How often should defibrillation pads be changed?: the effect of evaporative drying

In order to minimise transthoracic impedance (TTI) during defibrillation, water-based pads are used to improve conductivity between metal defibrillation paddles and skin. Subjectively, these pads appear to dry very quickly; an effect that may lead to an increase in TTI due to poorer conduction between paddles and skin. This study was carried out to assess the effect of evaporative drying of defibrillation pads on TTI.

TTI was measured at 5-10 min intervals in 20 adult male volunteers across 3M defibrillation pads (2346N) placed in the anterior-apical position. Measurements of TTI were made at 30 kHz using a Bodystat MultiScan 5000 monitor at end-expiration. A third pad was placed on the left precordium and its mass recorded each time a TTI measurement was made.

The median age of subjects was 22 years (range 21-52 years) and their median body mass index was 23.1 kg m(-2) (range 18.4-42.8 kg m(-2)). Median room temperature was 23.0 degrees C (range 19.0-24.0 degrees C) and the median humidity was 31.0% (range 28.0-48.0%). 3M defibrillation pads had an initial mean mass of 25.14 g (S.D. +/- 0.41 g). Changes in defibrillation pad mass due to evaporative loss occurred immediately and rapidly, with all measurements being significantly lower than the baseline value. Mean baseline TTI was 63.6 ohms (S.D. +/- 13.7 ohms). After 30 min a statistically significant (P = 0.012) rise of 1.4 ohms (2.2%), was observed corresponding to a 12.6% decrease in pad mass, after which TTI continued to increase in a linear fashion.

In the absence of a defibrillation current. 3M defibrillation pads can safely be left on the chest wall for at least 25 min in a typical hospital environment before evaporative drying results in a significant increase in transthoracic impedance.