BACKGROUND:Target-controlled infusion (TCI) optimizes drug delivery and hemodynamic stability using individualized pharmacokinetic models. We compared the effects of TCI versus manually controlled infusion (MCI) of propofol and remifentanil on postoperative recovery in elderly cardiac surgery patients. METHODS:An international retrospective cohort study was conducted using data from Union Hospital and the INSPIRE dataset. Elderly patients (≥65 years) receiving TCI were matched 1:1 with those receiving MCI using propensity scores to adjust for confounding variables. The primary outcome was the length of stay in the ICU. Secondary outcomes included postoperative complications and intraoperative hemodynamic stability. RESULTS:Of 1398 enrolled patients, propensity score matching yielded 468 in the Union cohort and 270 in the INSPIRE cohort. In the Union cohort, TCI was associated with a significantly shorter ICU stay (Mean Difference [MD] -0.92 days; 95% CI: -1.68 to -0.17; P = 0.017), lower odds of reintubation (OR 0.94; 95% CI: 0.90-0.99), reduced AKI incidence (OR 0.84; 95% CI: 0.78-0.91), and lower in-hospital mortality (OR 0.95; 95% CI: 0.92-0.99) compared with MCI. Validation in the INSPIRE cohort confirmed shorter ICU stays (MD -1.20 days; 95% CI: -2.32 to -0.80; P = 0.036) and lower reintubation rates (OR 0.96; 95% CI: 0.92-0.99). In both cohorts, TCI was associated with superior intraoperative hemodynamic stability, characterized by lower blood pressure variability and a reduced burden of hypotension. CONCLUSION:TCI is associated with superior intraoperative hemodynamic stability, significantly shorter ICU length of stay, and improved postoperative recovery profiles compared with MCI in elderly cardiac surgery patients.

ABSTRACT Hypoalbuminemia is a recognized risk factor for postoperative pulmonary complications (PPCs), but whether preoperative albumin infusion improves pulmonary outcomes after cardiac surgery remains unclear. In this single‐center, randomized, double‐blind, controlled pilot trial (ChiCTR2300076609), 80 elderly patients with preoperative albumin concentrations <40 g/L who underwent elective cardiac surgery with cardiopulmonary bypass were randomly assigned to receive 100 mL of 20% albumin (40 patients) or saline before surgery (40 patients). The primary outcomes were PPC severity and incidence during hospitalization, and perioperative immune cell changes were also explored. All randomized participants completed the trial. Compared with the controls, the patients in the albumin group had lower PPC severity (median [IQR], 1 [1–2] vs. 2 [2–3]; mean difference, −0.75; 95% CI, −1.15 to −0.35; p = 0.001) and a lower incidence of PPCs (30% vs. 57.5%; risk difference, −0.275; 95% CI, −0.48 to −0.07; p = 0.013). Additionally, the Th17 cell proportion and γδTreg cell proportion were significantly decreased and increased, respectively, in the albumin group after surgery (p < 0.001). These findings suggest that preoperative albumin supplementation may reduce PPC risk and modulate immune responses, warranting confirmation in larger multicenter trials.

Background: Acute kidney injury (AKI) is a common and serious complication after cardiac surgery that significantly affects patient outcomes. Given the limited treatment options available, identifying modifiable risk factors is critical. Frailty and obesity, two heterogeneous physiological states, have significant implications for identifying and preventing AKI. Our study investigated the interplay among frailty, body composition, and AKI risk after cardiac surgery to inform patient management strategies. Material and methods: This retrospective cohort study included three international cohorts. Primary analysis was conducted on adult patients who underwent cardiac surgery between 2014 and 2019 at Wuhan Union Hospital, China. We tested the generalizability of our findings with data from two independent international cohorts, the Medical Information Mart for Intensive Care IV (MIMIC-IV) and the eICU Collaborative Research Database. Frailty was assessed using a clinical lab-based frailty index (FI-LAB), while total body fat percentage (BF%) was calculated based on a formula accounting for BMI, sex, and age. Logistic regression models were used to analyze the associations between frailty, body fat, and AKI, adjusting for pertinent covariates. Results: A total of 8785 patients across three international cohorts were included in the study. In the primary analysis of 3569 patients from Wuhan Union Hospital, moderate and severe frailty were associated with an increased AKI risk after cardiac surgery. Moreover, a nonlinear relationship was observed between BF% and AKI risk. When stratified by the degree of frailty, lower body fat correlated with a decreased incidence of AKI. Extended analyses using the MIMIC-IV and eICU cohorts (n=3951 and n=1265, respectively) validated these findings and demonstrated that a lower total BF% was associated with decreased AKI incidence. Moderation analysis revealed that the effect of frailty on AKI risk was moderated by the BF%. Sensitivity analyses demonstrated results consistent with the main analyses. Conclusion: Higher degrees of frailty were associated with an elevated risk of AKI following cardiac surgery, and total BF% moderated this relationship. This research underscores the significance of integrating frailty and body fat assessments into routine cardiovascular care to identify high-risk patients for AKI and implement personalized interventions to improve patient outcomes.

