Achieving an adequate defibrillation safety margin can sometimes be troublesome. Today’s implantable defibrillators allow for the programming of vector and polarity to overcome some of these issues. Some devices also allow the programming of the pulse width of each phase of the defibrillation waveform. This case illustrates the value of proper programming of the first and second phase of the waveform as an aid in achieving adequate safety margins. The measurement of a surrogate cardiac membrane time constant can be utilized in the selection of the proper pulse widths.