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Quantitative inspiratory–expiratory chest CT findings in COVID-19 survivors at the 6-month follow-up

We evaluated pulmonary sequelae in COVID-19 survivors by quantitative inspiratory–expiratory chest CT (QCT) and explored abnormal pulmonary diffusion risk factors at the 6-month follow-up. This retrospective study enrolled 205 COVID-19 survivors with baseline CT data and QCT scans at 6-month follow-up. Patients without follow-up pulmonary function tests were excluded. All subjects were divided into group 1 (carbon monoxide diffusion capacity [DLCO] < 80% predicted, n = 88) and group 2 (DLCO ≥ 80% predicted, n = 117). Clinical characteristics and lung radiological changes were recorded. Semiquantitative total CT score (0–25) was calculated by adding five lobes scores (0–5) according to the range of lesion involvement (0: no involvement; 1: < 5%; 2: 5–25%; 3: 26–50%; 4: 51–75%; 5: > 75%). Data was analyzed by two-sample t-test, Spearman test, etc. 29% survivors showed air trapping by follow-up QCT. Semiquantitative CT score and QCT parameter of air trapping in group 1 were significantly greater than group 2 (p < 0.001). Decreased DLCO was negatively correlated with the follow-up CT score for ground-glass opacity (r = − 0.246, p = 0.003), reticulation (r = − 0.206, p = 0.002), air trapping (r = − 0.220, p = 0.002) and relative lung volume changes (r = − 0.265, p = 0.001). COVID-19 survivors with lung diffusion deficits at 6-month follow-up tended to develop air trapping, possibly due to small-airway impairment.


Materials and methods
Patient population and general information. This was a retrospective study. A total of 3792 patients with laboratory-confirmed COVID-19 were discharged from Jinyintan Hospital between January 7 and May 29, 2020. A total of 324 patients were included according to the following inclusion criteria: (1) older than 18 years of age; (2) available initial CT findings at admission; (3) without a history of lung cancer or lung surgery; and (4) able and willing to provide informed consent. A total of 119 patients were excluded because of the following exclusion criteria: (1) death in the hospital (n = 53); (2) inadequate CT image quality (n = 45); (3) inability to undergo PFT at follow-up due to their clinical status (n = 8); (4) declined to follow-up (n = 11); and (5) pregnancy (n = 2) (Fig. 1). The diagnostic criteria for severe pneumonia in adults were in accordance with WHO interim guidlines 21 and included fever or suspected respiratory tract infection plus one of the following: respiratory rate > 30 breaths/min; SpO 2 < 90% on room air; or severe respiratory distress. The discharge criteria for all included patients were consistent with the Chinese clinical guidance for COVID-19 pneumonia diagnosis and treatment issued by the National Health Commission 22 . Throat swab specimens from the upper respiratory tract were collected to confirm SARS-CoV-2 by real-time reverse transcription polymerase chain reaction (RT-PCR) using a protocol described previously 7,23 . Clinical data including demographic characteristics, clinical characteristics (onset symptoms, hospital stay duration, self-reported comorbidities, incidence of acute respiratory distress syndrome [ARDS]), peak laboratory findings (the maximum values of parameters reached during the acute phase of the disease), and treatments were collected from electronic medical records by physicians (YKC, XYH, XJ and YTZ, with 7, 5, 3 and 2 years of experience in radiology, respectively). Initial and follow-up CT scans and time from symptom onset to CT scans were also reviewed. We used the Berlin definition of ARDS as a judging reference 24 . CT image acquisition parameters. All patients received initial CT scans at admission and completed the QCT at the 6-month follow-up. Only the follow-up QCT was acquired in the expiratory and inspiratory phases, while the initial CT was performed in a single phase. The initial and follow-up CT images were acquired in the supine position using a SOMATOM Definition AS+ scanner or a SOMATOM Perspective scanner (Siemens Healthineers, Forchheim, Germany) with the following parameters: SOMATOM Definition AS+ scanner: pitch, 1.2; collimation, 64 × 0.6 mm; thickens of acquisition, 1.5 mm and mm gap, 1.5 mm, with a reconstruction kernel (B60f); SOMATOM Perspective scanner: pitch, 1/5; collimation, 128 × 0.6 mm/64 × 0.6 mm; thickens of acquisition, 1 mm/5 mm and mm gap, 1 mm/5 mm, with a reconstruction kernel (B80S). The initial CT images of 133/205 (64.9%) patients were reconstructed with a slice thickness of 1 mm and an interval of 1 mm, and those of the remaining 72/205 (35%) patients were reconstructed with a slice thickness of 5 mm and an interval of 5 mm. Due to the decrease in the workload of imaging examination, QCT was performed at the 6-month follow-up, and we deliberately minimized the scan thickness. All follow-up CT images were reconstructed at a 1-mm slice thickness and 1-mm intervals. Noncontrast chest CT scans were performed with acquisition from the thoracic inlet to the diaphragm. Other parameters used for the scanning protocol were as follows: a tube voltage of 120 kV with automatic tube current modulation and a matrix of 512 × 512. The tube current was regulated by an automatic exposure control system (CARE Dose 4D; Siemens Healthineers). All CT images, including the mediastinal window (center, 50; width, 350) and lung window (center, − 600; width, 1200), were obtained using picture archiving and communication systems (Vue PACS, version 11.3.5.8902, Carestream Health, Canada).
