Pulmonary Dysfunction in Patients Recovered from COVID-19 Pneumonia: A 6-Month Follow-up Study


 Objectives: This study investigates the clinical features and pulmonary functions of COVID-19 pneumonia survivors at 3 or 6 months after diagnosis in the Heilongjiang Province, China.Methods: Forty-six patients with COVID-19 pneumonia diagnosed since February 2020 were enrolled in this study for follow-up in July 2020. These patients were categorized into three groups: Group A (n=24) and Group B (n=11) who were diagnosed with moderate or severe pneumonia and followed up at three months after diagnosis; Group C (n=11) who were diagnosed with severe pneumonia and followed up at six months after diagnosis. Data on pulmonary function, arterial blood gas analysis, chest CT, blood test, antibody test, and health-related quality of life during hospitalization and at the follow-up visits were collected and analyzed. Results: Abnormal PO2 (A-a) was more prevalent in severe cases (Group B and C) than in moderate cases (Group A). Pulmonary dysfunction was common in this cohort. Abnormal CT scores of severe cases (Group B and C) were significantly higher than that of moderate cases (Group A). During the follow-up, lung abnormalities gradually resolved in the first 3 months (Group A and B), however, further resolution was not significant from 3 months to 6 months (Group B and C). Conclusion: Although pulmonary interstitial changes due to COVID-19 pneumonia gradually reverse over time, pulmonary dysfunction is common and appears to persist at least up to 6 months in patients recovered from COVID-19 pneumonia.

Because of its high rates of transmission and mortality, more than 50 million patients have been infected globally among whom 1,250,000 have died from COVID-19 pneumonia or other complications since its outbreak. 5% of symptomatic patients were classi ed as critically ill and needed to be treated in the ICU; the mortality rate of such patients was as high as 53% [1,2].
The severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) mainly attacks the lung and causes pneumonia. Patients often presented with fever and cough; severe cases may rapidly progress to acute respiratory distress syndrome (ARDS) and respiratory failure and need respiratory support. Although the long-term sequelae of COVID-19 are currently not fully understood, there is a possibility of pulmonary brosis based on previous experience with other coronaviral diseases such as the Middle East Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS) [3,4]. Such sequelae often lead to severe pulmonary dysfunction that signi cantly affects the quality of life [5].
While there has been a surge in research aiming to gain a deeper insight into the clinical pro le and pathophysiology of the disease, less is known regarding the long-term pulmonary function in survivors of COVID-19 pneumonia [6,7]. This study aims to follow up on patients who have recovered from COVID-19 pneumonia for three or six months to understand the impact of disease severity and time on the pulmonary function and clinical characteristics after recovery from COVID19 pneumonia.

Patients
According to the COVID-19 treatment guidelines from China and the WHO [8,9], the severity of COVID-19 pneumonia is categorized into four levels according to the condition at the time of admission: mild (no radiological evidence of pneumonia and mild clinical symptoms), moderate (pneumonia on chest radiograph with fever and evidence of respiratory symptoms), severe (pneumonia with any of the following indications: PaO2/FiO2≤300mmHg, oxygen saturation≤93% at rest, tachypnoea at RR≥30/min or respiratory distress), and critical (patients who either developed organ failure requiring ICU monitoring or respiratory failure requiring mechanical ventilation).
Forty-six patients with COVID-19 pneumonia treated at the First A liated Hospital, Harbin Medical University since February 2020 were enrolled in this study for follow-up in July 2020. These patients were categorized into three groups: Group A (n=24) who were diagnosed with moderate pneumonia in April 2020 and were followed up at three months after diagnosis; Group B (n=11) who were diagnosed with severe pneumonia in April 2020 and were followed up at three months after diagnosis; Group C (n=11) who were diagnosed with severe pneumonia in February 2020 and were followed up at six months after diagnosis. During hospitalization, all patients were treated in accordance with the COVID-19 treatment guidelines of China [8]. Patients were given oxygen therapy, high-ow nasal cannula, non-invasive ventilator, or invasive ventilator according to their pulmonary conditions. They were discharged from our hospital when the following criteria were met: oropharyngeal swab SARS-CoV-2 nucleic acid was negative twice from tests done at least 24 hours apart; body temperature has returned to normal for more than 3 days; respiratory symptoms have improved signi cantly; lung radiograph shows signi cant improvement in acute exudative lesion.
These patients were all followed up in the outpatient clinic in July 2020. Figure 1 shows a owchart of the study. They were assessed for their pulmonary function, chest high-resolution CT (HRCT), arterial blood gas analysis, blood tests, modi ed Medical Research Council (mMRC) dyspnea score, and healthrelated quality of life (HRQoL). The clinical research ethics committee of the First A liated Hospital, Harbin Medical University approved this study (Protocol No. IRB-AF/SC-04/01.0).

