Inclusion Criteria:

1. PSV initiated during the last 24 hours, whether transition from controlled modes or primary initiation;
2. Mechanical ventilation expected to be required for at least 24-48 hours by responsible physicians;
3. The partial pressure of oxygen in arterial blood (PaO2)/inspired oxygen fraction (FiO2) ≤ 300 mmHg (measuring at clinical FiO2 and positive end-expiratory pressure [PEEP]);
4. No sedation or stable sedation with Richmond Agitation Sedation Scale (RASS) of -2 to +1 or Riker's Sedation-Agitation Scale (SAS) of 3 to 4.

Exclusion Criteria:

1. Age younger than 18 years old;
2. Initiation of PSV before ICU admission;
3. Duration of mechanical ventilation longer than 7 days before enrollment;
4. History of neuromuscular diseases;
5. Clinical suspicion of increased intracranial pressure;
6. Presentation with pneumothorax and/or bronchopleural fistula;
7. Extracorporeal support;
8. Moribund conditions;
9. Refusal by the ICU physicians or the patient.