Measurement of fingertip blood flow using thermal clearance reflects anastomotic rather than nutrient blood flow

A thermal clearance probe was used to measure fingertip blood flow. When flow was occluded the baseline value was reproducible (coefficient of variation 2% between subjects), whereas basal and maximum flow were poorly reproducible even within subjects. Synchronous changes in thermal clearance were seen when two probes were used on fingers and toes of different limbs (r = 0.77 - 0.97), in keeping with a central control mechanism. Fingertip blood flow, as measured by thermal clearance, correlated with Po2 of venous blood draining from the dorsum of the hand (r = 0.72 - 0.92). Contralateral hand cooling caused a sharp reduction of fingertip thermal clearance by 44.2 +/- 3.7%. Thermal clearance traces were damped in fingers with heavy keratinization, and improved when keratin was removed. Comparisons with venous occlusion plethysmography (VOP) and photoelectric plethysmography (PPG) showed that thermal clearance correlates with both methods (r = 0.60 - 0.92 for VOP and 0.64 - 0.93 for PPG) but with much integration of the signal and a 10 s lag. It is concluded that the probes used measure predominantly anastomotic flow and not nutrient skin blood flow alone.