Influence of Ventilation Phase on Transthoracic Impedance and Defibrillation Effectiveness

Ewy GA, Hellman DA, McClung S, Taren D Critical Care Medicine 1980 Pages 164-166

The influence of the phase of ventilation on the transthoracic impedance and defibrillation success was studied in 6 mongrel dogs. Defibrillation was attempted after 1 min of ventricular fibrillation. The initial stored energy levels were 20 watt-sec for the first two shocks, one delivered in inspiration and one in expiration. If the shock at the initial energy level was not successful, the energy level was increased by 10 watt-sec before the next shock. Initial discharge was given at inspiration in half of the animals and at expiration in the other half. The transthoracic impedance to defibrillator discharge was measured with each shock. The study revealed a significantly higher transthoracic impedance with inspiration (76.3 +/- 13 ohms vs. 68.4 +/- 12 ohms expiration, P<0.01), and a significant decrease in defibrillation success rate when shocks were delivered in inspiration (10%) compared to expiration (50%). The authors conclude that the phase of ventilation is an important determinant of impedance and has a significant influence on defibrillation effectiveness particularly when marginal energy levels are used. Recent studies of external thoracic compression have shown that aortic systolic pressure and carotid blood flow significantly increases when intrathoracic pressures were increased by maintaining lungs fully inflated during chest compressions. If these observations result in altered recommendations for external cardiac massage, it will be important that the pressure be decreased and the patient be in forced expiration before defibrillation attempts.