We demonstrated here the distinct differences and prognostic factors between the early and late death groups and clarified two clinical phenotypes from fatal cases in our patients with ARDS due to pneumonia. Patients who died early accounted for 36% of deaths, and experienced more severe general condition caused by the so called “cytokine storm”, which was evidenced by higher APACHE II score, higher SOFA score, and higher DIC score as well as higher disease severity in spite of less extensive radiological features. Conversely, patients who died late accounted for 64% of deaths, and were characterized by more extensive radiographic infiltration, more severe lung fibroproliferation on HRCT scans, and typically experienced prolonged mechanical ventilation followed by secondary multiple organ failure. We also demonstrated here for the first time the radiological differences as well as other clinical differences between the two fatal groups. Similar to our study, previous studies of causes and timing of deaths among ARDS cases from the 1980s and 1990s showed that fatal cases were classified into early (< 72 hours after diagnosis) and late (> 72 h) death and emphasized the common cause of death as sepsis syndrome (6,7). However, our data suggest that different processes were involved in sepsis syndrome deaths early and late. Contrast that approximately 90% of patients who died early succumbed to primary sepsis syndrome, which was also the cause of ARDS itself, whereas 60% of those who died late suffered secondary sepsis syndrome following prolonged mechanical ventilation. Recently, ARDS has been classified into two clinical phenotypes by using a latent class analysis: hypoinflammation (phenotype 1) and hyperinflammation (phenotype 2) (4,5). Although personalized medicine for ARDS would be expected according to the phenotypes (5), two phenotypes from our fatal cases may be subgroup of these phenotype 1 and 2.
We reported that evidence of early fibroproliferation based on HRCT scans at diagnosis was independently associated with the ventilator-associated outcomes and subsequent multiple organ failure, as well as refractory respiratory failure (14,15). In a prospective cohort study evaluating 159 autopsy lungs from ARDS patients, Thille et al (10) described that early fibroproliferation occurred within one week and fibrosis formation was observed after one week at earliest. Interestingly, fibrosis was more frequent in ARDS of pulmonary origin compared to that of the other origins. Since our study patients’ ARDS was caused by pneumonia, early progression of lung fibro-proliferation evaluated by HRCT score was the most relevant risk factor for the late death and was considered the most crucial.
A new frontier in ARDS clinical trials, where phenotyping of patients before randomization has been proposed. Personalized mechanical ventilation tailored to the type of CT pattern of the patient (focal or non-focal) has already been reported in this field (29). This study was analyzed to plan the study design for a randomized, open-label multicentre phase II study to evaluate the efficacy and safety of MultiStem® cells [HLCM051], an allogeneic bone marrow-derived stem cell product, in patients with ARDS due to pneumonia (NCT03807804). Using the cut-off value of APACHE II score ≥ 27, patients who are likely to die of severe systemic organ failure in a few days without confirming the effect of the investigational new drug would be excluded. On the other hand, patients who are at high risk of progressive pulmonary fibroproliferation associated with secondary septic syndrome and could hardly be rescued by any conventional treatment, were extracted by using the value of HRCT score ≥ 211.
Coagulation and fibrinolytic abnormalities which result in DIC, and excessive systemic inflammation lead to multiple organ failure (28). Although these abnormalities have garnered relatively little attention among ARDS researchers previously (18, 20), there has been emerging concern around these abnormalities as coronavirus infections in 2019 (COVID-19) are reported to evoke prominent coagulopathy associated with an increased risk of death (29). Because DIC score was one of the independent predictive factors in our study, a new assessment of these abnormalities may be necessary in patients with ARDS caused by the other pneumonia pathogens as well as COVID-19.
Recent studies indicated that approximately one-half of patients with ARDS who met the Berlin definition, exhibit DAD, and that the prognosis of patients with DAD was inferior to those who do not have DAD (12,13). In our study, the definite DAD pattern was most frequently observed in the late death group, while the inconsistent with DAD pattern significantly dominated the early death group. Sarmiento et al (30) who studied 36 autopsy cases of ARDS due to pneumonia reported that pathological alterations of DAD were seen in less than 50% of patients who died within six days. The distinct difference in HRCT findings between the early and late death groups may reflect a difference of the underlying pathology. It is of critical value to identify DAD without using invasive procedures (31). Although HRCT patterns correlated well with HRCT scores and tended to be a prognostic value in our study, further study is needed to evaluate the relationship between HRCT patterns and prognosis.
In a study of 432 patients requiring mechanical ventilation for severe community-acquired pneumonia (CAP) including 125 (29%) cases of ARDS, multivariate logistic regression analyses showed that previous antibiotic use and inadequate antibiotic therapy were independently associated with 30-day mortality, respectively (8). In our study, “previous antibiotic use” and “indeterminate” sensitivity of initial antimicrobial agents were significantly more frequent in the late death group, and the latter was one of the predictive factors for the late death, but not for the early death. It was reported that duration of use of antibiotic therapy over 24 hours lead to lower sensitivity to detect significant pathogens (32). Longer use of broad-spectrum antibiotics without de-escalation according to the sensitivity for cultured pathogens could have resulted in inducing the antibiotics treated ventilator-associated infection which was the most often observed in the late death group.
Our study has several limitations. First, it was a retrospective evaluation using a prospective collected dataset. Compared with a typical retrospective design however, our study is strengthened by the use of a prospectively collected cohort including prospective identification of acute respiratory failure as a suspected ARDS. Second, this study included a relatively small number of patients and was conducted at a single center, which necessitates cautious extrapolation of the findings to other settings. Although we have previously reported the critical utility of HRCT findings and scoring, and the prognostic value of DIC score for care of ARDS patients (19,20), CT findings and coagulative and fibrinolytic abnormalities have only recently gained more widespread consideration during the current pandemic of severe acute respiratory syndrome of COVID-19. Third, the long period of recruitment (14 years) may have affected patient care.
We did introduce advances in ventilatory management (33) and other supportive care over time.
However, the fundamental management including lung protective ventilation strategy did not change during our cohort. Finally, respiratory viruses except for flu could not routinely be identified because of diagnostic techniques over the period. In cases for which no significant pathogen was identified in our study, these patients might have been infected with other respiratory viruses. Even if these viruses were involved in the disease, only supportive care could be taken in addition to lung protective ventilation.