Earlier defibrillators had a monophasic wave form, meaning that a single pulse of energy in one direction was delivered. Now it is known that biphasic wave form is more effective in defibrillating the heart. A biphasic wave form has two phases of opposite polarity. 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.