Myocardial contractile reserve and indices of contractility

A close examiniation of several indices of contractility derived from the ventricular pressure was made using an isolated canine heart preparation. The repsonses to single intracoronary injections of calcium chloride as well as to increasing doses of this agent were tested. From the latter, calcium dose-transient-response curves intended to reveal the extent of myocardial/contractile reserve were constructed. Indices included various extrapolations to maximal velocity of shortening at zero load, maximal value of the quotient of the first derivative and ventricular pressure, time-tension index, energy averaged power density, and power averaged rate of generation of power density. Indices were compared over the same cardiac cycles before and after administration of calcium. Most indices showed increments from 10 to 20%, except power density functions which had increments of 40 and 70%, respectively. Calcium dose-response curves were linear for most indices, but, again, the power density functions showed the steepest slopes. After severe coronary occlusion, the curves for most indices flattened and lost linearity and, presumably, this was due to loss of myocardial contractile reserve. For milder occlusions, only the power density functions showed significant flattening. The conceptual framework of a contractile myocardial reserve appears supported by these results.