Accuracy of a Cerebral Oximeter in Healthy Volunteers under Conditions of Isocapnic Hypoxia

Background: A cerebral oximeter measures oxygen saturation of brain tissue noninvasively by near infrared spectroscopy. The accuracy of a commercially available oximeter was tested in healthy volunteers by precisely controlling end-tidal oxygen (PET O2) and carbon dioxide (PET CO2) tensions to alter global cerebral oxygen saturation.

Methods: In 30 healthy volunteers, dynamic end-tidal forcing was used to produce step changes in PET O2 resulting in arterial saturation ranging from [approximately] 70% to 100% under conditions of controlled normocapnia (each person's resting PET CO2) or hypercapnia (resting plus 7-10 mmHg). Blood arterial (SaO2) and jugular bulb venous (Sjv with bar O2) saturations during each PET O2 interval were determined by co-oximetry. The cerebral oximeter reading (rSO2) and an estimated jugular venous saturation (Sjv with bar O2), derived from a combination of SaO2 and rSO2, were compared with the measured Sjv with bar O2.

Results: The Sjv with bar O2 was significantly higher with hypercapnia than with normocapnia for the same SaO2. The rSO sub 2 and Sjv with bar O2 were both highly correlated with S sub jv with bar O2 for individual volunteers (mean r2 = 0.91 for each relation); however, the slopes and intercepts varied widely among volunteers. In three of them, the cerebral oximeter substantially underestimated the measured Sjv O2.