A defibrillation system for treating cardiac arrhythmia comprising: a defibrillator having a charge storage device (304, 800) for administering electrotherapy to a patient through at least two electrodes (106, 808, 810); a memory associated with the defibrillator and configured to store instructions for operation of the defibrillator; and a processor associated with the defibrillator and configured to execute the instructions to perform operations comprising: providing (502, 504, 1302, 1402) an alternating electrical signal to a patient through the at least two electrodes (106, 808, 810), estimating (510, 512, 1304, 1404) the transthoracic impedance of the patient from the alternating electrical signal sampled immediately prior to charging the charge storage device (304, 800), charging (514, 606, 1310) the charge storage device to a target voltage level based on the estimated transthoracic impedance value, selecting (514, 1308) a schedule for applying resistance values during the discharge of the charge storage device located external to the patient, and executing the schedule for applying resistance values during the discharge of the charge storage device to control the shape of the waveform (700, 704) during application of electrotherapy to the patient. a defibrillator having a charge storage device (304, 800) for administering electrotherapy to a patient through at least two electrodes (106, 808, 810); a memory associated with the defibrillator and configured to store instructions for operation of the defibrillator; and a processor associated with the defibrillator and configured to execute the instructions to perform operations comprising: providing (502, 504, 1302, 1402) an alternating electrical signal to a patient through the at least two electrodes (106, 808, 810), estimating (510, 512, 1304, 1404) the transthoracic impedance of the patient from the alternating electrical signal sampled immediately prior to charging the charge storage device (304, 800), charging (514, 606, 1310) the charge storage device to a target voltage level based on the estimated transthoracic impedance value, selecting (514, 1308) a schedule for applying resistance values during the discharge of the charge storage device located external to the patient, and executing the schedule for applying resistance values during the discharge of the charge storage device to control the shape of the waveform (700, 704) during application of electrotherapy to the patient. providing (502, 504, 1302, 1402) an alternating electrical signal to a patient through the at least two electrodes (106, 808, 810), estimating (510, 512, 1304, 1404) the transthoracic impedance of the patient from the alternating electrical signal sampled immediately prior to charging the charge storage device (304, 800), charging (514, 606, 1310) the charge storage device to a target voltage level based on the estimated transthoracic impedance value, selecting (514, 1308) a schedule for applying resistance values during the discharge of the charge storage device located external to the patient, and executing the schedule for applying resistance values during the discharge of the charge storage device to control the shape of the waveform (700, 704) during application of electrotherapy to the patient.