Transthoracic defibrillation: does electrode adhesive pad position alter transthoracic impedance?

Successful termination of ventricular fibrillation by transthoracic shocks is dependent on achieving adequate current flow, which in turn is governed by transthoracic impedance (TTI). The American Heart Association (AHA) Advanced Cardiac Life Support textbook recommends three electrode positions for defibrillation: (1) anterior-apex, (2) apex-posterior and (3) anterior-posterior. However, there are few data available comparing TTI of these positions. To study this, we applied large (78 cm 2) self-adhesive monitor-defibrillator pads to 20 subjects (ten male, ten female, ages 21–79) and measured TTI using a validated test-pulse technique which does not require actual shocks. We performed two studies. In Study 1 (all 20 subjects) the electrode pads were applied in the three positions recommended by the AHA, with the posterior electrode placed in the right infrascapular location. All TTI measurements were made at end-expiration and body surface area (BSA) was recorded. The results (TTI, Ω, mean±S.D.) for the respective positions were the following: anterior-apex, 82.0±24.7; apex-posterior, 71.2±23.5; anterior-posterior, 77.0±24.7 ( P NS). In Study 2 (six subjects) we compared the effect of right vs. left infrascapular posterior electrode placement (TTI, Ω): apex-right infrascapular (RIS), 76.8±18.4; apex-left infrascapular (LIS), 72.1±18.7; anterior-RIS, 72.5±19.4; anterior-LIS, 71.6±18.6 ( P NS). Correlation of TTI (anterior-apex placement) with BSA: TTI=15.9 (BSA)+46.7, r=0.60, P