Diffusion, pericapillary distribution and clearance of Na-fluorescein in the human nailfold

An intravital fluorescence videomicroscopy technique is described in an attempt to develop non-invasive methods for the study of transcapillary exchange in human physiology and clinical medicine. The apparatus used consists of a Ploemopak incident light fluorescence microscope and a low light level television camera with linear output. After intravenous bolus injection of Na-fluorescein in 12 healthy subjects the movement of the dye particles in the nailfold was observed and stored on video-tape. Immediately after arrival the dye leaks into a halo-like section and later into the more remote parts of the pericapillary space. By moving a densitometric window on an axis transversal to the capillary loop (single frames of the tape), characteristic patterns of fluorescent light intensity distribution were obtained at different time intervals. At the edge to the halo the high pericapillary light intensity decreases abruptly moving further away from the capillary (mean: 10.0 micron) for 20 min and more in each individual indicating the presence of a diffusion barrier at this location between halo and more remote areas. Clearance of the dye is much slower than transcapillary diffusion and lasts approximately 2 h.