title: Changes in transthoracic impedance during sequential biphasic defibrillation.
abstract: BACKGROUND
  Sequential monophasic defibrillation reduces transthoracic impedance (TTI) and progressively increases current flow for any given energy level. The effect of sequential biphasic shocks on TTI is unknown. We therefore studied patients undergoing elective cardioversion using a biphasic waveform to establish whether this is a phenomenon seen in the clinical setting.
  
  
  METHODS
  Adults undergoing elective DC cardioversion for atrial flutter or fibrillation received sequential transthoracic shocks using an escalating protocol (70J, 100J, 150J, 200J, and 300J) with a truncated exponential biphasic waveform. TTI was calculated through the defibrillator circuit and recorded electronically. Successful cardioversion terminated further defibrillation shocks.
  
  
  RESULTS
  A total of 58 patients underwent elective cardioversion. Cardioversion was successful in 93.1% patients. First shock TTI was 92.2 [52.0-126.0]Omega (n=58) and decreased significantly with each sequential shock. Mean TTI in patients receiving five shocks (n=5) was 85.0Omega.
  
  
  CONCLUSIONS
  Sequential biphasic defibrillation decreases TTI in a similar manner to that seen with monophasic waveforms. The effect is likely during defibrillation during cardiac arrest by the quick succession in which shocks are delivered and the lack of cutaneous blood flow which limits the inflammatory response. The ability of biphasic defibrillators to adjust their waveform according to TTI is likely to minimise any effect of these findings on defibrillation efficacy.
authors.affiliation: Shackleton Department of Anaesthetics, Southampton University Hospital NHS Trust, Tremona Road, Southampton SO16 6YD, UK. charlesdeakin@doctors.org.uk
authors.forename: Charles D
authors.initials: CD
authors.lastname: Deakin
authors.forename: Jonathan J S
authors.initials: JJ
authors.lastname: Ambler
authors.forename: Steven
authors.initials: S
authors.lastname: Shaw
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000704:Analysis of Variance; D001281:Atrial Fibrillation / Q000503:physiopathology / Q000628:therapy*; D001282:Atrial Flutter / Q000503:physiopathology / Q000628:therapy*; D002307:Cardiography, Impedance / Q000379:methods*; D004554:Electric Countershock / Q000379:methods*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D016896:Treatment Outcome