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 arteriolar and venular plexus connected by arteriolovenular anastomoses. Capillary perfusion is of paramount importance for skin viability. Recently a new combined laser Doppler instrument has become available, featuring a combination of near-infrared (RL; 780 nm) and green (GL; 543 nm) laser light sources. Theoretically, the red laser will penetrate deeper, whereas the green laser will read fairly superficially. This may enable differentiation between the more superficial, i.e., capillary, and the deeper skin layers. To test this hypothesis, the combined laser Doppler technique was compared with 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, Perimed) was equipped with a special dual probe conducting both GL and RL. The probe was attached to the pulp of the big toe (with many AV-shunts) and to the nail fold, at the site where capillary microscopy was performed too. Laser Doppler and capillary perfusion was assessed at rest and during postocclusive reactive hyperemia. These measurements were performed both in the sitting and the supine positions to test the postural vasoconstriction response. 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 value in the sitting position on the dorsum of the toe. Plantar perfusion was 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, side 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 laser 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 reactivity similar to that measured with capillary microscopy. Thus, it is questionable whether the green laser exclusively measures capillary perfusion