Evaluation of systolic effectiveness and its determinants: pressure/midwall-volume relations

Abstract

It is generally agreed that systolic performance of a heart chamber is the fractional inward displacement of its wall during contraction and that this depends on preload, afterload, and characteristics of the relation between afterload and end-ejection dimensions. However, there is no consensus on the details of this statement. How can one define and identify the wall element, the displacement of which best expresses performance? What is preload? What parameters best characterize the relation between afterload and end-ejection dimensions? Dividing a thick-wall compliance equation by a thick-wall pressure equation reveals the midwall element, the normalized displacements of which depend consistently on normalized pressure changes according to wall properties regardless of wall-to-cavity ratio. This midwall element's reference dimensions best express chamber size, its reference-normalized dimensions best express wall stretch or distension, and its fractional displacements best express systolic performance. The hydraulically unloaded state is a poor reference for expressing size, normalizing dimensions, and defining chamber characteristics; it is inaccessible, immeasurable, acutely variable, and not a mechanically unloaded state. Therefore stiffness is neither a characteristic nor an expression of systolic vigor. A better reference state is the "average basal end-diastolic distension to which the chamber is accustomed"; it is accessible, measurable, and a state where stretches throughout the wall are near a characteristic value. End-diastolic midwall dimension relative to its average basal value is a valid expression of preload. There are two main hydrodynamic characteristics expressing systolic vigor: 1) peak isovolumic pressure at reference distension and 2) displacement from reference distension if afterload were zero. An additional characteristic is shape of the pressure-volume relation. It appears possible to account for systolic performance in terms of preload, afterload, and these hydrodynamic characteristics.