Current thresholds and liminal size in excitation of heart muscle

End-diastolic current thresholds have been measured in 12 open-chested dogs with epicardial disc electrodes ranging in radius from 0.01 to 9.0 mm. The current threshold for cathodal 1 ms impulses proved to be independent of electrode size for radii smaller than 0.2 mm and proportional to the radius to the power 1.5 for radii greater than 0.4 mm. This relationship can be explained on the assumption that excitation occurs if the current passing through a 'liminal area' with dimensions of about 0.3 mm exceeds a critical value. This result is in agreement with the liminal length concept used to describe excitation of cardiac Purkinje fibres. The existance of a liminal area or liminal lenght for excitation implies that there will be a limit to increasing stimulation efficiency by applying smaller stimulation electrodes.