Variations in left and right ventricular oxygen balance produced paired electrical stimulations

The possible differential effect of positive inotropic stimulation upon regional myocardial oxygen balance in the two ventricles was investigated during tachycardia and paired electrical stimulation. Isometric contractile force was measured by strain gauge arches; local coronary blood supply was measured by thermistor probes and intracellular NADH redox level was recorded using surface fluorometry. It was found that when contractility was increased by paired stimulation at a basic rate of 140 bpm, the inotropic response was more pronounced in the right ventricle (97.2 +/- 11.5%) than in the left (63.1 +/- 12.6%). Coronary blood supply to the left ventricle increased by 117.8 +/- 30.4% and the corresponding NADH redox level increased by 54.3 +/- 19.9%. When the contractile force was increased to the same extent (64.1 +/- 8.9%) by single stimulation at a rate of 210 bpm, the coronary flow to the left ventricle was increased by only 36.4 +/- 11.0% and the NADH state rose by 67.1 +/- 12.1%. It is concluded that paired stimulation reduced the mechanical limitation to flow during tachycardia, thus allowing coronary blood supply to increase in response to positive inotropic stimulation, thereby preserving a relatively improved oxygen state. It was also observed that the ratio contractile force/blood supply (contraction efficiency) was usually proportional to the NADH redox level (oxygen balance). Nevertheless, variations observed in the force/supply ratio for the left ventricle indicate that the NADH redox level cannot be predicted quantitatively by the force/supply ratio.