An external defibrillator system is provided. The system includes: graphical display; one or more sensors for obtaining CO2 data for a patient over a time interval; one or more sensors for obtaining information about a patient's heart activity over the time interval; and a controller. The controller can be configured to: determine a CO2 waveform over the time interval from the obtained CO2 data; determine a filtered electrocardiogram trace from the obtained information about the patient's heart activity; and display on the graphical display the CO2 waveform and the filtered electrocardiogram trace. A method for monitoring patient activity using an external defibrillator is also provided.