Sepsis is a leading cause of in-hospital mortality, with more than 50% of patients developing Acute Lung Injury (ALI). Alveolar macrophages are the primary immune cells in lung tissue and play a crucial role in the pathogenesis of ALI. In this study, we investigated the potential therapeutic effects of dexmedetomidine (DEX) in both sepsis mice models of ALI and Lipopolysaccharide & Interferon gamma (LPS&IFN-γ)- stimulated macrophages, elucidating its underlying mechanism. In vivo experiments were conducted using the cecal ligation and puncture (CLP) technique to induce sepsis in mice, and intraperitoneal administration of DEX was initiated at 3, 6, 12 h post-CLP. Results showed that DEX significantly ameliorated CLP-induced lung tissue injury, impeded proinflammatory cytokine release, and reduced neutrophil infiltration. Additionally, DEX decreased M1 macrophage polarization in the lung tissue of septic mice. In vitro, experiments showed that DEX effectively attenuated the elevation of macrophage glycolysis levels induced by LPS & IFN-γ. Mechanistically, DEX treatment suppressed the phosphorylation of Pyruvate kinase M2 (PKM2) at the Y105 site and its nuclear translocation levels, thus disrupting the Warburg effect to reduce M1 cell polarization. Therefore, this work has demonstrated the role of DEX in macrophage polarization and sepsis-induced acute lung injury, providing valuable guidance for the treatment of critically ill patients with clinical sepsis.

Postoperative pulmonary infection (POI) is strongly associated with a poor prognosis and has a high incidence in elderly patients undergoing major surgery. Machine learning (ML) algorithms are increasingly being used in medicine, but the predictive role of logistic regression (LR) and ML algorithms for POI in high-risk populations remains unclear. We conducted a retrospective cohort study of older adults undergoing major surgery over a period of six years. The included patients were randomly divided into training and validation sets at a ratio of 7:3. The features selected by the least absolute shrinkage and selection operator regression algorithm were used as the input variables of the ML and LR models. The random forest of multiple interpretable methods was used to interpret the ML models. Of the 9481 older adults in our study, 951 developed POI. Among the different algorithms, LR performed the best with an AUC of 0.80, whereas the decision tree performed the worst with an AUC of 0.75. Furthermore, the LR model outperformed the other ML models in terms of accuracy (88.22

Background Although blood flow is restored after treatment of myocardial infarction (MI), myocardial ischemia and reperfusion (I/R) can cause cardiac injury, which is a leading cause of heart failure. Gastrodin (GAS) exerts protective effects against brain, heart, and kidney I/R. However, its pharmacological mechanism in myocardial I/R injury (MIRI) remains unclear. Purpose GAS regulates autophagy in various diseases, such as acute hepatitis, vascular dementia, and stroke. We hypothesized that GAS could repair mitochondrial damage and regulate autophagy to protect against MIRI. Study design Male C57BL/6 mice and H9C2 cells were subjected to I/R and hypoxia-reoxygenation (H/R) injury after GAS administration, respectively, to assess the impact of GAS on cardiomyocyte phenotypes, heart, and mitochondrial structure and function. The effect of GAS on cardiac function and mitochondrial structure in patients undergoing cardiac surgery has been observed in clinical practice. Methods The effects of GAS on cardiac structure and function, mitochondrial structure, and expression of related molecules in an animal model of MIRI were evaluated using immunohistochemical staining, enzyme-linked immunosorbent assay (ELISA), transmission electron microscopy, western blotting, and gene sequencing. Its effects on the morphological, molecular, and functional phenotypes of cardiomyocytes undergoing H/R were observed using immunohistochemical staining, real-time quantitative PCR, and western blotting. Results GAS significantly reduces myocardial infarct size and improves cardiac function in MIRI mice in animal models and increases cardiomyocyte viability and reduces cardiomyocyte damage in cellular models. In clinical practice, myocardial injury was alleviated with better cardiac function in patients undergoing cardiac surgery after the application of GAS; improvements in mitochondria and autophagy activation were also observed. GAS primarily exerts cardioprotective effects through activation of the PINK1/Parkin pathway, which promotes mitochondrial autophagy to clear damaged mitochondria. Conclusion GAS can promote mitophagy and preserve mitochondria through PINK1/Parkin, thus indicating its tremendous potential as an effective perioperative myocardial protective agent.

