The Influence of Alterations in the Cardiac Loading Conditions on Indices of Myocardial Contractility in the Anaesthetized Dog

SUMMARY 1. An online analogue computer was used to measure myocardial contractility as the ratio dP/dt/IIT where dP/dt is maximum rate of change of left ventricular pressure and IIT is integrated isometric tension. 2. In the open thorax anaesthetized dog, max (dP/dt) and dP/dt/IIT were compared during alterations in preload, by partial vena caval occlusion, and changes in after‐load obtained by acetylcholine injections and by partial occlusion of the descending thoracic aorta. 3. A fall in preload lowered max (dP/dt) but did not alter dP/dt/IIT A rise and fall in afterload produced respectively an increase and decrease in max (dP/dt) but dp/dt/IIT was unaltered. 4. From expanded time base recordings, the time from the start of aortic flow (EM flowmeter) to max (dP/dt) was measured to indicate the relationship between valve opening and rnax (dP/dt). At low preload or elevated afterload the aortic valve opened well after max (dP/dt) had been reached. When the afterload fell, max (dP/dt) occurred after the opening of the aortic valve. 5. Positive inotropic stimulation following intravenous isoprenaline caused a marked elevation in dP/dt/IIT. However, the rise in max (dP/dt) was attenuated in comparison to dP/dt/IIT by a marked fall in afterload and thus early opening of the aortic valve. 6. It is concluded that max (dP/dt) is very sensitive to alterations in preload or afterload but the index——‐dP/dt/IIT is normalized so that changes in end diastolic fibre length do not alter this index. When the afterload falls max (dP/dt) is no longer determined in the isovolumic phase of contraction so that magnitude of rnax (dP/dt) is reduced. However, it is an empirical finding that IIT also falls so that the ratio is unaltered. dP/dt/IIT is a useful index of myocardial contractility that is insensitive to changes in preload or afterload.