title: Biphasic waveforms prevent the chronic rise of defibrillation thresholds with a transvenous lead system.
abstract: OBJECTIVE
  The purpose of this study was to compare chronic changes in monophasic and biphasic defibrillation thresholds using a uniform transvenous lead system and testing protocol.
  
  
  BACKGROUND
  Defibrillation thresholds increase over time in patients with nonthoracotomy lead systems. This increase can result in an inadequate chronic defibrillation safety margin and could limit the safety of smaller pulse generators, which have a reduced maximal output. However, previous studies of the temporal changes of defibrillation thresholds evaluated complex lead systems or monophasic shock waveforms, neither of which are used with current technology.
  
  
  METHODS
  This study was a prospective, randomized assessment of the effects of shock waveforms on the changes of transvenous defibrillation thresholds over time. Paired monophasic and biphasic thresholds were measured both at implantation and at follow-up (250 +/- 105 days) in 24 consecutive patients who were not receiving antiarrhythmic drugs. The lead system was a dual-coil Endotak C lead, and reverse polarity shocks (distal coil = anode) were delivered.
  
  
  RESULTS
  Monophasic defibrillation thresholds increased from (mean +/- SD) 13.7 +/- 6.0 J to 16.8 +/- 6.7 J (p = 0.02), whereas biphasic thresholds were unchanged (10.4 +/- 4.3 J to 10.2 +/- 4.8 J, p = 0.86) in the same patients. Shock impedance chronically increased (47.0 omega to 50.5 omega, p = 0.02) and was unaffected by waveform.
  
  
  CONCLUSIONS
  These results indicate that biphasic shocks prevent the chronic increase in defibrillation thresholds with a transvenous lead system.
authors.affiliation: Department of Medicine, Division of Cardiology, University of Maryland School of Medicine, Baltimore, USA. MGold@heart.ab.umd.edu
authors.forename: M R
authors.initials: MR
authors.lastname: Gold
authors.forename: N G
authors.initials: NG
authors.lastname: Kavesh
authors.forename: R W
authors.initials: RW
authors.lastname: Peters
authors.forename: S R
authors.initials: SR
authors.lastname: Shorofsky
mesh_terms: D001281:Atrial Fibrillation / Q000503:physiopathology / Q000628:therapy; D002908:Chronic Disease; D017147:Defibrillators, Implantable; D004554:Electric Countershock / Q000379:methods*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D013997:Time Factors; D014693:Ventricular Fibrillation / Q000503:physiopathology / Q000628:therapy