Alteration in transthoracic impedance following cardiac surgery

Introduction Haemodynamically significant ventricular tachyarrhythmias are a frequent complication in the immediate post-operative period after cardiac surgery. Successful cardioversion depends on delivery of sufficient current, which in turn is dependent on transthoracic impedance (TTI). However, it is uncertain if there is a change in TTI immediately following cardiac surgery using cardiopulmonary bypass (CPB). Methods TTI was measured on 40 patients undergoing first time isolated cardiac surgery using CPB. TTI was recorded at 30 kHz using Bodystat® Multiscan 5000 equipment before operation (with and without a positive end-expiratory pressure (PEEP) of 5 cm of H2O) and then at 1, 4 and 24 h  after the operation. Data was analyzed to determine the relationship between pre- and post-operative variables and TTI values. Results Mean pre-operative TTI was 54.5 ± 10.55 Ω without PEEP and 61.8 ± 15.4 Ω on a PEEP of 5 cm of H2O. TTI dropped significantly (p < 0.001) after the operation to 47.2 ± 10.6 Ω at 1 h, 42.6 ± 10.2 Ω at 4 h and 41.8 ± 10.4 Ω at 24 h. A positive correlation was noted between duration of operation and TTI change at 1 h (r = 0.38; p = 0.016). There was no significant correlation between the duration of bypass and change in TTI. Conclusion TTI decreases by more than 30% in the immediate post-operative period following cardiac surgery. This state may favour defibrillation at lower energy levels.