Systems, methods, and devices for detecting or confirming fibrillation are discussed. In one example, a method for detecting a cardiac arrhythmia of a patients' heart comprises receiving, by a leadless cardiac pacemaker fixed in the patients' heart, an indication from a remote device that a cardiac arrhythmia is detected, monitoring by the leadless cardiac pacemaker a signal generated by a sensor that is located within the patients' heart, and based at least in part on the monitored signal, confirming whether a cardiac arrhythmia is occurring or not. In some embodiments, the method may further comprise, if a cardiac arrhythmia is confirmed, delivering a therapy to treat the cardiac arrhythmia.