Elevated potassium concentration converts excitation mechanism from make to break

We used optical fluorescence imaging to study the mechanism of unipolar stimulation of cardiac tissue under elevated potassium (K⁺) conditions. We examined K⁺ concentrations of 4, 8, 10, and 12 mM for a ±6 mA, 10 ms stimulus, approximately 20× the cathodal threshold with 4 mM K⁺. For 4 and 8 mM K⁺, make stimulation was observed. Raising the concentration to 10 mM, however, resulted in break stimulation. When the K⁺ concentration was increased to 12 mM, the tissue failed to respond to stimuli as large as 9 mA. We conclude that elevated potassium, as might occur in myocardial ischemia or fibrillation, alters not only stimulation threshold but also the excitation mechanism for point stimulation.