MEASUREMENT OF SYSTEMIC RESISTANCES IN AORTIC REGURGITATION

Peripheral resistance is usually measured by dividing mean aortic pres sure by mean aortic flow. This statement holds true as long as resista nce is constant throughout the heart cycle. This is not the case in ao rtic regurgitation, because during diastole, but not in systole, a con duit is opened to blood flow through the regurgitating valve. Peripher al resistance was measured in 11 patients with aortic regurgitation an d in 23 normal subjects by solving for Ri in the ''windkessel'' equati on. We compared this resistance (R1) with that measured by standard me thods (RES). In normal subjects, R1 and RES are almost identical [R1 = 0.96 (RES) +/- 0.12, r = .95], while in aortic regurgitation there is no correlation [R1 = 0.64 (RES +/- 1.4, r = 0.2]. RES in normal subje cts is increased with respect to RES in aortic regurgitation (32 vs 22 , p = 0.0019), while R1 in aortic regurgitation is decreased compared to both R1 and RES in normal subjects (13.5 vs 21 and 22, p = 0.0063). The difference between R1 and RES in aortic regurgitation is related to the regurgitating volume. Compliance, calculated by assuming a mono exponential diastolic aortic pressure decay, is markedly decreased in aortic insufficiency, while it is increased if it is calculated by div iding the time constant of aortic pressure decay by R1. Thus, in sever e aortic regurgitation peripheral resistance is usually less than norm al, and standard methods of measurement fail to detect this fact. Corr ect evaluation of resistance and compliance may be useful to evaluate ventriculoarterial coupling and to titrate vasodilator therapy in this disease.