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The aim of our study was to assess the arrhythmic profile in patients with primary pulmonary hypertension (PPH) and its correlation with autonomic features, echocardiographic indexes and pulmonary function. We studied 9 subjects with a mean age of 42 ± 11 years. All underwent echocardiography, 24-hour Holter monitoring, and cardiopulmonary exercise testing. Left ventricle ejection fraction was normal (65 ± 6%). The right ventricle end diastolic volume was increased (108 ± 32 ml/m²) with a slight reduction of ejection fraction (49 ± 5%). Right ventricle systolic pressure was increased (91 ± 25 mmHg). Heart rate variability analysis showed evidence of a reduced standard deviation of all NN intervals (SDNN) compared with the control group (102.8 ± 32 versus 156.1 ± 32, p < 0.005). Patients with significant ventricular arrhythmias had a lower SDNN, and lower baseline and effort PO[sub 2] (SDNN: 87.0 ± 15 versus 115.4 ± 38; baseline PO[sub 2]:63.2 ± 12% versus 78.8 ± 7%; effort PO[sub 2]: 50.7 ± 13% versus 68.7 ± 19%). The patients with SDNN lower than 90 ms were characterized by a higher right ventricle systolic pressure (115.0 ± 22.9 mmHg versus 79.2 ± 17.8 mmHg, p = 0.035). The patients who experienced syncope had higher SDNN (131.7 ± 36 versus 88.4 ± 20, p < 0.05), higher effort PO[sub 2] (77.5 ± 14 mmHg versus 52.3 ± 14 mmHg, p < 0.03). The patients with PPH evidenced an increased sympathetic activity. Premature ventricular beats were more frequent in those subjects with higher adrenergic drive and lower oxygen saturation. Patients with episodes of syncope seem to have a relatively higher vagal activity, and effective mechanisms of adjustment in blood oxygenation during effort. Key words: Primary pulmonary hypertension Autonomic nervous system;Heart rate variability;Ventricular arrhythmias. [ABSTRACT FROM AUTHOR]