning begins half an hour before the cross-clamp comes off, with rewarming: by the time the heart is ready to eject, the patient must be warm (core 36 degrees C or more), ventilated, de-aired, in a stable rhythm, with potassium corrected and haematocrit adequate — because the pump cannot hand over a workload to a heart and lungs that are not ready to receive it. The mechanical part is a graded descent: pump flow halves stepwise while reservoir blood is transfused to fill the heart, watched on the transoesophageal echo and the pressures, until the ejected stroke volume sustains a cardiac index above 2.2 L/min/m2 and a mean pressure above 65 mmHg. Only after stability does protamine go in — slowly, through a line, never the circuit — and decannulation follows venous first, arterial last.
What you must remember
- Readiness checklist: core temperature 36 degrees C or more (rewarm slowly, water-to-blood gradient under 10 degrees C), haematocrit about 20-24% off pump, potassium roughly 4.5-5.5 mmol/L, acid-base normal, ventilation resumed with a recruitment manoeuvre.
- Hyperkalaemia from repeated cardioplegia is treated at weaning with calcium chloride 0.5-1 g, and insulin-dextrose if severe; calcium also boosts contractility as the heart resumes.
- De-airing: vents off, the heart agitated and needle-vented, and transoesophageal echocardiography is the standard proof that the left chambers are clear — residual air embolises to the coronaries and brain.
- Rhythm: sinus restored or epicardial pacing at about 90-100 per minute; internal defibrillation typically 10-20 joules (lower with biphasic devices) for ventricular fibrillation.
- Flow reduction: halve flows stepwise while transfusing from the reservoir — watch for filling without distension on echo, rising systemic pressure, and a venous saturation above 60-65%.
- A low mean pressure during weaning is treated with a vasopressor, not with more pump flow — chasing pressure with flow defeats the wean.
- Stability criteria off pump: cardiac index above 2.2 L/min/m2, mean arterial pressure 65 mmHg or more, venous saturation above 60-65%, improving lactate, no ventricular distension.
- Sequencing after wean: stable 15-20 minutes, then protamine slowly (never into the circuit), venous decannulation, transfusion of reservoir blood through the arterial cannula, arterial decannulation last.
- Failed wean ladder: return to bypass, inotropes or vasodilators by haemodynamics, intra-aortic balloon pump (contraindicated in aortic regurgitation and dissection), VA-ECMO for myocardial stun; paediatric programmes add modified ultrafiltration.
The last thirty minutes, step by step
The cross-clamp comes off and the heart reperfuses, often fibrillating briefly before reverting — or being reverted — to sinus. Rewarming completes to 36.5 degrees C while the anaesthetist reinflates the lungs, resumes ventilation and moves the volatile agent from oxygenator to lungs. The surgeon and the echo sweep for air; the vent comes off. Pacing wires are placed at 90 per minute in a valve case. Now the descent: the perfusionist announces each step — "half flow, two litres" — transfusing 100 mL aliquots from the reservoir as the echo shows the ventricles filling; "eight hundred"; "off". The heart ejects, the mean pressure holds at 68 with low-dose noradrenaline, the cardiac index reads 2.4, and the team waits, cannulae still in, because the first quarter hour of stability decides everything. Then protamine by slow infusion with the titrated dose, the venous cannula out, the remaining pump blood transfused through the arterial cannula, and finally the arterial cannula out.
Where students slip
Overfilling is the signature error: transfusing "until the pressure looks nice" while the echo shows a distending, under-ejecting ventricle — the correct response to a falling cardiac index with a full heart is to stop filling, unload, and add inotropy, not volume. The second error is protamine timing and route: given while unstable it removes the rescue of returning to pump, and given into the circuit it is catastrophic — acute pulmonary vasoconstriction; it must run slowly through a patient line with pulmonary pressures watched. Third, the un-restarted ventilator: the pump oxygenated the patient for an hour, and weaning is exactly when a silent ventilator becomes lethal — the readback includes "ventilation on" before flow comes down. Fourth, conceptual: chasing mean pressure with pump flow during the descent. The pump is being retired; pressure now belongs to the vasculature and a vasopressor.
Frequently asked questions
What core temperature is required before weaning?
At least 36 degrees C, achieved with a water-to-blood gradient under 10 degrees C to avoid gaseous emboli from rapidly warmed blood.
What haematocrit is targeted off pump?
Roughly 20-24% acceptable, aiming near 24%, using reservoir blood or diuresis/ultrafiltration as needed.
Why is calcium given at weaning?
To counter cardioplegia hyperkalaemia and citrated transfusion, and to support contractility as the heart resumes work.
What energy is used for internal defibrillation?
Typically 10-20 joules between internal paddles (lower with biphasic devices).
How is protamine correctly administered?
Slowly over 10-15 minutes through a patient line after stable weaning — never into the circuit, with pulmonary pressures monitored.
What are the options if the patient cannot be weaned from bypass?
Return to bypass for reassessment, inotropes and vasodilators by haemodynamic profile, intra-aortic balloon counterpulsation (barring aortic regurgitation or dissection), and VA-ECMO for refractory myocardial failure.