Cardioplegia flow dynamics in an in vitro model

The flow of fluids in extracorporeal circuits does not conform to conventional Poiseuille mechanics which confounds calculating cardioplegia (CP) flow distribution. The purpose of this study was to quantify CP flow dynamics in a model simulating coronary atherosclerosis across varying sized restrictions. An in vitro preparation was designed to assess hydraulic fluid movement across paired restrictions of 51, 81 and 98% lumen reductions. Volume data were obtained at variable flow, temperature, viscosity and pressure conditions. CP delivered through 14- and 18-gauge (GA) conduits at 8°C and 100 mmHg infusion pressure revealed that both four to one and crystalloid CP solutions had significantly less total percentage flow through the 14-GA conduit, p < 0.0001 and p < 0.001, respectively. Overall, 4:1 CP exhibited the most favorable fluid dynamics at 8°C in that it delivered the highest percentages of total CP flow through the smaller lumen conduit. At both 8°C and 37°C delivery, blood CP resulted in the least homogeneous fluid distribution at all delivery parameters. The results in relation to blood viscosity indicate that, although the 8°C blood CP had a significantly greater viscosity than 37°C blood CP, it did not produce an effect in fluid distribution. These data show that increasing the cardioplegic solution hematocrit causes an inhomogeneous fluid distribution regardless of delivery temperature or infusion pressure.