Higher hematocrit improves liver blood flow and metabolism during cardiopulmonary bypass in piglets

Hemodilution has been applied conventionally during cardiopulmonary bypass (CPB) and deep hypothermic circulatory arrest (DHCA) to counteract the increase in viscosity and deleterious rheological effects caused by hypothermia. However, liver dysfunction after low-flow bypass and DHCA is common, and little is known about the effects of hematocrit (Hct). The purpose of the present study is to evaluate the impact of two hemodilution priming protocols used clinically on liver perfusion and metabolism.

Ten piglets were randomized into 2 groups. One group (n = 5) had a crystalloid prime resulting in an Hct of about 15 % (low hematocrit; group L), the other (n = 5) a total-blood prime (Hct = 25 %; high hematocrit; group H). All animals underwent 70 min cooling at full flow (150 ml/kg/min), 30 min of low flow (50 ml/kg/min) at 15 degrees C followed by 45 min of DHCA and 75 min of rewarming at full flow. Liver blood flow (LBF) was assessed at the beginning of CPB at 34 degrees C, at the end of cooling at 15 degrees C, at the end of low flow, 5 min after the start of warming, and at the end of rewarming at 34 degrees C by injections of radioactive microspheres. Liver function was evaluated at the same time using the MEGX test, which measures the metabolism of lidocaine.

LBF was insignificantly reduced during cooling, decreased during low flow (p = 0.001), and increased again after DHCA with the highest flow at the end of rewarming. LBF tended to be lower at all times in group L (p = 0.096). The liver lidocaine metabolic rate did not significantly decrease during cooling and low flow, but was increased at the end of rewarming (p = 0.01); the metabolism was higher in group H (p = 0.025). Multiregression analysis revealed liver blood flow (p = 0.003) and hematocrit (p < 0.001) as independent determinants of the liver lidocaine metabolism; arterial blood pressure and temperature did not have significant influence in this model.

Hemodilution results in a tendency towards reduced liver blood flow during CPB; much worse is the resulting impaired liver metabolism, independent of reduced blood flow and pressure. Avoidance of low hematocrit during CPB may be a useful adjunct to preserve liver function in patients undergoing cardiac surgery with long duration CPB and DHCA.