Qualitative CT image evaluation. Three senior cardiothoracic radiologists (HSS, YQF, and JG, with 31, 13 and 10 years of experience in thoracic radiology, respectively) analyzed the CT characteristics without knowing anything about the clinical data, laboratory findings or patient outcomes. Different opinions from the three readers were discussed until a consensus was reached. According to the standard definitions recommended by the Fleischner Society 15 , the predominant pattern on CT scans was categorized as (1) pure GGO, which was defined as increased lung density with no obscuration of the underlying lung marks; (2) fine reticular pattern, which was defined as GGO with reticulation or intralobular networks that were regular, more uniform than crazy-paving pattern, and of the same size; (3) GGO with consolidation, which was defined as increased lung density with obscuration of the underlying lung marks; and (4) mixed pattern, which refers to a combination of consolidation, GGO, and reticular opacities in the presence of architectural distortion and bronchiectasis; (5) air trapping, which was seen on end-expiration CT scans as parenchymal areas with less than a normal increase in attenuation and a lack of volume reduction. To estimate the extent of lung involvement of all these abnormalities, we assigned a semiquantitative CT score to each of five lung lobes. Each lobe was assigned a score from 0 to 5 (0: no involvement; 1: < 5% involvement; 2: 5-25% involvement; 3: 26-50% involvement; 4: 51-75% involvement; 5: > 75% involvement) 25 . The CT scores for the five lung lobes were added to obtain the total CT score, which measured the overall lung involvement, ranging from 0 (no involvement) to 25 (maximum involvement). www.nature.com/scientificreports/ QCT assessment of air trapping. All follow-up CT images were transferred to IntelliSpacePortal software (Version 9.0) for posttreatment. Lung parenchyma was automatically segmented from the chest wall, mediastinum and airways and then analyzed using threshold techniques. The segmentation was adjusted by two physicians (XYH and XJ, with 5 and 3 years of experience in radiology, respectively). Air trapping was quantified using two measures suggested in the current literature. QCT measures (1) the relative inspiratory to expiratory volume change in voxels with attenuation values from − 860 to − 950 HU (RVC −860 to −950 HU ). Studies 26,27 reported that the volume with HU-values below − 950 on inspiratory and expiratory CT was excluded to correct for emphysematous and cystic lesions. RVC −860 to −950 HU is calculated according to the formula expiratory relative lung volume below − 860 HU-inspiratory relative lung volume below − 860 HU, with relative lung volume below − 860 HU defined as the lung volume between − 860 and − 950 divided by the total lung volume over − 950 HU 27 . Increased air trapping causes a higher RVC −860 to −950 HU value.
QCT measures (2) the expiratory to inspiratory ratio of mean lung density (E/I-ratio MLD ) 28 . Increased air trapping causes a higher E/I-ratio MLD .
Pulmonary function tests. Within 1 week after the 6-month follow-up, PFT was performed and evaluated according to the American Thoracic Society standards on the following items: maximum vital capacity (VC www.nature.com/scientificreports/ max); forced vital capacity (FVC); forced expiratory volume in one second (FEV1); FEV1/FVC ratio; maximal voluntary ventilation (MVV); DL CO ; and DL CO divided by the alveolar volume (DL CO /VA). All PFTs were measured as a percentage of the predicted value. A measured DL CO < 80% of the predicted value indicated pulmonary diffusion impairment. Patients were divided into group 1 with DL CO < 80% predicted and group 2 with DL CO > 80% predicted. Spearman's rank correlation coefficient was used to evaluate factors associated with DL CO . To explore the risk factors associated with abnormal pulmonary diffusion, multiple logistic regression analysis was performed. To prevent data overload, we chose eleven variables that had significant between-group differences as the variables included in the final multiple logistic regression analysis. We included HR, duration of hospital stay, the presence of ARDS, invasive mechanical ventilation and the initial total lesion CT score because there is evidence that these variables are independent predictive factors of fibrotic-like changes in severe COVID-19 survivors 8 . We included d-dimer concentrations because there is emerging evidence of coagulopathy in patients with severe COVID-19 29 . We included the lowest oxygen saturation on room air and the use of glucocorticoids and lactate dehydrogenase (LDH), as these variables were predictors of a 3-month mortality rate of acute exacerbation of idiopathic pulmonary fibrosis 30 . We also included the peak level of leukocyte count and hypersensitive C-reactive protein because of their significant correlation with DL CO . A stepwise logistic regression model with a significance level of 0.05 was used in multivariate analysis. The thresholds of each selected variable were based on the medians or the normal medical range, as appropriate. All statistical analyses were two-sided with a significance level of 0.05.
