Lung Ultrasound Score Predicts Extubation Failure in Elderly Surgical Patients-- a Multicenterprospective Cohort Study

1.1. Research significance mechanical ventilation (MV) is an important technique for the treatment of acute respiratory failure, and about 20% of patients have difficulty in ventilation due to respiratory muscle related diaphragm dysfunction . The duration of mechanical ventilation in elderly patients after large-scale abdominal surgery is longer, the lung function reserve is significantly decreased, and the incidence of postoperative lung complications is as high as 53.1%, which is easy to cause offline difficulties, thus prolongating the hospital stay and seriously threatening the life safety of patients . Therefore, it is of great clinical significance and social value to accurately grasp the ventilator withdrawal time of elderly patients after surgery and reduce ventilator-related lung injury.

1.2. Research status at home and abroad 1.2.1 Research status of mechanical ventilation lung injury Mechanical ventilation provides important respiratory and circulatory support for critically ill patients, but prolonged mechanical ventilation is prone to ventilator-associated lung injury. Studies have shown that with the extension of mechanical ventilation time, the incidence of pulmonary complications increases, such as ventilator-associated pneumonia, which extends the length of ICU stay of patients and increases hospitalization costs . Ventilator-associated diaphragm dysfunction, which has received increasing attention in recent years, is directly related to the duration of mechanical ventilation. Animal studies have shown that the maximum tonic contractile tension and monocontractile tension of diaphragm gradually decrease with the extension of mechanical ventilation . Clinical studies have shown that mechanical ventilation for 18-24 h can lead to diaphragmatic dysfunction . Due to the complicated condition, organ insufficiency and complications of severe patients, it is often very difficult to withdraw the machine. Therefore, the appropriate time of withdrawal is of great significance for the prognosis of elderly critically ill patients.

1.2.2 Characteristics of lung structure and postoperative lung complications in the elderly With the increase of age, the main structure and function of the lung have progressive changes, the lung elastic retraction force is reduced, the defense mechanism is impaired, the respiratory muscle strength is weakened, and the sensitivity to the respiratory depression caused by anesthesia and analgesics is increased, which leads to the decline of the respiratory reserve function. After the trauma stimulation of major surgery for a long time, the lung functional reserve, respiratory muscle strength, lung compliance, respiratory filtration function, gas exchange function and so on decreased significantly in elderly patients. Smetana et al. have shown that the incidence of pulmonary complications in elderly patients after major operations is 33%, the mortality rate of patients within 30 days is 14%30%, the length of hospital stay is prolonged, the medical expenses are increased, and the postoperative quality of life of patients is seriously affected .

Studies have shown that the incidence of postoperative pulmonary complications is negatively correlated with the distance between the surgical site and the diaphragm. The incidence of complications after upper abdominal surgery was 17%76%, and the forced vital capacity after open surgery was reduced by 50% on average, which was positively correlated with the operation duration. Lung complications were 8% if the operation time was less than 2 hours. The complication rate of 34h operation time will increase to 40%. Zuo Mingzhang et al. found that the incidence of pulmonary complications in elderly patients after major abdominal operations was 53.1%. In conclusion, the risk of pulmonary complications after large-scale gastrointestinal abdominal surgery is higher in elderly patients, and the postoperative complications seriously affect the long-term prognosis of patients, prolong the length of hospital stay, and increase hospitalization costs and mortality.

1.2.3 Lung protective ventilation strategies Lung function reserve decreased significantly in elderly patients, and a variety of lung protective ventilation strategies have been applied clinically to reduce the occurrence of postoperative lung complications. Studies have found that the lung protective ventilation strategy of small tidal volume combined with lung reexpansion manipulation can improve lung ventilation function and intrapulmonary oxygenation in elderly patients after major abdominal surgery, reduce postoperative lung complications, and shorten the average length of hospital stay . TAO et al. found that small tidal volume lung ventilation can reduce the occurrence of postoperative lung infection, lung injury and other complications, and to a certain extent reduce the postoperative mortality of patients with general anesthesia . Positive end expiratory pressure (PEEP) reduces the incidence of postoperative atelectasis. Expert consensus on the clinical application of perioperative lung protective ventilation strategy points out that high inhalation oxygen concentration (FiO2) is easy to cause absorptive alveolar collapse and increase the incidence of postoperative lung complications. Helmer horst et al. found that selecting appropriate FiO2 could not only satisfy the oxygen supply of the body and maintain the normal oxygenation index, but also reduce the lung injury caused by high FiO2 .