Phenylephrine decreases frontal lobe oxygenation at rest but not during moderately intense exercise

Abstract

Whether sympathetic activity influences cerebral blood flow (CBF) and oxygenation remains controversial. The influence of sympathetic activity on CBF and oxygenation was evaluated by the effect of phenylephrine on middle cerebral artery (MCA) mean flow velocity (Vmean) and the near-infrared spectroscopy-derived frontal lobe oxygenation (ScO2) at rest and during exercise. At rest, nine healthy male subjects received bolus injections of phenylephrine (0.1, 0.25, and 0.4 mg), and changes in mean arterial pressure (MAP), MCA Vmean, internal jugular venous O2 saturation (SjvO2), ScO2), and arterial PCO2 (PaCO2) were measured and the cerebral metabolic rate for O2 (CMRO2) was calculated. In randomized order, a bolus of saline or 0.3 mg of phenylephrine was then injected during semisupine cycling, eliciting a low (approximately 110 beats/min) or a high (approximately 150 beats/min) heart rate. At rest, MAP and MCA Vmean increased approximately 20% (P<0.001) and approximately 10% (P<0.001 for 0.25 mg of phenylephrine and P<0.05 for 0.4 mg of phenylephrine), respectively. ScO2 then decreased approximately 7% (P<0.001). Phenylephrine had no effect on SjvO2, PaCO2, or CMRO2. MAP increased after the administration of phenylephrine during low-intensity exercise (approximately 15%), but this was attenuated (approximately 10%) during high-intensity exercise (P<0.001). The reduction in ScO2 after administration of phenylephrine was attenuated during low-intensity exercise (-5%, P<0.001) and abolished during high-intensity exercise (-3%, P=not significant), where PaCO2 decreased 7% (P<0.05) and CMRO2 increased 17% (P<0.05). These results suggest that the administration of phenylephrine reduced ScO2 but that the increased cerebral metabolism needed for moderately intense exercise eliminated that effect.

Effect of Phenylephrine and Ephedrine Bolus Treatment on Cerebral Oxygenation in Anaesthetized Patients

“…Owing to the fact that the maximal change in Sct O 2 lagged the maximal change in MAP (an observation in both this study and a previous study), 11 Sct O 2 measurements were recorded when corresponding changes reached the maximum level. The mean value of three successive recordings for each parameter was used for analysis.…”

Effects of Positioning on Cerebral Oxygenation

“…Phenylephrine decreases ScO 2 by 7 % in conscious subjects [112] despite an increase in MAP and MCAv. This discordance between CPP and ScO 2 was abolished by increasing cerebral metabolic rate with exercise.…”

Utility of transcranial Doppler ultrasound for the integrative assessment of cerebrovascular function

“…This is to say, a slope of zero would imply perfect autoregulation where CBF remains constant across the entire range of MAP, while a gain equal to 1 would reflect the total absence of autoregulation (Tzeng et al, 2010b). The limitations inherent to any pharmacological approach remain manifest with this technique, and although there is supportive evidence (Greenfield and Tindall, 1968) the assumption that there is no direct drug-effect on the cerebral vasculature has been questioned (Brassard et al, 2010). However, direct effects of PE and SNP on cerebral vasculature are considered unlikely given that the blood-brain barrier normally prevents endogenous circulating catecholamine from binding to ␣ 1 -adrenoreceptors in small cerebral vessels (Mackenzie et al, 1976;McCalden et al, 1977;Ainslie and Tzeng, 2010).…”