Pump thrombosis (PT) is a relatively uncommon but serious complication of a left ventricular assist device (LVAD). We believe that de-airing of the HeartMate II (HMII) with the pump turned on and a clamp across the outflow graft may lead to early thrombus formation due to heat generation on the bearings. Methods Patients who underwent HMII implantation from November 2012 to February 2016 were retrospectively reviewed. Patients were separated into two groups depending on the timing of removing

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... e clamp from the outflow graft. Patients in Group 1 underwent de-airing by turning on the pump with the vascular clamp on the outflow graft and patients in Group 2 were completely de-aired with the pump off and the pump was only activated after removing the clamp. Results There were 45 patients in Group 1 ("clamp on") and 33 patients in Group 2 ("clamp off"). Five patients had PT in Group 1 but none in Group 2 (p=0.07). Average LDH levels in the early postoperative period were similar (404±168 IU/L vs 425±267 IU/L; p=0.71). However, average LDH levels in the late postoperative period were significantly higher in Group 1 (388±214 IU/L vs 313±73 IU/L; p=0.045). Conclusion De-airing a running HMII with the outflow graft clamped increases LDH levels, suggesting that the bearings may act as a nidus for early thrombus formation caused by the lack of heat dissipation. Delaying pump activation until removal of the clamp on the outflow graft may affect the incidence of PT after HMII implantation. This may have potential importance in any VAD where there are mechanical bearings or other areas susceptible to heat generation.