Two mechanisms of cardiac arrest produced by potassium.

• Cardiac arrest, produced by an increased extracellular potassium concentration, was first observed by Ilering in 1907. Recent studies with microelectrodes have revealed that an increased potassium concentration decreases the transmembrane potential of fibers of atrium,' ventricle' and the Purkinje system. Sensitivity to potassium, however, differs for the several tissues. Conduction fails earlier in atrial fibers than in ventricular tissue. The specialized fibers of the sino-atrial and atrio-ventrieular node and the crista terminalis are less sensitive to potassium than the atrial fibers. The most resistant fibers are apparently those of the bundle of His, which remained excitable when other types of cardiac fibers are inexcitable." The loss of excitability and the failure of conduction which lead to cardiac arrest have been attributed to the fall of the resting potential below a critical level.