Respiratory muscle dysfunction in long-COVID patients

Symptoms often persistent for more than 4 weeks after COVID-19-now commonly referred to as 'Long COVID'. Independent of initial disease severity or pathological pulmonary functions tests, fatigue, exertional intolerance and dyspnea are among the most common COVID-19 sequelae. We hypothesized that respiratory muscle dysfunction might be prevalent in persistently symptomatic patients after COVID-19 with self-reported exercise intolerance.

In a small cross-sectional pilot study (n = 67) of mild-to-moderate (nonhospitalized) and moderate-to-critical convalescent (formerly hospitalized) patients presenting to our outpatient clinic approx. 5 months after acute infection, we measured neuroventilatory activity P_0.1, inspiratory muscle strength (PI_max) and total respiratory muscle strain (P_0.1/PI_max) in addition to standard pulmonary functions tests, capillary blood gas analysis, 6 min walking tests and functional questionnaires.

Pathological P_0.1/PI_max was found in 88% of symptomatic patients. Mean PI_max was reduced in hospitalized patients, but reduced PI_max was also found in 65% of nonhospitalized patients. Mean P_0.1 was pathologically increased in both groups. Increased P_0.1 was associated with exercise-induced deoxygenation, impaired exercise tolerance, decreased activity and productivity and worse Post-COVID-19 functional status scale. Pathological changes in P_0.1, PI_max or P_0.1/PI_max were not associated with pre-existing conditions.

Our findings point towards respiratory muscle dysfunction as a novel aspect of COVID-19 sequelae. Thus, we strongly advocate for systematic respiratory muscle testing during the diagnostic workup of persistently symptomatic, convalescent COVID-19 patients.