Pharmacologic alterations of the isovolumic index: a study of the effects of isoproterenol, propranolol and verapamil.

The isovolumic index has been proposed as a noninvasive index of left ventricular function which, unlike the traditional systolic time index, incorporates the period of isovolumic relaxation. The responses of the isovolumic index and the systolic time index to three cardioactive drugs (isoproterenol, propranolol and verapamil) were assessed by measurements of ejection time (ET), isovolumic contraction time (IVC) and isovolumic relaxation time (IVR) in 23 dogs instrumented with high fidelity micromanometers and ultrasonic crystals. Isoproterenol infusions resulted in improvements in both the isovolumic and systolic time indices as a result of significant shortening of ET, IVC and IVR. Propranolol infusions caused significant and parallel increases of IVC (P less than 0.01) and ET (P less than 0.05) but failed to cause prolongation of either the systolic time index, defined as (IVC/ET) or deterioration of regional function. A higher propranolol dose caused significant increases in ET, IVC (P less than 0.001) and IVR (P less than 0.05), yet neither the systolic time index or the isovolumic index (IVC + IVR/ET) were prolonged, and regional function remained normal. Verapamil infusion caused an upward, nonsignificant trend in the isovolumic index and no change in the systolic time index. Stepwise multiple linear regression analysis demonstrated a similar load dependency of both indices and a higher inverse correlation of the isovolumic index with fractional shortening. The systolic time index showed a higher dependency on peak positive dP/dt whereas only the isovolumic index showed dependency on the isovolumic relaxation time constant. When altered loading conditions are accounted for, the isovolumic index shows directional changes that reflect changes in peak positive dP/dt, the isovolumic relaxation time constant and regional shortening.