BACKGROUND: High rotational pump speeds in percutaneous mechanical circulatory support (pMCS) devices frequently result in hemolysis. Consequently, in vitro hemolysis testing for newly developed devices is essential to mitigate this significant risk. This study compared the degree of hemolysis induced by the Second Heart Assist Whisper pMCS at its maximum pump speed, representing the worst-case scenario, with that of the Impella CP, following American society for testing and materials standards. METHODS: Five Whisper devices were tested at 10000 RPM, while a comparator Impella CP was operated at 44 133 ± 606 RPM to match the Whisper's flow rate. Both devices were evaluated in two identical in vitro blood circulatory loops using citrated bovine blood. Hemolysis was analyzed by the tetramethylbenzidine method. RESULTS: The change in plasma free hemoglobin (ΔpfHb) was significantly greater with the Impella CP than with the Whisper (p <
0.01). Both devices caused a steady increase in pfHb, with significant differences after 60 min of in vitro testing (p <
0.01). The Whisper and Impella CP differed significantly in normalized index of hemolysis (0.088 ± 0.022 vs. 0.194 ± 0.029 g/100, p <
0.01) and modified index of hemolysis (9.72 ± 2.4 vs. 20.83 ± 3.5, p <
0.01). CONCLUSION: The Whisper exhibited superior hemolytic performance to that of the Impella CP under identical hemodynamic conditions. Our newly designed blood circulatory loop and these benchmark values will aid future in vitro hemolysis testing for pMCS devices.