The Effect of Sildenafil on Diffusion Capacity Measurements in Patients With Pulmonary Hypertension and Parenchymal Lung Disease

Sildenafil is a cyclic GMP selective phosphodiesterase type 5 inhibitor approved for use by the FDA in patients with pulmonary arterial hypertension. There has been recent interest in the use of the drug in patients with diffuse parenchymal lung diseases. This is largely based on sildenafil's salutary effects observed in small studies (Collard et al. 897-99;Ghofrani et al. 895-900;Collard et al. 897-99) and the frequent coexistence of pulmonary hypertension in these patients (Nathan et al. 657-63). In eight patients with pulmonary fibrosis and pulmonary hypertension, sildenafil reduced pulmonary artery pressure and improved shunt fraction as determined by multiple inert gas elimination technique (Ghofrani et al. 895-900;Nathan et al. 657-63). Collard et al demonstrated a 49 meter improvement in 6 minute walk distance in 11 patients with idiopathic pulmonary fibrosis and pulmonary hypertension after 3 months of therapy with sildenafil.

Main theoretical concern with the use of pulmonary arterial vasodilator therapy has been worsening of ventilation perfusion matching due to release of hypoxic vasoconstriction. Such a phenomenon is well recognized during prostacyclin infusion(Ghofrani et al. 895-900;Walmrath et al. 1084-92). Inhaled route seems to circumvent this side effect by preferentially delivering the drug to the alveoli with high V/Q ratios and augmenting blood flow to these regions (Ghofrani et al. 895-900;Walmrath et al. 1084-92). In the only study to address possible effect of systemically administered sildenafil on gas exchange, Ghofrani et al observed improvement in V/Q matching in eight patients using the multiple inert gas elimination technique (MIGET). The authors postulated an enhancement of local defense mechanisms against hypoxia i.e. increased nitric oxide availability to hypoxic alveoli despite oral route of administration(Ghofrani et al. 895-900). In a recent study of 14 normal subjects, oral sildenafil did not effect DLCO measurements at rest, after exercise and under hypoxic conditions(Snyder et al. 421-30).

Diffusion capacity of the lung for carbon monoxide (DLCO) is the clinically available method of assessing gas exchange for practitioners. It is frequently used as a tool for serial monitoring of diffuse parenchymal lung disease. American Thoracic Society has endorsed its use in the assessment of patients with idiopathic interstitial pneumonias(American Thoracic Society/European Respiratory Society International Multidisciplinary Consensus Classification of the Idiopathic Interstitial Pneumonias . This Joint Statement of the American Thoracic Society (ATS), and the European Respiratory Society (ERS) was adopted by the ATS Board of Directors, June 2001 and by The ERS Executive Committee, June 2001 277-304).

The aim of our study was to delineate the effects of orally administered sildenafil on diffusion capacity (absolute value). The significance of this information was twofold. First, it would be helpful clinically to determine the contribution of a pulmonary vasodilator to the change in DLCO to separate this effect from a true clinical change. Second, sildenafil's effect on gas exchange may lead to modifications in therapy such as changes in oxygen flow delivered to patients.

This is not a study that measures clinical outcome or safety of sildenafil. A single dose was administered and effect on diffusion capacity and 6 minute walk were assessed compared to baseline. While we would caution clinicians for rare exceptions to these findings, we believe our data exclude a significant confounding effect in interpretation of diffusion capacity in patients with parenchymal lung disease who are treated for pulmonary hypertension with oral sildenafil. Whether this conclusion holds true at higher doses of sildenafil and in patients with vasodilator response may offer further venues for research.