An electrically non-conductive back plate for supporting the back of a patient in a supine position comprises on its front face facing the back of the patient a pair or more of ECG electrodes capable of maintaining electrically conductive skin contact with the patient's back. The pair of ECG electrodes is disposed at a sharp angle with the mean heart vector, in particular so as to form an angle β of about 45°±25° with the projection S of the patient's spine on the back plate. Also disclosed is an ECG electrode for mounting in a recess of the back plate and an apparatus for treating cardiac arrest by compression of the sternum comprising the back plate.