COVID-19 is a novel infectious disease, characterized by high transmissibility and serious harmfulness. A few patients with severe course of disease tend to have severe clinical symptoms, who may rapidly progress into ARDS and need the aids of intensive care unit . Hence, it is essential to closely monitor the condition of patients, by dynamically monitoring the alteration of symptoms and laboratory examinations, the change of the chest imaging performances, which are helpful for the evaluation of the disease severity and to adjust treatment plan timely.
There were some characteristic clinical features pertaining to the severe disease course of SARS-COV-2 infected. The past medical history had an effect on disease mortality, which confirmed by the reports from Sohrabi et al , Guan et al  and Jordan et al . In present study, the mean age of death cases was approximately 10 years older than that of survivors, which was similar to the previous study . The gender prevailing of patients with severe COVID-19 was obvious, almost 3: 2 for male-female ratio in present study. This was in consistence with Chen’s study, suggested that older men were more likely to be infected with SARS-COV-2, resulting in severe and even fatal respiratory diseases such as ARDS . In the death group, the duration of symptoms prior to admission was longer than that of survival group, reflecting that the prolonged duration of symptom onset to hospitalization tended to poorer outcomes, which was in consistence with Liang’s study .
In present study, the main initial symptoms of the severe patients were fever and/or cough. The dyspnea was frequently seen in the severe course of the patients with COVID-19 pneumonia, especially in critically severe patients, due to the severe lung lesions of the pneumonia. The incidence of ARDS in critically severe and death group was significantly higher than that in severe and survival group, respectively. The RR in critically severe patients was significantly higher than that in severe patients, as well as the SaO2 and FiO2, which may be due to mechanical ventilation. As to the blood routine, increased leukocyte and neutrophil counts, decreased lymphocytes count and ratio were remarkable features, especially for critically severe group and death group. Wang et al firstly uncovered the continuous increase of neutrophil counts in dead cases . It may be related to cytokine storm induced by virus invasion. And lymphopenia suggested SARS-COV-2 might mainly target at lymphocytes and lead to the progression of the disease .
The infection related factors, including CRP, ESR, procalcitonin, IL-6, IL-8 and IL-10, were increased in the severe patients, especially in critically severe patients and death cases. The study from Ulhaq et al suggested that continuous measurement of circulating IL-6 levels may be of great significance in identifying disease progression in patients infected with COVID-19 . A retrospective study suggested that elevated levels of IL-6 was related to the high mortality of COVID-19 infection . A significantly higher incidence of septic shock and DIC was seen in critically severe and death group. This may be due to the imbalance of thrombin production caused by the activation of vascular endothelium, platelets and white blood cells, which occurred locally and systematically in the lung system of patients with severe pneumonia, resulting in fibrin deposition, tissue damage and microangiopathy . It could be aggravated by the occurrence of septic shock [27, 28]. It was reported that most of death cases and very few survivors have evidence of DIC, which occurred frequently in the deterioration of COVID-19 pneumonia and was often associated with mortality . It also suggested that clinicians needed to be vigilant to identify the presence of DIC, especially in patients who had already experienced septic shock.
There was some significant relationship between multiple organs injury and mortality. In critically severe and death patients, myoglobin, troponin, LDH and the incidence of cardiac injury were more higher than those in non-death patients, which was similar to the results of some previous studies on the relationship between the severity of illness and myocardial injury in patients with COVID-19, and was consistent with the correlation study between heart injury and death after SARS-CoV-2 infection [30, 31]. Recent studies on COVID-19 had shown that the incidence of liver injury ranges from 14.8–53%, with the decreased albumin level in critically ill patients, and the incidence of liver injury might reach as high as 78.0% in the death cases of COVID-19 . In this study, the incidence of liver injury in critically severe and death group was significantly increased compared to severe and survival group. This demonstrated that liver injury was related to the severity of the disease and mortality, which may be due to the cytokine storm, or the drug-induced liver damage [32, 33]. In the present study, the eGFR, serum creatinine and serum urea nitrogen levels in the death group were significantly higher than those in the survival group, and there was a significant prevalence with AKI of patients in both the critically severe group and the death group. It was consistent with the study of Cheng et al, which showed that the development of AKI during hospitalization in patients with COVID-19 was related to in-hospital mortality .
The coagulation function and the serum Na+ concentration changed in the severe course of COVID-19 pneumonia. Recently, the coagulation function was concerned and some related indices were studied between severe and non-severe patients [18, 35]. In this study, these indices were further compared between severe and critically severe patients, and between survival and death patients. PT, APTT, INR and fibrinogen level were related to the severity of the disease, and the former three might be related to the mortality. According to previous study , hypernatremia was a common electrolyte disorder, which was related to long-term hospitalization and death, and was more common in critically ill patients. Abnormal changes in the central nervous system and mental state may be the causes of hypernatremia, while the digestive tract or urinary system disorder cannot be ruled out . In addition, it may also be related to a large number of intravenous supplements of sodium-containing fluids.
As to the imaging performances, multiple lung lobes were involved in 98.6% patients, and whole lung lobes were involved in 84.06% patients. The proportion of patients in stage III increased significantly in death group, as well as comprehensive CT imaging scores in critically severe and death group. Our results showed that the severity of CT findings was consistent with the severity of clinical course of the disease, as suggested by previous study . Li et al  found that the development pattern of COVID-19 on CT images was similar to that of SARS or MERS. There were some common imaging features, so the final diagnosis had to be combined with the clinical manifestation, epidemic history and laboratory examination. However, the advantage of convenient and rapid CT examination was irreplaceable. A study about critically ill patients with SARS-CoV-2 pneumonia demonstrated that early or repeated radiological examination is helpful to screen patients with SARS-CoV-2 pneumonia .
The previous studies referred to the mortality risk, calculated overall probability based on the infection and confirmed population [40, 41]. However, the individual rough risk of death was important, especially in severe and critically severe patients, which might influence the treatment plan and the response of clinicians or medical institutions. In the univariate logistic regression analysis, DIC showed as the best predictor (nearly 59 times of the death risk for the patients without DIC), followed by septic shock and cardiac injury. The prediction model included evidence of patient’s age, cardiac injury, AKI and ARDS, among which the evidence of ARDS was the most powerful predictor. In the current COVID-19 epidemic, this prediction model might be a promising method to help clinicians to quickly identify and screen potential individuals who had a high-risk of death.
There were several limitations of this study. First, the clinical and imaging data of patients were from multiple centers, hence the data were heterogeneous which might affect the statistical results. Additionally, some indices were missing too many values, which lead to that the P value could not be calculated in the test of group differences. Second, the initial imaging and follow-up imaging of the patients were lack of uniform standard. Some patients were only with chest X-ray because of the disease severity, and the follow-up interval was not identical. Finally, although both of the percent concordant and the area under curve of the prediction model were in a high level, a larger cohort study might be warranted to validate the accuracy and application value of the prediction model.