Isochronal behavior in left ventricular systolic pressure-wall thickness relations

Regional left ventricular systolic pressure-thickness relations have been used to assess regional load-insensitive contractility with the assumption that they possess linear isochrones that are fundamental to the time-varying elastance model of global pressure-volume relations. We examined the shape and time-varying behavior of pressure-thickness isochrones in six open-chest canine preparations. Transmural wall thickening (sonomicrometry) and ventricular pressure were altered by abrupt preload alterations during control, dobutamine, and propranolol. In all dogs and interventions, linear isochrones (r2 mean +/- SE = 0.91 +/- 0.11) were found at 5-ms intervals. During control, linear isochrone slope rose monotonically from onset to end of systole. Thickness-axis intercepts also varied continuously in time, but peak intercept and maximal slope were asynchronous. Dobutamine caused a steeper earlier maximum slope and increased slope-intercept asynchrony. Propranolol reduced maximum slope and slope-intercept asynchrony. Isochronal data during early systole were better fitted to a parabolic than to the linear model; however, fits to linear and parabolic models were equally good near end of systole. Linear isochronal behavior exists in systolic pressure-thickness relations especially near end systole and is maintained during modest inotropic alterations.