Functional versus structural changes of forearm vascular resistance in hypertension.

Abstract

SUMMARY Structural changes in resistance vessels have been considered an important factor in triggering and maintaining chronic hypertension in humans and in experimental animals. To determine whether the increased forearm vascular resistance observed following vasodilator maneuvers in hypertensive patients is predominantly due to structural or to functional changes, we examined the influence of different vasodilator stimuli on forearm blood flow and blood pressure in 22 male patients with established essential hypertension and in 22 age-matched normotensive men (age range, 28-52 years). Blood pressure was measured directly, and blood flow was measured by venous occlusion plethysmography. The maneuvers applied were 1) arterial occlusion combined with handgrip exercise and local heating, 2) intra-arterial infusion of the calcium entry blocker nifedipine, 3) intra-arterial infusion of the nonspecific vasodilator sodium nitroprusside, 4) arterial occlusion initiated after intraarterial infusion of nifedipine. Vascular resistance during vasodilation induced by arterial occlusion or infusion of nifedipine or sodium nitroprusside remained significantly higher in the hypertensive than in the normotensive subjects. However, the maximal vasodilation achieved by the combination of arterial occlusion and nifedipine resulted in a similar resistance in both groups (1.6 ± 0.2 in the hypertensive vs 1.4 ± 0.2 nun Hg/ml/min/100 ml tissue in the normotensive subjects). These data suggest that there is an important functional component of the elevated resistance in patients with essential hypertension. (Hypertension 11: 320-325, 1988) KEY WORDS nitroprusside hypertension nifedipine• forearm vascular resistance arterial occlusion sodium I NCREASED peripheral vascular resistance is a cardinal feature of any form of persistently elevated blood pressure (BP). 1 It is still a matter of debate whether the increased resistance is predominantly due to a functional abnormality 2 ' 3 or to structural changes 4 " 6 concomitant with decreased lumen of the resistance vessels.Folkow et al. 4 have used venous occlusion plethysmography to show that the minimal forearm vascular resistance achieved by 10 minutes of arterial occlusion combined with handgrip exercise and local heating and subsequent reperfusion is higher in patients with established hypertension than in normotensive subjects. They ascribed this observation to an increased wall/lumen ratio caused by hypertrophy of the walls of the resistance vessels, which according to Hagen-

Resistance to blood flow at maximal vasodilatation.

“…First, the procedures used in our study were really able to induce complete vascular relaxation. Second, as this was the case in Schulte et al's study 1 only after the addition of nifedipine, they evidently did not push the ischemia-exerciseheating procedures for vasodilatation quite as far as we did in our study.…”

“…The addition of nifedipine increased blood flow up to 30% in both their normotensive and hypertensive subjects and resulted in R^,, values that were apparently lower than those reached in our study. Schulte et al 1 therefore concluded that 1) complete vascular relaxation was only achieved when calcium channel blockers were added, and 2) R^,, was not significantly elevated in their hypertensive group. On closer scrutiny, however, their results seem to confirm the results of our group.…”

“…In the April issue of Hypertension, a study by Distler's group (Schulte et al 1 ), explored whether minimal forearm resistance to blood flow at maximal vasodilatation (R^J is elevated in human primary hypertension by repeating and critically extending the studies published in 1956 and 1958 by Folkow et al 2 -3 In addition to the methods used by our group (ischemia, exercise, and local heating) to provoke maximal dilatation, they added an infusion of the calcium channel blocker nifedipine. The addition of nifedipine increased blood flow up to 30% in both their normotensive and hypertensive subjects and resulted in R^,, values that were apparently lower than those reached in our study.…”

Resistance to blood flow at maximal vasodilation.

“…1 questioned some of the methods used in our study 2 and cast doubts on our conclusions. We will comment here on the methodology only and leave it to the reader to draw conclusions from the data published.…”

“…Statistical analysis. As detailed in our study, 2 we have chosen a univariate approach by comparison of carefully selected matched pairs, which is recognized as a valid method of analyzing such data. When we analyzed our data as suggested by Folkow, 1 although resting blood pressure values did not form a continuum, we obtained a weak but significant correlation between mean arterial pressure and resistance at maximal dilation (r=0.363; p<0.05).…”