Estimation of jugular venous O2 saturation from cerebral oximetry or arterial O2 saturation during isocapnic hypoxia.
Near-infrared spectroscopy (NIRS) has the potential for providing valuable information about oxygen delivery to the brain. However, questions have been raised about the accuracy of these measurements. This study was undertaken to compare noninvasive cerebral saturation measurements to jugular venous saturation under conditions of hypoxia and hypercapnia. Data was obtain on forty-two subjects. Cerebral oxygenation was measured with a Somanetics INVOS 4100-SSA placed on the forehead of the subjects. PETCO2 was controlled to approximately 2 and 7 mmHg above resting values and PETO2 was controlled to 80, 45, 60 and 41 mmHg consecutively for four of five minutes each. Internal jugular blood gas measurements were made via a retrograde catheter. Both the cerebral oximetry measured saturation (rSO2) and the jugular venous saturation (SjvO2) were significantly increased by increasing the PETCO2 at all levels of hypoxia. The increase in the rSO2 was less than the increase in SjvO2. The rSO2 had a bias of 5.2% and a precision of 10.7% compared to the measured SjvO2. Cerebral oxygen saturation measured by cerebral oximetry compares well to the measured SjvO2 in normal subjects, despite multiple physiological reasons for differences. The closer relationship of SjvO2 to rSO2 than SaO2 under the conditions of these experiments indicates that the measurement reflects primarily intracranial saturation. However, outcome studies under clinical conditions are needed to determine the clinical utility of cerebral oximetry.