Background: Reduced gas transfer in patients with pulmonary arterial hypertension (PAH) is traditionally attributed\r\nto remodeling and progressive loss of pulmonary arterial vasculature that results in decreased capillary blood\r\nvolume available for gas exchange.\r\nMethods: We tested this hypothesis by determination of lung diffusing capacity (DL) and its components, the\r\nalveolar capillary membrane diffusing capacity (Dm) and lung capillary blood volume (Vc) in 28 individuals with PAH\r\nin comparison to 41 healthy individuals, and in 19 PAH patients over time. Using single breath simultaneous\r\nmeasure of diffusion of carbon monoxide (DLCO) and nitric oxide (DLNO), DL and Dm were respectively determined,\r\nand Vc calculated. Dm and Vc were evaluated over time in relation to standard clinical indicators of disease severity,\r\nincluding brain natriuretic peptide (BNP), 6-minute walk distance (6MWD) and right ventricular systolic pressure\r\n(RVSP) by echocardiography.\r\nResults: Both DLCO and DLNO were reduced in PAH as compared to controls and the lower DL in PAH was due to\r\nloss of both Dm and Vc (all p < 0.01). While DLCO of PAH patients did not change over time, DLNO decreased by\r\n24 ml/min/mmHg/year (p = 0.01). Consequently, Dm decreased and Vc tended to increase over time, which led to\r\ndeterioration of the Dm/Vc ratio, a measure of alveolar-capillary membrane functional efficiency without changes in\r\nclinical markers.\r\nConclusions: The findings indicate that lower than normal gas transfer in PAH is due to loss of both Dm and Vc,\r\nbut that deterioration of Dm/Vc over time is related to worsening membrane diffusion.
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