Accuracy of Response of Six Pulse Oximeters to Profound Hypoxia

Abstract

Oxygen saturation, SpO2%, was recorded during rapidly induced 42.5 .+-. 7.2-s plateaus of profound hypoxia at 40-70% saturation by 1 or 2 pulse oximeters from each of six manufacturers (NE - Nellcor N100, OH = Ohmeda 3700, NO = Novametrix 500 versions 2.2 and 3.3 (revised instrumentation), CR = Criticare CSI 501+ version .27 and version .28 in 501 and 502 (revised instrumentation), PC = PhysioControl Lifestat 1600, and MQ = Marquest/Minolta PulseOx 7). Usually, one probe of each pair was mounted on the ear, the other on a finger. Semi-recumbent, healthy, nomotensive, non-smoking caucasian or asian volunteers (age range 18-64 yr) performed the test six to seven times each. After insertion of a radial artery catheter, subjects hyperventilated 3% CO2, 0-5% O2, balance N2. Saturation ScO2, computed on-line from mass spectrometer end-tidal PO2 and PCO2, was used to manually adjust FIO2 breath by breath to obtain a rapid fall to a hypoxic plateau lasting 30-45s, followed by rapid resaturation. Arterial HbO2% (Radiometer OSM-3) sampled near the end of the plateau averaged 55.5 .+-. 7.5%. ScO2% (from the mass spectrometer) and SaO2% (from pH and PO2, by Corning 178) differed from HbO2% by +0.2 .+-. 3.6% and 0.4 .+-. 2.8%, respectively. The mean and SD errors of pulse oximeters (vs. HbO2%) were determined. The plateaus were always long enough to permit instruments to demonstrate a plateau with ear probes, but finger probes sometimes failed to provide plateaus in subjects with peripheral vasoconstriction. Nonetheless, SpO2 read significantly too low with finger probes at 55% mean SaO2. The mean error with ear probes was not significant. Several instruments occasionally defaulted to zero sataration during rapid desaturation. Precision was independent of probe location, but differed widely between instruments. The studies provided data with which manufacturers could improve function, as illustrated by subsequent series with CR and NO. The authors conclude that square-waves of hypoxia can assess both the transient and the steady-state profound hypoxic responses of pulse oximeters, disclosing a variety of problems, and facilitating their resolution. An addendum follows the article.