in an icd , a highly efficient biphasic defibrillation pulse is generated by switching at least two charged capacitors from a parallel connection to various combinations of a parallel / series connection or a series connection during the first phase of the defibrillation pulse . such mid - stream parallel / series connection changes of the capacitors and steps up the voltage applied to the cardiac tissue during the first phase . a stepped - up voltage during the first phase , in turn , gives an extra boost to , and thereby forces additional charge into , the cardiac tissue cells , and thereby transfers more charge to the membrane of the excitable cardiac cell than if the capacitors were continuously discharged in series . phase reversal is timed with the cell membrane reaching its maximum value at the end of the first phase .