More than half of patients who recover from acute Covid-19 continue to experience dyspnea, which remains otherwise unexplained. A new study published in the American Journal of Respiratory and Critical Care Medicine has attempted to explain the persistence of dyspnea in Covid recovered patients.¹ For the first time, this study has demonstrated an association between diaphragm muscle weakness and persistent dyspnea in post-Covid patients with normal lung and cardiac function.

This study enrolled 50 patients who had been hospitalized due to Covid-19 from February 2020 and April 2021. Half of these patients were administered invasive mechanical ventilation, while the remaining half required just supplemental oxygen. The mean age of the study subjects was 58 years. The objective of the study was to investigate diaphragm muscle strength and explore its association with unexplained dyspnea on exertion. Lung function test, echocardiography, 6-minute walk test, twitch transdiaphragmatic pressure following cervical magnetic stimulation of the phrenic nerve roots and diaphragm ultrasound were performed to assess the patients.

At follow-up after 15 months post-discharge, around two-thirds of patients continued to have moderate or severe dyspnea on exertion; 28% patients had mild to no exertional dyspnea, 48% had moderate exertional dyspnea, while 32% reported severe exertional dyspnea. PFT and echo revealed no cardiorespiratory abnormalities. However, significant impairment in the twitch transdiaphragmatic pressure was noted vs the healthy controls; 14 vs 21 cmH2O in mechanically ventilated patients and 15 vs 21 cmH2O in non-ventilated patients indicating diaphragmatic muscle weakness. This impairment was independent of the severity of the infection. A significant association was observed between twitch transdiaphragmatic pressure and the severity of dyspnea on exertion.

In this prospective non-interventional study, diaphragmatic muscle weakness was noted even in patients who did not require invasive ventilation. By demonstrating a correlation between persistent dyspnea and weakness of the diaphragmatic muscle, this study has implicated diaphragm muscle weakness as a likely explanation for dyspnea in patients with long Covid in whom lung and cardiac function are normal. This finding may have clinical significance as a potential therapeutic intervention “since respiratory muscle training has been shown to be effective in other groups of patients with diaphragm muscle weakness”, write the authors. However, they also caution that theirs is an observational study and many other factors such as antiviral drugs, phrenic nerve neuropathy could also have contributed to diaphragm weakness in these patients.

Reference

1. Regmi B, et al. Diaphragm muscle weakness might explain exertional dyspnea fifteen months after hospitalization for COVID-19. Am J Respir Crit Care Med. 2023 Jan 3. doi: 10.1164/rccm.202206-1243OC.