Blood tests
Blood analyses included blood cell counts, renal and liver function tests, coagulation pro le, and immunoglobulin test for SARS-CoV-2. Serum SAR-CoV-2 IgG and IgM antibody titers (AU/mL) were analyzed with a chemiluminescent immunoassay (Shenzhen Yahuilong Biotechnology Co., Ltd, Shenzhen, China), with a reference level of 10 AU/mL. A blood gas analyzer (GEM Premier 3000; Instrumentation Laboratory, New York, USA) was used to quantify arterial oxygen partial pressure and the alveolar-arterial oxygen pressure gradient (PO 2 (A-a)). Arterial blood gas analysis was not performed for two patients because of their disconsent.

Pulmonary function
All patients rst were assessed with an oropharyngeal swab SARS-CoV-2 nucleic acid test to exclude active viral infection. Standard single-breath pulmonary function testing (MasterScreen Body/Diff, Jaeger Co., Germany) was then carried out to determine total lung capacity (TLC), forced vital capacity (FVC), vital capacity (VC), forced expiratory volume at rst second (FEV1), and diffusing lung capacity for carbon monoxide single-breath (DLCO SB). DLCO SB values were corrected for individual hemoglobin levels. A DLCO SB of less than 80% was interpreted as diffusion de cit, while all other results were depicted in terms of percentages of predicted normal values. Pulmonary function test was not done for one patient because of positive SARS-CoV-2 IgM.
Chest high-resolution CT Scan (HRCT) All COVID-19 patients underwent chest HRCT with a 256-slice multi-detector CT scanner (Brilliance iCT, Philips Healthcare, Holland) when hospitalized, and chest HRCT at the follow-up visit was performed with a 40-row multi-detector CT scanner (uCT 528, Shanghai United Imaging Healthcare, Shanghai, China). Imaging parameters were set as follows: slice thickness, 1 mm; tube voltage, 100 kV; 125 mas; 0.30second gantry rotation time, automatic. All images were analyzed with the Extended Brilliance Workspace Two experienced radiologists without prior knowledge of the clinical pro les reviewed and graded CT images independently. A consensus had to be reached between these two radiologists about the abnormalities. When there was a disagreement, the nal decision would be made by a third senior radiologist with more than 10 years of experience. A scoring system was adapted for this study [10]. Each image was assigned a score ranging from 0 to 5 based on the presence of air trapping, brosis, consolidation, and ground-glass opacity (GGO). Score 0 indicated a normal lung; Score 1 was scored if <5% of a lobe presented GGO; Score 2 if 6-25% was involved; Score 3 if 26-50% was involved; 4 points if 51-75% was involved; Score 5 if more than 75% was involved. All 5 lung lobes were scored and an abnormal CT score (range 0-25) was generated by adding scores of individual lobes.
Health-related quality of life (HRQoL) assessment and mMRC dyspnea score The HRQoL of patients was determined using the St. George's Respiratory Questionnaire (SGRQ) [11]. This questionnaire encompassed information regarding symptom severity, activity tolerance, and impact on daily life. A higher score was indicative of worse overall functional status. The mMRC dyspnea scale was used to assess dyspnea (score 0-4, with 4 indicating the worst dyspnea) [12].

Statistical analysis
Statistical analysis was performed using SPSS 25.0, For normally distributed variables, the data were expressed as mean ± standard deviation (SD); the differences among these three groups were analyzed with one-way ANOVA and then Fisher's LSD tests. For variables that are not normally distributed, data were presented as medians (interquartile range) and analyzed with the Kruskal-Wallis H test and then Nemenyi tests. Categorical variables were presented as frequencies or percentages and statistically analyzed with the Chi-square test or Fisher's exact test. A p-value of <0.05 was taken to indicate statistical signi cance.

