Effects of respiratory gas composition on transthoracic impedance

The transthoracic current generated during defibrillation comprises a cardiac and extracardiac component. Changes in impedance of transthoracic pathways will alter the transmyocardial current and may thus affect the outcome from defibrillation. The lungs comprise a large component of extracardiac tissue but the effects of different respiratory gases on transthoracic impedance has not been documented. We therefore measured transthoracic impedance (TTI) using different respiratory gas mixtures. TTI across self-adhesive defibrillation pads placed in the antero-apical position was measured at end-expiration using a 30 kHz low amplitude AC current. Ten healthy subjects aged 22-34 years (eight male) were studied whilst breathing alternate mixtures of air, 100% oxygen. 70% helium in 30% oxygen, and 70% nitrous oxide in 30% oxygen administered in a random sequence. There was no significant difference in TTI between any of the four respiratory gases. Therefore transthoracic current during defibrillation is unlikely to be affected by different respiratory gases. This is the first study to document that these respiratory gases do not change the impedance of transthoracic pathways.