, Volume 35, Issue 4, pp 425-427

Optical assessment of recovery of tissue blood supply after removal of externally applied pressure

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The authors use photoelectric plethysmography to determine the external occlusion pressure for blood vessels in human tissue in vivo. Three wavelengths are employed; 950 nm (infra-red), 640 nm (red) and 583 nm (yellow). Each probe is applied in turn to one finger of each subject. Pressure is applied, using a neonatal blood pressure cuff, to the finger via the probe. This pressure is increased linearly to 20 kPa (150 mmHg) over 15 s and then decreased linearly to zero over 15 s. The pressure at which perfusion returns is obtained for four repeat measurements at each wavelength. The mean (±standard deviation) occlusion pressures for all 13 subjects investigated are 7.1(±1.9) kPa for infra-red, 6.3(±1.7) kPa for red and 5.8(±1.8) kPa for yellow. The pressure is 0.79(±0.83) kPa lower for red compared with infra-red (P<0.01), 0.54(±0.60) kPa lower for yellow compared with red (P<0.002) and 1.3(±1.0) kPa lower for yellow compared with infra-red (P<0.005). The reduced penetration of shorter optical wavelengths can be used to detect the lower occlusion pressures of the smaller blood vessels nearer the skin surface.