Lung function – respiratory evaluation: Spirometry, bodyplethismography, CO diffusion capacity, arterial blood gases and overnight oximetry
Simple office spirometry is helpful to exclude significant obstructive or restrictive lung disease as causes of the pulmonary hypertension.
Pulmonary function tests including diffusion capacity for carbon monoxide and blood gas analysis will identify the contribution of underlying parenchymal lung disease and are therefore mandatory for the evaluation of patients with pulmonary hypertension.
Patients with PH usually have decreased lung diffusion capacity for carbon monoxide (typically in the range of 40–80% predicted) and mild to moderate reduction of lung volumes. Peripheral airway obstruction can also be detected. Arterial oxygen tension is normal or only slightly lower than normal at rest and arterial carbon dioxide tension is decreased because of alveolar hyperventilation.
COPD as a cause of hypoxic PH is diagnosed on the evidence of irreversible airflow obstruction together with increased residual volumes and reduced diffusion capacity for carbon monoxide and normal or increased carbon dioxide tension. The severity of emphysema and of interstitial lung disease can be diagnosed using high-resolution computed tomography (CT).
A decrease in lung volume together with a decrease in diffusion capacity for carbon monoxide may indicate a diagnosis of interstitial lung disease.
If clinically suspected, screening overnight oximetry will exclude significant obstructive sleep apnoea/hypopnoea. Resting hypercapnia suggesting alveolar hypoventilation warrants further evaluation (polygraphy / polysomnography). Severe hypoxemia suggests either intracardiac shunting due to a patent foramen ovale or a severely reduced cardiac output resulting in low venous oxygen saturation.