Objective:The risk factors of postoperative pulmonary complications (PPCs) have been extensively investigated in non-cardiac surgery and non-elderly adult patients undergoing cardiac surgery. However, data on elderly patients after cardiopulmonary bypass (CPB) is limited. This study aimed to evaluate the risk factors and short-term outcomes for PPCs in elderly patients undergoing CPB procedures. Patients and Methods:Data from 660 patients who underwent CPB over a six-year period at a tertiary care hospital were collected. The primary outcome encompassed the incidence of PPCs, including re-intubation, postoperative mechanical ventilation exceeding 48 hours, pulmonary infection, pleural effusion requiring thoracic drainage, and acute respiratory distress syndrome. Missing data were managed using multiple imputation. Univariate analysis and the multiple logistic regression method were utilized to ascertain independent risk factors for PPCs. Results:Among the 660 patients, PPCs were observed in 375 individuals (56.82%). Multiple logistic regression identified serum albumin levels <40 g/L, type of surgery, CPB duration >150 minutes, blood transfusion, and intra-aortic balloon pump use before extubation as independent risk factors for PPCs. Patients experiencing PPCs had prolonged mechanical ventilation, extended hospitalization and ICU stays, elevated postoperative mortality, and higher tracheotomy rates compared to those without PPCs. Conclusion:Elderly patients following CPB displayed a substantially high incidence of PPCs, significantly impacting their prognosis. Additionally, this study identified five prominent risk factors associated with PPCs in this population. These findings enable clinicians to better recognize patients who may benefit from perioperative prevention strategies based on these risk factors.

Background:The predictive effect of systemic inflammatory factors on postoperative pulmonary complications in elderly patients remains unclear. In addition, machine learning models are rarely used in prediction models for elderly patients.Patients and Methods:We retrospectively evaluated elderly patients who underwent general anesthesia during a 6-year period. Eligible patients were randomly assigned in a 7:3 ratio to the development group and validation group. The Least logistic absolute shrinkage and selection operator (LASSO) regression model and multiple logistic regression analysis were used to select the optimal feature. The discrimination, calibration and net reclassification improvement (NRI) of the final model were compared with "the Assess Respiratory Risk in Surgical Patients in Catalonia" (ARISCAT) model.Results:Of the 9775 patients analyzed, 8.31% developed PPCs. The final model included age, preoperative SpO2, ANS (the Albumin/NLR Score), operation time, and red blood cells (RBC) transfusion. The concordance index (C-index) values of the model for the development cohort and the validation cohort were 0.740 and 0.748, respectively. The P values of the Hosmer-Lemeshow test in two cohorts were insignificant. Our model outperformed ARISCAT model, with C-index (0.740 VS 0.717, P = 0.003) and NRI (0.117, P < 0.001).Conclusion:Based on LASSO machine learning algorithm, we constructed a prediction model superior to ARISCAT model in predicting the risk of PPCs. Clinicians could utilize these predictors to optimize prospective and preventive interventions in this patient population.

