Difference Between a Biphasic and Monophasic Defibrillators

The direct current shock given can have a monophasic or biphasic waveform. In monophasic shock, the shock is given in only one direction from one electrode to the other. In a biphasic shock, initial direction of shock is reversed by changing the polarity of the electrodes in the latter part of the shock. Usually the initial voltage applied is higher than the reversed polarity shock. Biphasic waveforms were initially developed for use in implantable cardioverter defibrillators (ICD) and later adapted to external defibrillators. Biphasic truncated exponential waveform and rectilinear biphasic waveform are two types of biphasic waveforms used by different manufacturers. Defibrillators can sense the thoracic impedance and increase or decrease their internal resistance so that the selected level of energy is delivered to the subject. Biphasic shocks are more effective than monophasic shocks and need lesser energy. Typically, when 360 Joules are delivered for defibrillation in a monophasic defibrillator, 200 Joules are given in a biphasic defibrillator. This could theoretically reduce the potential damage to the heart muscle by the high voltage shock. The proposed mechanism is that a single monophasic wave of energy is not able to depolarize all the myocardial cells. Some cells close to the electrode gets too much energy while those away from the electrode gets too little. Reversing the polarity helps to sweep off these cells as well. This response is sometimes called a ‘burping’ response of a biphasic waveform.