Study design and participants
COVID-FIBROTIC (clinicaltrials.gov NCT04409275) is a prospective, multicenter, observational follow-up study of patients admitted for bilateral COVID-19 pneumonia in 12 hospitals in Spain. Diagnosis of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was based on centers of disease control (CDC) criteria, with all patients confirmed by reverse transcription polymerase chain reaction (PCR). Diagnosis of COVID-19 pneumonia was established in accordance with World Health Organization (WHO) interim guidance if patients met any of the following criteria: oxygen saturation (SpO2) < 94% in room air at sea level, arterial partial pressure of oxygen to fraction of inspired oxygen (PaO2/FiO2) ratio < 300 mmHg, respiratory frequency > 30 breaths/min, or lung infiltrates > 50% [16]. The extent of pneumonia at the time of emergency room diagnosis was quantified by adapting the Radiographic Assessment of Lung Edema (RALE) score to COVID-19 (minimum 0–maximum 8) [17]. When there was no prior history of pulmonary diseases (except for asthma or sleep apnea) or uncontrolled cardiac or renal failure, findings were attributed to SARS-CoV-2 infection.
All patients discharged from respiratory services aged over 18 with a life expectancy > 1 year were invited to participate. Patients with unilateral COVID-19 pneumonia, a previous diagnosis of interstitial lung disease (ILD) or chronic obstructive pulmonary disease (COPD) and/or with difficulties in attending the centers for follow-up visits were excluded. Patients who experienced a pulmonary embolism during admission were not excluded if they were properly anticoagulated and had shown embolism resolution in a previous angioCT.
Pulmonary function testing (PFT) and 6-m walk test (6MWT)
Pulmonary function tests (PFT) were performed in the respiratory functional testing laboratory in all participating centers and included determination of lung volumes [total lung capacity (TLC), residual volume (RV), functional residual capacity (FRC) using plethysmography], and spirometry [forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1) and FEV1/FVC ratio]. DLCO was determined by the single breath technique using an infrared analyzer, correcting for hemoglobin values. All procedures were performed according to American Thoracic Society (ATS) and European Respiratory Society (ERS) guidelines [18,19,20,21].
The 6-min walk test (6MWT) measures the distance that a patient can walk quickly on a flat, hard surface in a period of 6 min. It evaluates the global and integrated responses of all systems involved during exercise, including pulmonary and cardiovascular systems, systemic circulation, peripheral circulation, blood, neuromuscular units and muscle metabolism [22, 23].
Imaging tests
Control chest X-rays (CXRs) were performed in all patients 2 months after discharge using standardized techniques with computed radiography equipment. Pulmonary damage was quantified using the RALE score. HRCT scans (SOMATOM, Siemens, Germany; AQUILION, Toshiba, Japan; OPTIMA, General Electric, USA) were obtained with subjects in the supine position during breath hold at end-inspiration. Axial reconstructions were performed with a slice thickness of 1 mm, with 1 mm increment, 512 mm × 512 mm (resolution 0.625 mm/10). The same protocol was used in each center, adjusted to the different CT machines. HRCT images were evaluated for presence of ground-glass opacities (GGO), consolidations, bronchiectasis, parenchymal bands and reticulations as defined by the Fleischner society glossary of terms [24]. Fibrotic-like changes on HRCT were defined as presence of traction bronchiectasis, parenchymal bands and/or reticular pattern [25, 26]. Experienced chest radiologists in each centers, blinded to all clinical and functional data, evaluated the images. CT scans performed at 1 year were also compared with the 2-month scan.
Procedures
During the screening visit (30 days after hospital discharge), demographic data (age, sex, ethnicity) and medical history (smoking, hypertension, diabetes, previous respiratory, cardiac or renal history and concomitant medication) were collected. Data associated with the acute episode were also collected (days of symptom onset, intensity of dyspnea, extent of pneumonia on diagnostic X-ray in the emergency room, maximum radiological extent during admission, days of admission, maximum respiratory support and inflammatory laboratory values). All data were retrieved from electronic medical records and de-identified data were entered into an electronic database (Veridata™ EDC). Patients were assessed 2 months after discharge (visit 1, V1) by collecting residual dyspnea using the modified British Medical Research Council (mMRC) [27], CXR and PFT. Patients with impairment in PFT (FVC < 80% without FEV1/FVC < 70 and/or DLCO < 80%) and/or persistent radiological alteration on CXR underwent thoracic HRCT [28]. Functional changes, exercise capacity using the 6MWT and evolution of dyspnea were assessed at 2 (V1), 6 (visit 2, V2) and 12 months (visit 3, V3) after discharge, repeating chest HRCT in patients without complete resolution at 2-month CT scan. No further treatment was indicated.
For further analysis, patients were stratified according to WHO Ordinal Outcomes Scale [29] into three groups, depending on the maximum respiratory support needed:
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Group 1: hospitalized mild disease (scale 4): hospitalized patients who required supplemental oxygen by mask or nasal prongs.
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Group 2: hospitalized severe disease (scale 5): hospitalized patients who required non-invasive ventilation (non-IMV) or high flow nasal oxygen cannula (HFNC).
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Group 3: hospitalized critical disease (scale 6–7): hospitalized patients who required respiratory support by intubation and invasive mechanical ventilation (IMV).
Statistical analysis
We followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) [30] guidelines for reporting observational studies. We calculated the required sample size using the ‘nsize’ command in Stata 12.1 based on data published by Hui [31], who describe a proportion of fibrotic radiological abnormalities on chest X-ray on 27.8% patients (SARS survivors in 2003) assuming a maximum error of 5% and 95% confidence.
Qualitative variables were described using frequencies and percentages, and quantitative variables by means and standard deviation. Normality for continuous variables was checked using the Shapiro-Wilks test and if their normal distribution was not confirmed, variables were expressed with median and interquartile range.
Mean comparison was carried out using the Student t-test in the presence of normality, and if otherwise, using the Mann–Whitney U test. For qualitative variables, comparison of percentages between groups was carried out using Fisher's exact test for dichotomous variables or chi-square test for contingency tables with more than two categories. Patient pulmonary function at follow-up was assessed with mixed linear models for quantitative outcomes, with individual identification key as random effect and time and/or severity status and interaction as independent factors. Cochran-Q test was used in the case of dichotomous outcomes. Factors associated with diffusion impairment and fibrotic pattern at 12 months were studied using multivariable logistic regression. The variables for the multivariable analysis were selected using Akaike’s Information Criterion in a backward-forward stepwise procedure.
Effect of time and severity in DLCO, 6MM and FVC was evaluated using linear mixed models with time (V1, V2, V3) and severity (mild, moderate, severe) as fixed effects and individuals as random effect; interaction between severity and time was also included. Compound symmetry was used for correlation between time measures of same individual. Similarly dyspnea was dichotomized into 2 or more versus less than 2, model proposed was a generalized linear mixed model with time, severity and interaction between both fixed effects and individuals as random effect. Post-hoc analysis was carried out in the interaction term with p value adjustment according to Bonferroni method for 36 tests.