title: Intrathoracic impedance preceding ventricular tachyarrhythmia episodes.
abstract: BACKGROUND
  Heart failure is associated with ventricular tachyarrhythmias (VT/VF). Fluid accumulation during worsened heart failure may trigger VT/VF. Increased intrathoracic impedance has been correlated with fluid accumulation during heart failure. Implanted defibrillators capable of daily measures of intrathoracic impedance allow correlation of impedance with occurrence of VT/VF. We hypothesized that VT/VF episodes are preceded by decreases in intrathoracic impedance. The goal was to identify the relationship of intrathoracic impedance measured by implanted cardioverter defibrillators to the occurrence of VT/VF.
  
  
  METHODS
  Implanted defibrillator follow-up data were obtained retrospectively. Those with Medtronic OptiVol (Medtronic Inc., Minneapolis, MN, USA), storing averaged daily and reference impedance values, were reviewed for VT/VF episodes. Impedance changes in the week leading up to VT/VF were analyzed.
  
  
  RESULTS
  A total of 317 VT/VF episodes in a cohort of 121 patients' follow-up data were evaluated. Averaged daily intrathoracic impedance declined preceding 64% of VT/VF episodes, with an average decline of 0.46 +/- 0.35 Ohms from the day before the VT/VF episodes. However, the mean values of the averaged daily and reference impedance did not change significantly. A novel measure, DeltaTI, the sum of the daily differences between the averaged daily and reference impedance, was negative preceding 66% of VT/VF episodes (P < 0.001). The mean DeltaTI was -4.0 +/- 1.3 Ohms, which was significantly lower than the theoretically expected value of zero Ohms (P < 0.01).
  
  
  CONCLUSIONS
  (1) Averaged daily impedance declined preceding 64% of VT/VF episodes, but the overall decline was of small magnitude; (2) a novel measure, DeltaTI, was negative preceding 66% of VT/VF episodes, and significantly below zero.
authors.affiliation: Washington Veterans Affairs Medical Center, Washington, DC, USA. Hans.Moore@VA.Gov
authors.forename: Hans J
authors.initials: HJ
authors.lastname: Moore
authors.forename: Matthew N
authors.initials: MN
authors.lastname: Peters
authors.forename: Michael R
authors.initials: MR
authors.lastname: Franz
authors.forename: Pamela E
authors.initials: PE
authors.lastname: Karasik
authors.forename: Steven N
authors.initials: SN
authors.lastname: Singh
authors.forename: Ross D
authors.initials: RD
authors.lastname: Fletcher
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D002307:Cardiography, Impedance; D017147:Defibrillators, Implantable; D005260:Female; D006333:Heart Failure / Q000175:diagnosis / Q000209:etiology; D006801:Humans; D008297:Male; D008875:Middle Aged; D017180:Tachycardia, Ventricular / Q000150:complications / Q000503:physiopathology* / Q000628:therapy; D014693:Ventricular Fibrillation / Q000503:physiopathology / Q000628:therapy