During a 3 day period this unit experienced 2 incidents of abnormally high trans-membrane pressure gradients (TMP) during the early stages of cardiopulmonary bypass. The department routinely utilises roller pumps for bypass and measures pre- and post-membrane pressures for all cases. Abnormal gradients are defined within the unit as P >300mmHg, with attention to related factors e.g. flow etc. Consideration for oxygenator change out is only made when perfusion becomes limited and potentially inadequate. For the cases there were no observed common factors e.g. surgeon or perfusion scientist; however the same oxygenator model (Sorin Inspire 6F) was utilised but each had a different batch number.


• Case 1: The patient was an average sized 18 year old male scheduled for redo sternotomy for an aortic root replacement having previously undergone a valvotomy in childhood. After sternotomy and heparinisation, a small bleed occurred requiring quicker than expected bypass initiation however a longer than adequate ACT had already been achieved. Initially full flow was reached, with a normal TMP 150mmHg. Cooling to 30C was commenced. More dissection of the heart was required for mobilisation and therefore the aortic cross clamp was not applied. After about 20mins on bypass the TMP gradually increased to >300mmHg. The circuit was checked for physical obstructions, a second perfusion scientist was consulted, the surgeon was informed and the case perfusion scientist requested to rewarm the patient hoping this may resolve the situation. The pressure however continued to rise, the pre-membrane pressure reaching in excess of 600mmHg (TMP>450mmHg). After discussion, and as the bleeding issue had been resolved but knowing a prolonged bypass was expected the decision was made to come off bypass and change the oxygenator. The patient was ventilated, filled and normal ejection was established for change out. This occurred without any untoward event, bypass was re-established, and normal pressures were observed and maintained for the duration of the case. The patient was weaned successfully and underwent an uneventful post-op course. There was no observed effect to the patient from this event.


• Case 2: A 66year old unremarkable male was scheduled for a primary mitral valve repair. Bypass was initiated after an ACT=541 and the patient cooled to 34C according to specific surgeon request, TMP 200mmHg. After about 30mins on bypass and the application of the aortic cross clamp and opening of the heart chamber increasing TMP was noted, again the circuit was checked for obstruction or error and another Perfusion Scientists opinion requested. The pre-membrane pressure continued to rise to >600mmHg , TMP>400mmHg. The situation was discussed with the surgeon and as the heart was open cooling of the patient commenced. The TMP increased to >500mmHg despite reducing arterial flow (and therefore ABP) and the decision was made to replace the oxygenator. Change out occurred at 31C, CPB was interrupted for 1.5 minutes and re-established without incident. Normal pressures returned and the bypass, weaning and post-op period occurred uneventfully. No negative effects were observed in the patient, neurological or otherwise.