Severe pulmonary artery hypertension leads to acute dilation of both the right atrium and ventricle. Consequently, in the ECG we observe a prominent P-wave (lead II) and a shift of the heart axis to the right (S wave in lead I, Q-wave in lead III, R-S wave transition in V4-V5). Right ventricular wall tension (dysfunction) may cause ST-elevation in V1-V2 and/or T-wave inversion in V1-V4.