Comparison of peak laboratory findings. The comparison of laboratory examination data between the two groups is shown in Supplementary Comparison of initial and follow-up CT findings and scores. All patients received an initial CT scan 24 ± 16 days after symptom onset, and the CT findings and scores are summarized in Table 2. Compared with patients in group 2, those in group 1 had much higher CT scores for total lesions (15 ± 8 vs. 12 ± 7, p = 0.004), GGOs (14 ± 8 vs. 11 ± 7, p = 0.001) and reticular lesions (5 ± 5 vs. 4 ± 4, p = 0.002). As shown in Table 3 www.nature.com/scientificreports/ with complete radiological resolution in group 1 was significantly lower than that in group 2 (34% vs. 59%, p < 0.001). However, the percentage of patients with complete radiological resolution who had residual air trapping ( Fig. 2) was significantly lower than the percentage of patients without air trapping ( Fig. 3) (27% vs. 73%).
Correlation coefficient for DL CO . Spearman's rank correlation analysis (Table 4) Table S3, the differences in follow-up pulmonary function indicated that the proportions of patients with VC max%, FVC%, FEV1% and DLCO/V) < 80% predicted in group 1 (DLCO < 80% predicted) were markedly higher than those in group 2 (DLCO ≥ 80% predicted) (p < 0.05). However, no other predicted differences in FEV1/FVC and MVV < 80% were found between the two groups.

Discussion
As the number of COVID-19 survivors increased, there was growing concern about the pulmonary sequelae of COVID-19 survivors. The present study indicated that 43% of COVID-19 survivors had abnormal lung diffusion capacity (DL CO < 80%) at the 6-month follow-up. The semiquantitative CT scores for air trapping and the quantitative air trapping parameters in patients with DLCO < 80% were obviously higher than in patients with DLCO ≥ 80% at follow-up. Multivariate analysis showed that oxygen saturation on room air < 95%, ARDS and leukocyte count > 10 × 10 9 /L at admission were independent risk factors for abnormal pulmonary diffusion at follow-up, which negatively correlated with the follow-up CT score of GGOs, reticulation and air trapping. In our study, the frequency of complete radiological resolution at the 6-month follow-up was 48%, higher than 28-38% in other 6-month follow-up studies 8,31 . The discrepancy may be due to the fact that other studies enrolled moderate to severe COVID-19 patients while this study also included mild patients. GGOs and fibrotic-like changes were the top two most frequent findings of follow-up CT abnormalities in this study, which was consistent with other studies 8,31,32 . Furthermore, fibrotic-like changes (like reticulation, honeycombing and bronchiectasis) increased while GGOs reduced on the 6-month follow-up CT when compared with the baseline CT. This variation trend was also consistent with other studies 8,31 at the 6-month follow-up. It is suggested that fibrotic-like changes maybe the most common CT abnormalities at long-term follow-up. Whether these lesions are reversible needs further research. In addition, several studies [33][34][35] in chronic obstructive pulmonary disease (COPD) or chronic airway disease proposed indicators of air trapping by expiratory chest CT scans. Inspiratory and expiratory CT expose patients to additional radiation, but more research is needed to optimize the radiation dose for the quantification of air trapping 36 . However, QCT scans can distinguish air trapping due to emphysema from air trapping due to small airway diseases (SAD) 33 . QCT is superior to expiratory CT imaging alone to define indicators of SAD as predictors of lung function. Not all patients who receive CT scans complete PFT. Patients who recognize SAD can be recommended to undergo PFT for further confirmation.
