Pulsatile extracorporeal circulation: fluidmechanic considerations

Abstract

With a simple mechanical mock circulation it was possible to measure the pressure wave and flow in the arterial line and the pressure in the mock circulation and their dependence upon the pulse characteristics, aortic cannula size (inner diameter between 5.4 mm and 12.7 mm) and the use of a hollow fibre membrane oxygenator during nonpulsatile as well as pulsatile perfusion. In the arterial line, pressure peaks up to 750 mmHg have been registered, resulting in peak flow rates of 12.4 I/min. Due to the mechanical construction of the roller pump and the use of silicon tubing in the head, negative pressure peaks of 240 mmHg resulting in retrograde flow peaks up to 5 I/min were measured. The pressure in the mock circulation was dependent on the inner diameter and shape of the aortic cannula. Pulse pressure up to 100 mmHg, systolic pressure up to 130 mmHg and dp/dt up to 1250 mmHg/s could be achieved by using a cannula with an internal diameter of 12.7 mm. A cannula, however, with an internal diameter of only 5.4 mm produced a reduced peak pulse pressure of 65 mmHg, a systolic pressure of 100 mmHg and a dp/dt of 450 mmHg/s. By calculating the shear stress at the wall and, most importantly, in the free mixing layer, it was possible to estimate the resulting haemolysis. Haemolysis occurs when a shear stress over 100 Pa is present for over 100 ms. This destroys platelets as well as erythrocytes. The calculations showed shear stresses up to 1500 Pa while using a cannula with an internal diameter of 5.4 mm in comparison to 50 Pa with the internal diameter of 12.7 mm.