Hypocapnia enhances the pressor effect of phenylephrine during isoflurane anesthesia in monkeys.

Phenylephrine was administered to increase arterial blood pressure in 6 monkeys anesthetized with isoflurane during both normocapnia (arterial partial pressure of CO2 35 to 44 mm Hg) and hypocapnia (arterial partial pressure of CO2 23 to 29 mm Hg). The doses of phenylephrine required to increase mean blood pressure to 33% and 66% above control pressure during hypocapnia [1.7+/-0.9 and 3.1+/-1.7 microg/kg/min (mean+/-SD), respectively] were significantly less than the doses required to achieve the same changes in blood pressure during normocapnia (2.4+/-0.9 and 4.9+/-2.4 microg/kg/min, respectively, P<0.05). In patients with intracranial pathology, for whom hypocapnia is frequently induced, phenylephrine dosage may need to be appropriately reduced.