A method and system for monitoring atrial activation in a patient is provided. In the method, a moment of occurrence and a wave form are predicted for a subsequent P wave based on a sequence of P waves already received in ECG signal data obtained from the patient. The process then checks whether the subsequent P wave really occurs in the ECG signal data within a first time window around the predicted moment of occurrence. If this is the case, the process further monitors whether the P wave is followed by a QRS complex in the ECG signal data.