An electrotherapy apparatus performs a low level impedance measurement upon the patient to determine the initial charge level on the capacitor used to deliver an electrotherapy waveform to the patient. In addition, the waveform applied to the patient is dynamically controlled to compensate for patient impedance variability. Determining the initial charge level in this manner prevents unnecessarily high peak currents from flowing in low impedance patients while maintaining peak current in high impedance patients at therapeutically beneficial levels. The electrotherapy apparatus includes a measuring device used for measuring a parameter related to the impedance of the patient. The parameter is used for determining low level patient impedance. The measuring device provides a voltage output used by a controller for determining the initial charge level of the capacitor. A first embodiment of a first electrotherapy apparatus includes four electronic switches to deliver a bi-phasic waveform to the patient. The first embodiment of the first electrotherapy apparatus further includes a comparator coupled to a timer to monitor the voltage across the capacitor and control the termination or extension of the waveform applied to the patient. A first embodiment of a second electrotherapy apparatus measures the charge delivered to the patient and sets the time of the first phase of the bi-phasic pulse based upon the time required to deliver a predetermined amount of charge to the patient.