Effects of Chronic Heart Failure on the Inotropic Response of the Right Ventricle of the Conscious Dog to a Cardiac Glycoside and to Tachycardia

The effects of ouabain, 20 µg/kg, and elevation of cardiac rate from 90 to 240 beats/min (BQwditch phenomenon) were examined on right ventricular (RV) segment length (SL), pressure (P), velocity of shortening (V), dP/dt, and dP/dt/P in conscious dogs before and after chronic right heart failure was induced by tricuspid avulsion and pulmonary stenosis, or by pulmonary stenosis alone. Chronic pulmonary stenosis and tricuspid regurgitation increased RV systolic P from 25 ± 3 to 59 ± 7 mm Hg, end-diastolic P from 3 ± 1 to 16 ± 2 mm Hg and reduced dP/dt/P from 48 ± 6 to 24 ± 4 sec –1 and V from 26 ± 3 to 15 ± 3 mm/sec. Both ouabain and tachycardia exerted relatively minor inotropic effects in normal, healthy, conscious dogs; dP/dt/P and V rose by 18 ± 2 and 17 ± 2% with ouabain, and by 15 ± 3 and 16 ± 3% with tachycardia. When the tachycardia-induced reductions in end-diastolic SL were prevented by saline infusion, the increases in dP/dt/P and V were 19 ± 3% and 21 ± 3%. After chronic heart failure had been induced, the positive inotropic increases, with ouabain and tachycardia expressed on an absolute or relative basis, were significantly greater than had occurred in the dogs prior to heart failure; dP/dt/P and V rose by 47 ± 5 and 48 ± 6% with ouabain and by 41 ± 4% and 51 ± 3% with tachycardia. Thus, cardiac glycosides and the Bowditch phenomenon exert relatively minor positive inotropic responses in the nonfailing heart of the conscious dog when compared with the responses to these stimuli in the failing heart.