BACKGROUND Two mechanisms by which firm external paddle force decreases transthoracic impedance (TTI) have been proposed. Decreased impedance at the paddle-skin interface has been assumed to be the primary mechanism, but expulsion of air from the lungs, reducing lung volume is also likely to contribute. The relative contribution of each mechanism is unknown. RESULTS Thirty five intubated patients undergoing general anaesthesia for cardiac surgery were studied. TTI across external defibrillation paddles was measured as paddle force was increased to 12kgf. Measurements were performed twice; once allowing the volume of the lungs to change and once with lung volume held at functional residual capacity. TTI with constant lung volume was significantly higher at (P< 0.001), confirming that a reduction in lung volume contributes to the decrease in TTI. At an optimal paddle force of 8kg, the reduction in lung volume contributed to 16.2% of the overall decrease in TTI. CONCLUSIONS The decrease in TTI seen with increasing external paddle force is due primarily to improved electrical contact at the paddle-skin interface, with a decrease in thoracic volume accounting for no more than 16% of the overall decrease at forces used clinically.