Limitations of R-average as an index of left ventricular isovolumic relaxation

Freeman et al. (1993) have recently introduced a new index measuring isovolumic relaxation in the in situ left ventricle. This index, called the R-average, shows less variability than the traditionally used monoexponential time constant (tau), and could therefore represent an alternative measure of isovolumic relaxation during different physiological or pathophysiological interventions. However, the R-average represents the average pressure fall during isovolumic relaxation (isovolumic pressure fall divided by the isovolumic time period), and is therefore highly influenced by the end-systolic pressure level. The present study was done in order to assess whether small increments in loading conditions would alter the R-average without changes in the isovolumic relaxation period or the monoexponential pressure decay tau. We used a right heart bypass porcine model, and end-systolic pressure was altered between 92 and 140 mmHg by pre-loading (servo-pump) or after-loading (phenylephrine) in spontaneously beating and paced hearts. During loading, we found a significant increase in the R-average and a close correlation (r = 0.72 - 0.99) between R-average and en-systolic pressure. However, no alterations were found in the duration of the isovolumic relaxation time period or monoexponential pressure decay (tau) during these loading conditions. In our view, the R-average indicates the systolic loading level for the ventricle, but does not necessarily reflect alterations in the process of active relaxation.