From the Guidelines
The answer to the question is A. Asthma, as it is the only condition listed that can have a normal diffusion capacity of carbon monoxide (DLCO) due to its primary effect on the airways rather than the alveolar-capillary interface. Among the conditions listed, asthma is distinct because it primarily affects the airways, leading to bronchial hyperresponsiveness, inflammation, and reversible airflow obstruction, without directly impairing the alveolar membrane and pulmonary capillary blood volume. In contrast, pulmonary hypertension, pulmonary fibrosis, and severe COPD involve structural changes that directly impair gas exchange across the alveolar-capillary membrane, resulting in reduced DLCO values. Pulmonary hypertension reduces the vascular bed available for gas exchange 1, pulmonary fibrosis thickens the alveolar-capillary membrane, and severe COPD often involves destruction of alveolar walls and reduced capillary blood volume, particularly in emphysema. Understanding this distinction is crucial for differentiating asthma from other pulmonary conditions when interpreting pulmonary function tests, as noted in guidelines for the diagnosis and treatment of pulmonary hypertension 1. Asthma's characteristic high or high normal DLCO value is highlighted in the evaluation of patients with refractory asthma, where a low diffusing capacity suggests alternative diagnoses such as emphysema, pulmonary vascular disease, or interstitial lung disease 1. Therefore, when considering the conditions listed, asthma stands out as the one that can present with a normal DLCO, making it the correct answer based on the pathophysiology of these diseases and their effects on lung function.
From the Research
Conditions with Normal Diffusion Capacity of CO (DLCO)
The following conditions can have a normal DLCO:
- Asthma: According to 2, single breath diffusing capacity for carbon monoxide (Dco) is commonly used as a simple method of assessing overall pulmonary gas exchange properties, and studies of Dco in bronchial asthma have yielded conflicting results. However, the mean value of Dco was increased to 117% of predicted values in patients with uncomplicated stable asthma.
- Pulmonary hypertension: Although 3 suggests that the FVC/DLCO ratio can be used to predict pulmonary hypertension in patients with COPD, it does not necessarily imply that DLCO is abnormal in all cases of pulmonary hypertension.
Conditions with Abnormal DLCO
The following conditions are associated with abnormal DLCO:
- Pulmonary fibrosis: While not directly mentioned in the provided studies, pulmonary fibrosis is known to affect gas exchange and can lead to reduced DLCO.
- Severe COPD: According to 4 and 5, DLCO is significantly reduced in patients with COPD, especially in those with severe disease.