['Thoracic impedance (TI) influences the success of external cardioversion (ECV) or defibrillation because current intensity traversing the heart is inversely related to TI. Experimental data suggest that TI decreases after multiple shocks. We undertook a clinical study to determine changes of TI values in patients with atrial fibrillation or flutter requiring ECV.', 'We enrolled 222 consecutive patients (age 73 +/- 11 years; males 67%; body weight 75 +/- 13 kg) who underwent ECV between January 2004 and February 2007. Biphasic shocks were delivered through adhesive pads placed in the anteroposterior position. The initial energy was set at 1 J/kg, with progressive increases up to a maximum of 180 J in case of failure. In the last 39 elective patients, plasma concentration of interleukin-6 (IL-6) and tumor necrosis factor (TNF)-alpha were determined before and 6 hours after ECV.', 'Sinus rhythm was restored in 202 patients (91.0%). Of these, 155 (69.8%) required more than one shock (on average, 2.5 +/- 1.5 shocks/patient). Final values of energy and peak current intensity were 136 +/- 47 J and 50 +/- 14 A, respectively. TI decreased significantly by 6.2% from baseline after > or =2 shocks (P < 0.001). The absolute reduction was correlated with baseline TI, number of delivered shocks, and hemoglobin oxygen saturation. IL-6 and TNF-alpha increased with ECV (P < 0.001 and P = 0.014, respectively).'] Do multiple shocks affect thoracic electrical impedance during external cardioversion of atrial fibrillation?