Bovine regional brain blood flow during sojourn at a simulated altitude of 3500 m.

Regional distribution of brain blood flow (15 micron diameter radionuclide labelled microspheres injected into the left atrium) was studied in 6 unanesthetized calves during the 7th-8th weeks of exposure to a simulated altitude of 3500 m (PB = 500 mm Hg). Measurements were made during chronic hypoxemia (PaO2 = 48 +/- 1 mm Hg) and acute normoxemia (PaO2 = 91 +/- 1 mm Hg). Five calves, born and raised at sea level, were also studied in a similar manner during normoxemia (PaO2 = 86 +/- 2 mm Hg) and at 12 and 22 min of acute hypoxemia (PaO2 = 49 +/- 1 mm Hg) to serve as controls. Acute hypoxemia in sea level calves resulted in a marked uniform increase in blood flow to all regions of the brain and the brain O2 delivery remained similar to its normoxemic value. By comparison, however, blood flow in all regions of the brain in calves sojourning at 3500 m remained unchanged between hypoxemia and normoxemia. In these calves brain O2 delivery decreased during hypoxemia. These experiments demonstrated that blood flow in the bovine brain-stem and cerebellum behaved in the same manner as in the cerebrum during exposure to acute as well as chronic hypoxia. It is suggested that there are likely to be differences in adaptation of bovine cerebral circulation to acute vs chronic hypoxia.