The aim of the study was to evaluate electrocardiographic changes in patients with pulmonary arterial hypertension (PAH) and to refer them to the heart structure and hemodynamics of pulmonary circulation. In the study, the novel mechanism of qR in V1 was proposed, that is a change in heart geometry. In the era of modern therapy of pulmonary arterial hypertension, the presence of qR in V1 is still associated with a higher risk of death in long-term observation. The prevalence of supraventricular arrhythmias in patients with PAH is high and is associated with the size of the right atrium. Index of the right atrium area was a predictor of the occurrence of clinically significant arrhythmias requiring hospitalization. In addition, it was shown that 12-lead ECG can be used in the assessment of the effectiveness of the PAH specific treatment. The reduction of the RV1, max RV1,2 + max S1,aV1. - SV1 and PII amplitudes reflects the improvement in pulmonary hemodynamics. Among these changes, reduction of the R v1 amplitude is associated with a better survival.