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(O2)) is decreased after phenylephrine treatment. We hypothesized that the negative impact of phenylephrine administration on Sct(O2) is affected by arterial blood carbon dioxide partial pressure (Pa-CO2) because CO2 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 similar to 20-30% during hypocapnia, normocapnia, and hypercapnia. Sct(O2) 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(O2) during hypocapnia [DSct(O2) 23.4 (1.5)%, P < 0.001], normocapnia [Delta Sct(O2) 22.4 (1.5)%, P<0.001], and hypercapnia [Delta Sct(O2) 21.4 (1.5)%, P < 0.01]. Decreases in Sct(O2) 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(O2) and CBV is intensified during hypocapnia while blunted during hypercapnia.