Pulmonary function tests are performed to identify underlying lung disease and assess severity of Pulmonary hypertension. The lungs of the Pulmonary hypertension patient are stiffer, smaller, and have a decreased lung diffusion capacity for carbon monoxide. The carbon monoxide diffusing test (DLCO) measures the efficiency of oxygen diffusion from the lungs to the alveoli. Pulmonary hypertension patients tend to have a decrease in oxygen diffusion across the capillary membrane.