Circulation, Vol 89, 1829-1838, Copyright © 1994 by American Heart Association
BD Hoit, Y Shao, M Gabel and RA Walsh
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 micrograms 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
(Emax) occurred 29 +/- 16 milliseconds before atrial end systole (minimal
LA volume). Emax was not significantly different than the slopes of either
the nonisochronal end- systolic P-V relation (Ees) or the nonisochronal
maximal P-to-V relation (EmaxPV): 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). Emax increased
significantly with a calcium-induced increase in inotropic state (4.5 +/-
1.6 to 5.7 +/- 1.8 mm Hg/mL, P < .01), but the volume axis intercept was
unchanged (3.6 +/- 0.7 versus 3.4 +/- 1.9, P = NS). In 4 additional dogs
with heart failure (mean LA pressure, 26 +/- 6 mm Hg) produced by 3 weeks
of rapid right ventricular pacing, LA stroke volume was significantly
greater than and elastance determinations were similar to those of normal
dogs. However, the effects of calcium infusion on LA function were
attenuated in these animals. CONCLUSIONS: We conclude that (1) in the
intact heart, LA contraction may be approximated by time-varying elastance
with time-dependent changes in E(t) and that (2) LA systolic P-V relations
using either the nonisochronal maximum P-to-V ratio or end systole may be
useful as an estimate of Emax, are highly linear and sensitive to
calcium-induced changes in inotropic state, and may be useful in
identifying LA chamber adaptation to chronic hemodynamic loads.
ARTICLES
In vivo assessment of left atrial contractile performance in normal and pathological conditions using a time-varying elastance model
Department of Internal Medicine, University of Cincinnati Medical Center, OH 45267-0542.
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