Effect of ouabain on the relation between left ventricular oxygen consumption and systolic pressure-volume area (PVA) in dog heart

Summary We studied the effect of ouabain (digitalis) on the relation between left ventricular (LV) O₂ consumption (Vo₂) and pressure-volume (P-V) area (PVA) in 7 excised cross-circulated canine heart preparations. PVA is a measure of the total mechanical energy generated by LV contraction and was obtained as the specific area in the P-V diagram circumscribed by the end-systolic P-V line, end-diastolic P-V curve, and the systolic P-V trajectory. Ouabain (0.11 mg, intracoronary-arterially) increased Emax (LV contractility index) by 58±44% (mean ± SD) from 7.8±3.4 to 12.0±4.8 mmHg/(ml/100 g LV). PVA correlated linearly with LV Vo₂ per beat in either the control (r>0.97) or the ouabain run (r>0.96) in individual hearts. Ouabain increased the Vo₂-axis intercept of the regression line of Vo₂ on PVA from 0.029±0.004 in the control run to 0.036±0.009 ml O₂/beat/100 g LV without significantly changing the slope [(1.53±0.24)·10^–5 ml O₂/(mmHg/ml)] of the regression line. This slope is equivalent to the contractile efficiency value of 44±6% from the excess Vo₂ above unloaded Vo₂ to PVA. The parallel elevation of the Vo₂-PVA relation with ouabain was similar to the results produced by epinephrine and Ca²⁺ in our previous studies. Ouabain, like epinephrine and Ca²⁺, did not change the contractile efficiency from the PVA-dependent fraction of Vo₂ to PVA.