Can transcutaneous oximetry detect nutritive perfusion disturbances in patients with lower limb ischemia?

Transcutaneous oximetry (TcpO2) performed at 37 and 44 degrees on the dorsum of the foot and capillary microscopy of the nailfold of the big toe were applied to 85 patients with various (including asymptomatic) stages of lower limb ischemia to appreciate the relationship between (disturbances in) capillary perfusion and skin oxygen tension. In mildly diseased patients, capillary perfusion as measured by direct observation, was preserved. In critically ischemic patients in the supine position, red blood cell-perfused capillary density was reduced. Nutritive perfusion was severely reduced and showed an absent reactive hyperemia after a 1-min arterial occlusion. Also, postural vasoconstrictive activity was reduced. TcpO2 measured at 37 degrees was very low already in mildly diseased patients, illustrating the poor oxygen diffusion toward the skin. At 44 degrees, TcpO2 was severely reduced in critically ischemic patients. Reactive hyperemic response and postural vasoconstriction were suppressed, due to local heating of the skin. Measurement of the TcpO2 has limitations in the assessment of nutritive perfusion, as opposed to capillary microscopy, since it is an indirect measure of skin perfusion, not necessarily derived from capillaries only. The obligatory local skin heating impairs physiological studies as to hyperemic reserve capacity or postural constriction mechanisms. Thus, transcutaneous oximetry is a poor method of characterizing pathophysiological mechanisms occurring in skin nutritive microcirculation. However, capillary microscopy and transcutaneous oximetry can give additive information as to the severity of peripheral ischemia. Peak red blood cell velocity during reactive hyperemia using capillary microscopy and the resting TcpO2 at 44 degrees, both measured in the supine position, appeared to be valuable microcirculatory parameters in detecting critical limb ischemia.