Quantification of the contractile state of the intact human heart. Maximal velocity of contractile element shortening determined by the instantaneous relation between the rate of pressure rise and pressure in the left ventricle during isovolumic systole

A new and easily applicable means based on force-velocity principles within the framework of muscle mechanics is presented for quantification of ventricular contractility, independent of loading, in conscious patients. In the intact heart during isovolumic contraction, alterations in ventricular geometry are small, and thus contractile element velocity (V_CE) can be assumed to equal series elastic elongation. It was therefore considered in this study that V_CE during isovolumic portion of ejecting beats could be determined entirely from isovolumic pressure (IP) and its rate of rise ( dP dt) without requiring knowledge of tension; this provided the rational basis for accurate calculation of instantaneous isovolumic V_CE in the spherical or ellipsoidal ventricle as ( dP dt)/(K × IP), where K is series elastic constant of 32 per muscle length (ML) at body temperature. Further, it was recognized that V_CE at the ordinate is identical when derived from pressure-velocity or tension-velocity curves, and since the left ventricular pressure-velocity relation can be defined precisely, construction of the isovolumic IP to V_CE curve and extrapolation to zero load allowed determination of maximal V_CE (v_max), the independent numerical measure of contractility. High fidelity left ventricular pressures and corresponding dP dt were continuously recorded in 23 studies in 8 conscious patients without valvular regurgitation or ventricular segmental disease. Increasing contractility by leg exercise raised V_MAX from 1.19 to 1.58 ML/sec and by isoproterenol to 2.03 ML/sec (P < 0.01). In contrast, elevations of left ventricular end-diastolic or aortic diastolic pressure levels did not increase V_MAX. Thus, determination of instantaneous V_CE and V_MAX solely from IP and dP dt during isovolumic systole affords a new, practical, sensitive and valid means of quantifying left ventricular contractility Without influence of preload and afterload from beat to beat in individual patients. In addition, since V_MAX is expressed in terms of muscle units and is free of variables of left ventricular loading and wall thickness, this method permits specific comparison of inotropic state among different patients.