THE ACTION OF SOME HEART TONICS EMIL LEYKO

Starling’s heart lung preparation is very convenient for the study of the direct effect of a drug upon the heart under conditions which closely approach the normal: it excludes many complicat-ing factors such as the influence of the nervous system which might change the heart rate with resulting change in output, and it excludes effects on the vessels which might change the venous inflow and ventricular output and confuse the direct action of a “tonic ” drug on the heart muscle. In this preparation all the environmental conditions of the heart such as temperature, volume of blood inflow, and arterial resistance can be exactly controlled and kept unaltered and thus the work of the heart maintained constant during the whole experiment. As the heart rate in this preparation depends only on the temperature, pro-vided that the other conditions are unchangeable, by keeping the temperature of the blood ‘at the same level, we can be sure of a regular rhythm of the heart under investigation. The meaning of the term heart-tone in this paper is that used by Bodo (1) in his work performed in this Laboratory and defined by Starling (2) as the mechanical efficiency of the muscle fiber, i.e. the relation of the mechanical energy to the total energy liberated. The total energy set free in the living heart, main-tained under constant chemical and temperature conditions as measured by the oxygen consumption, is determined on the basis of the experiments of Starling and Visscher (3), by the initial length of its muscular fibers, i.e. by its diastolic volume. That means that if the same work is done by. two hearts, one with a small and the other with a large diastolic volume, the mechanical efficiency of the first is greater and its tone is bette