Effects of hyperventilation, CO2, and CSF pressure on internal carotid blood flow in the baboon

The combined effect upon cerebral blood flow (CBF) of an elevation of cerebrospinal fluid pressure (CSFP) and changes in respiratory CO2 was studied in nine baboons under chloralose anesthesia. The animals were mildly hyperventilated and provided with increasing amounts of CO2 in O2-air. Arterial CO2 tensions (PaCO2) increased from 17 to 58 mm Hg. Internal carotid blood flow (ICBF) was measured at normal CSFP and at hydrostatically maintained 50 mm Hg CSFP. It was found that: 1) end-tidal CO2 may be used as a substitute for arterial PaCO2 determinations; 2) this elevation of CSFP has little effect on ICBF during hypercapnia and normocapnia; however, 3) during hypocapnia the ICBF is reduced an additional 20% when CSFP is elevated; that is, ICBF is reduced 50% from normal when end-tidal CO2 is reduced to 2% at this elevated level of CSFP. Caution should be exercised during hyperventilation therapy particularly if the elevated CSFP or intracranial pressure (ICP) is not reduced to approach normal levels; in these conditions, the combination of decreasing PaCO2 and elevated ICP may reduce CBF below critical levels and thus lead to cerebral hypoxia.