Pressure support ventilation (PSV) is an assistant mechanical ventilation mode, that is widely implemented in mechanical ventilation treatment but there are no exact guidelines to guide PS setting. Traditional PS setting strategy (VT/PBW 6-8ml/kg and RR 20-30 breaths/min)has risks of excessive or insufficient assistance. Inspiratory muscle pressure index (PMI) is a noninvasive respiratory mechanical indicator and is available at the bedside. PMI was correlated with inspiratory effort and has the potential ability to predict low inspiratory effort and high inspiratory effort. The primary objective of this study is to investigate the clinical validity of a PMI-guided PS setting strategy. Specifically, the investigators aim to evaluate its impact on inspiratory effort as well as its potential for lung and diaphragm protection. Additionally, the investigators seek to assess the effect of this ventilation strategy on mechanical ventilation outcomes while evaluating the feasibility of our trial protocol.

Pressure support ventilation (PSV) is an assistant mechanical ventilation mode to provide synchronous inspiratory support for patients with spontaneous breathing and to efficiently reduce the workload imposed on the respiratory muscle. PSV is widely implemented in mechanical ventilation treatment but there are no exact guidelines to guide PS setting. Clinicians and respiratory therapists usually adjust ventilator parameters based on tidal volume/predicted body weight (VT/PBW, 6-8ml/kg) and current respiratory rate (RR, 20-30 breaths/min). This strategy has risks of excessive or insufficient assistance because the PS setting cannot be modulated dynamically based on the requirements of ventilated patients.

Inspiratory muscle pressure index (Pmus index, PMI) is defined as the difference between plateau pressure (Pplat) and airway peak pressure (Ppeak) during end-inspiratory occlusion (EIO). PMI is a noninvasive respiratory mechanical indicator and is available at the bedside like airway occlusion pressure (P0.1) because holding operations were integrated into the majority ventilator. Several studies showed PMI was correlated with inspiratory effort. Our previous study showed PMI has the potential ability to predict low inspiratory effort and high inspiratory effort, and the optimal cut-off PMI value was approximately 0 cmH2O and 2 cmH2O.

The primary objective of this study is to investigate the clinical validity of a PMI-guided PS setting strategy. Specifically, the investigators aim to evaluate its impact on inspiratory effort as well as its potential for lung and diaphragm protection. Additionally, the investigators seek to assess the effect of this ventilation strategy on mechanical ventilation outcomes while evaluating the feasibility of our trial protocol.

Inclusion Criteria:

• Patients under PSV
• Predicted ventilated time of more than 48 hours.

Exclusion Criteria:

• Age less than 18 years old;
• Known pregnancy;
• BMI>35kg/m2;
• Brain-stem or cervical cord injury;
• Known or clinically suspected elevated intracranial pressure (ICP>18 mmHg);
• Nervus phrenicus or diaphragm injury;
• Broncho-pleural fistula;
• Chronic obstructive pulmonary disease (COPD);
• Pressure support ventilated time more than 48 hours before randomization.