A defibrillator (AED) using an ECG analysis model or algorithm which can function in two different modes. The ECG analysis model is particularly suited for analysis during periods of CPR. Both modes of operation reach a shock decision in essentially the same way, where one or more ECG data segments indicate a shockable cardiac condition. In one mode of operation, the shock decision is irreversible once made. In another mode of operation, the shock decision is reversible if one or more subsequent ECG data segments indicate a no-shock decision. Improved specificity of the model is obtained without undue degradation of sensitivity to shockable ECG.