Effects of changes in arterial O2 content on cerebral blood flow in the lamb.

The effect of variations in arterial O2 content (CaO2) on the cerebrovascular bed of seven unanesthetized newborn lambs was studied as the hematocrit and arterial PO2 (PaO2) were varied. Each subject was studied at a high hematocrit [44 +/- 3% (SD)] and a low hematocrit [24 +/- 3%]. At each hematocrit level the PaO2 was changed over a range of 30-150 mmHg. The relationship between cerebral blood flow and PO2 depended on hematocrit and vice versa. To the contrary, the relationship of blood flow to CaO2 was independent of hematocrit and/or PO/. As CaO2 fell, regardless of whether this was due to a fall in PO2 hematocrit or both, there was a reciprocal increase in cerebral blood flow such that cerebral O2 delivery (cerebral blood flow x CaO2) was constant. These data show that CaO2 is a variable of fundamental importance to the regulation of cerebral blood flow. Changes in CaO2 are accompanied by reciprocal changes in cerebral blood flow to maintain constant cerebral O2 delivery. Data among species with differing cerebral O2 consumption show that cerebral O2 delivery, in turn, is regulated according to cerebral O2 consumption.