Study design and patients
This is a prospective cohort study of patients attending the COVID-19 Research Clinic at the Faculty of Medicine, University Technology MARA (UITM) in Malaysia, from March 2021 to December 2022. The inclusion criteria were Malaysians aged eighteen years and above, with confirmed COVID-19 via validated reverse transcription-polymerase chain reaction method, who had moderate-to-critical illness according to the WHO classification,13 and were at least three months post-discharge from either the Sungai Buloh Hospital or the UITM Medical Centre.14 Patients with pre-existing chronic lung diseases before COVID-19, including bronchial asthma, as well as individuals who were pregnant, completely immobilized, had uncontrolled psychiatric illness, or were contraindicated for spirometry were excluded.
A minimum sample size of 386 subjects was determined using the formula for a cross-sectional study - sample size = Z1-α 2p(1 - p)/d2.15 Z represented the confidence interval at 95%, d denoted the margin of error at 5%, and p referred to the proportion of abnormal lung function (52.7%) among SARS survivors in a previous study.8 All patients provided written informed consent before participating in the study. The study received ethics approval from the Medical Research Ethics Committee of the Ministry of Health Malaysia (NMRR-20-2011-56330 (IIR) and the respective hospitals, and it was conducted in adherence to the Declaration of Helsinki.
Procedure and outcomes
Eligible patients were consecutively identified from the COVID-19 registry of Sungai Buloh Hospital and UITM Medical Centre. Those meeting all inclusion criteria and having none of the exclusion criteria were scheduled for early physical appointments at the COVID-19 Research Clinic.
1. Demographic, clinical, and hospitalization data:
Demographic, clinical, and hospitalization data were gathered through face-to-face interviews and the electronic records. Demographic information included age, gender, and ethnicity, while clinical details included smoking status, obesity, and the presence of underlying chronic diseases. Hospitalization data included the duration of illness before admission, length of hospital stays, COVID-19 severity at presentation, the most severe COVID-19 episode during hospitalization, pharmacotherapy administrated, respiratory support provided, the occurrence of respiratory complications, and details regarding intensive care unit (ICU) admission, including its length of stay.
The severity of COVID-19 was defined according to the WHO classification as: asymptomatic, mild (symptomatic without pneumonia), moderate (pneumonia without hypoxia), severe (pneumonia with hypoxia requiring oxygen supplementation), and critical (critically ill, such as ARDS, sepsis, or septic shock).13 Available treatments for COVID-19 during the study period included corticosteroids, hydroxychloroquine, immunomodulators (tocilizumab, interferon beta, and interferon alpha), and antivirals (favipiravir, lopinavir-ritonavir, ritonavir, and atazanavir).16 Respiratory support was categorized into oxygen supplementation by nasal cannulae, venti-mask, or high-flow mask, non-invasive mechanical ventilation (NIV) or nasal high flow (NHF), and invasive mechanical ventilation (IMV).17,18 Common respiratory complications of COVID-19 that were recorded included ARDS, pulmonary embolism, pneumothorax, and pleural effusion.19,20
2. Patients reported outcomes (PROs):
Patients were instructed to independently complete the modified Medical Research Council (mMRC) dyspnea scale and the post-COVID-19 Functional Status (PCFS) scale with minimal assistance from investigators. The mMRC and PCFS was interpreted as per the original validation of the questionnaire.21,22 A higher score indicates a greater degree of symptom severity and impairment, respectively.
3. Lung function tests:
Spirometry was conducted using SpiroUSBTM (Vyaire Medical, Chicago, IL) to obtain dynamic lung volumes, including the forced expiratory volume in one second (FEV1) and forced vital capacity (FVC). The cut-off value of ≥ 80% of the predicted was deemed normal for both parameters. Spirometry results were categorized into four groups: normal spirometry – normal FEV1, normal FVC, and FEV1/FVC > 0.7; restrictive pattern – reduced or normal FEV1, reduced FVC, and FEV1/FVC > 0.7; obstructive pattern – reduced FEV1, reduced or normal FVC, and FEV1/FVC < 0.7; and preserved ratio impaired spirometry (PRISm) – reduced FEV1, normal FVC¸ and FEV1/FVC > 0.7.23,24 For patients with an obstructive pattern, post-bronchodilator spirometry was performed to identify reversible airflow obstruction. Those with a restrictive pattern were scheduled for static lung volumes and diffusion capacity measurement within two weeks using PFT Vyntus BodyboxTM (Vyaire Medical, Chicago, IL). The parameters measured included residual volume (RV), total lung capacity (TLC), diffusion capacity for carbon monoxide (DLCO), and carbon monoxide transfer coefficient (DLCO/Va). All lung function tests were conducted by certified respiratory technicians following the American Thoracic Society (ATS) and European Respiratory Society guidelines.25,26
4. Cardiopulmonary functional tests:
Patients underwent a 6-minute walk test (6MWT) under the guidance of a certified respiratory physiotherapist, following the ATS guideline.27 Their pulses and oxygen saturation were continuously monitored using the Nonin® WristOx2 ™ 3150 Bluetooth Pulse Oximeter. A 1-minute sit-to-stand test (1MSTS) guided by the same respiratory physiotherapist followed and in accordance with the procedure outlined in a previous study.28 Both assessments utilized a digital stopwatch for time measurement, and the Borg scale was employed to assess the severity of dyspnea and fatigue.
5. Radio-imaging:
All patients underwent a standard posterior-anterior chest X-ray examination. Only those demonstrating a restrictive pattern in spirometry were scheduled for high-resolution computed tomography (HRCT) of the lungs within one month. Radiologists, blinded to patients' information, independently reviewed these images to identify consolidation, ground glass opacity (GGO), and lung parenchymal reticulation. Additionally, the HRCT also documented the presence of organizing pneumonia (OP) and other findings, such as lung nodules, atelectasis, pleural effusion or thickening, diaphragmatic elevation, cardiomegaly, and fractures, if any. Lung involvement severity was assessed using the CT-score method developed by Kunhua Li et al.29 Each lobe received a score ranging from 0 to 5 based on its level of involvement: 0 (0%), 1 (< 5%), 2 (5 – 25%), 3 (26 – 49%), 4 (50 – 75%), and 5 (> 75%). The total score, representing cumulative involvement across all lobes, ranged from 0 to 25 points.
Statistical analyses
Categorical variables are presented as percentages, while continuous variables are presented as mean ± standard deviation (SD). Patients were categorized into those with normal versus those with abnormal spirometry for two-group comparisons, as well as normal versus restrictive pattern or PRISm/obstructive pattern spirometry for three-group comparisons. Between-group differences were assessed using an independent t-test for continuous variables and a Chi-Square test for categorical variables. A two-sided p-value of less than 0.05 was considered statistically significant.
For multivariate analyses, variables exhibiting significant two-sided p-values in the univariate analyses were included as covariates in binary logistic regression and multinomial logistic regression. The latter analysis excluded variables showing multicollinearity (variance inflation factor > 5). The analysis aimed to derive odds ratios (OR), 95% confidence intervals (95% CI), and two-sided p-values. Statistical analysis was conducted using the Statistical Package for the Social Sciences (SPSS for Windows version 25.0, SPSS Inc, Chicago, IL, USA).