The main manifestation of ARDS is stubborn hypoxemia due to the pathophysiological changes of decreased lung volume, decreased compliance and decreased ratio of ventilatory blood flow in it(5). Although more and more clinical studies in recent years have confirmed that some biomarkers are associated with ARDS, there is no evidence that they can be used in the diagnosis of ARDS(6, 7). The diagnosis of ARDS still focuses on the function of the lung, including the time of onset, oxygenation index and imaging findings(8). Oxygenation index is the most important method to judge the severity of ARDS(9). However, as mentioned earlier, the high altitude area has the characteristics of low oxygen pressure, strong radiation, cold climate and so on(10). With the increase of altitude, the atmospheric pressure and the partial pressure of inhaled oxygen decrease obviously(3). The low oxygen environment is one of the greatest effects on the human body in the high altitude area(11). Some studies have pointed out that the arterial blood oxygen partial pressure of healthy adults at high altitude is significantly lower than that of healthy adults of the same age group in the plain area(12). Acute and chronic hypoxia at high altitude can induce a variety of diseases and affect the quality of life and working ability of people at high altitude. People living at high altitude for a long time have different tolerance to hypoxic environment, resulting in pathophysiological changes of ARDS patients in high altitude area, which are different from those in plain area(13).
Studies have confirmed that high altitude acute respiratory distress syndrome is characterized by hypoxemia that is not easy to correct, pulmonary artery pressure is significantly increased, and the inflammatory response is more serious than the plain. We need to put forward the plateau criteria.
According to the AECC criteria, some mild patients are diagnosed as ALI, which makes clinicians lack awareness of this subgroup and fail to give appropriate support and treatment. In 2001, Zhang et al put forward the diagnostic criteria of ALI/ARDS in high altitude areas of western China on the basis of AECC diagnostic criteria. Therefore, we need to update the Zhang criteria(4). From this study, we can know that when diagnosing ARDS patients in Xining, Qinghai Province according to the diagnostic criteria of ALI/ARDS in high altitude areas of western China, the case fatality rate of ARDS is significantly lower than that of ARDS in China and other parts of the world. This may be due to the fact that the criteria excludes some patients with mild ARDS, that is, oxygen and index between 200–253 mmHg, resulting in no significant difference between groups.
In recent years, the case fatality rate and mortality rate of large-scale ARDS epidemiological survey in the world vary greatly from region to region. 2016 LUNGSAFE study involving 50 countries and regions and 459 ICU shows that even though the case fatality rate of ARDS is different in different regions, the global case fatality rate of ARDS is still as high as 40%(1). In 2018, Liu Ling and others completed the ARDS epidemiological survey of 20 ICU in China. In China, the ICU fatality rate of ARDS is about 32%, and the hospital mortality rate of severe ARDS is as high as 60%(14).
In this study, in Xining, Qinghai, the ICU mortality of patients diagnosed as ARDS according to the plateau criteria for high altitude areas was 24.02%, and the ICU mortality of mild, moderate and severe ARDS were 17.76%, 21.43% and 47.37%, respectively. The 28-days mortality was 37.56%, 23.36% for mild, 44.05% for moderate, and 63.16% for severe ARDS. The results are similar to the results of international and domestic large-scale epidemiological studies of ARDS in recent years. In contrast, according to the Berlin criteria to classify the severity of ARDS in Xining, Qinghai, the ICU mortality of mild ARDS patients is only 6.12%, which is significantly lower than that of epidemiological studies. At high altitude, the oxygen content and partial pressure of oxygen in the air were lower than those in the plain area, and when the partial pressure of oxygen < 300 mmHg in patients at high altitude, there were no other serious pulmonary pathological changes, even if the patients had lung infiltration shadow and other characteristics consistent with the diagnostic criteria of ARDS, it could not be regarded as ARDS. When the Berlin Definition was included in patients according to 300 mmHg criteria, false positives occurred, resulting in a significant reduction in mortality in the mild ARDS group.
This study preliminarily confirmed that the plateau criteria can be used to classify the severity of ARDS in Xining area of Qinghai Province. Zhang criteria may cause clinicians to fail to realize the seriousness of some patients, resulting in delay in diagnosis, while the application of Berlin criteria in high altitude areas may include a large number of patients who do not belong to ARDS, resulting in false positive and overtreatment.
This study also has some limitations: (1) This study is a single-center retrospective study, the sample size is small, the sample source is limited, resulting in the sample is not very representative, it still needs to be verified by multicenter clinical studies in the later stage. (2) According to the results of this study, the plateau criteria of ARDS in Xining, Qinghai, but it is not clear whether this criteria is applicable to other areas at the same altitude and other altitude areas, which still needs to be verified by later multicenter clinical trials. (3) As there is no other strong basis for the diagnosis of ARDS at present, the mortality is regarded as the main prognostic end point in this study, and in the process of disease progression, a variety of clinical intervention measures are often given, which may affect the prognosis of patients to a certain extent.(4) In the experiment, we observed that there is no difference in the setting of PEEP in different severity levels. The severe ARDS FiO2 setting is higher. We cannot judge whether PEEP has an effect on the improvement of oxygenation index.