Effect of the Ouabain-Quinidine Interaction

The effect of the ouabain-quinidine interaction was examined in 10 conscious dogs. Left ventricular (LV) pressure, LV dP/dt, LV diameter and left atrial (LA) diameter were measured with highfidelity micromanometers and sonomicrometer crystals. Ouabain, 0.025 mg/kg, significantly (p < 0.05) increased LV dP/dt, LV and LA fractional shortening and LV and LA velocity of circumferential fiber shortening (Vcf). In a separate experiment, quinidine was administered as a bolus dose, 3.85 mg/kg, followed by an infusion, 0.28 mg/kg/min. This resulted in steady-state quinidine concentrations that produced no change in wall motion or hemodynamics. When ouabain was given 1 hour into the quinidine infusion, only LV dP/dt increased significantly (p < 0.05). Ouabain alone increased LV dP/dt 26.4 + 3.5%, whereas ouabain during the quinidine infusion increased it by 9.5 ± 2.3%. Similar differences were seen in the responses to ouabain in the absence and presence of quinidine: LV Vcf, 22.4 ± 4.9% vs 6.0 ± 2.1%, LV fractional shortening, 23.1 ± 4.6% vs 5.8 ± 2.1%, LAVcf, 22.7 ± 5.9 vs 4.6 ± 2.0% and LA fractional shortening, 21.8 7% vs 7.8 + 3.3%. Thus, in the presence of quinidine the increase in intropy usually seen with ouabain was markedly attenuated. These data suggest that the quinidine-induced increase in digoxin serum concentrations is accompanied by a decrease in the contractile response of the heart to