In vivo comparison of the nailfold capillary diameter as determined by using the erythrocyte column and FITC-labelled albumin.

The diameter of nailfold capillaries in conventional intravital microscopy is estimated by measuring th width of the erythrocyte-columns. We determined the diameter of the fluorescence-marked microscopy in 33 capillary loops in the nailfold of 12 subjects and compared it with that of the ec-columns. 400-500 mg FITC-albumin were injected into the brachial artery through cannulas inserted for bloodgas analysis. The FITC-marked capillary lumen exceeded the width of the ec-columns of 10.8 +/- 3.0 microns by 4.2 +/- 0.7 microns or by 38% on the arteriolar side, and that of 12.0 +/- 2.7 microns by 4.6 +/- 0.8 microns (p less than 0.01), on the venular side. Venous occlusion produced a significant enlargement of the ec-columns by 13% (p less than 0.01), but no significant increase in the FITC-marked capillary lumen. The difference of 405 microns between the full capillary lumen as represented by the fluorescent plasma space and the width of the ec-columns corresponds presumably to the plasmatic zone between red cells and the vessel wall. No extravasation of FITC-HA was observed neither before nor after venous occlusion.