Different left ventricular relaxation parameters in isolated working rat and guinea pig hearts. Influence of preload, afterload, temperature, and isoprenaline.

In isolated ejecting rat and guinea pig hearts, the sensitivity of the time constant tau of left ventricular isovolumic pressure fall, the maximum pressure fall velocity min LVdP/dt, and the relaxation time to different hemodynamic conditions, temperature, and isoprenaline were investigated. Tau was obtained by fitting the isovolumic pressure fall three-parametrically to the exponential p(t) = p infinity + (p0-p infinity) exp (-t/tau) which was found to be superior to semilogarithmic estimation. The influence of different working conditions on the relaxation parameters was tested by a rank correlation test and quantified by calculating standardized regression coefficients. Hemodynamic conditions were altered by changing left ventricular end-diastolic pressure (increasing inflow to the heart) and peak pressure (max LVP, varying aortic outflow resistance), and by atrial pacing (variation of interbeat interval). Lusitropic sensitivity was investigated by changing temperature and by applying isoprenaline. All regression parameters were only moderately sensitive to changes in end-diastolic pressure, max LVP, or heart rate, with the exception of a considerable afterload dependence of min LVdP/dt in rat hearts. This dependence, however, can be overcome to a large extent by dividing min LVdP/dt by mean aortic pressure. Isoprenaline strongly influenced all relaxation parameters, and so did temperature, except for relaxation time in guinea pig hearts. We conclude that tau serves as a reliable relaxation parameter, also in the hearts of small animals with heart rates up to 450 beats/min. In isolated hearts, min LVdP/dt, corrected for afterload dependence, is also suitable as a complementary index of the early relaxation phase.