title: The resetting response of ventricular tachycardia to single and double extrastimuli: implications for an excitable gap.
abstract: To evaluate the influence of local tissue refractoriness and delay in intervening tissue on the ability of single ventricular extrastimuli to reset and characterize a resetting response pattern in ventricular tachycardia (VT), single ventricular extrastimuli were delivered during 81 VTs and double ventricular extrastimuli in 45 of the 81 VTs. Resetting of VT was recognized as a less than fully compensatory pause after stimulation and was seen in 43 of 81 VTs (53%) with single ventricular extrastimuli and 35 of 45 (78%) with double ventricular extrastimuli. Double ventricular extrastimuli reset 16 VTs not reset by single ventricular extrastimuli. The return cycle, the interval from the extrastimulus to the first VT beat after extrastimuli, has 1 of 3 distinct response patterns: flat, increasing, and flat plus increasing. In 19 VTs, resetting was seen with both single ventricular extrastimuli and double ventricular extrastimuli; 4 flat responses with single ventricular extrastimuli became flat plus increasing with double ventricular extrastimuli. All other patterns were unchanged. In the 19 VTs reset by both single and double ventricular extrastimuli, the estimate of both the total reset zone (94 +/- 36 vs 56 +/- 32 ms) and the flat portion of the reset zone (52 +/- 42 vs 42 +/- 28 ms) was significantly longer with double ventricular extrastimuli (p less than 0.001 and p less than 0.02, respectively).
  
  
  CONCLUSIONS
  (1) when single ventricular extrastimuli failed to reset a VT, double ventricular extrastimuli from the same site may reset the VT.
mesh_terms: D002304:Cardiac Pacing, Artificial; D004562:Electrocardiography; D004594:Electrophysiology; D006329:Heart Conduction System / Q000503:physiopathology*; D006801:Humans; D013610:Tachycardia / Q000503:physiopathology / Q000628:therapy*