Impact of phenylephrine administration on cerebral tissue oxygen saturation and blood volume is modulated by carbon dioxide in anaesthetized patients.

BACKGROUND Multiple studies have shown that cerebral tissue oxygen saturation (Sct(O(2))) is decreased after phenylephrine treatment. We hypothesized that the negative impact of phenylephrine administration on Sct(O(2)) is affected by arterial blood carbon dioxide partial pressure (Pa(CO(2))) because CO(2) is a powerful modulator of cerebrovascular tone. METHODS In 14 anaesthetized healthy patients, i.v. phenylephrine bolus was administered to increase the mean arterial pressure ~20-30% during hypocapnia, normocapnia, and hypercapnia. Sct(O(2)) and cerebral blood volume (CBV) were measured using frequency domain near-infrared spectroscopy, a quantitative technology. Data collection occurred before and after each treatment. RESULTS Phenylephrine caused a significant decrease in Sct(O(2)) during hypocapnia [ΔSct(O(2)) =-3.4 (1.5)%, P<0.001], normocapnia [ΔSct(O(2)) =-2.4 (1.5)%, P<0.001], and hypercapnia [ΔSct(O(2)) =-1.4 (1.5)%, P<0.01]. Decreases in Sct(O(2)) were significantly different between hypocapnia, normocapnia, and hypercapnia (P<0.001). Phenylephrine also caused a significant decrease in CBV during hypocapnia (P<0.01), but not during normocapnia or hypercapnia. CONCLUSION The negative impact of phenylephrine treatment on Sct(O(2)) and CBV is intensified during hypocapnia while blunted during hypercapnia.