Response of skin photoplethysmography, laser Doppler flowmetry and transcutaneous oxygen tensiometry to stenosis-induced reductions in limb blood flow.

Photoplethysmography (PPG), transcutaneous oxygen tensiometry (TCpO2) and laser Doppler flowmetry (LDF) were used to measure changes in perfusion of the distal lower limb of the pig, in response to a stenosis created in the external iliac artery. The severity of stenosis was quantitated by the ratio of peak systolic distal pressure to peak systolic proximal pressure (distal systolic pressure index, DSPI). PPG and LDF were immediately responsive to changes in perfusion pressure while TCpO2 responded much more slowly. PPG dropped precipitously in response to even a small reduction in perfusion pressure, reaching zero at a DSPI of approximately 0.3. TCpO2 was insensitive to the presence of a mild stenosis (DSPI greater than 0.6), but was highly sensitive in the range 0.6 greater than DSPI greater than 0.3. At lower DSPI values, TCpO2 also became zero, due in part to electrode consumption of oxygen. LDF readings were sensitive to changes in blood flow throughout the entire range of stenosis, down to the point of total occlusion, although changes were not significant at DSPI values greater than 0.6. Because LDF measures not only nutritional blood flow, but flow in AV shunts and subdermal plexuses as well, high LDF readings could erroneously suggest tissue viability and good healing potential. Resting levels of TCpO2 and LDF can be used to follow the progress of peripheral vascular disease, and used complementarily, in many cases, to evaluate the severity of flow impairment.