Oxygen Reserve Index: A Novel Noninvasive Measure of Oxygen Reserve--A Pilot Study

Pulse oximetry provides no indication of downward trends in PaO2 until saturation begins to decrease. The Oxygen Reserve Index (ORI) is a novel pulse oximeter-based nondimensional index that ranges from 1 to 0 as PaO2 decreases from about 200 to 80 mmHg and is measured by optically detecting changes in SvO2 after SaO2 saturates to the maximum. The authors tested the hypothesis that the ORI provides a clinically important warning of impending desaturation in pediatric patients during induction of anesthesia.

After preoxygenation, anesthesia induction, and tracheal intubation, the anesthesia circuit was disconnected and oxygen saturation was allowed to decrease to 90% before ventilation recommenced. The ORI and SpO2 values were recorded from a Masimo Pulse Co-Oximeter Sensor at the beginning of apnea, beginning and end of intubation, beginning and end of the ORI alarm, and 2 min after reoxygenation.

Data from 25 healthy children, aged 7.6 ± 4.6 yr, were included in the analysis. During apnea, the ORI slowly and progressively decreased over a mean of 5.9 ± 3.1 min from 0.73 ± 0.16 at the beginning of apnea to 0.37 ± 0.11. SpO2 remained 100% throughout this initial period. Concurrently with alarm activation, the ORI began to decrease rapidly, and in median of 31.5 s (interquartile range, 19 to 34.3 s), saturation decreased to 98%.

In this pilot study, the ORI detected impending desaturation in median of 31.5 s (interquartile range, 19-34.3 s) before noticeable changes in SpO2 occurred. This represents a clinically important warning time, which might give clinicians time for corrective actions.