Failure to wean a patient from CPB at the first attempt after routine cardiac surgery is a relatively uncommon occurrence. Following prolonged, complex or emergency surgery, however, failure to wean is relatively common. In the majority of cases, weaning difficulty can be attributed to myocardial ischaemia secondary to a prolonged AXC time, inadequate myocardial protection, frank MI or coronary embolism. Less common causes include extremes of prosthetic valve malfunction, anastomotic strictures, extremes of vascular resistance and cardiac compression from retained surgical swabs.