Results
Demographics and clinical characteristics of patients at the baseline Forty-six COVID-19 pneumonia patients participated in this investigation. Our study cohort involved 24 moderate cases diagnosed for 3 months (Group A), 11 severe COVID-19 cases diagnosed for 3 months (Group B), and 11 severe COVID-19 cases diagnosed for 6 months (Group C). There were no differences among these three groups in major clinical characteristics such as body mass index (BMI), comorbidities, symptoms such as dyspnea, sputum, and fatigue, except that in contrast to moderate cases severe cases were older; the proportion of male cases in Group C were more than that in Group A (81.82% vs 25.00%, p = 0.006); Group B had more smokers than other groups. All patients had a negative result of SARS-CoV-2 nucleic acid oropharyngeal swab test, while all were positive for SARS-CoV-2 IgG and only one patient in Group C was positive for SARS-CoV-2 IgM (Table 1). Blood tests at follow-up All blood test results (leucocytes, lymphocytes, coagulation function, liver, and kidney function) were in the normal range, and most were not markedly different among these three groups. However, the level of creatinine in Group C was signi cantly higher than that in Group B, while the level of lactic dehydrogenase (LDH) in Group B was higher than that in Group A (Table 2). Blood level of SARS-CoV-2 IgG antibody during hospitalization and at follow-up SARS-CoV-2 IgG antibody in blood was measured in all patients when hospitalized and at the follow-up visit. The level of SARS-CoV-2 IgG kept increasing signi cantly at 3 months (Group A and B; Fig. 2A Fig. 2C).

Arterial blood gas analysis at follow-up
Forty-four patients underwent arterial blood gas analysis at follow-up (Table 3). Although the average oxygenation indexes of each group were within the normal range, severe cases (Group B and C) had signi cantly lower oxygenation indices compared to moderate cases (Group A). Group B had a higher difference of PO 2 (A-a) than Group A, but this parameter was equivalent in Group B and Group C. 36.36% patients of Group A had their PO 2 (A-a) higher than the normal predicted value, while 45.46% in Group B and 54.55% in Group C had values higher than the normal predicted value. Severe cases (Group B and C) seemed to have higher hemoglobin than moderate cases (Group A) although the difference did not reach statistical signi cance. These data suggest that severe cases had worse blood oxygen exchange than moderate cases, and such damage in severe cases persisted up to 6 months at follow-up.  (Table 4). However, on average, the DLCO SB of all three groups was less than the predicted 80%. The DLCO SB of Group B was slightly lower than that of Group A without statistical signi cance, yet the DLCO SB of Group C was signi cantly higher than that of Group B (75.56 ± 13.95 vs 61.01 ± 15.04 P < 0.05) ( Table 4). The TLC of Group B was signi cantly lower that of Group A; but the TLC of Group C was higher than that of Group B (88.35 ± 9.48 vs 76.90 ± 11.46; P 0.05). Additionally, the FRC was markedly reduced in Group B compared to that in Group C. Abnormal ventilation (FEV1 < 80% predicted or FEV1/FVC < 70% predicted or MEF50, MEF25, MMEF75/25 < 70% predicted) were present in 52.17%, 72.73%, and 45.45% of the cases, in three groups, respectively. Abnormal diffusion (DLCO SB < 80% predicted or DLCO/VA < 80% predicted) was present in 65.22%, 81.82%, 72.73% of the cases. All together, pulmonary dysfunction (abnormal ventilation or diffusion) was observed in 82.61%, 81.82%, 72.73% of the cases in these three groups, respectively. These data suggest that at 3 and 6 months of follow-up, most patients still had pulmonary dysfunction. Dynamic changes of chest HRCT Images and data of chest HRCT were available for each patient at four stages: admission, progression, discharge, and follow-up (Fig. 3A-L). Severe cases (Group B and C) had worse signs of pneumonia than moderate cases (Group A). While the HRCT image was exacerbated initially and then improved after the patients being discharged, severe cases (Group B and C) tended to have some residual changes in the lung at follow-up. The abnormal CT score at follow-up of each group was lower than that at the time of hospital discharge (Fig. 3M). At 3 months of follow-up, severe cases (Group B) had a higher score than moderate cases (Group A). For severe cases, the abnormal CT score remained high at 6 months of followup (Group C) as compared with 3 months of follow-up (Group B).
HRQoL score and mMRC dyspnea score SGRQ scores of most patients remained abnormal at follow-up. Although there was no statistical difference among the SGRQ scores of these three groups at follow-up, severe cases at 6 months (Group C) had a better quality of life than moderate cases at 3 months (Group A) and severe cases at 3 months (Group B). Similarly, the mMRC apnea score of severe cases at 6 months (Group C) improved as compared with severe cases at 3 months (Group B) ( Table 5).

