If a patient's DLCO is elevated, which of the following conditions could be suspected?

• A. Chronic bronchitis
• B. Interstitial lung disease
• C. Asthma
• D. Pneumothorax

Answer: (C)

An elevated diffusing capacity of the lungs for carbon monoxide (DLCO) can indicate conditions where there is an increased surface area for gas exchange or increased pulmonary blood flow. Asthma is characterized by airway hyperreactivity and inflammation, which can lead to an increase in perfusion and surface area for gas exchange due to the dilation of pulmonary vessels in response to the increased demands during an asthmatic episode.

In asthma, the presence of airway obstruction can sometimes also lead to increased recruitment of perfused lung units as there is a compensatory mechanism that enhances surface area for gas exchange in response to hypoxemia, often resulting in a higher DLCO. This occurs especially during exacerbations when airflow limitation is present, but the lung parenchyma itself remains relatively intact, allowing for adequate gas exchange and contributing to the elevated DLCO.

In contrast, chronic bronchitis, interstitial lung disease, and pneumothorax generally lead to a reduced DLCO. Chronic bronchitis results in airflow obstruction without significant alteration to the lung parenchyma affecting diffusion, while interstitial lung disease leads to thickened alveolar membranes that impede gas exchange, thus decreasing DLCO. A pneumothorax may affect lung volume and compliance, also typically leading