Site of initial excitation and current threshold as a function of electrode radius in heart muscle

End-diastolic current thresholds have been measured in 13 open chested dogs as a function of electrode radius by stimulating the left ventricle with epicardial disc electrodes ranging in a radius from 0-3 mm to 9 mm. Thresholds for cathodal rectangular short stimuli as well as specifically for cathodal make stimulation, proved to be proportional to the electrode radius to the power 1-5. This relationship between radius and threshold can be explained theoretically, assuming that electrical stimulation results in a propagated depolarization front if a critical current density is reached somewhere in the myocardium. The current distribution measured over the electrode and the site of initial depolarization in the tissue are in accordance with this theoretical explanation.