With the continuous understanding of COVID-19, it has been determined that many patients infected with the novel coronavirus have different degrees of symptoms in different disease periods. A meta-analysis[21] showed that dyspnea, chest pain and cough are the most common respiratory symptoms after COVID-19 infection, with 22.9–53.0% of patients still having dyspnea 2 months after infection[22]. In this study, a retrospective analysis was conducted of individuals who had negative nucleic acid conversion and even reached the criteria for clinical recovery, comprehensively assessing general clinical data, lung function indexes and lung CT images.
In terms of clinical baseline data, age was slightly higher in the dyspnea cases versus the non-dyspnea group, but statistical significance was not achieved (P > 0.05), indicating that age is not a risk factor for dyspnea persistence. In addition, there were no significant differences in hospitalization duration or disease severity between the dyspnea and non-dyspnea groups. In contrast, Yin X et al.[23] found that the longer the hospital stay and the more severe the clinical classification, the higher the risk of dyspnea. This discrepancy may be explained by the distinct evaluation times. The patients selected by YIN et al. were infected by the Delta variant that was epidemic in 2020, while the patients included in the present study were infected by the variant strain Omicron that has been prevalent since December 2022. In terms of laboratory findings, the biomarkers of COVID-19 include CRP, LYM and NLR[24][25]. These indicators can be used as predictors of prognosis in critically ill patients[26]. In this study, CRP, PCT and WBC were slightly reduced in dyspnea cases versus the non-dyspnea group, but the differences were not statistically significant (all P > 0.05).
A meta-analysis[27] showed altered diffusion capacity, restrictive pattern and obstructive pattern in 39%, 15% and 7% of COVID-19 cases post-infection, respectively, corroborating previous findings[28][29][30]. In this study, altered diffusion capacity, ventilation dysfunction and small airway dysfunction at discharge were found in 31,03%, 24.14% and 72.41% of patients, respectively, and showed no statistically significant differences between the two groups. Relevant imaging studies have found that some patients have imaging abnormalities at 3 months after infection accompanied by abnormal lung function. As an important examination tool for lung diseases, CT accurately reveals the nature and scope of lung lesions, with a critical role in severity and treatment efficacy evaluations in COVID-19 patients[20]. In this study, the VVS method and CT quantitative parameter score were combined to analyze lung CT scans. As shown above, the main CT sign in COVID-19 infected patients was GGO, which was consistent with previous studies[31][32]. Most patients still showed residual disease in the lungs on CT images after they were discharged from the hospital. Studies have shown that the absorption of lesions mainly occurs after discharge and is more obvious in the first 3 months[33].
A long-term follow-up study[34] found that quantitative CT parameters related to lesion volume are highly accurate in predicting persistent dyspnea in discharged COVID-19 cases, with survivors developing dyspnea showing enhanced reticulation, which might be the factor causing dyspnea. Guinto E et al.[35] suggested that the prevalence of imaging abnormalities is related to the proportion of patients with dyspnea. Cortés-Telles A et al.[36] suggested that individuals with persistent dyspnea have an elevated number of abnormalities in comparison with well-matched non-dyspnea cases, including enhanced restriction on spirometry, decreased DLCO and suppressed functional capacity. The present study found no factors that affect dyspnea recovery in patients, among the examined laboratory indexes, lung function or lung CT signs and visual scores. In addition to temporal and spatial differences, we believe that the following factors can also explain the inconsistency between this study and previous reports. On the one hand, as the Omicron strain continues to mutate, its virulence also changes. In addition, milder symptoms are detected after infection by Omicron compared with previous strains. Shuai et al.[37], Trunfio et al.[38] reported that compared with the WT variant, Omicron variants are less pathogenic, have lower replication efficiency in vivo, and induce relatively lower levels of cell damage in infected cells. The cases examined in this study were all diagnosed after 2022, when Omicron was considered a variant of concern by the WHO[39]. On the other hand, patients became ill after widespread vaccination, which is a key strategy to prevent broad viral infection and reduce morbidity and mortality. Furthermore, we consider that the persistence of dyspnea symptoms is related to mental factors; in other words, some dyspnea symptoms in these cases may be due to anxiety. Previous studies have shown that COVID-19 has increased the incidence of severe depression worldwide by 27.6%[40]. Although the toxicity of mutant strains has decreased for most individuals, COVID-19 remains a major public health issue. Current evidence indicates that breathing difficulties are a multidimensional subjective feeling; difficulty in breathing generally results in a state of fear, anxiety, etc., in patients[41].
This study had some limitations. First of all, due to the lack of baseline PFT data, it was impossible to compare pre-disease PFT data with post-disease values. Secondly, all cases in this study were Chinese individuals from a single institution; additionally, the sample size was small, and we did not classify cases according to disease severity and the number of vaccinations, indicating potential selection bias. Finally, this study only analyzed a short disease course from hospitalization to discharge, and the long-term dynamic changes of lung function still need further investigation.