Thermodynamic characterization of the contractile state of the myocardium.

Definition of the contractile state of the myocardium from force, length, and velocity relationships and measurements of ventricular pressure is a continuum mechanics approach that requires a detailed knowledge of the system under consideration or, in the absence of detailed knowledge, the use of suitable conceptual models. In this paper we proposed a simpler, though still rigorous, approach that required only a general, rather than a detailed, a priori knowledge of the system. Using the theory of thermodynamics, we gained some insight into the physical meaning of the time derivatives of the left ventricular pressure pulse. From integrations and normalizations we obtained power-density functions (energy-averaged power density and power-averaged rate of generation of power density). These functions relating the free energy in the myocardium to the pressure in the blood did not require geometric considerations, and they furnished, to a first approximation, a quantification of the contractile state in a population of 26 normal dogs under anesthesia. Moreover, the functions clearly separated the normal state from abnormal ones created by drug intervention, not only in the same dog, but also in different dogs. The functions appeared to be insensitive to volume loading. In two patients studied before and after surgery, these functions also showed significant changes.