title: In vitro assessment of induced phrenic nerve cryothermal injury.
abstract: BACKGROUND
  Phrenic nerve injury, both left and right, is considered a significant complication of cryoballoon ablation for treatment of drug-refractory atrial fibrillation, and functional recovery of the phrenic nerve can take anywhere from hours to months.
  
  
  OBJECTIVE
  The purpose of this study was to focus on short periods of cooling to determine the minimal amount of cooling that may terminate nerve function related to cryo ablation.
  
  
  METHODS
  Left and/or right phrenic nerves were dissected from the pericardium and connective tissue of swine (n = 35 preparations). Nerves were placed in a recording chamber modified with a thermocouple array. This apparatus was placed in a digital water bath to maintain an internal chamber temperature of 37°C. Nerves were stimulated proximally with a 1-V, 0.1-ms square wave. Bipolar compound action potentials were recorded proximal and distal to the site of ablation both before and after ablation, then analyzed to determine changes in latency, amplitude, and duration. Temperatures were recorded at a rate of 5 Hz, and maximum cooling rates were calculated.
  
  
  RESULTS
  Phrenic nerves were found to elicit compound action potentials upon stimulation for periods up to 4 hours minimum. Average conduction velocity was 56.7 ± 14.7 m/s preablation and 49.8 ± 16.6 m/s postablation (P = .17). Cooling to mild subzero temperatures ceased production of action potentials for >1 hour.
  
  
  CONCLUSIONS
  Taking into account the data presented here, previous publications, and a conservative stance, during cryotherapy applications, cooling of the nerve to below 4°C should be avoided whenever possible.
authors: Department of Biomedical Engineering; Department of Surgery.
authors: Ryan P
authors: RP
authors: Goff
authors: Department of Genetics, Cell Biology and Development.
authors: Stephanie M
authors: SM
authors: Bersie
authors: Department of Surgery; Institute for Engineering in Medicine at the University of Minnesota, Minneapolis, Minnesota. Electronic address: iaizz001@umn.edu.
authors: Paul A
authors: PA
authors: Iaizzo
mesh_terms: D000818:Animals; D001281:Atrial Fibrillation / Q000601:surgery*; D003452:Cryosurgery / Q000009:adverse effects*; D004195:Disease Models, Animal; D006329:Heart Conduction System / Q000601:surgery*; D008297:Male; D010791:Phrenic Nerve / Q000293:injuries* / Q000503:physiopathology; D011183:Postoperative Complications; D011379:Prognosis; D020127:Recovery of Function; D013552:Swine
keywords: Atrial fibrillation; Compound action potential; Cryoballoon ablation; Phrenic nerve injury; Phrenic thermal tolerance