An external defibrillator system is configured with at least two different algorithms for determining the duration of a shock administered to a patient being treated and selects the algorithm based on one or more patient parameters such as, for example, the patient's TTI. The patient's TTI can be measured prior to or while the shock is being administered to the patient. The shock can be, for example, a multiphasic defibrillation or a multiphasic cardioversion shock. The charge voltage of the system's energy storage device can additionally be varied depending on the one or more patient parameters. For example, the system may charge the energy storage device so that the charge voltage is higher or lower than a nominal charge voltage responsive to the patient's TTI is higher or lower compared to an average TTI, respectively.