Abstract: Features of the ECG P-wave remain largely unexplained. P-wave notching is thought to relate to distinct events of right and left atrial activations. Wandering pacemaker is known to give rise to distinct P wave morphologies. In this study, we develop a 3D computational model of the human atria that is incorporated into a 3D human torso model which can simulate realistic P-waves across the torso. The atrial model considered detailed anatomical structure and electrophysiological heterogeneity. The torso model is used to produce ECG P-waves corresponding to those of the standard 12 lead ECG as well as a more detailed 64 lead electrode vest. The model is validated both in terms of the atrial activation sequence and the P-waves produced. The 12 lead ECG is then used to detect the presence of acetylcholine and isoprenaline on the atria. The 64 lead vest simulation is used to confirm that the presence of distinct P-wave modes in a clinical ECG is likely due to the presence of a leading pacemaker site shift, which may be caused by acetylcholine.