Conventional methods to assess cardiac contractility have been focused on the mechanical properties of the myocardium. Many of these have suffered from theoretical and practical drawbacks. In this study, we have attempted to evaluate the left ventricular contractile status using the thermodynamic principle and compared these indices with the conventional index (dP/dt). A total of 8 mongrel dogs were anesthetized and artificially ventilated. A Millar catheter with high-fidelity multiple sensors was inserted into the aorta to record the simultaneous changes in aortic flow and pressure waves. Cardiac inotropy was increased by graded doses (2-32 micrograms/kg/min) of dobutamine. Angiotensin injection (100-300 micrograms) to achieve a blood pressure elevation of 30 mmHg was employed for afterload augmentation. Preload was increased by rapid infusion of 6% dextran solution (20 ml/kg). Several thermodynamic parameters were calculated at steady state during the control period and after drug interventions. These included power-averaged rate of power density generation (ARPD), peak ejection rate of change of power (PREP), energy-averaged power density (APD) and frequency-normalized ARPD (FARPD). PREP and APD were unchanged by afterload increment while FARPD, ARPD and dP/dt decreased significantly with an increase in afterload, ARPD was highly sensitive to an increased preload (p < 0.001), and PREP was also changed significantly by an increase in preload. dP/dt was boarderlinely affected by Dextran infusion (p = 0.05), and APD was independent of preload (P > 0.05). Inotropic stimulation increased ARPD, PREP, APD, FARPD and dP/dt by 139.0 +/- 48.0%, 98.0 +/- 51.4%, 74.0 +/- 49.7%, 60.0 +/- 30.4% and 44.6 +/- 10.4%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)