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

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.

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.

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%).

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.