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 (VCE) can be assumed to equal series elastic elongation. It was therefore considered in this study that VCE 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 VCE 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 VCE 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 VCE curve and extrapolation to zero load allowed determination of maximal VCE (vmax), 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 VMAX 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 VMAX. Thus, determination of instantaneous VCE and VMAX 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 VMAX 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.
All Science Journal Classification (ASJC) codes
- Cardiology and Cardiovascular Medicine