Impact of hyperoxia and phenylephrine on cerebral oxygenation: An experimental clinical study

Background: Oxygen supply to the brain is of special importance during intracranial surgery because it may be compromised by intracranial pathology. A high arterial blood pressure (mean arterial pressure above 80 mmHg) and a high arterial oxygen tension (PaO₂ above 12 kPa) is therefore often targeted in these patients, when for example intracranial pressure is increased or when a mass effect on brain tissue from a tumour is present, and it is pursued by administering vasopressors such as phenylephrine and by increasing inspiratory oxygen fraction (FiO₂ ). However, whether these interventions increase cerebral oxygenation remains uncertain. We aimed to investigate the effect of hyperoxia and phenylephrine on brain tissue oxygen tension (PbtO₂ ) in patients undergoing craniotomy.

Methods: In this experimental study, we included 17 adult patients scheduled for elective craniotomy. After securing a stable baseline of the oxygen probe, PbtO₂ was measured in white matter peripherally in the surgical field during general anaesthesia. Primary comparisons were PbtO₂ before versus after an increase in FiO₂ from 0.30 to 0.80 as well as before versus after a bolus dose of phenylephrine (0.1-0.2 mg depending on patient haemodynamics). Data were analysed with the Wilcoxon signed rank test.

Results: We obtained complete data sets in 11 patients undergoing the FiO₂ increase and six patients receiving the phenylephrine bolus. PbtO₂ was 22 (median; 5%-95% range, 4.6-54) mmHg during 30% oxygen, 68 (8.4-99) mmHg during 80% oxygen (p = .004 compared to 30% oxygen), 21 (4.5-81) mmHg before phenylephrine, and 19 (4.2-56) mmHg after phenylephrine (p = .56 compared to before phenylephrine).

Conclusion: In patients undergoing craniotomy under general anaesthesia, brain tissue oxygen tension increased with a high inspiratory oxygen fraction but remained unchanged after a bolus dose of phenylephrine.