Preload-independent index of contractility for the assessment of left ventricular function

We describe a method for the assessment of myocardial contractile force; it is based on the measurement of left ventricular pressure P and its first derivative dP/dt at two well-defined points on the isovolumic pressure curve. The slope Vs relating rise of dP/dt to rise of P is a measure of contractile element velocity. Vs values for normal children and adults, varying in body surface area from 0.18 to 2.14 m2 and in age from 1 day to 60 years, were in close agreement, indicating that this parameter is substantially independent of heart size. Ten children with myocardial disease all had lower Vs values than any of 24 children with normal left ventricular function. Within each of these two groups, the increase of dP/dt during isovolumic contraction was highly correlated with the increase of P, but the regression line for the normal group had a significantly steeper slope than that for the impaired group. The near zero intercepts of the two regression lines on both delta P and the delta dP/dt axes gave additional evidence that alterations of preload did not affect Vs.