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 μg/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 μg/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.