Effects of hemodilution on hypotention during cardiopulmonary bypass

Arterial pressure, systemic vascular resistance, and viscosity measurements were made on 20 patients, during cardiopulmonary bypass with hemodilution to an average hematocrit reduction of 48 plus or minus 12 percent. The arterial pressure and blood viscosity decreased an average of 56 plus or minus 16 percent and 40 plus or minus 12 percent, respectively. The total peripheral resistance following the start of bypass was significantly below normal (p smaller than 0.005), but not when corrected for the change in viscosity. There was a strong correlation between the extent of pressure reduction and the viscosity decrease with hemodilution (p smaller than 0.01), although not all the changes in pressure could be accounted for in terms of viscosity. There was no apparent correlation between the extent of hypotension as measured by the area of the pressure-time curve below 50 mm. Hg and the presence or absence of postoperative cerebral or cardiac complications, in contrast to other recent reports.