Evidence for skin microvascular compartmentalization by laser-Doppler and photoplethysmographic techniques.

Indices of perfusion were obtained using commercial laser-Doppler instruments with differing wavelengths on metatarsal skin of sheep (which is abundant with AVAs) and compared with quantitative measurements of AVA and capillary blood flow using microspheres; an infra-red photoplethysmograph was also compared. The data provide physiological evidence that laser-Doppler instruments of longer wavelength sense to a greater depth, and our photoplethysmograph behaves similarly to a "near-infra-red" laser. Thus, we suggest that near-infra-red light, either laser-Doppler or photoplethysmograph, should be used to measure a parameter most representative of total local blood flow, and light of a very much shorter wavelength, probably green light, would allow more confidence in estimates of true capillary perfusion. Because the commonly used visible red light can, but does not necessarily detect deep AVA perfusion, the influence depending upon the magnitude of the AVA perfusion, measurements by an instrument based on such red light is difficult to define.