AUGMENTED EFFICIENCY OF REGIONAL MYOCARDIAL WORK BY OUABAIN

Study objective - The effect of ouabain on regional myocardial mechanics and corresponding regional myocardial oxygen consumption was investigated. Design - Regional myocardial segment length (ultrasonic dimension crystals) and force development (miniature strain gauge) were measured. The integrated multiples of myocardial shortening by corresponding force during an averaged beat was used to express regional segment work (= area under the systolic portion of the length-force loop). External cardiac work was calculated from aortic blood pressure and cardiac output. Global and regional myocardial O2 consumption (MVO2) were evaluated at baseline and during intravenous infusion of ouabain (7-mu-g.kg-1.min-1). Regional coronary blood flow was measured with radioactive microspheres, and microspectrophotometry of frozen myocardial biopsies was used to evaluate O2 saturation in small arteries and veins. These variables were used to calculate regional myocardial MVO2. Arterial and coronary sinus O2 saturation was used to calculate global left ventricular O2 consumption. Myocardial efficiency was expressed as regional work/regional myocardial MVO2. Subjects - Ten open chest anaesthetised dogs, weight about 20 kg, were used. Measurements and main results - Ouabain increased regional myocardial segment work [from 5291(SEM 1067) to 8916(2338) mm.g-1.min-1] to a greater extent than regional myocardial MVO2 [from 6.33(0.75) to 8.54(0.75) ml O2.min-1 100 g-1]. Regional efficiency was increased from 734(78) to 1036(160) g-mm.ml O2(-1).100 g-1. External efficiency was not raised. Percent shortening was increased to a greater extent than peak force development (+ 76% v +38%), possibly indicating increased myocardial compliance. The length-force loop was shifted upward and to the right. Conclusions - Ouabain increases regional efficiency of myocardial contraction.