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Impaired Blood Rheology in Pulmonary Arterial Hypertension. BACKGROUND: Understanding of the pathophysiologic manifestations of pulmonary arterial hypertension (PAH) is still evolving. The aims of the present study were to determine the alterations in blood rheology, and to investigate the relationship between those alterations and laboratory parameters in PAH. METHODS: The study included 21 consecutive treatment-naive patients with PAH and 32 age and sex-matched healthy controls. Patients were categorised in class II (n=6), class III (n=13), and class IV (n=2). All subjects underwent right-heart catheterisation. Erythrocyte deformability and aggregation were measured by an ektacytometer. RESULTS: Haemodynamic variables were as follows: the mean right atrial pressure: 9.94±5.76mmHg; the average pulmonary vascular resistance: 5.66±3 WU; Fick cardiac index: 4.15±2.75l/min/m2; and mixed venous O2 saturation: 64.59±12.53%. The average 6-minute walk distance was 351.09±133.08m. Erythrocyte deformability measured at 0.95, 3.00, and 5.33Pa was significantly lower, erythrocyte aggregation index (AI) was higher, and aggregation half-time (t1/2) was lower in PAH. AI and fibrinogen were positively correlated with NT pro-BNP (AI-NT pro-BNP: r=0.579; fibrinogen-NT pro-BNP: r=0.591). t1/2 was negatively correlated with NT pro-BNP (t1/2-NT pro-BNP: r=-0.648). CONCLUSIONS: The increase in erythrocyte aggregation and the decrease in deformability may theoretically increase the flow resistance and may be of haemodynamic significance. The association between erythrocyte aggregation and NT pro-BNP may indicate that erythrocyte aggregation increases with disease progression. These alterations contribute to the understanding of the pathophysiology and could serve as markers of disease presence.