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What is optimal flow using a mini-bypass system?

The purpose of this study is to determine the appropriate arterial pump flow /cardiac index (CI) utilizing a mini-bypass system. The unique feature of most mini-bypass systems is that the centrifugal pump combines the function of kinetic venous drainage with arterial pump flow. Therefore, if drainage is reduced, arterial pump flow is also reduced. Managing this system can present challenges to the clinical perfusionist. We reviewed fifteen cases, using the Medtronic Resting Heart System (RHS). This retrospective study examined the arterial pump flow, measured as cardiac index (CI), mean arterial pressure, inlet venous saturation, urine output, vasopressor use, and lactate production during routine cardiac surgery. The mean cardiac index for all patients was 1.90 +/- 0.14, range 1.63-2.08 L/min/m2. The mean hemoglobin on cardiopulmonary bypass (CPB) was 10.6 +/- 1.2, with a range of 9.2-13.3 g/dL. The lactate produced on CPB was 2.03 +/-0.67 with a range of 1.5-3.5 mmol/L. The mean change in lactate measured from pre CPB to post CPB was 0.85 +/- 0.71 with a median lactate of 0.6 mmol/L. The venous saturation was 65.53 +/- 6.03% with a median of 65% and a range of 57-82%. The mean arterial pressure was 67.04 +/- 10.45 mmHg with a median of 62.5 mmHg. The median urine output was 125 mls. The vasopressor median was 2200 µg. All patients were maintained on CPB with a mean nasopharyngeal temperature of 35.43° Celsius. Despite using lower than predicted flows, it would appear that adequate perfusion is provided. The higher hemoglobin content achieved through reduced hemodilution and reduced inflammation appears to enable this system to deliver adequate flow and perfusion at reduced cardiac indices. This research provides evidence which challenges longstanding beliefs that a cardiac index of 2.4 L/min/m2 is required for all cases.