Bradford B. Chew

Temperature threshold for burn injury: An oximeter safety study

Pulse oximeters have become essential devices for evaluating and monitoring patient oxygenation. The probe emits a small amount of heat into the skin in the process of signal detection. Regulations set by the Food and Drug Administration currently limit the maximum allowable temperature of an oximeter probe to 41 degrees C. As a result of the prolonged exposure of extremities to these devices, we sought to determine the actual temperature threshold for burn injury in patients. Eighteen patients undergoing surgery for removal of redundant skin (abdominoplasty, breast reduction) consented to the application of a temperature-controlled custom probe with four light-emitting diodes that had temperatures set randomly at the expected threshold for burn injury (42.5 degrees C, 43 degrees C, 43.5 degrees C, and 44 degrees C). The probe was left in place for 8 hours (or less if significant pain was noted). The sites covered by the probes were then checked for signs of injury. On the next day, the redundant skin was removed as a scheduled procedure, and histopathology was performed to detect the extent of burn injury. The study was approved by the local institutional research board. Two patients were excluded because of technical problems with the probe, one of whom had the probe turned off because of pain. The only observed sign of injury was either erythema or a superficial blister that was usually unobservable or slightly red at operation. These subtle signs of a burn were noted in one patient at 43 degrees C, four at 43.5 degrees C, and nine at 44 degrees C. No burns were noted in two patients. Minimal or no signs of injury frequently were noted by histopathology. Pulse oximeter probes are safe up to a temperature of 43 degrees C for at least 8 hours in well-perfused skin. Above that temperature, there is a risk of burn injury. Performing temperature threshold tests in redundant skin that is planned for excision is a potential method for testing the safety of devices or materials.

Evaluation of optical methods of detecting dental pulp vitality.

An attempt to develop an optical probe to assess dental pulp vitality led to the conclusion that it would be difficult to construct an effective optical probe for the middle-aged and older patient who is a common candidate for pulpal assessment. In such patients, vascularity of the gingiva is so much greater than that of the pulp that signals originating in the gingiva overwhelm pulpal effects. This conclusion applies to photoplethysmography, pulse oximetry, and laser Doppler flowmetry, and probably to spectrophotometry as well. The effect of the gingiva was first demonstrated by placing opaque plastic film in the gingival sulcus, which caused attenuation of light passing through molar and bicuspid teeth. More dramatically, the remaining pulsatile signal could be much further attenuated by vasoconstricting the gingiva. An unusually sensitive photoplethysmograph was developed (noise floor, with signal averaging, was near 10 fA), as were a variety of optical probes (including two that approached the tooth below the gingival margin). The tools, methods, and conceptual models that were developed are intended also to be of use to investigators who are developing optical probes for use in younger subjects, in whom such probes appear to be useful.