In an aspect, a system for treating a patient in cardiac arrest is described and includes memory, one or more electronic ports for receiving signals from sensors for obtaining indications of an electrocardiogram (ECG) of the patient, one or more sensors for obtaining a transthoracic impedance of the patient, and a patient treatment module executable on one or more processing devices that is configured to generate, from the ECG, transform values that represent magnitudes of two or more frequency components of the ECG, and modify, based on at least one transform value, at least one shock delivery parameter.
