Controversy surrounds the use of regional cerebral oximetry (rSO2) as a measure of true cerebral oxygenation because of extracranial signal contamination and unmeasured confounding of cerebral a:v ratio. The measurement of brain tissue oxygen (PbrO2) has been used in routine neurosurgery and has been shown to reliably demonstrate cerebral hypoxia following severe head injury. It is the most direct measure of cerebral oxygenation. Here, we test the hypothesis that there is a correlation between PbrO2 and rSO2 under conditions of varying inspired oxygen fraction and the varying partial pressure of carbon dioxide in arterial blood in uninjured, normal human brain.

Patients who are scheduled for elective removal of secondary cerebral metastases under general anesthesia will be recruited following written informed consent obtained by a study team member during their preoperative evaluation. BIS and rSO2 optodes will be applied, before induction of anesthesia, by a single researcher on both sides of the patient's forehead, as recommended by the manufacturer. General anesthesia will be maintained by total intravenous anesthesia (TIVA) with a combination of propofol (80-150 mcg/kg/min) and remifentanil (0.05-0.1 mcg/kg/min) targeted to a Bispectral Index range 40-60 (BIS; Covidien, Boulder, CO). Following craniotomy, the LICOX probe will be placed under direct vision into an area of normal brain within the tumor excision canal by the attending neurosurgeon. During a pause in surgery FIO2 and minute ventilation will be sequentially adjusted to achieve the following pairs of ventilation set points: 1) FIO2 0.3 and paCO2 30mmHg, 2) FIO2 1.0 and paCO2 40mmHg. After ≥5 minutes at each set point FIO2, PaCO2, rSO2 and PbrO2 will be recorded as a "snap-shot".

A sample size of 15 achieves an 80% power with a one-sided type I error of 5% to detect a positive correlation of 0.6 (from the null hypothesis of no correlation) between changes in PbrO2 and changes in rSO2 subsequent on alterations made in ventilation strategy. Correlation will be measured using Pearson's Correlation. P values < 0.05 will be considered statistically significant.

Inclusion Criteria:

• Patients who are scheduled for elective removal of secondary cerebral metastases under general anesthesia.

Exclusion Criteria:

• Patients will be excluded if they refuse to give consent, have evidence of elevated intracranial pressure on preoperative CT scan, have coagulopathy, are taking therapeutic agents known to increase bleeding risk, have a history of cardiovascular disease, cerebrovascular disease, suffer from respiratory failure, or are not fluent English speakers.

Data were collected intraoperatively, and postoperatively for 24 hours.