Intravital capillary video-microscopy is a dynamic method for studying skin capillaries. The technique of direct intravital microscopy (without dyes) depends on the presence of red blood cells inside capillaries for their identification. The aim of the present study was to compare different techniques to try to establish the best method for maximizing the number of visible perfused capillaries during intravital capillary microscopy. We compared the effects of venous congestion with those of post-occlusive reactive hyperaemia (Study 1). We also investigated venous congestion followed first by post-occlusive reactive hyperaemia and then by a core heat load test (Study 2). Finally we investigated venous congestion followed by post-occlusive reactive hyperaemia combined with venous congestion (Study 3). In Study 1, capillary density increased with venous congestion from a baseline value of 74±2 (mean±S.E.M.) per field to 82±3 per field (P< 0.0001; analysis of variance). With reactive hyperaemia, there was an apparent decrease in visible capillary density to 69±2 per field. In Study 2, baseline capillary density was 69±4 per field, and this increased significantly with venous congestion to 74±4 per field (P = 0.01). With both reactive hyperaemia and core heat load, the apparent density was 62±4 per field. In Study 3 the baseline density was 70±2 per field, and this increased significantly with venous congestion to 80±3 per field (P< 0.0001). With reactive hyperaemia combined with venous congestion, the density was 81±3 per field (P = 0.328 compared with venous congestion alone). The results show that venous congestion at 60 mmHg for 2 min is the most effective method for visualization of the maximal number of perfused skin capillaries during intravital video-microscopy.

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