Studies on digitalis. XVI. Effects on myocardial oxygen consumption.

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Abstract
There has been considerable dispute concerning the effects of digitalis on myocardial O2 consumption (MVO2) and efficiency. The effects of acetylstrophanthidin %vg. dose =0.26 cat units/kg) were studied in 6 nonfalling, canine, right heart bypass preparations in which heart rate, stroke volume, and mean aortic pressure were held constant. MVO2 increased in all experiments, by an average of 2.56 ml/min., while calculated cardiac efficiency declined by an average of 24.4% of control. Even though mean arterial pressure was held constant, the glycoside reduced the integrated systolic tension by an average of 39%, chiefly as a consequence of a small decline in left ventricular end-diastolic volume. However, the velocity of myocardial fiber shortening increased considerably, the peak left ventricular ejection rate rising an average of 36% and the peak ventricular dp/dt increasing by an average of 82%. Acetylstrophanthidin did not alter MVO2 in hearts studied in an identical manner, but in which left ventricular end-diastolic pressure was initially elevated. However, the large fall in end-diastolic volume in the failing hearts resulted in a marked fall in systolic tension.