Development of an index of the rate of left ventricular isovolumic pressure fall that accurately and consistently reflects alterations in relaxation has been elusive. Considerations with the use of any index include avoiding potential artifacts due to linear data shifts and loss of monoexponentiality. This report details the effects of an intervention that significantly alters the value of the variable asymptote on the calculation of certain indexes of left ventricular isovolumic relaxation. Our results indicate that in the closed-chest canine, in the normal sinus beats immediately following short-coupled extrasystoles, the magnitude and the direction of the trends of relaxation can be a function of the method of index calculation. Our results also emphasize the utility of biexponential approaches in the delineation of subtle trends in the form of isovolumic pressure decay that may reflect the complex interplay between active and passive components of the diastolic process.