title: Effect of shock timing on efficacy and safety of internal cardioversion for ventricular tachycardia.
abstract: OBJECTIVE
  We examined the effect of shock timing within the QRS complex on cardioversion efficacy in a randomized crossover test of shocks delivered at two timing intervals relative to QRS onset.
  
  
  BACKGROUND
  The local ventricular electrogram is used in implantable cardioverter-defibrillators to synchronize cardioversion shocks to terminate ventricular tachycardia. However, the timing of the local electrogram relative to global ventricular depolarization is variable, depending on the site of ventricular tachycardia origin.
  
  
  METHODS
  Transvenous defibrillation leads were positioned in the right ventricular apex (cathode), coronary sinus and superior vena cava (anodes) of patients with sustained monomorphic ventricular tachycardia. After repeat ventricular tachycardia induction, sequential shocks with energy settings of 0.5 to 22 J were delivered simultaneously with QRS onset (QRS + 0 shock) or 100 ms after QRS onset (QRS + 100 shock). QRS onset was determined from the surface electrocardiogram. Cardioversion threshold, defined as the lowest shock energy for successful ventricular tachycardia termination, was measured for these two timings.
  
  
  RESULTS
  Fifteen patients (13 men, 2 women; mean [+/- SD] age 60.5 +/- 7.7 years) completed testing. Cardioversion threshold was significantly lower with QRS + 100 shocks than QRS + 0 shocks (3.1 +/- 3.5 vs. 10.5 +/- 7.4 J, p < 0.01). Thirteen patients (87%) experienced ventricular tachycardia acceleration with QRS + 0 shocks, but only three patients (20%) had ventricular tachycardia acceleration using QRS + 100 shocks (p < 0.01). Of the 32 failed QRS + 0 shocks, 25 (78%) caused ventricular tachycardia acceleration, whereas only 5 (36%) of the 14 failed QRS + 100 shocks caused ventricular tachycardia acceleration (p < 0.05). Cardioversion threshold was not correlated with ventricular tachycardia cycle length, QRS duration, left ventricular ejection fraction or left ventricular diastolic volume (p = NS).
  
  
  CONCLUSIONS
  Internal cardioversion shocks delivered late in the QRS complex during ventricular tachycardia are more effective and have a lower risk of ventricular tachycardia acceleration than those delivered near QRS onset.
authors.affiliation: Department of Medicine, University of Western Ontario, London, Canada.
authors.forename: H G
authors.initials: HG
authors.lastname: Li
authors.forename: R
authors.initials: R
authors.lastname: Yee
authors.forename: R
authors.initials: R
authors.lastname: Mehra
authors.forename: P
authors.initials: P
authors.lastname: DeGroot
authors.forename: G J
authors.initials: GJ
authors.lastname: Klein
authors.forename: M
authors.initials: M
authors.lastname: Zardini
authors.forename: R K
authors.initials: RK
authors.lastname: Thakur
authors.forename: C A
authors.initials: CA
authors.lastname: Morillo
mesh_terms: D000368:Aged; D016009:Chi-Square Distribution; D004554:Electric Countershock / Q000009:adverse effects / Q000379:methods*; D004562:Electrocardiography; D004594:Electrophysiology; D005260:Female; D006352:Heart Ventricles / Q000503:physiopathology; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D012044:Regression Analysis; D017180:Tachycardia, Ventricular / Q000175:diagnosis / Q000503:physiopathology / Q000628:therapy*; D013997:Time Factors; D016896:Treatment Outcome; D014693:Ventricular Fibrillation / Q000453:epidemiology / Q000209:etiology