A cerebral oximeter is a device that uses light to measure the amount of oxygen within the brain. It is similar to the device that measures the level of oxygen in the tip of the finger, known as a pulse oximeter. The cerebral oximeter consists of a sensor placed on the forehead that both emits and detects the amount of light absorbed. This study will determine how accurate the device is by comparing the displayed value on the monitor with blood samples taken simultaneously from the arterial blood in the wrist and venous blood in the neck. In order to test the device over a suitable range, the level of oxygen within the blood will be reduced in a controlled manner by reduction of the inspired oxygen concentration. This is the equivalent of ascending to an altitude of 16,000 feet. The study will be conducted in healthy volunteers.

This is a calibration and validation study of a near-infrared spectroscopy (NIRS) device designed to measure the cerebral tissue oxygen saturation non-invasively. This is achieved by comparing NIRS-derived cerebral tissue oxygen saturation with a calculated value derived from simultaneous arterial and jugular venous blood samples.

At present the FDA have adopted the standards published in 2005 by the International Organization for Standardization (ISO), entitled ISO 9919. This is a set of technical specifications and guidelines for pulse oximeters, which share certain technical similarities to cerebral oximeters. In particular, Annex EE details the conduct of a controlled desaturation study for the calibration of pulse oximeter equipment. Specifically, the fraction of inspired oxygen delivered to test subjects is varied to achieve a series of targeted steady state saturation periods over a range of arterial oxygen saturation of 70 - 100%.

While cerebral oximeters differ from pulse oximeters in terms of the what is being measured (brain tissue v arterial blood) the FDA have maintained the requirement to examine data from human volunteer studies in which the arterial oxygen saturation ranges from 70 - 100%. Two FDA-approved cerebral oximeters were validated in a similar manner.

The device controlling the inspired gas concentration is the RespirAct, which permits precise reduction in the arterial oxygen saturation while maintaining the arterial carbon dioxide level at 40 mmHg.

The study consists of 3 sequences:

• First sequence: reduction in arterial oxygen saturation in approximately 5% increments from 100 to 70% followed by return to room air and then a period of supplemental oxygen.
• Second sequence: reduction in arterial oxygen saturation in a single drop from 100 to 70% followed by return to room air and then a period of supplemental oxygen.
• Third sequence: reduction in arterial oxygen saturation in approximately 10% increments from 100 to 70% with alteration of carbon dioxide level from 35 to 45 mmHg at each interval followed by return to room air.

Inclusion Criteria:

• 21 to 35 years
• American Society of Anesthesiologists health assessment level 1
• Body Mass Index (BMI) 18 to 30

Exclusion Criteria:

• anemia
• hemoglobinopathy (e.g. sickle cell disease, thalassemia)
• positive pregnancy test (females)
• significant cardiac or pulmonary disease
• history of sleep apnea
• tobacco, drug or alcohol abuse
• difficult airway
• abnormal EKG / pulmonary function test / room air saturation
• intolerance to breathing mask apparatus