Firm paddle pressure during defibrillation is important to minimize transthoracic impedance (TTI) and optimize trans-myocardial current. Previous European Resuscitation Council (ERC) guidelines recommended that 12 kg force should be used [1] and the more recent International Liaison Committee on Resuscitation (ILCOR) guidelines recommend 'firm' pressure [2]. The actual relationship between force and transthoracic impedance has not been established.

55 patients (36 male, 19 female) undergoing general anaesthesia for cardiac surgery were investigated. Male chests were shaved. A pair of gel pads and paddles were placed in the anterior-apical position according to the ILCOR guidelines. The endotracheal tube was opened to air and paddles instrumented with force sensors were applied to the chest wall with progressive force to a maximum of 12 kgf. The resultant TTI was recorded at 10 measurements per second to a hard drive.

Light pressure results in a high TTI that rapidly decreases as paddle force increases (Fig. 1). An 18.7% fall in overall TTI occurs when applying force from 0.5 to 6.0 kg. Increasing paddle force from 6 to 12 kg results in a further decrease in TTI of 3.2%.

A minimum of 6 kg force applied to each paddle is necessary to achieve 83% of the overall decrease in TTI seen at 12 kgf. Further decrease in TTI as force is applied in excess of 6 kgf is small and a force of 12 kgf only reduces TTI by a further 3.2% compared with TTI at 6 kgf. The additional clinical benefit of applying 12 kgf as opposed to 6 kgf is questionable.