A medical device and method conserve electrical power used in monitoring cardiac arrhythmias. The device includes a sensing circuit configured to sense a cardiac signal, a power source and a control circuit having a processor powered by the power source. The control circuit is configured to operate in a normal state by waking up the processor to analyze the cardiac electrical signal for determining a state of an arrhythmia. The control circuit switches from the normal state to a power saving state that includes waking up the processor at a lower rate than during the normal state.