Red and green laser Doppler compared with capillary microscopy to assess skin microcirculation in the feet of healthy subjects

Skin microvasculature consists of nutritive capillaries and subpapillary ar teriolar and venular plexus connected by arteriolovenular anastomoses. Capi llary perfusion is of paramount importance for skin viability. Recently a n ew combined laser Doppler instrument has become available, featuring a comb ination of near-infrared (RL; 780 nm) and green (GL; 543 mm) laser light so urces. Theoretically, the red laser will penetrate deeper, whereas the gree n laser will read fairly superficially. This may enable differentiation bet ween the more superficial, i.e., capillary, and the deeper skin layers. To test this hypothesis, the combined laser Doppler technique was compared wit h nail fold capillary microscopy in the feet of 10 healthy subjects. Seven males and 3 females with a median age of 26 (range 20-42) years and without arterial pathology were investigated. The laser Doppler (Periflux 4001, Pe rimed) was equipped with a special dual probe conducting both GL and RL. Th e probe was attached to the pulp of the big toe (with many AV-shunts) and t o the nail fold, at the site where capillary microscopy was performed too. Laser Doppler and capillary perfusion was assessed at rest and during posto cclusive reactive hyperemia. These measurements were performed both in the sitting and the supine positions to test the postural vasoconstriction resp onse. Median resting and hyperemic skin perfusion with GL were lower (P < 0 .01) than with the RL in both areas and positions, except for the resting v alue in the sitting position on the dorsum of the toe. Plantar perfusion wa s found significantly higher than dorsal perfusion only with the RL in the supine position (P < 0.01). GL and RL on the plantar, but not the dorsal, s ide showed a significantly decreased perfusion upon dependency (P < 0.05), both at rest and during hyperemia. In contrast, resting and peak capillary velocity did show a decrease on dependency (P < 0.05). Although the green l aser measures a lower perfusion than does the red laser, which is likely to be derived from more superficial skin layers, it does not show a reactivit y similar to that measured with capillary microscopy. Thus, it is questiona ble whether the green laser exclusively measures capillary perfusion. (C) 1 999 Academic Press.