THE INFLUENCE OF ALTERATIONS IN THE CARDIAC LOADING CONDITIONS ON INDICES OF MYOCARDIAL CONTRACTILITY IN THE ANAESTHETIZED DOG |
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Authors: | J. A. Angus D. R. Richmond L. B. Cobbin A. H. Goodman |
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Affiliation: | Department of Pharmacology, University of Sydney, Sydney, New South Wales, Australia |
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Abstract: | 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. |
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Keywords: | cardiac output, haemodynamics, heart function tests, isoprenaline, myocardium. |
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