THE FIBRILLATION THRESHOLD AFTER ADMINISTRATION OF DIGITALIS AND OUABAIN

¹ From the Department of Physiology, Western Reserve University Medical School, Cleveland, O.

1. As a result of previous investigations, a critical method for determining the sensitivity of the dog ventricles to fibrillation is presented. It consists fundamentally in measuring the strength of a short D.C. stimulus applied in late systole, just sufficient to induce fibrillation.

2. Studied by this method, digitalis and ouabain (g-strophanthin) in doses sufficient to elicit clear signs of action, do not alter significantly Such a fibrillation threshold of the dog ventricles.

3. Since toxic doses of digitalis and ouabain induce spontaneous fibrillation of the ventricles, some other mechanism must be involved. Results are presented which lead to the conclusions (a) that the type of fibrillation induced differs significantly from that caused by electrical currents and coronary occlusion in normal hearts and (b) that its onset and development depend on favorable development of localized blocks and changes in myocardial conductivity and does not require the advent of an effective stimulus during the vulnerable period, as in electrocution and coronary occlusion.

4. The results are of fundamental importance in showing (a) that ventricular fibrillation may result as a consequence of more than one mechanism, and (b) that the slow and somewhat variable progression of the fibrillating process which leads to death from digitalis, is one of the factors which makes dogs unfavorable animals for the bio-assay of digitalis.