title: Effect of first-phase polarity of biphasic shocks on defibrillation threshold with a single transvenous lead system.
abstract: OBJECTIVE
  The purpose of this study was to determine whether the polarity of the first phase of a biphasic shock affects the defibrillation threshold.
  
  
  BACKGROUND
  The polarity of a monophasic shock has been shown to affect the defibrillation threshold.
  
  
  METHODS
  A transvenous defibrillation lead with distal and proximal shocking electrodes was used in this study. In 15 consecutive patients, the defibrillation threshold was determined twice using a step-down protocol, in random order: with the distal coil as the anode for the initial phase (anodal biphasic shock) and with the polarity reversed (cathodal biphasic shock). The power to detect a 5.0-J difference in this study is 0.96. These patients were 61 +/- 11 years old (mean +/- SD), and the mean left ventricular ejection fraction was 0.32 +/- 0.10.
  
  
  RESULTS
  Mean defibrillation threshold using anodal biphasic shocks was 9.9 +/- 4.8 J, compared with 9.5 +/- 4.2 J using cathodal biphasic shocks (p = 0.8). In three patients the defibrillation threshold was lower by a mean of 6.3 +/- 2.9 J with the former configuration; in three patients the defibrillation threshold was lower by a mean of 6.7 +/- 2.5 J with the latter configuration; and in nine patients it was the same. Using the standard cathodal configuration, a defibrillation threshold < or = 10 J was obtained in approximately 70% of patients, and a subcutaneous patch was not required in any patient.
  
  
  CONCLUSIONS
  The polarity of the first phase of a biphasic shock used with a single transvenous lead does not affect the defibrillation threshold.
authors.affiliation: Department of Internal Medicine, University of Michigan Medical Center, Ann Arbor 48109-0022, USA.
authors.forename: S A
authors.initials: SA
authors.lastname: Strickberger
authors.forename: K C
authors.initials: KC
authors.lastname: Man
authors.forename: E
authors.initials: E
authors.lastname: Daoud
authors.forename: M P
authors.initials: MP
authors.lastname: Neary
authors.forename: L E
authors.initials: LE
authors.lastname: Horwood
authors.forename: M
authors.initials: M
authors.lastname: Niebauer
authors.forename: J D
authors.initials: JD
authors.lastname: Hummel
authors.forename: F
authors.initials: F
authors.lastname: Morady
mesh_terms: D017147:Defibrillators, Implantable; D004554:Electric Countershock / Q000379:methods*; D004567:Electrodes, Implanted; D004867:Equipment Design; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D014693:Ventricular Fibrillation / Q000503:physiopathology / Q000628:therapy*