The goal of this observational study is to investigate whether intravenous polymyxin B combined with nebulisation achieves better antimicrobial efficacy and clinical outcomes than intravenous use alone in patients with multidrug-resistant gram-negative bacilli infected with ventilator-associated pneumonia. The main questions it aims to answer are:

• When using intravenous polymyxin B to treat patients with ventilator-associated pneumonia caused by multidrug-resistant bacteria in clinical practice, is it necessary to assist with polymyxin B nebulization therapy？
• If necessary, how much dose of nebulization is better? Participants will be divided into two groups based on whether they have received nebulization treatment with polymyxin B in clinical practice. Blood and alveolar lavage fluid samples will be collected after the first dose injection and reaching the steady-state dose, and the drug concentration differences in blood and ELF will be measured in patients who have received intravenous injection of polymyxin B alone and those who have received adjuvant nebulization of polymyxin B, as well as differences in clinical outcomes and side effects.

Researchers will compare the differences in blood and ELF drug concentrations, clinical outcomes, and incidence of side effects between two groups of patients, to see if is it necessary to assist with polymyxin B nebulization therapy in patients with multidrug-resistant gram-negative bacilli infected with ventilator-associated pneumonia.

Inclusion Criteria:

• 18-80 years old;
• ICU patients;
• Patients with MDR-infected VAP were diagnosed by etiological culture results;
• Expect to receive at least 6 doses (3 days) of polymyxin B;
• Obtain informed consent;

Exclusion Criteria:

• The patient has a history of severe hypersensitivity to polymyxin B;
• Patients cannot tolerate alveolar lavage;
• Oncology patients,includes hematologic malignancies;
• Pregnant or lactating women;
• Patients with other conditions that the investigators consider unsuitable for enrollment;

Screening of patients with multiple drug-resistant bacterial infections and ventilator-associated pneumonia admitted to the Intensive Care Department of Affiliated Zhongda Hospital of Southeast University. Based on the medical decisions of clinical physicians, they received intravenous administration of polymyxin B alone and intravenous combination nebulization of polymyxin B. Patients were divided into two groups according to the actual treatment methods of polymyxin B.