Remote Ischemic Preconditioning (RIPC) can reduce myocardial ischemia-reperfusion injury, but its mechanism is not clear. In order to explore the mechanism of RIPC in myocardial protection, we collected myocardial specimens during cardiac surgery in children with tetralogy of Fallot for sequencing. Our study found RIPC reduces the expression of the calcium channel subunit cacna2d3, thereby impacting the function of calcium channels. As a result, calcium overload during ischemia-reperfusion is reduced, and the activation of calpain 1 is inhibited. This ultimately leads to a decrease in calpain 1 cleavage of Bax, consequently inhibiting increased mitochondrial permeability-mediated apoptosis. Notably, in both murine and human models of myocardial ischemia-reperfusion injury, RIPC inhibiting the expression of the calcium channel subunit cacna2d3 and the activation of calpain 1, improving cardiac function and histological outcomes. Overall, our findings put forth a proposed mechanism that elucidates how RIPC reduces myocardial ischemia-reperfusion injury, ultimately providing a solid theoretical foundation for the widespread clinic application of RIPC.

Calpain is a conserved cysteine protease readily expressed in several mammalian tissues, which is usually activated by Ca2+ and with maximum activity at neutral pH. The activity of calpain is tightly regulated because its aberrant activation will nonspecifically cleave various proteins in cells. Abnormally elevation of Ca2+ promotes the abnormal activation of calpain during myocardial ischemia-reperfusion, resulting in myocardial injury and cardiac dysfunction. In this paper, we mainly reviewed the effects of calpain in various programmed cell death (such as apoptosis, mitochondrial-mediated necrosis, autophagy-dependent cell death, and parthanatos) in myocardial ischemia-reperfusion. In addition, we also discussed the abnormal activation of calpain during myocardial ischemia-reperfusion, the effect of calpain on myocardial repair, and the possible future research directions of calpain.

BACKGROUND:The pathogenesis of neuropathic pain and the reasons for the prolonged unhealing remain unknown. Increasing evidence suggests that sex oestrogen differences play a role in pain sensitivity, but few studies have focused on the oestrogen receptor which may be an important molecular component contributing to peripheral pain transduction. We aimed to investigate the impact of oestrogen receptors on the nociceptive neuronal response in the dorsal root ganglion (DRG) and spinal dorsal horn using a spared nerve injury (SNI) rat model of chronic pain.METHODS:We intrathecally (i.t.) administered a class of oestrogen receptor antagonists and agonists intrathecal (i.t.) administrated to male rats with SNI or normal rats to identify the main receptor. Moreover, we assessed genes identified through genomic metabolic analysis to determine the key metabolism point and elucidate potential mechanisms mediating continuous neuronal sensitization and neuroinflammatory responses in neuropathic pain. The excitability of DRG neurons was detected using the patch-clamp technique. Primary culture was used to extract microglia and DRG neurons, and siRNA transfection was used to silence receptor protein expression. Immunofluorescence, Western blotting, RT-PCR and behavioural testing were used to assess the expression, cellular distribution, and actions of the main receptor and its related signalling molecules.RESULTS:Increasing the expression and function of G protein-coupled oestrogen receptor (GPER), but not oestrogen receptor-α (ERα) and oestrogen receptor-β (ERβ), in the DRG neuron and microglia, but not the dorsal spinal cord, contributed to SNI-induced neuronal sensitization. Inhibiting GPER expression in the DRG alleviated SNI-induced pain behaviours and neuroinflammation by simultaneously downregulating iNOS, IL-1β and IL-6 expression and restoring GABAα2 expression. Additionally, the positive interaction between GPER and β-alanine and subsequent β-alanine accumulation enhances pain sensation and promotes chronic pain development.CONCLUSION:GPER activation in the DRG induces a positive association between β-alanine with iNOS, IL-1β and IL-6 expression and represses GABAα2 involved in post-SNI neuropathic pain development. Blocking GPER and eliminating β-alanine in the DRG neurons and microglia may prevent neuropathic pain development.

