External carotid artery flow maintains near infrared spectroscopy-determined frontal lobe oxygenation during ephedrine administration

Background: Phenylephrine and ephedrine affect frontal lobe oxygenation ([Formula: see text]) differently when assessed by spatially resolved near infrared spectroscopy. We evaluated the effect of phenylephrine and ephedrine on extra- vs intra-cerebral blood flow and on [Formula: see text].

Methods: In 10 healthy males (age 20-54 yr), phenylephrine or ephedrine was infused for an ∼20 mm Hg increase in mean arterial pressure. Cerebral oxygenation (SavO₂) was calculated from the arterial and jugular bulb oxygen saturations. Blood flow in the internal carotid artery (ICAf) and blood flow in the external carotid artery (ECAf) were assessed by duplex ultrasonography. Invos-5100c (SinvosO₂) and Foresight (SforeO₂) determined [Formula: see text] while forehead skin oxygenation (SskinO₂) was assessed.

Results: Phenylephrine reduced SforeO₂ by 6.9% (95% confidence interval: 4.8-9.0%; P<0.0001), SinvosO₂ by 10.5 (8.2-12.9%; P<0.0001), and ECAf (6-28%; P=0.0001), but increased ICAf (5-21%; P=0.003) albeit with no consequence for SskinO₂ or SavO₂. In contrast, SforeO₂ was maintained with administration of ephedrine while SinvosO₂ and SavO₂ decreased [by 3.1 (0.7-4.5%; P=0.017) and 2.1 (0.5-3.3%; P=0.012)] as arterial carbon dioxide pressure decreased (P=0.003). ICAf was stable and ECAf increased by 11 (4-18%; P=0.005) with administration of ephedrine while SskinO₂ did not change.

Conclusions: The effect of phenylephrine on ScO₂ is governed by a decrease in external carotid blood flow since it increases cerebral blood flow as determined by flow in the internal carotid artery. In contrast, ScO₂ is largely maintained with administration of ephedrine because blood flow to extracerebral tissue increases.