The hemodynamic determinants of the time constant of left ventricular (LV) isovolumic pressure (P) decline were studied in 32 anesthetized dogs. The time constant, tau (an index of LV relaxation), was determined from the best exponential fit of the equation P = Poe-t/r, to LVP measured at 5-ms intervals during isovolumic relaxation; Po = LVP at maximum negative dP/dt and t = time. At a constant heart rate of 120 beats/min, tau was determined during steady-state increases in preload (volume expansion), increases in afterload (methoxamine infusion), reductions in afterload (nitroprusside infusion), and in variably afterloaded beats at a constant preload (single-beat interventions). tau was directly related to LV systolic pressure and length during the alterations in LV loading conditions, but tau was not closely related to the extent of fiber shortening. During isoproterenol infusion, relaxation was more rapid (tau), and following the administration of propranolol, relaxation was prolonged (tau). While data from the variably afterloaded contractions indicate the presence of systolic load-dependent LV relaxation velocity, the steady-state studies do not exclude the possibility that altered contractility through reflex or other mechanisms contributes to the observed changes in tau.

CHRONIC AORTIC REGURGITATION/Gaasch et al. with the technical assistance ofL. M. Woodbury SUMMARY Serial echocardiographic left ventricular (LV) studies were performed in 19 patients before (preop) and after (postop) aortic valve replacement (AVR) for chronic aortic regurgitation (AR); the effect of AVR on LV volume, mass and function was determined from the echocardiographic data. In the 12 patients who were considered to have successful surgical results, the average LV end-diastolic dimension fell from a preop value of 6.9 ± 0.2 cm to 5.5 0.2 cm (P < 0.01) at the time of the early postop study (seven to 10 days). Muscle cross-sectional area (CSA) derived from dimension and wall thickness data was used as an index of LV muscle mass (preop CSA = 26 ± 1.3 cm2); CSA was unchanged at the early postop study, but subsequently fell to near normal within six months after AVR (20 1 cm2, P < 0.01). There was a trend toward improvement in systolic performance by the late postop studies (12+ months). In two out of three patients with postop paravalvular AR, LV dimension increased after an initial fall. Four patients without paravalvular AR failed to show a significant reduction in LV dimension in the postop studies. In this group the preop studies showed a tendency toward a large end-diastolic dimension and decreased fractional shortening, but the single preop parameter which differentiated these four from the successfully treated group was an end-diastolic radius-towall thickness (R/Th) ratio _ 4.Thus, successful AVR for chronic AR results in the normalization of LV volume and a decrease in LV muscle mass to near normal. The R/Th ratio has important prognostic value which appears to be independent of fractional shortening in some patients with chronic AR.OPTIMUM TIMING of valve replacement in patients with chronic left ventricular (LV) volume overload is a difficult and challenging problem. While aortic valve replacement often results in striking