Chest
Validity of Ear Oximetry in Clinical Exercise Testing
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MATERIAL AND METHODS
This analysis includes 14 patients, all recently referred to our laboratory for clinical exercise testing, in whom we obtained concurrent data from both a brachial arterial catheter and an ear oximeter (Biox IIA). Prior to exercise, each patient was interviewed, examined by a physician, and received a chest roentgenogram, resting test of respiratory function, and resting 12-lead electrocardiogram. Informed consent was obtained.
After rubbing the external ear briskly for 15 to 20 seconds with an
RESULTS
During rest and unloaded cycling, the 28 simultaneous ear oximetric and blood oximetric values for oxyhemoglobin saturation were not significantly different (96.6±0.28 percent vs 96.5±0.37 percent; t = 0.333; p>0.5); however, comparison of the blood and ear oximetric values obtained during the highest tolerated work rates revealed striking discrepancies in some patients. We divided the patients into three groups, based on the pattern of late-exercise ear oximetric response (Table 1). Group A
DISCUSSION
We obtained good agreement between the estimated and measured blood oxyhemoglobin saturation. This occurred despite the relatively high concentration of carboxyhemoglobin found in many of our patients. When the blood oximetric measures are properly corrected for the hemoglobin unavailable for oxygenation because of carbon monoxide, the direct blood estimates and measures of oxyhemoglobin saturation show no systematic differences. Previous studies have identified the importance of
ACKNOWLEDGMENT
We thank Dr. Stan Siu and Ms. Concepcion Enriquez for professional and technical assistance and Ms. Barbara Young for preparation of the manuscript.
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Manuscript received June 23; revision accepted September 6.
Estimation of oxyhemoglobin saturation by measurement of arterial oxygen pressure and pH with use of the Severinghaus15 equation has been determined to be, in general, less accurate than direct blood oxygen saturation measurement. Nevertheless, comparison of the estimated (by Radiometer blood gas analyzer) and measured (by IL CO-oximeter) oxygen saturation of available hemoglobin in the 64 samples of arterial blood showed a mean difference of only 0.4 percent. The differences at measured saturation of carboxyhemoglobin of 5.0 to 7.1, 4.0 to 4.9, 3.0 to 3.9, 2.0 to 2.9, 1.0 to 1.9, and 0.0 to 0.9 percent were 0.4, 0.5, 0.5, 0.5, 0.5, and 0 percent, respectively. This comparison, with a correlation coefficient of 0.95, confirms the accuracy of the direct measurement of oxygen saturation of available hemoglobin in these patients' arterial blood. In contrast, there was a mean difference of 2.3 percent when the comparison between estimated and measured oxyhemoglobin saturation was made without correcting for carboxyhemoglobin.