LACK OF DEPENDENCE OF CEREBRAL BLOOD-FLOW ON BLOOD-VISCOSITY AFTER BLOOD-EXCHANGE WITH A NEWTONIAN O-2 CARRIER

Whether the increase in cerebral blood flow measured after hemodilutio n is mediated by a decrease in blood viscosity or in oxygen delivery t o the brain is debated. In the present study, blood was replaced by an oxygen-carrying blood substitute, ultrapurified, polymerized, bovine hemoglobin (UPBHB). In contrast to normal blood, UPBHB yields a consta nt and defined viscosity in the brain circulation, since its viscosity is not dependent on the shear rate. CBF was determined after blood ex change with UPBHB in one group of conscious rats (UPBHB group) and in another group of blood-exchanged conscious rats in which viscosity was increased fourfold by the addition of 2% polyvinylpyrrolidone (PVP), mw 750,000 (UPBHB-PVP group). Local CBF (LCBF) was measured in 34 brai n structures by means of the quantitative iodo(C-14)antipyrine method. After blood replacement, systemic parameters such as cardiac index, a rterial blood pressure, blood gases, and acid-base status were not dif ferent between the UPBHB and the UPBHB-PVP groups. In particular, arte rial oxygen content was similar in both groups. Compared with a contro l group without blood exchange, LCBF was increased after blood exchang e in the different brain structures by 60-102% (UPBHB group) and by 33 -101% (UPBHB-PVP group). Mean CBF was increased by 77% in the UPBHB gr oup and by 69% in the UPBHB-PVP group. No significant differences were observed in the values of LCBF or mean CBF between the UPBHB group an d the UPBHB-PVP group. The results show that a fourfold variation in t he viscosity of a Newtonian blood substitute does not result in differ ences in CBF values. It is concluded that blood viscosity is less impo rtant to CBF than hitherto postulated.