Pressure effects on roller pump blood flow during hemodialysis

Accurate measurement of blood flow during hemodialysis is essential to avoid underdialysis. When blood roller pumps are pushed to the high-flow rates demanded by high performance dialyzers, flow may be overestimated. This study examined the effect of inflow (Pa) and outflow (Pv) pressures induced by hemodialysis roller pumps on flow in vitro and in vivo. Blood flow was measured volumetrically, and with an ultrasonic flow probe, whereas Pa was adjusted from -50 to -400 torr and Pv from 50 to 300 torr. Only Pa influenced flow. At -200 torr, true flow measured volumetrically averaged 8.5% +/- 1.3% less than the blood pump revolutions per minute (RPM) meter reading. At -400 torr, the difference was 33.0% +/- 1.9%. There was no visible indication that flow was reduced to less than pump meter readings. Pv, hematocrit, and the source of pump tubing had no significant effect. Flow measured with an ultrasonic transit-time probe during routine hemodialysis in 64 patients was 9.0% +/- 2.7% less than pump meter readings when Pa varied from -180 to -220 torr. Blood pump meter readings greater than 400 ml/min were usually inaccurate because of low Pa. Prepump monitoring of arterial inflow pressure can prevent hidden reductions in blood flow that decrease dialysis efficiency.