Assessment of isovolumic and ejection phase indices of left ventricular performance

Variables derived from isovolumic phase pressure tracings and from the characteristics of wall shortening during ejection may be used for defining the basal levels of contractility in normal subjects and in patients with cardiac disease. The problem of identifying an acute change in contractility appears to be relatively straightforward, and both isovolumic and ejection phase indices of left ventricular performance seem to be useful for detecting such acute alterations. Under chronic conditions, however, the heart may compensate for long-term stress by hypertrophy, dilatation and maximum use of the Frank-Starling reserve mechanism. Geometrical and wall thickness changes may serve to maintain preload and afterload relatively constant, and if basal contractility is maintained, myocardial performance per unit of circumference during ejection will remain normal. Isovolumic indices, which are derived from the preejection phase of contraction, are relatively insensitive measures of left ventricular performance. Therefore, when myocardial contractility becomes depressed in the basal state, the isovolumic indices may not detect the inability of the heart to sustain performance and in these circumstances it seems preferable to measure shortening characteristics in order to detect the depressed level of contractility. This concept may provide at least a partial explanation of why ejection phase indices have proved more reliable than isovolumic measures for detecting reduced contractility in the basal state.