Measurement of changes in optical pathlength through human muscle during cuff occlusion on the arm

Concentration changes of haemoglobin and cytochromes indicative of tissue perfusion and oxygenation can be quantitatively evaluated from near-infrared (NIR) measurements of absorption and optical pathlength through tissue. The accuracy of such measurements is limited as current bedside instrumentation cannot measure optical pathlength. Using a recently developed phase-resolved spectroscopic technique we can produce continuous and simultaneous measurements of changes in both attenuation and optical pathlength at four wavelengths in the NIR in a bedside instrument. The change in optical pathlength through arm muscle during a cuff occlusion is compared with the absolute pathlength estimated from time resolved spectroscopy measurements in the laboratory. These show that the pathlength varies by 4-10% during the occlusion, demonstrating the need for continuous measurements of optical pathlength.