Absolute quantification (ml blood/sec ∗ mm2 tissue) of normal vs. diabetic foot skin microvascular blood perfusion: Feasibility of FM-PPG measurements under clinical conditions.

Fluorescence-mediated photoplethysmography (FM-PPG) is the first routine clinical methodology by which to quantifiably measure tissue blood perfusion in absolute terms (mL blood/sec ∗ mm2 tissue). The FM-PPG methodology has been described in detail previously in this journal (MVR 114, 2017, 92-100), along with initial proof-of-concept measurements of blood perfusion in both ocular and forearm skin tissues. The motivation for the current study was to investigate whether FM-PPG can be used readily and routinely under realistic clinical conditions. The vehicle for doing this was to measure medial foot capillary blood flow, i.e., tissue perfusion, in 7 normal subjects, mean = 6.76 ± 2.29 E-005 mL/(sec ∙ mm2), and lesion-free areas of 8 type-2 diabetic patients with skin ulceration, mean = 4.67 + 3.15 E-005 mL/(sec ∙ mm2). Thus, perfusion in the diabetics was found to be moderately lower than that in the normal control subjects. Earlier skin perfusion measurements in medial forearms of 4 normal subjects, mean = 2.64 + 0.22 E-005 mL/(sec ∙ mm2), were lower than both the normal and diabetic foot perfusion measurements. Variability in the heartbeat-to-heartbeat blood perfusion pulses in the skin capillaries, defined as the ratio of the standard deviation among beat-to-beat pulses divided by the mean perfusion of those pulses, was determined for each subject. Average variability in foot skin was 21% in the diabetic population, versus 16% for normal subjects; and it was 18% in forearm skin. We conclude that absolute quantitative FM-PPG measurement of skin blood perfusion at the level of nutritive capillaries is feasible routinely under clinical conditions, allowing for quantitative measurement of skin tissue blood perfusion in absolute terms.