CT scans demonstrated that significant air trapping existed in approximately one-third of COVID-19 survivors at the 6-month follow-up. Unfortunately, baseline quantitative inspiratory-expiratory chest CT data of all subjects were unavailable. Therefore, patients who had air trapping before being infected with SARS-CoV-2 cannot be excluded. Several studies [37][38][39] have suggested that air trapping is mainly caused by emphysema or SAD. In the current study, 5 (2.6%) participants self-reported emphysema at admission. Residual air trapping caused by emphysema cannot be excluded. However, the proportion of patients with emphysema (2.6%) in this Table 2. Comparison of initial CT findings and scores between groups. The data are presented as medians (interquartile ranges) or n/N (%). p values comparing patients with DLco < 80%(group 1) and patients with DL CO ≥ 80% (group 2) are from χ 2 , Fisher's exact test, independent-samples T test, or Wilcoxon rank-sum test. GGO ground-glass opacities.

Characteristics
All patients (n = 205) Group 1 (n = 88) Group 2 (n = 117) p value    27,48 have shown that the RVC −860 to −950 HU value can be used to evaluate the extent of air trapping, excluding emphysematous and cystic lesions. However, the E/I-ratio MLD values were not significantly different between the two groups. This result may be explained by the hypothesis that increased density in some regions at follow-up (for example, due to residual GGOs) could make the E/I ratio a nonsuitable parameter for the quantification of air trapping. This finding may also be due to the E/I-ratio MLD cannot reliably distinguish air trapping from emphysema, which was also reflected by a higher extent of emphysema in people with higher E/I ratios 17 . The results revealed that patients with persisting DL CO deficits were more inclined to develop air trapping. Nevertheless, the correlation between the RVC value and lung diffusion function was relatively weak. Some research 49,50 reported that anemia, smoking and pulmonary vascular diseases, such as pulmonary hypertension, can also cause a decrease in DL CO , which may explain the weak correlation.
Multivariate analysis showed that oxygen saturation on room air < 95% was an independent predictor of abnormal pulmonary diffusion function. One study 51 demonstrated that the poor oxygenation of COVID-19  www.nature.com/scientificreports/ patients might be directly related to impaired lung diffusion capacity caused by parenchymal destruction and increased alveolar-capillary distances. Additionally, a higher incidence of ARDS (26.7%) was another independent predictor of lung diffusion dysfunction. Studies 52,53 have shown that ARDS during acute episodes may lead to the development of chronic lung changes and impaired lung diffusion function. Regarding laboratory tests, the present study found that a leukocyte count > 10 × 10 9 /L was also a risk factor for abnormal pulmonary diffusion function. There is some evidence 54,55 to suggest that an elevated leukocyte count could be the result of excessive inflammation of lung tissue caused by SARS-CoV-2, subsequently leading to chronic lung disease and abnormal pulmonary diffusion. The current study revealed that an initial total lesion CT score ≥ 13 was not an independent risk factor for decreased DL CO at follow-up. However, Han et al. 8 found that a higher CT score (≥ 18) on the initial CT was an independent prognostic factor for the presence of fibrotic-like changes at the 6-month follow-up exam. This disparity in the results could be due to differences in demographics and study criteria. The grouping criteria of Han's study included fibrotic-like changes in CT scans, which may overestimate the population with true fibrotic lung disease, and only severe COVID-19 patients were included in Han's research. Of the 205 patients enrolled in our study, 80 had severe disease, and 125 had mild disease. Nevertheless, negative correlations between DL CO and the initial CT scores of total lesions, GGOs and reticulation of COVID-19 survivors were found in the current study. Similarly, Hui et al. 56,57 reported significant negative correlations between abnormal chest radiograph (CXR) scores and DL CO in SARS survivors, reflecting the physiologic effects of lung parenchymal inflammation and fibrosis. Thus, COVID-19 survivors may also have residual lung fibrosis that induces abnormal pulmonary diffusion.
However, there were several limitations in this study. First, the study sample was small, and this study was only a 6-month follow-up study. A larger sample size and longer follow-up would be more ideal to judge the reversibility of lung abnormalities. Second, baseline pulmonary function data and quantitative inspiratory-expiratory chest CT data were unavailable. Patients who had air trapping before being infected with SARS-CoV-2 cannot be excluded. The observed impaired pulmonary function and air trapping cannot be directly attributed to COVID-19. However, the proportion of patients with chronic pulmonary disease in this cohort was very low, which may have had a small effect on the results. Third, 72/205 (35%) patients had a slice thickness of 5 mm in the initial scan, which may prevent subtle findings from being easily noticed. However, all follow-up CT scans Table 4. Correlation coefficient for DL CO . All data were analyzed using Spearman correlation. HR heart rate, ARDS acute respiratory distress syndrome, GGO ground-glass opacities.