A non-linear elastic model of contraction of ischaemic segments

Previous studies have characterised the motion of the myocardium using a linear time varying elastance model, ie, they have sought to characterise the relationship between left ventricular volume and internal pressure as linear, but with time varying slopes over the cardiac cycle. However, the motion of myocardium during regional ischaemia has not been characterised by such models. Studies of totally ischaemic tissue and of myocardium in diastole have characterised the relationship between tension or stress and segment length as exponential. It is the purpose of this study to present a new model in which myocardial contraction is expressed as an exponential, but time varying elastic relationship. In this model tension, T, is related to segment length according to the formula T=eα(t)L+β, where α(t) rises with systole and falls in diastole. This model was applied to the motion of hypokinetic segments noted in a series of conscious dogs studied for other purposes. Hypokinetic segments display early systolic bulging, decreased systolic shortening, and early diastolic recoil. These particular types of segment motion are naturally predicted by this model. Furthermore, the motion of myocardial segments as they become increasingly ischaemic may be predicted, including a gradual shift to the right and narrowing of the tension-length loop. α was noted to be independent of loading change, and thus may be viewed as an index of contractility. This model thus predicts the pattern of motion of hypokinetic segments and provides new insight into myocardial contractility.