Structural Abnormalities of the Resistance Vasculature in Hypertension

Accessible online at: www.karger.com/jvr In this number, Bund and Lee [1] provide a timely review concerning some of the problems associated with measurement and discussion of the structural abnormalities in the resistance vessel which are associated with hypertension. The review has three parts. First, the authors describe the various methods which have been used to measure the structure of resistance vessels, and the limitations involved. Second, the authors review current ways of classifying the structural abnormalities which are found. Third, the authors discuss the extent to which the structural changes have functional consequences. In the first part, the authors identify the paradox that although there is general agreement that resistance vessel structure is altered in hypertension compared to normotensive controls, it is a difficult quantity to measure. In vivo studies of vascular resistance measured under conditions of complete relaxation provide a sensitive comparison of apparent resistance vessel diameter, but are difficult to interpret, since any differences could be due to differences in vascular architecture rather than structure of the individual vessels. In vitro studies of isolated resistance vessels have the advantage of precise measurements of vascular dimensions without fixation artefacts, but suffer from the difficulty that there is no clear agreement as to the length to which vessels should be set. Histological studies have the advantage of allowing global analysis, but may be compromised by unintended activation of vessels during fixation, and lack of knowledge of the intravascular pressure during the process. None of the methods is therefore perfect, and the authors conclude correctly that it is unwise to draw strong conclusions alone on the basis of results from one method. Nevertheless, if measurements are confined to a comparison of the ratio of wall thickness to lumen diameter ‘wall:lumen’ (or, with measurements of tunica media thickness, ‘media:lumen’), ratios at a given lumen, all methods are in agreement that this parameter is increased in hypertension, at least in the more proximal resistance vessels. The difficulty arises as soon as comparison is to be made of lumen diameters, or of the wall (media) cross-sectional areas, since here all of the methods are faced with the fundamental problem of comparing vessels from different individuals. Vascular architecture differs between individuals even of the same strain, and substantial differences in architecture between strains have been documented [2]. Thus, for example, a ‘third branch’ in one animal may be at a different level in another. Bund and Lee allude to the problem of vessel comparison between individuals, but do not give an indication of how this could be solved beyond reference to the 1965 work of Short [3], who studied the entire human mesentery. A less time-consuming method can, however, be envisaged to obtain a global and reliable measurement. It would, for example, be possible to perfusion-fix a whole