MRI assessment of LV relaxation by untwisting rate: a new isovolumic phasemeasure of tau

Most noninvasive measures of diastolic function are made during left ventri cular (LV) filling and are therefore subject to "pseudonormalization," beca use variation in left atrial (LA) pressure may confound the estimation of r elaxation rate. Counterclockwise twist of the LV develops during ejection, but untwisting occurs rapidly during isovolumic relaxation, before mitral o pening. We hypothesized that the rate of untwisting might reflect the proce ss of relaxation independent of LA pressure. Recoil rate (RR), the velocity of LV untwisting, was measured by tagged magnetic resonance imaging and re gressed against the relaxation time constant (tau), recorded by catheteriza tion, in 10 dogs at baseline and after dobutamine, saline, esmolol, and met hoxamine treatment. RR correlated closely (average r = -0.86) with tau and was unaffected by elevated LA pressure. Multiple regression showed that tau , but not LA or aortic pressure, was an independent predictor of RR (P < 0. 0001, P = 0.99, and P = 0.18, respectively). The rate of recoil of torsion, determined wholly noninvasively, provides an isovolumic phase, preload-ind ependent assessment of LV relaxation. Use of this novel parameter should al low the detailed study of diastolic function in states known to affect fill ing rates, such as aging, hypertension, and congestive heart failure.