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

Abstract

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 2+-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.