Accuracy of a cerebral oximeter in healthy volunteers under conditions of isocapnic hypoxia

Abstract

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 (P(ET)O2) and carbon dioxide (P(ET)CO2) tensions to alter global cerebral oxygen saturation. Methods: In 30 healthy volunteers, dynamic end-tidal forcing was used to produce step changes in P(ET)O2 resulting in arterial saturation ranging from ~70% to 100% under conditions of controlled normocapnia (each person's resting P(ET)CO2) or hypercapnia (resting plus 7-10 mmHg). Blood arterial (SaO2) and jugular bulb venous (S(jv)O2) saturations during each P(ET)O2 interval were determined by co-oximetry. The cerebral oximeter reading (rSO2) and an estimated jugular venous saturation (S(jv)O2), derived from a combination of SaO2 and rSO2, were compared with the measured S(jv)O2. Results: The S(jv)O2 was significantly higher with hypercapnia than with normocapnia for the same SaO2. The rSO2 and S(jv)O2 were both highly correlated with S(jv)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 S(jv)O2. Conclusions: During isocapnic hypoxia in healthy persons, cerebral oxygenation as estimated by near infrared spectroscopy precisely tracks changes in measured S(jv)O2 within individuals, but the relation exhibits a wide range of slopes and intercepts. Therefore the clinical utility of the device is limited to situations in which tracking trends in cerebral oxygenation would be acceptable.