Abstract: Introduction: pulmonary arterial hypertension (PAH) is a pathophysiological syndrome that can occur in a variety of clinical conditions. Percutaneous balloon dilatation and stent implantation are methods for creating or expanding atrial communication in a variety of conditions to improve cardiac output. It should be kept in mind that creation of an inadequate size of the shunt leads to an excess of right-left shunt, worsening of pulmonary blood flow, severe hypoxemia, and acute left ventricular failure. Possibility of a calculated determination of required size of shunt in the interatrial septum will increase the effectiveness and safety of atrioseptostomy, which is especially important in this severe category of patients. Aim: to substantiate a method of determining of optimal diameter of the atrial communication during atrioseptostomy in patients with PAH for increase of exercise tolerance, prevention of syncope and reducing the risk of sudden death. Materials and methods: the choice of the diameter of the interatrial communication during atrioseptostomy operation in patients with PAH is as follows: before the operation, patient undergoes an invasive measurement of pressure in right and left atrium and determination of stroke volume of left ventricle. Then calculation the diameter of the interatrial communication according to the formula is performed. We performed calculation according to presented formula in 4 patients with PAH. In 2 patients, a fenestrated occluder was implanted, in 1 patient atrial septum stenting was performed, and 1 patient underwent open atrioseptostomy. Results: in all patients after atrioseptostomy, an improvement in quality of life was observed: decreased dyspnea, increased exercise tolerance, decreased edema of lower limbs, and the absence of syncopal conditions. Thus, after the operation, there was a positive dynamics in clinical status of patients, indicators of test with a six-minute walk, as well as changes in echocardiographic indicators: a decrease in the size of the right ventricle and square area of right atrium, an increase in the end-diastolic size of the left ventricle, which indicates an improvement in function of both ventricles. Conclusion: a mathematical model based on principles of intracardiac hemodynamics, demonstrates the importance of choosing of size of foramen to create a certain Qp/Qs. Size of foramen, depending on the pressure in atrium, in conditions of high pulmonary hypertension has a small range of values (from 6 to 8 mm). Therefore, the use of the 7 mm size, previously obtained empirically by other authors, is physically justified. 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