Thoracic electric impedance and its changes after multiple shocks for external cardioversion of atrial fibrillation. A role for acute inflammatory response?

Background: Thoracic impedance (TI), according to current Resuscitation Guidelines, is one of the major determinants of success of external cardioversion (ECV) or defibrillation. In fact, the current intensity crossing heart chambers is inversely associated with TI. Moreover, old experimental data seem to suggest that TI decreases after multiple shocks, allowing an easier flow of current through the thorax. With this study we wanted to evaluate normal baseline values of TI and its changes in patients (pts) undergoing ECV. Methods: All the 193 pts (age: 73±9 years; men: 66%, BMI: 26.3±3.7 Kg/m2) who consecutively underwent ECV of atrial fibrillation or flutter between January 2004 and September 2006 were enrolled in the study. Biphasic shocks (Multipulse Biowave, Schiller/ESAOTE) were delivered trough adhesive pads placed in antero-posterior position. Initial energy was set at 1 Joule/Kg, with progressive increases at 130 and 180 Joule in case of failure of previous attempts of ECV. In a subgroup of patients, plasma concentrations of Interleukin-6 (IL-6) and Tumor Necrosis Factor (TNF) - alpha were evaluated at baseline and after 5 hours from ECV. Results: Mean TI was 57±12 Ohm; the variable had a normal distribution (p=0.624). Delivered shocks were 2.4±1.4, with 131 pts (67.9%) needing more than 1 shock. Final values of energy and of current intensity were 135±49 Joule and of 50±15 Ampere, respectively. TI significantly decreased after multiple shocks (57±12 vs 54±11 Ohm, - 5.7±3.3%, p<0.001), with the absolute reduction directly associated (R=0.640) with baseline TI (p<0.001) and the total amount of delivered energy (p<0.001). IL-6 (0.16±0.03 vs 1.00±1.70 pg/mL, p=0.016) and TNF-alpha (1.39±0.71 vs 2.48±4.13 pg/mL, p=0.015) significantly increased with ECV. Conclusions: TI, one of the determinants of success of ECV, significantly decreases after multiple shocks. At the same time, IL-6 and TNF-alpha increase their concentrations, possibly linking the changes in physical properties of thorax to acute inflammation.