Background Successful weaning and extubation after cardiac surgery is an important step of postoperative recovery. Delayed extubation is associated with poor prognosis and high mortality, thereby contributing to a substantial economic burden. The aim of this study was to develop and validate a prediction model estimate the risk of delayed extubation after cardiac surgery based on perioperative risk factors. Methods We performed a retrospective cohort study of adult patients undergoing cardiac surgery from 2014 to 2019. Eligible participants were randomly assigned into the development and validation cohorts, with a ratio of 7:3. Variables were selected using least absolute shrinkage and selection operator (LASSO) logistic regression model with 10-fold cross-validation. Multivariable logistic regression was applied to develop a predictive model by introducing the predictors selected from the LASSO regression. Receiver operating characteristic (ROC) curve, calibration plot, decision curve analysis (DCA) and clinical impact curve were used to evaluate the performance of the predictive risk score model. Results Among the 3,919 adults included in our study, 533 patients (13.6%) experienced delayed extubation. The median ventilation time was 68 h in the group with delayed extubation and 21 h in the group without delayed extubation. A predictive scoring system was derived based on 10 identified risk factors based on 10 identified risk factors including age, BMI ≥ 28 kg/m2, EF < 50%, history of cardiac surgery, type of operation, emergency surgery, CPB ≥ 120 min, duration of surgery, IABP and eGFR < 60 mL/min/1.73 m2. According to the scoring system, the patients were classified into three risk intervals: low, medium and high risk. The model performed well in the validation set with AUC of 0.782 and a non-significant p-value of 0.901 in the Hosmer-Lemeshow test. The DCA curve and clinical impact curve showed a good clinical utility of this model. Conclusions We developed and validated a prediction score model to predict the risk of delayed extubation after cardiac surgery, which may help identify high-risk patients to target with potential preventive measures.

Fra-1(Fos-related antigen1), a member of transcription factor activator protein (AP-1), plays an important role in cell proliferation, apoptosis, differentiation, inflammation, oncogenesis and tumor metastasis. Accumulating evidence suggest that the malignancy and invasive ability of tumors can be significantly changed by directly targeting Fra-1. Besides, the effects of Fra-1 are gradually revealed in immune and inflammatory settings, such as arthritis, pneumonia, psoriasis and cardiovascular disease. These regulatory mechanisms that orchestrate immune and non-immune cells underlie Fra-1 as a potential therapeutic target for a variety of human diseases. In this review, we focus on the current knowledge of Fra-1 in immune system, highlighting its unique importance in regulating tissue homeostasis. In addition, we also discuss the possible critical intervention strategy in diseases, which also outline future research and development avenues.

Alveolar macrophages (AMs) play an essential role in ventilator-induced lung injury (VILI). Exosomes and their cargo, including microRNAs (miRNAs/miRs) serve as regulators of the intercellular communications between macrophages and epithelial cells (ECs), and are involved in maintaining homeostasis in lung tissue. The present study found that exosomes released by ECs subjected to cyclic stretching promoted M2 macrophage polarization. It was demonstrated that miR-21a-5p, upregulated in epithelial-derived exosomes, increased the percentage of M2 macrophages by suppressing the expression of Notch2 and the suppressor of cytokine signaling 1 (SOCS1). The overexpression of Notch2 decreased the percentage of M2 macrophages. However, these effects were reversed by the downregulation of SOCS1. The percentage of M2 macrophages was increased in both short-term high- and low-tidal-volume mechanical ventilation, and the administration of exosomes-derived from cyclically stretched ECs had the same function. However, the administration of miR-21a-5p antagomir decreased M2 macrophage activation induced by cyclically stretched ECs or ventilation. Thus, the present study demonstrates that the intercellular transferring of exosomes from ECs to AMs promotes M2 macrophage polarization. Exosomes may prove to be a novel treatment for VILI.

Abstract Background: Postoperative delirium (POD) not only increases the medical burden but also adversely affects patient prognosis. Although some cases of delirium can be avoided by early intervention, there is no clear evidence indicating whether any of these measures can effectively prevent POD in specific patient groups. Objective: The aim of this meta-analysis was to compare the efficacy and safety of the existing preventive measures for managing POD. Methods: The PubMed, OVID (Embase and MEDLINE), Web of Science, and the Cochrane Library databases were searched for articles published before January 2020. The relevant randomized controlled trials (RCTs) were selected based on the inclusion and exclusion criteria. Data extraction and methodological quality assessment were performed according to a predesigned data extraction form and scoring system, respectively. The interventions were compared on the basis of the primary outcome like incidence of POD, and secondary outcomes like duration of delirium and the length of intensive care unit and hospital stay. Results: Sixty-three RCTs were included in the study, covering interventions like surgery, anesthesia, analgesics, intraoperative blood glucose control, cholinesterase inhibitors, anticonvulsant drugs, antipsychotic drugs, sleep rhythmic regulation, and multi-modal nursing. The occurrence of POD was low in 4 trials that monitored the depth of anesthesia with bispectral index during the operation (P < .0001). Two studies showed that supplementary analgesia was useful for delirium prevention (P = .002). Seventeen studies showed that perioperative sedation with α2-adrenergic receptor agonists prevented POD (P = .0006). Six studies showed that both typical and atypical antipsychotic drugs can reduce the incidence of POD (P = .002). Multimodal nursing during the perioperative period effectively reduced POD in 6 studies (P < .00001). Furthermore, these preventive measures can reduce the duration of delirium, as well as the total and postoperative length of hospitalized stay for non-cardiac surgery patients. For patients undergoing cardiac surgery, effective prevention can only reduce the length of intensive care unit stay. Conclusion: Measures including intraoperative monitoring of bispectral index, supplemental analgesia, α2-adrenergic receptor agonists, antipsychotic drugs, and multimodal care are helpful to prevent POD effectively. However, larger, high-quality RCTs are needed to verify these findings and develop more interventions and drugs for preventing postoperative delirium.

