Pulse oximetry to detect hypoxemia during apnea: comparison of finger and ear probes

INTRODUCTION: When investigating apnea, for example in diving or altitude studies, hypoxemia is a variable that must be monitored to reduce the risk of hypoxic syncope. Pulse oximetry is a simple technique that measures arterial oxygen saturation (SpO2). As apnea induces a peripheral vasoconstriction, we hypothesized that it would be better to measure hypoxia using more centrally placed ear lobe oximetry probes rather than peripheral finger probes. METHODS: Seven men were studied, ages 18-35. Two pulse oximeters were used, a Satlite Trans (Ox-1) and Ohmeda Biox (Ox-2), both with ear and finger probes. Subjects carried out a sub-maximal breath hold for 60 s while performing dynamic leg exercise on a cycle ergometer at 50 W. Subjects performed the maneuver six times in total, in a crossover design. RESULTS: The Ox-1 finger probe showed 6.0 +/- 3.7% higher values than the ear-lobe probe at their respective nadirs. The Ox-2 probes differed in the same manner by 6.5 +/- 4.2%. The average delay between the nadir shown by the ear and finger probes was 15 s (+/- 3.5). When the ear-probes were at their nadir (SpO2 78 +/- 3.5%), the finger probes had considerably higher SpO2 levels (94.6 +/- 3.5%). DISCUSSION: Apneic induced hypoxemia was monitored poorly by finger probe pulse oximetry. The delay in response may jeopardize safety, for example in breath-hold diving studies. Hypoxemia does not seem to be accurately reflected by finger measurements in situations where peripheral vasoconstriction may occur.