Hypersensitivity pneumonitis and pulmonary hypertension: how the breeze affects the squeeze

Chronic hypersensitivity pneumonitis is commonly complicated by PH, which results in increased functional limitation http://ow.ly/vGBOSThe current World Health Organization (WHO) classification of pulmonary hypertension (PH) categorises PH due to chronic lung disease under group 3 [1].There are many reports of PH complicating the course of the more common interstitial lung diseases (ILD), such as idiopathic pulmonary fibrosis (IPF), connective tissue disease associated-ILD and conditions within WHO group 5, such as sarcoidosis.What is surprising though is the relative paucity of data on PH complicating the course of other forms of ILD.The study in this issue of the European Respiratory Journal by OLIVEIRA et al. [2] is the largest to date of right heart catheterisation (RHC) documented PH associated with chronic hypersensitivity pneumonitis (CHP).The authors documented the presence of PH in 50% of their cohort, most of whom (22 out of 25) had group 3 PH, with the three remaining patients having evidence of group 2 PH.Similarly to IPF, they found that the presence of PH was associated with worse functional impairment [3].This study raised a number of important issues.The first point of interest does not pertain to PH, but rather the broad spectrum of ILD.Specifically, the authors evaluated 1023 consecutive patients with ILD, of whom 95 were documented to have CHP.This equates to a prevalence of just below 10%, which is informative to the ILD community; although, this prevalence is likely to vary regionally and internationally based on many economic, environmental and cultural differences.The somewhat high prevalence of PH is in keeping with what has been described previously in IPF as is the spectrum of severity, with most patients having mild PH [3,4].Are the causative mechanisms for PH likely to be the same?Certainly fibrosis is common to both diseases, yet this aspect of the disease is thought to be only one component of the pathogenic process in IPF.Many other factors including comorbidities, hypoxaemia and the cytokine milieu are probably contributory to the genesis of PH in IPF.Whether all the same factors play a role in CHP is uncertain and remains to be determined.Indeed, it is also possible that elements unique to the disease may play a role.For example, the anatomic location and distribution of disease in CHP is distinctly different to that of IPF, with a more bronchiolocentric pattern to both inflammation and fibrosis.The proximity of pathological changes within the bronchovascular bundles might heighten the propensity for PH by involvement of larger more proximal vessels.This anatomic predilection might be one of the reasons for the link demonstrated in this current study between the degree of lung function impairment and PH [2].Interestingly, a similar association in IPF, specifically between the severity of restriction and lung fibrosis with PH, has been found to be lacking [5,6].What of the role of the inhaled antigen itself?Is there an acute vasospastic response, akin to hypoxic vasoconstriction, that over time results in vascular remodelling?The study by OLIVEIRA et al.[2] lays the foundation and hopefully provides a stimulus for further studies into the pathogenesis of PH in CHP.An interesting confirmatory detail from this study is the limitation of echocardiography as a screening tool for PH in patients with advanced lung disease [7].In this study over half the patients would have had an