Discussion
The findings of this study demonstrated high mortality rates in patients with PARDS; only a third of patients survived, and nearly a quarter of survivors exhibited new morbidities, with respiratory, feeding, and motor functions being predominantly affected. Additionally, severe PARDS was an independent risk factor for mortality.
The 64.4% mortality rate observed among patients in this study was more than double the previously reported rate of 24% [15]. A multicenter study from China found a similar mortality rate of 61% in patients with PARDS; however, the large variability in mortality rates could be due to various factors, such as differences in referral infrastructure, PICU admission and discharge policies, illness severity, patient composition, level of patient care, and accessibility of resources [16]. The present study also excluded patients who were admitted for <24 h, most of whom were in a near-death state upon admission. As a result, mortality rates may have been underestimated. Several factors could explain the high mortality rate in our center. The mean PELOD-2 score of 9 upon admission was higher than that reported for the mortality groups in other centers (mean PELOD-2 score of 7.9) [2]. Additionally, every patient in this study was treated with invasive ventilation. Patients undergoing mechanical ventilation have greater disease severity than those undergoing noninvasive ventilation. In addition, more than half of the patients were diagnosed with severe PARDS, which would be expected to contribute to the high mortality rate. A previous study reported a mortality rate of 10–15% in patients with mild/moderate PARDS and 33% in patients with severe PARDS [17]. Moreover, patients who withdrew from treatment midway would be expected to have a higher mortality rate.
Consistent with the findings of other studies, multi-organ dysfunction was the leading cause of death in patients with PARDS [18]. In this study, severe PARDS was found to be an independent risk factor for mortality, which is also consistent with previous studies [19]. However, the present results demonstrated associations between both the OI and P/F and survival rates. At the onset of PARDS, both the prevalence and severity of non-pulmonary organ failure were shown to increase with severity of hypoxemia [20]. When assessing severe PARDS, the OI was consistent with the P/F ratio. Consequently, our study’s results further support the utility of the P/F ratio as a reliable and practical metric for assessing the prognosis of patients with severe PARDS.
Multi-organ failure, including sepsis-associated delirium, septic encephalopathy, capillary leakage-induced edema of the extremities, and intestinal failure leading to feeding intolerance, affects mortality rates and functional status at discharge. In addition to dysfunction caused by the disease itself, treatment factors can also cause new morbidity. For example, in the current study, lower extremity deep vein thrombosis, jugular vein thrombosis, intracranial hemorrhage, and cerebral infarction, which are adverse events that are being increasingly investigated, could have developed during ECMO treatment. Two patients developed lower extremity deep vein thromboses after this treatment, leading to a motor domain FSS score of 2 at discharge. All the included patients were in critical condition at the time of admission. Although survivors showed greatly improved functional status after discharge, 24.5% experienced residual dysfunction. A recent study reported similar residual dysfunction among survivors of PARDS, with 24.5% of patients being discharged to a rehabilitation facility [12]. Similarly, consistent with the present findings, another study revealed new morbidities in 15.6% of patients with PARDS upon discharge with an FSS score of 7 [5].
The results demonstrated that respiratory, feeding, and motor functions were the domains most affected at the time of PICU discharge. More than half the survivors exhibited respiratory dysfunction, most likely due to 71.7% (38/53) of them having direct PARDS. Most cases of mild respiratory dysfunction (respiratory FSS score = 2) were due to localized lung injuries, including pleural effusion, lobar pneumonia, pneumothorax, and pulmonary fibrosis; patients with these injuries required oxygen therapy or suction after discharge. Further, respiratory function may affect the quality of life after discharge. One study reported respiratory dysfunction in 36.8% of patients with PARDS, 33% of whom continued to experience mild-to-moderate impairment of lung function 3 months after discharge [21].
Furthermore, feeding dysfunction is a common factor affecting the long-term outcomes of PARDS and is likely to be altered in the PICU. One study revealed a significant increase in FSS feeding domain scores compared with scores measured upon admission [12]. Another study found that 37.8% of patients with PARDS still required enteral assistance [5] which was higher than the 17% rate of feeding dysfunction in the present study. Most of those patients required preterm milk, special formulas, liquid meals, feeding assistance, and nasogastric tube feeding after discharge, although there was minimal need for total parenteral nutrition at discharge.
Motor dysfunction was one of the main complications experienced by survivors, with the most common causes being mental disorders, edema or venous thrombosis of the extremities, and finger ischemia or necrosis. Thus, special attention should be paid to the respiratory, feeding, and motor domains of PARDS survivors.

Strengths

This is one of a few studies that provide data from an Asian population on the use of the FSS to assess illness severity upon PICU admission and discharge, with a focus on factors affecting mortality rate and functional status at discharge. Additionally, this study describes the functional status of a subgroup of patients with PARDS who underwent invasive ventilation. Finally, respiratory, feeding, and motor dysfunctions were identified as the most common areas requiring rehabilitation.

Limitations

This study had a small sample size; thus, the true incidence of the disease could not be accurately estimated. Additionally, the study was based on data from a single pediatric center. Therefore, there is a risk of missing data and information bias. Moreover, because this was a retrospective study, patients who had nearly drowned or had severe pneumonia, aspiration pneumonia, lung contusions, or severe sepsis but had not obtained a PARDS diagnosis may have been missed. Further, the FSS has not been validated for retrospective assessment of PARDS, and its ability to predict outcomes in patients in the PICU has not been investigated. Thus, it is unclear whether FSS dysfunction upon hospital discharge reflects baseline functional disability or new morbidities associated with PARDS. Additional limitations include the fact that the study did not list modifiable risk factors for functional status dysfunction (FSS > 8) at hospital discharge in patients with PARDS.
Further research should focus on investigating the functional status of patients from multiple centers, and follow-up clinical trials involving patients with PARDS are warranted. In addition, future studies should establish an objective outcome measure of mortality or significant functional morbidity upon hospital discharge.