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 V_s relating rise of dP dt to rise of P is a measure of contractile element velocity. V_s 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 V_s 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 ΔP and the ΔdP dt axes gave additional evidence that alterations of preload did not affect V_s.