Infrared fluorescence videomicroscopy of skin capillaries with indocyanine green

A new intravital capillary microscopic technique is described using intravenous indocyanine green (Cardiogreen) in combination with a special filter set, an infrared sensitive microchip videocamera and a television-recording system. The technique was used to measure arterial and venous limb capillary diameters in 29 nailfold capillaries from 11 healthy volunteers. The results were compared with measurements of erythrocyte column diameters from the same capillaries. Erythrocyte column diameters were 68% of arterial and venous limb capillary diameters determined with indocyanine green fluorescence videomicroscopy. (Arterial limb: 12.1 +/- 3.0 micron versus 17.8 +/- 3.9 micron; Venous limb: 13.7 +/- 4.1 micron versus 20.1 +/- 4.4 micron) (p less than 0.01). As indocyanine green is nearly 100% protein bound, these differences of 5-7 micron presumably correspond to the plasmatic zone interposed between the erythrocytes and the capillary wall. Other potential uses for the technique in clinical investigations of microcirculatory disturbances are briefly discussed.