Discussion
At present, SARS-CoV-2 is still in a global pandemic, infecting tens of millions of people and severely affecting global health and economy. The long-term consequences of COVID-19 pneumonia have not been fully documented. In this study, our data have shown that severe cases had worse pulmonary function than moderate cases at three months of follow-up, and severe cases had residual lesions in the lung even at six months of follow-up. This observation is reminiscent of SARS, MERS, and ARDS due to other causes. Pulmonary dysfunction due to residual pulmonary brosis persisted in these situations and caused high mortality after recovery [13] [14] [15]. COVID-19 pneumonia may present a similar pattern of disease progression.
GGO or grid-like changes are typical changes on HRCT of COVID-19 pneumonia. The lung parenchyma of severe cases may be extensively exuded; 25% of patients still had ber-stripe shadows and bronchial structural distortions after discharge [16]. This is similar to CT changes in SARS patients during recovery. Such structural changes may remain for a long period, resulting in pulmonary dysfunction in ventilation and diffusion. We have reported previously that the rate of abnormal ventilation in COVID-19 pneumonia was high at the time of hospital discharge, with restrictive ventilatory defect and small airway dysfunction in 50% of the cases [17]. Zhao et al. indicated that 25.45% of recovered COVID-19 patients still had signi cantly impaired pulmonary function at 3 months after discharge [18]. But so far, there is no follow-up study on the pulmonary function of COVID-19 pneumonia at 6 months of follow-up. It also remains unknown whether initial disease severity and follow-up time may have signi cant impacts on pulmonary function during the recovery of COVID-19 pneumonia.
Previous studies have noted that impaired DLCO SB and reduced total lung capacity appeared to persist for months or even years in survivors of coronaviral pulmonary infections [19][20][21]. Some studies also reported that SARS and H1N1 patients also had small airway dysfunction during recovery [22,23]. In our study, we observed that the value of PO 2 (A-a) in severe cases at 3 months of follow-up (Group B) was higher than that in moderate cases at 3 months of follow-up (Group A), but was not different from that in severe cases at 6 months of follow-up (Group C). The proportions of patients with abnormal PO 2 (A-a) in Groups A, B, and C were 36.36%, 45.46%, and 54.55%, respectively (Table 3). Obviously, PO 2 (A-a) increased with the severity of the disease; it did not recover at 6 months of follow-up. Pulmonary dysfunction was common in our cases, and pulmonary function data matched the arterial gas analysis data, for example, DLCO SB in severe cases at 3 months of follow-up (Group B) was lower than that in moderate cases at 3 months of follow-up (Group A). In severe cases, DLCO SB at 6 months of follow-up (Group C) was higher than that at 3 months (Table 4). These results suggest that decreased diffusion and restrictive ventilatory defect are common in patients recovered from COVID-19 pneumonia, and the severity of pulmonary dysfunction is related to the severity of the initial disease. Although pulmonary function improved over time, at 6 months many severe cases still had pulmonary dysfunction.
In this study, chest HRCT of COVID-19 pneumonia at admission was signi cant with GGO and interstitial changes. As the disease progressed, consolidations occurred in some patients (Fig. 3), a development that corresponded with disease severity. Lung lesions appeared to gradually resolve on subsequent chest HRCT images taken at follow-up, with a decrease in density (Fig. 3M). Evidence from patients with other coronavirus infections such as SARS and MERS has shown that interstitial lung damage is followed by parenchymal lesions and then pulmonary brosis that causes signi cantly reduced pulmonary function lasting months to years after hospital discharge [4,24,25]. CT ndings of COVID-19 pneumonia were similar to those noted in MERS and SARS which were all related to in ammatory damage of the lower respiratory tract and alveoli. The rst change that occurs at the early stage was excessive in ammatory exudate and edema in the pulmonary interstitium manifested as GGO. 'Crazy-paving' was seen on later CT images, a feature due to increased exudate and thickened interlobular septum. At the most severe stage, cellular brous mucus-like organizing exudates appeared in the alveolar cavity and the density of the lesions increased, showing paving stone-like changes combined with consolidation [26,27].
Subsequent follow-up of chest HRCT during recovery was signi cant with partial resolution of the lung lesions [28]. As expected, at the time of follow-up, the abnormal CT score was signi cantly lower than that at discharge. At 3 months of follow-up, severe cases (Group B) had higher abnormal CT scores than moderate cases (Group A), but their scores did not further improve at 6 months of follow-up (Group C) (Fig. 3M). Pathological studies of SARS, MERS, and COVID-19 have con rmed these changes [29]. These previous studies and our follow-up results suggest that COVID-19 lesions may involve the formation of thickened alveolar sacs and alveolar walls, rather than complete pulmonary brosis.
Given the key role of IgG in modulating the immune response, further efforts in developing vaccines and preventing reinfection are dependent on a better understanding of IgG changes in patients who have recovered from COVID-19 infection. Liu et al. tested 484 patients for SARS-CoV-2 IgG antibodies at 100 days after symptom onset and noted that 18% were tested negative [30]. In our study, the level of SARS-CoV-2 IgG rose signi cantly in patients at 3 months of follow-up (Group A and B) ( Fig. 2A and B). However, the level of SARS-CoV-2 IgG at 6 months of follow-up (Group C) dropped signi cantly from 123.81 ± 15.02 AU/ml (at hospital admission) to 58.84 ± 33.74 AU/ml (Fig. 2C). The underlying factors and mechanisms contributing to the dynamics of IgG level in COVID-19 require further investigation.
Pulmonary dysfunction, fatigue, anxiety, and depression are associated with the decline of patient's quality of life [22,31]. In our study, SGRQ and mMRC scores deteriorated with the severity of the initial disease, but as time goes by, they tended to improve (Table 5).
While several reports have delineated the outcomes of COVID-19 pneumonia at 3 months after diagnosis [18], ours is the rst to correlate clinical features, HRCT imaging, and pulmonary function at 3 and 6 months of follow-up. Additionally, there appears to be a permanent impairment in pulmonary function despite apparent resolution of lung pathology seen on sequential HRCT images, resulting in poorer quality of life. Long-term follow-up of a larger cohort of COVID-19 pneumonia is warranted in future efforts to tackle this debilitating condition.

