LIMITATIONS OF R-AVERAGE AS AN INDEX OF LEFT-VENTRICULAR ISOVOLUMIC RELAXATION

Freeman et al. (1993) have recently introduced a new index measuring i sovolumic relaxation in the in situ left ventricle. This index, called the R-average, shows less variability than the traditionally used mon oexponential time constant (tau, tau), and could therefore represent a n alternative measure of isovolumic relaxation during different physio logical or pathophysiological interventions. However, the R-average re presents the average pressure fall during isovolumic relaxation (isovo lumic pressure fall divided by the isovolumic time period), and is the refore highly influenced by the end-systolic pressure level. The prese nt study was done in order to assess whether small increments in loadi ng conditions would alter the R-average without changes in the isovolu mic relaxation period or the monoexponential pressure decay tau. We us ed a right heart bypass porcine model, and end-systolic pressure was a ltered between 92 and 140 mmHg by pre-loading (servo-pump) or after-lo ading (phenylephrine) in spontaneously beating and paced hearts. Durin g loading, we found a significant increase in the R-average and a clos e correlation (r=0.72-0.99) between R-average and end-systolic pressur e, However, no alterations were found in the duration of the isovolumi c relaxation time period or monoexponential pressure decay (tau) durin g these loading conditions. In our view, the R-average indicates the s ystolic loading level for the ventricle, but does not necessarily refl ect alterations in the process of active relaxation.