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.