Fibrillation Threshold of Premature Ventricular Responses

• Asynchrony of recovery from the refrac- tory state has been emphasized repeatedly as a major factor in the induction of ventricular fibrillation by strong stimuli applied during the "vulnerable" period. The role of asym- metry of refractoriness, excitability, and con- duction has been appraised thoroughly by Brooks et al. 1 Some of the postulates proposed in their discussion have been tested and sup- ported by recent studies in this laboratory; a number of agencies which initiate or predis- pose to the induction of ventricular fibrillation were shown to increase the temporal dis- persion of recovery from the refractory state. 2 " 4 When an early premature response is evoked in an irregularly excitable field, its propaga- tion from the site of origin must also be ir- regular, and the recovery of excitability from the premature response should be even more greatly dispersed. Increased temporal disper- sion of the refractory period duration deter- mined at numerous points close to the site of origin of early premature ventricular responses has been demonstrated. 2 If vulnerability to fibrillation during the rel- atively refractory period is due to the non- uniform excitability of the tissue, then pre- mature responses should be characterized by a lower fibrillation threshold and a longer duration of the vulnerable period. This pos- sibility was tested long ago by Wegria, Moe, and Wiggers, 5 who found that ". . . the vul-