The aim of this study is to investigate the impact of different diluent volumes for the same dosage of colistin in mechanically ventilated respiratory patients receiving colistin inhalation therapy. The study aims to determine whether the diluent volume has an effect on clinical outcomes, including ICU length of stay, total hospitalization duration, duration of respiratory support within 28 days, mortality rates within 28 days and 90 days, as well as analyze the pharmacokinetic profile of colistin in blood and bronchoalveolar lavage fluid. Additionally, the study will evaluate the incidence of nebulizer malfunctions and blockages. The findings of this research will help identify the optimal diluent volume for colistin inhalation therapy in clinical practice.

Background： Colistin is an antibiotic used to treat infections caused by multidrug-resistant Gram-negative bacteria. In recent years, it has been widely utilized as an inhaled antibiotic for the treatment of respiratory infections in critically ill patients on mechanical ventilation. Despite the increasing use of colistin inhalation therapy for ventilator-associated pneumonia, the clinical impact of diluent volume on nebulization efficiency remains unclear.

Study Design： A single-center, prospective, randomized controlled trial.

Methods： The study plans to enroll 60 participants. Colistin inhalation therapy will be administered using a blocked stratified random allocation. The control group will receive colistin 6 MU diluted in 6 ml of normal saline (1 MU per 1 ml), while the experimental group will receive colistin 6 MU diluted in 12 ml of normal saline (1 MU per 2 ml). The dosage, frequency, and administration method of colistin inhalation will follow the clinical standard of 6 MU, administered every 8 hours, continuously for 7 days.

Effect： The investigators anticipate that the clinical outcomes and pharmacokinetics of colistin inhalation therapy will not differ significantly between different diluent volumes. Based on comprehensive evaluation, a diluent volume of 12 ml is considered more suitable than 6 ml for the dilution of colistin.

Inclusion Criteria:

• Admitted to the Intensive Care Unit.
• Age greater than 18 years and using a ventilator due to acute respiratory failure.
• Requires inhalation colistin treatment as determined by the disease.
• Sign the written informed consent form.

Exclusion Criteria:

• Renal failure requiring renal dialysis.
• Receiving both inhaled colistin and intravenous colistin treatments.