Functional small airway disease with air trapping may explain the persistence of respiratory symptoms in patients with long COVID, suggests a new study published in the journal Radiology.¹

This prospective single center study recruited 100 COVID-19 adult patients who had symptoms more than a month after their initial diagnosis and were attending the post-acute sequelae of COVID-19 (PASC) between June and December 2020; 100 matched healthy participants, nonsmokers with no past history of cardiac or lung disease, were selected between March and August 2018 to act as controls.

Based on the level of care administered during the acute infection, the patients were categorized as ambulatory (67%), hospitalized (17%) or those requiring intensive care (16%). Compared to the ambulatory group, hospitalized or ICU patients were likely to be older; 44 vs 64 vs 60 years, respectively. Seventy-six percent of patients with post-Covid symptoms had at least one comorbid condition; of these 59 were obese, 27 were hypertensive, 26 had asthma, six had COPD, while 4 had interstitial lung disease.  Hospitalized patients were also more likely to be current or past smokers than the ambulatory patients; 53% vs 15%, respectively.

Patients underwent quantitative CT analysis using supervised machine-learning to estimate the regional ground glass opacities (GGO); air trapping was assessed via inspiratory and expiratory image-matching. The findings on CT were compared with the controls.

Analysis shows air trapping (58%) to be the most common feature on qualitative analysis of chest CT and ground glass opacities (GGOs) were present in 51% of patients. The total lung affected by air trapping was 25.4% among ambulatory patients, 34.6% in hospitalized patients, 27.3% in those requiring intensive care, while it was detected in only 7.2% of controls. Air trapping persisted beyond 200 days in 8 of 9 participants who underwent imaging. Regional GGOs were present in 13.2% of hospitalized patients, 28.7% of those admitted to ICU, 3.7% of ambulatory patients and just 0.06% in healthy controls. A correlation between air trapping and ratio of residual volume to total lung capacity was noted. None of the study participants had obstruction in airways on spirometry.

This study has shown high prevalence of air trapping in patients who had recovered from acute Covid-19 but continued to have symptoms post-Covid. The severity of infection did not affect the percentage of lung affected by air trapping. Since no airflow obstruction was noted in any patient, the authors suggest that air trapping does not involve large airways; instead, small airways, which are non-cartilaginous airways with an internal diameter <2mm, are affected. Any pathology affecting small airways is usually not detected on spirometry until more than 75% of them are involved. Spirometry may often miss air trapping, which can be detected using inspiratory and expiratory CT imaging and plethysmography. Further long-term studies are required to study the course of functional small airway disease in these patients.

Reference

1. Cho JL, et al. Quantitative chest CT assessment of small airways disease in post-acute SARS-CoV-2 infection. Radiology. 2022 March 15, https://doi.org/10.1148/radiol.212170/.