Impaired skin capillary recruitment in essential hypertension is caused by both functional and structural capillary rarefaction

Capillary rarefaction occurs in many tissues in patients with essential hypertension and may contribute to an increased vascular resistance and impaired muscle metabolism. Rarefaction may be caused by a structural (anatomic) absence of capillaries, functional nonperfusion, or both. The aim of this study was to assess the extent of structural versus functional capillary rarefaction in the skin of subjects with essential hypertension. We examined skin capillary density with video microscopy before and during maximization of the number of perfused capillaries by venous congestion (structural capillary number) and before and during postocclusive reactive hyperemia (capillary recruitment, which may have a structural and/or functional basis). The study group was composed of 26 patients with never-treated essential hypertension and 26 normotensive control subjects. In both groups, intermittently perfused capillaries in the resting state were an important functional reserve for recruitment during postocclusive hyperemia. Recruitment of perfused capillaries during postocclusive reactive hyperemia was decreased in the hypertensive subjects compared with normotensive control subjects (47.9±6.8 versus 57.2±8.6 capillaries/mm², respectively; P<0.01). During venous occlusion, maximal capillary density was significantly lower in the hypertensive subjects than in the control subjects (52.5±6.6 versus 57.2±8.6 capillaries/mm², respectively; P<0.05), suggesting structural rarefaction. However, in the hypertensive subjects compared with the normotensive subjects, a smaller proportion of the maximal number of capillaries was perfused during postocclusive hyperemia (91.6±7.5% versus 97.2±2.7%, respectively; P < 0.05), suggesting an additional functional impairment of capillary recruitment. If the difference in capillary numbers during venous congestion (≈4.6 capillaries/mm²) truly reflects the structural difference between the normotensive and hypertensive subjects, then, at most, 62% (4.6/7.4×100%) of the difference in capillary numbers during postocclusive hyperemia (≈7.4 capillaries/mm²) can be explained by structural defects, and at least 38% can be explained by functional defects. In conclusion, in patients with essential hypertension, recruitment of perfused capillaries is impaired, which can be explained by both functional and structural rarefaction.