In vivo assessment of left atrial contractile performance in normal and pathological conditions using a time-varying elastance model

Abstract

Background: Contractile function of the ex vivo, isolated left atrium (LA) has been described by a time-varying elastance, but this atrial chamber property has not been shown in vivo. Methods and Results: Instantaneous LA pressure-volume (P-V) relations were studied in 12 anesthetized, autonomically blocked, atrially paced dogs. LA volume was calculated from orthogonal sonomicrometer pairs using a cast-validated formula. Data were collected during increases in LA pressure produced by a phenylephrine bolus (200 to 400 μg IV). Isochronal P-V points from 5 beats, representing a wide range of atrial pressures, were fitted by linear regression analysis (range of R2, .92 to .99). There were significant time-dependent increases in the slopes [E(t)] and small but statistically insignificant decreases in the volume axis intercepts [Vo(t)] of the instantaneous LA P-V relations during atrial contraction; maximal elastance (E(max)) occurred 29±16 milliseconds before atrial end systole (minimal LA volume). E(max) was not significantly different than the slopes of either the nonisochronal end-systolic P-V relation (E(es)) or the nonisochronal maximal P-to-V relation (E(maxPV)); 5.5±2.8, 4.3±1.5, and 5.4±4.2 mm Hg/mL, respectively. In 7 dogs, data were collected both before and after a rapid infusion of calcium gluconate (1 to 2 g IV). E(max) increased significantly with a calcium-induced increased in inotropic state (4.5±1.6 to 5.7±1.8 mm Hg/mL, P