A system and method for using an implantable cardiac stimulation device to monitor a patient for the progress of an existing condition and/or early detection of an emerging condition based, at least in part, on measuring and evaluating the timing characteristics of the patient's atrial activity. The atrial timing characteristics are used as indicators or predictors of conditions of interest, such as heart failure (HF) and atrial fibrillation (AF). In certain implementations, the system can determine discriminating indicators of a predominant underlying cause of a condition, such as between vagal and non-vagal AF, as an indicator of a suggested therapy. The system can store data corresponding to the observed atrial timing for trending analysis as well as transmit data for offline analysis, such as via an external device.