Photo-plethysmographic assessment of skin viability using reflectance of multiple wavelengths.

Abstract

We have developed a multiple-wavelength photo-plethysmograph (MWP) to study skin blood flow. Focused incandescent light is reflected from tissue near the surface and through narrow bandwidth optical filters. The intensity of the reflected light is modulated by the pulsing changes in blood volume; the amount of modulation corresponds to the absorption of blood at particular wavelengths. Four narrow bandwidth optical filters were used during the study (560, 580, 810, and 950 nm). Force was applied (in increments), through the instrument, to the skin, in an effort to extinguish the pulsations. A localized hyperemic response was induced by the reduced blood flow to the tissue. Clinical and non-clinical use of the MWP has been demonstrated. In the nonclinical setting we collected waveforms from bony and soft tissues of a normal healthy subject population. The clinical study took place in the Podiatry Clinic at the Veteran's Administration Hospital in Tucson, Az., on an all male, age matched normal, diabetic and vascularly compromised population. Clinical data were collected from the tips of the right and left great toes after systolic pressures were measured with doppler ultrasound. Data consists of maximum amplitudes, maximum rates of change in the systolic rise, systolic slopes, and spectral power in several frequency bands of detected waveforms. Statistical tests for significance were performed on all the data, and included tests between: hyperemic and non-hyperemic data, clinical population groups, left and right great toe data of the clinical populations, and between data derived from different wavelengths of light. Significant differences (t < .05) were most consistently found in light filtered at 580 nm. Significant differences for the non-clinical population existed between hyperemic and non-hyperemic data at soft and bony locations. Significance existed between the clinical groups of general and compromised patients, diabetic and compromised patients, but not between general and diabetic patients. Correlation between measured toe pressures and data of the three clinical groups showed a slight positive bias. No significance was found to exist for the three clinical groups between data derived with light of different wavelengths (580 and 810 nm.).