Ambient Light Interference with Pulse Oximeters
User Experience Network™ [Health Devices Sep-Oct 1987;16(9-10):346-7]
One brand of pulse oximeters when used with a disposable infant flex
probe started displaying "Probe Off Patient" when under a hospital-constructed
radiant warming unit. Covering the probes with a towel or blanket did not prevent the
problem. As soon as the warmer was turned off, the oximeters returned to normal operation.
The radiant warming unit included two 250 W infrared (IR) heat lamps and a household
dimmer switch, which was used to control the power to the lamps and, thus, the radiated
heat. We initially suspected high-frequency electrical interference from the dimmer switch
as the source of the problem.
The hospital informed us of its problem, and we believe that the
"Probe Off Patient" message was caused by saturation of the detector component
of the probe by the intense light from the heat lamps. The operator's manual states that
detector saturation will elicit this message. The small probes used in this instance are
difficult to shield from ambient light. We are continuing to investigate probe designs
that will help eliminate this kind of interference.
We previously reported on the effects of ambient light on the operation of
pulse oximeters (see Health Devices 16:50-1, February 1987). In this case, we are
reasonably certain that ambient IR radiation from the lamps was saturating the detector of
the oximeter probe. Under our direction, the hospital performed certain tests that ruled
out electromagnetic interference, either radiated or conducted, as the cause of the
"Probe Off Patient" message. Bypassing the dimmer switch did not eliminate the
problem, nor did replacing the heat lamps with 150 W white incandescent bulbs. Only
multiple layers of dark cloth or boards of opaque material prevented the problem.
Oximeters are electro-optical systems that are subject to electrical,
optical, and mechanical interference. Protection from interference can be accomplished in
several ways (e.g., optical filtration, electrical filtration) and varies from one
manufacturer to another. Hospitals that plan to purchase pulse oximeters should perform
clinical trials on test units in the presence of the surgical lights, radiant warmers, and
the general ambient light in their institution. Ideally, pulse oximeters should not be
affected; but if they are, an error message should be displayed, as in this case.
Erroneous pulse or O2-Hg saturations mimicking normal values should not be
displayed, as discussed in our previous report.
Oximeters, Pulse [17-148]
Cause of Device-Related
Device factors: Design/labeling error; Device interaction
External factor: Environmental
Mechanism of Injury or
Misdiagnosis; Monitoring failure