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Steady-state effects of preload and afterload on isovolumic indices of contractility in autonomically blocked dogs.

The steady-state effects of independently altering left atrial pressure (LAP), aortic pressure (AP), and heart rate (HR) on left ventricular isovolumic indices were studied in autonomically blocked open-chested dogs. With mean AP and HR constant (dP/dt)max and (dP/dt)DP40 (that is dP/dt at a developed LVP of 5.3 kPa [40 mmHg]) rose to an approximate plateau above LAP 1.6 kPa (12 mmHg) implying optimum diastolic LV fibre length above this LAP. However, the index (dP/dt/TP)max (TP = total LVP above atmospheric) fell progressively at LAP > 1.6 kPa (12 mmHg). Increasing HR (LAP constant) increased (dP/dt)max by 5.6 ± 0.7% of basal value for every 10 beats.min−1 over HR range 120 to 200 beats·min−1. With LAP and HR constant changing AP in the aortic diastolic pressures (ADP) range 6.7 to 20 kPa (50 to 150 mmHg) had no effect on (dP/dt)DP40 and (dP/dt/TP)max, which occur early in systole. (dP/dt)max was afterload independent at ADP > 12 kPa (90 mmHg), but fell by about 5% (P <0.005) in the ADP range 6.8 to 9.3 kPa (51 to 70 mmHg) (basal conditions) and by 12% (P < 0.005) in the range 9.5 to 12 kPa (71 to 90 mmHg) (enhanced contractility), probably due to aortic valve opening. Under the present steady-state conditions direct afterload-mediated modulation of inotropic state could not be demonstrated. During Ca2+-mediated enhanced contractility the relative increases in the three indices differed, with the effects on (dP/dt)max, (dP/dt)DP40 and (dP/dt/TP)max respectively 235%, 176%, and 148% of their basal value. These changes were inversely related to the timing of each index in isovolumic systole and may reflect differences in activation of muscle fibres.