Serum Potassium Levels and Myocardial Threshold of Excitability

To the Editor.—  The electrocardiographic tracings or the title must be mislabeled in the article, "Transvenous Pacemaker Failure Induced by Hyperkalemia," by O'Reilly, Murnaghan, and Williams (228:336, 1974), since the tracings show the pacemaker functioning best at highest potassium levels. The text is also confusing. The patient's pacemaker functioned intermittently, regardless of serum potassium levels and the use of numerous agents known to alter myocardial threshold, until it was removed. The removed pacemaker tested as normal. A replacement pacemaker also functioned intermittently. This patient apparently had such extensive endocardial fibrosis as to largely insulate the myocardium from a normal transvenous electrode. Hence, it was a poor case from which to draw conclusions concerning factors influencing myocardial excitability, and the authors' conclusions were contrary to those of most investigators.Siebens et al¹ in 1953 demonstrated in dogs a parallel increase of myocardial responsiveness by increasing extracellular potassium levels from 3