We evaluated the values and changes of EELV in patients with CARDS and found certain association between EELV and their prognosis, as well as a significant correlation with AI-analyzed CT lung volumes. However, in both the survivor group and non-survivor group, solely observing changes in CT lesion volume or the PaO2/FiO2 ratio did not consistently yield differences at every measurement point. While there are some reports on pulmonary function changes post-discharge, literature is limited regarding EELV and its variations in CARDS patients under invasive mechanical ventilation. To our knowledge, this study is pioneering in demonstrating that EELV can be an effective indicator of lung damage extent in CARDS patients and provide valuable insights into their prognosis. Our analysis includes comparisons of EELV differences and trends in COVID-19 patients, potentially informing assessments and prognoses for patients with ARDS from other causes. Monitoring EELV could potentially serve as an alternative to repetitive CT scans for tracking lung lesion progression in patients with CARDS, offering a quicker and more convenient method for follow-up.
COVID-19 can progress to ARDS, necessitating mechanical ventilation in approximately one-third of critically ill patients[19]. Notably, during the initial wave of the pandemic, the mortality rates among patients receiving invasive mechanical ventilation varied widely, ranging from 23.3–81% [20, 21]. In our study, we investigated the ICU mortality rate of patients with CARDS and invasive mechanical ventilation after Shanghai’s first lockdown ended. The ICU mortality rate for these patients was 73.7%. Previous studies have indicated that ARDS typically develops around 8–9 days after the onset of COVID-19 symptoms. In our cohort, the average time from symptom onset to hospital admission was 11 days, with no significant difference between the survivor and non-survivor groups. This timeline could be attributed to the overwhelming surge of COVID-19 cases, which strained healthcare resources, leading to hospital bed shortages, personnel constraints, and limited availability of medications and equipment. Consistent with previous studies, factors like advanced age, comorbidities, and obesity were associated with poorer outcomes and prognosis in our patient groups. The average age of our patients was 72 years, and they generally exhibited overweight status, with a mean BMI of 25 (kg/m2). Although both the survivor and non-survivor groups had a Charlson Comorbidity Index score of 2, we observed a higher proportion of non-survivors with comorbidities such as kidney disease, cardiovascular disorders, and pulmonary diseases. Additionally, the APACHE II score tended to be higher in the non-survivor group, though it did not reach statistical significance. It should be noted, however, that the limited sample size of our study may have influenced these findings.
Low lung function is recognized as a strong and independent risk factor for all-cause mortality [22, 23]. However, previous studies have primarily focused on general populations or chronic disease cohorts, emphasizing FEV1 and FVC as the primary indicators [24]. Yet, there seems to be hesitancy in acknowledging lung function as an independent marker of disease severity. In patients discharged after severe or critical COVID-19, reduced respiratory function is a notable issue [25]. While blood gas analysis and CT scans are useful in assessing a patient's oxygenation capacity and detecting structural changes in the lungs, they fall short of providing a comprehensive evaluation of lung function. In this study, we propose that measures associated with EELV offer a more direct assessment of residual lung function, with potential correlations to patient prognosis. Our findings reveal a significant decline in EELV among patients with CARDS receiving mechanical ventilation. Dilken et al. conducted a study on 40 intubated COVID-19 patients to examine the variations in EELV while in supine and prone positions. Their study monitored changes over a single day, and reported median values of 1444 ml for EELV, 23.4 ml/kg for EELV/PBW, and 0.31 for strain in the supine position, but did not assess patient outcomes [26]. In our study, we found median values of 1287 ml for EELV, 19.96 ml/kg for EELV/PBW, and 0.30 for strain. Notably, EELV, EELV/PBW, and EELV/preFRC were consistently lower in the non-survivor group compared to the survivor group. Furthermore, the established cutoff values for EELV, EELV/PBW, and EELV/preFRC effectively differentiated patients into two distinct groups with varying survival times and prognoses. These findings suggest that EELV and its associated parameters could be vital in determining the prognostic outcomes of patients with CARDS.
In our study, although EELV and its associated parameters demonstrated a strong correlation with CT-measured lung volumes (including total lung volume and residual lung volume), no significant differences were observed between the survivor and non-survivor groups based on the CT measurements alone, except in lesion volume. Additionally, although there was an overall difference in the PaO2/FiO2 ratio between the groups, this difference was not statistically significant during the third follow-up measurement. Interestingly, both EELV/PBW and EELV/preFRC exhibited statistically significant differences between the survivor and non-survivor groups, both in the overall analysis and across the three measurement points. Lieuwe Bos et al. reported that while the PaO2/FiO2 ratio is an important prognostic indicator for patients with CARDS, the related mechanical ventilation parameters such as mechanical power and ventilatory ratio hold greater significance in guiding patient prognosis and classification over time [27]. Consistent with these observations, our study also noted that while PaO2/FiO2 ratio did not vary significantly between the survivor and non-survivor groups over time, a growing disparity was evident in EELV-related indicators. These findings suggest that EELV measurement may offer a more effective evaluation and follow-up indicator compared to PaO2/FiO2 and CT scans for assessing lung function and prognosis in CARDS patients. However, further studies are required to validate these results and understand their clinical implications.
This study has some limitations. Firstly, as a single center study, its findings necessitate further validation through broader research. Secondly, although data collection was prospective, the study's retrospective nature may impact the robustness of the conclusions. The study also had a relatively small sample size. Moreover, CT and EELV measurements were not conducted in real-time but rather within a 24-hour window surrounding each intervention. This approach may not accurately reflect the rapid and dynamic changes in patient conditions. Finally, while CT scans are the gold standard for assessing functional residual capacity, in this study, patients underwent CT imaging using a transport ventilator, which raises concerns about the consistency of capturing scans at end expiration, and could potentially affect lung volume evaluations.