Reentrant and focal arrhythmias in low potassium in isolated rabbit atrium

In isolated superfused left atria of the rabbit, tachyarrhythmias became highly inducible with a decrease of the extracellular potassium concentration to 2.0 mM. The nature of the arrhythmias was determined with a high-resolution mapping system. In several cases, abnormal impulse formation was found, but the majority of arrhythmias (75%) was caused by a circus movement of the impulse. Furthermore, circus-movement tachyarrhythmias often degenerated into fibrillatory activity, and activation maps revealed the presence of multiple wavelets during this chaotic rhythm. The occurrence of reentrant arrhythmias may be caused either by an increase in inhomogeneity in conduction or by a shortening of the wavelength in low potassium. Inhomogeneity in conduction was determined by calculating the difference in activation times between neighboring electrodes. In low potassium, premature activation significantly increased the inhomogeneity index compared with slow rhythm (from 2.2 to 3.9; P < 0.001) but was not significantly different from the inhomogeneity measured at normal potassium concentrations. Low potassium, however, did shorten significantly the wavelength of the impulse by ~ 40%. The increased inducibility of reentry in low potassium is therefore caused by a reduction of the length of the excitation wave and not by an increase in inhomogeneity in conduction.