缺血性心脏病是目前全球首位致病和致残的疾病之一,给公共卫生系统和病人生活都带来巨大的负担,缺血/再灌注损伤是其中最常见的病理生理机制.微小核糖核酸(miRNAs)是近年来心肌缺血再灌注损伤中研究较多的调控因子,对于线粒体的功能有重要的调节作用,尤其是定位于线粒体中的miRNAs (mitomiRNAs)可通过改变关键信号通路来影响线粒体功能.在这篇综述中,我们列举了一些影响线粒体功能的miRNAs,并提出了其目前在临床转化中面临的问题与挑战,为心肌缺血再灌注损伤的后续研究提供理论参考.

Background: Postoperative delirium (POD) not only increases the medical burden but also adversely affects patient prognosis. Although some cases of delirium can be avoided by early intervention, there is no clear evidence indicating whether any of these measures can effectively prevent POD in specific patient groups. Objective: The aim of this meta-analysis was to compare the efficacy and safety of the existing preventive measures for managing POD. Methods: The PubMed, OVID (Embase and MEDLINE), Web of Science, and the Cochrane Library databases were searched for articles published before January 2020. The relevant randomized controlled trials (RCTs) were selected based on the inclusion and exclusion criteria. Data extraction and methodological quality assessment were performed according to a predesigned data extraction form and scoring system, respectively. The interventions were compared on the basis of the primary outcome like incidence of POD, and secondary outcomes like duration of delirium and the length of intensive care unit and hospital stay. Results: Sixty-three RCTs were included in the study, covering interventions like surgery, anesthesia, analgesics, intraoperative blood glucose control, cholinesterase inhibitors, anticonvulsant drugs, antipsychotic drugs, sleep rhythmic regulation, and multi-modal nursing. The occurrence of POD was low in 4 trials that monitored the depth of anesthesia with bispectral index during the operation (P < .0001). Two studies showed that supplementary analgesia was useful for delirium prevention (P = .002). Seventeen studies showed that perioperative sedation with alpha(2)-adrenergic receptor agonists prevented POD (P = .0006). Six studies showed that both typical and atypical antipsychotic drugs can reduce the incidence of POD (P = .002). Multimodal nursing during the perioperative period effectively reduced POD in 6 studies (P < .00001). Furthermore, these preventive measures can reduce the duration of delirium, as well as the total and postoperative length of hospitalized stay for non-cardiac surgery patients. For patients undergoing cardiac surgery, effective prevention can only reduce the length of intensive care unit stay. Conclusion: Measures including intraoperative monitoring of bispectral index, supplemental analgesia, alpha(2)-adrenergic receptor agonists, antipsychotic drugs, and multimodal care are helpful to prevent POD effectively. However, larger, high-quality RCTs are needed to verify these findings and develop more interventions and drugs for preventing postoperative delirium.

Following the severe acute respiratory syndrome coronavirus (SARS-CoV) and the Middle East respiratory syndrome coronavirus (MERS-CoV), a third, highly pathogenic coronavirus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) appearing at end of 2019 led to a pandemic, increased panic and attracted global attention. This review analyzes the epidemiology, etiology, clinical characteristics, treatment and sequelae of the severe acute respiratory syndrome (SARS), the Middle East respiratory syndrome (MERS) and the 2019 novel coronavirus disease (COVID-19) to help provide direction for further studies that can help understand COVID-19.