The rate of force generation by the myocardium is not influenced by afterload

Abstract

The influence of afterload on the rate of force generation by the myocardium was investigated using two types of preparations: the in situ dog heart (dP/dt) and isolated papillary muscle of rats (dT/dt). Thirteen anesthetized, mechanically ventilated and thoracotomized dogs were submitted to pharmacological autonomic blockade (3.0 mg/ kg oxprenolol plus 0.5 mg/kg atropine). A reservoir connected to the left atrium permitted the control of left ventricular end-diastolic pressure (LVEDP). A mechanical constriction of the descending thoracic aorta allowed to increase the systolic pressure in two steps of 20 mmHg (conditions H 1 and H 2 ) above control values (condition C). After arterial pressure elevations (systolic pressure C: 119 ± 8.1; H 1 : 142 ± 7.9; H 2 : 166 ± 7.7 mmHg; P<0.01), there were no significant differences in heart rate (C: 125 ± 13.9; H 1 : 125 ± 13.5; H 2 : 123 ± 14.1 bpm; P>0.05) or LVEDP (C: 6.2 ± 2.48; H 1 : 6.3 ± 2.43; H 2 : 6.1 ± 2.51 mmHg; P>0.05). The values of dP/dt did not change after each elevation of arterial pressure (C: 3,068 ± 1,057; H 1 : 3,112 ± 996; H 2 : 3,086 ± 980 mmHg/s; P>0.05). In isolated rat papillary muscle, an afterload corresponding to 50% and 75% of the maximal developed tension did not alter the values of the maximum rate of tension development (100%: 78 ± 13; 75%: 80 ± 13; 50%: 79 ± 11 g mm -2 s -1 , P>0.05). The results show that the rise in afterload per se does not cause changes in dP/dt or dT/dt.

Influence of the elevation of the left ventricular diastolic pressure on the values of the first temporal derivative of the ventricular pressure (dP/dt)

“…The authors who assessed the relationship between these 2 variables considered that, during oscillations of BP, the values of dP/dt may be modified as a result of variations in the afterload or in the coronary perfusion. Studies conducted in our laboratory [29][30][31] , whose methods were similar to those used in the present study, showed that, in the experimental conditions that we used, the sudden increase in the aortic pressure did not affect the values of dP/dtmax.…”