A method for managing care of a person receiving emergency cardiac is disclosed and involves monitoring, with an external defibrillator, multiple parameters of the person receiving emergency cardiac assistance; determining from at least one of the parameters, an indication of trans-thoracic impedance of the person receiving emergency cardiac care; determining, from at least one of the parameters corresponding to an electrocardiogram of the person receiving emergency cardiac assistance, an initial indication of likely shock effectiveness; determining, as a function of at least the indication of trans-thoracic impedance and the initial indication of likely shock effectiveness, an indication of whether a shock provided to the person receiving emergency medical assistance will be effective; and affecting control of the defibrillator by a caregiver as a result of determining the indication of whether a shock will be effective.