Limitations
This study has several limitations. Firstly, a relatively small cohort of 46 patients with COVID-19 pneumonia was enrolled in this study. Secondly, due to the sudden outbreak of the epidemic, we were unable to evaluate those early severe cases (Group C) at the 3 months of follow-up; thus we were not able to produce longitudinal data of this group. The comparison between severe cases at 3 months of followup (Group B) and those at six months (Group C) was not ideal. Further longitudinal follow-up of a larger cohort would be necessary to increase our knowledge about pulmonary dysfunction after recovery from COVID-19 pneumonia.

Conclusions
To our best knowledge, this is the rst follow-up study to describe the pulmonary function and clinical characteristics of COVID-19 pneumonia at 3 and 6 months. Our data indicated that pulmonary dysfunction was common at these time points, particularly in those severe cases. Although pulmonary interstitial changes due to COVID-19 pneumonia gradually reversed over time, pulmonary dysfunction appears to persist at least up to 6 months and such patients require further follow-up and treatment. written informed consent for publication was obtained from all participants.
Availability of supporting data: the authors con rm that the data supporting the ndings of this study are available within the article. Follow-up of 46 cases of COVID-19 pneumonia. In this cohort of 46 cases, 24 cases with moderate pneumonia (Group A) were followed at 3 months after being diagnosed in April 2020, 11 cases with severe pneumonia (Group B) at 3 months after being diagnosed in April 2020, and 11 cases with severe pneumonia (Group C) at 6 months after being diagnosed in February 2020. Time 0 indicates the time of diagnosis. All follow-up was carried out in July 2020. Pulmonary function test and arterial blood gas analysis were performed in 45 cases and 44 cases, respectively.

Figure 1
Follow-up of 46 cases of COVID-19 pneumonia. In this cohort of 46 cases, 24 cases with moderate pneumonia (Group A) were followed at 3 months after being diagnosed in April 2020, 11 cases with severe pneumonia (Group B) at 3 months after being diagnosed in April 2020, and 11 cases with severe pneumonia (Group C) at 6 months after being diagnosed in February 2020. Time 0 indicates the time of diagnosis. All follow-up was carried out in July 2020. Pulmonary function test and arterial blood gas analysis were performed in 45 cases and 44 cases, respectively.