1. A medical device for diagnosing a cardiac rhythm during a performance of a cardio-pulmonary resuscitation (CPR) comprising: a front end operable to obtain two or more time-sequential electrocardiogram (ECG) data sets comprised of a first unfiltered ECG data set and a second unfiltered ECG data set; an input operable to acquire two or more time-sequential CPR reference signal data sets which correspond in time to the time-sequential ECG data sets; a filter in communication with both of the front end and the input, operable to obtain a first filtered ECG data set and a second filtered ECG data set; a shock classifier operable to classify each of the filtered and unfiltered ECG data sets as a “shock” advice or a “no-shock” advice; a comparator operable to generate a decision based on the classified filtered and unfiltered ECG data sets; an output generator for issuing an operational command based on the decision; and a reliability analyzer operable to generate an indication of the reliability of each of the shock classifier advices based on the respective ECG data set; wherein the comparator is further operable to generate the decision based on the reliability analyzer indications, and wherein the reliability indicator indicates that the respective ECG data set is either reliable or unreliable, and wherein the issuing act issues an arm operational command only if at least one reliability indicator indicates a reliable ECG data set.