An elastance characterization of the normal and failing heart
Baldridge, David Lawrence
Clark, John W.
Master of Science
The contractile state of the left ventricle of the heart is shown to be uniquely characterized by the ventricle's instantaneous pressure-volume ratio or elastance (Ev(t)). This unique characterization enables various disease states to be distinguished by respective parameters derived from the elastance function, the respective parameters being used to form indices of the myocardial contractile state (myocardial contractility). Previous methods of characterizing the contractile state of the left ventricle (which utilize a microscopic view of individual cardiac muscle fibers) are examined and their strengths and weaknesses are presented. The relation ship of these methods to the elastance characterization (which utilizes a macroscopic view of muscle) is shown and the advantages of the Ev(t) method are given. Although an historically controversial topic, the dependence of elastance on the preload (end-diastolic volume, Ve(j) of the heart is first shown to exist in the normal dog. This dependence is expressed in terms of the relationship of Ve(j to peak elastance and time to Ev,max from start of systole. After fitting human elastance data with a new and improved elastance curvefitting method utilizing spline functions, the same relationship to Vecj as in the dog is shown to exist in man, both for the normal and the failing heart. In addition to the normal heart, the disease states studied include asynergism (the state of partial wall akinesis or dyskinesis during contraction) and coronary artery disease (CAD). Characteristic elastance functions (dependent on Ved) are shown to exist for the normal, asynergic, and CAD hearts, with the CAD hearts demonstrating two distinct elastance functions, one being associated with a more advanced deterioration of the myocardial contractile state than the other. Using these 'characteristic elastance functions, various parameters derived from the elastance functions are shown to distinguish the normal contractile state from that of the asynergic and CAD patients, the CAD patients existing at a lower state of myocardial contractility than the normal patients but at a higher state than the asynergic patients. Being a preliminary study, areas of extension and additional applications of the elastance characterization of disease states are presented.