Neurogenic Control of Cerebral Blood Flow in the Baboon. Effects of Alpha Adrenergic Blockade With Phenoxybenzamine on Cerebral Autoregulation and Vasomotor Reactivity to Changes in Pa CO 2

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Abstract
Cerebral autoregulation was studied in the baboon by increasing and decreasing cerebral perfusion pressure (CPP) before and after intravenous administration (1.5 mg per kilogram) of a long-acting alpha adrenergic blocker, phenoxybenzamine (PBZ). Likewise, cerebral vasomotor reactivity to changes of arterial carbon dioxide tension (Pa CO 2 ) was examined before and after PBZ. In order to permit quantitative analysis, cerebral autoregulation (A.I.) and chemical vasomotor reactivity (C.I.) were expressed as indices where A.I.=δCBF/δCPP and C.I.=δCBF/δPa CO 2 . Following the intravenous injection of PBZ, cerebral autoregulatory vasoconstriction was impaired as CPP was increased. Cerebral vasomotor reactivity to changes in Pa CO 2 was altered both during hyperventilation hypocapnia (HV) and hypercarbia induced by inhalation of 5% carbon dioxide if alterations of CPP brought about by these procedures were taken into consideration. During hypocapnia C.I. was reduced 30% and during hypercarbia C.I. was increased 10%. It is concluded that PBZ reduces the vasoconstrictor tonus of cerebral vessels during hypocapnia and raised CPP. It also enhances the vasodilator response to CO 2 inhalation.