Participants in the control group will receive standard airway clearance therapy only, including postural drainage, humidification, and suctioning or fiberoptic bronchoscopy when necessary. In addition, they will undergo high-frequency chest wall oscillation therapy for 15 minutes per session, twice daily (with an interval of at least 2 hours between sessions). The MetaNeb system will not be used.

ICU length of stay was defined as the number of days from the date of first ICU admission to the date of final ICU discharge or death, calculated as calendar days.

Hospital length of stay was defined as the number of days from hospital admission to discharge or death, calculated as calendar days.

Score according to the following aspects:

1. body temperature (12 hour average, ℃): 0 point: 36.1-38.4 ℃; 1 point: 38.5-38.9 ℃.
2. white blood cell count (× 10 ⁹ /l): 0 point: 4.0-11.0; 1 point: 11.1-17.0; 2 points: ≤ 3.9 or ≥ 17.1.
3. secretion (24-hour aspirate characteristics and quantity): 0 point: no sputum or a little; 1 point: moderate to massive, non purulent; 2 points: moderate to massive, purulent.
4. oxygenation index (mmHg): 0 point: >240; 2 points: ≤240 without ARDS.
5. chest X-ray infiltrating shadow: 0 point: no infiltrating shadow; 1 point: patchy infiltrating shadow; 2 points: fusion of patchy infiltrating shadow.
6. sputum or airway aspirate culture: 0 point: no pathogenic bacteria cultured; 1 point: ≥ 1 pathogen; 2 points: the same bacteria are cultured for ≥ 2 times, or the smear is consistent with the cultured pathogen.

The lungs are divided into six zones: the upper zone above the level of the carina, the middle zone between the carina and the inferior pulmonary veins, and the lower zone below the level of the inferior pulmonary veins. Within each zone, the predominant CT pattern is identified and assigned a weighted score based on the severity of lung damage, where a score of 1 indicates normal lung tissue, 2 indicates ground-glass opacity, 3 indicates consolidation, 4 indicates ground-glass opacity with traction bronchiectasis or bronchiolectasis, 5 indicates consolidation with traction bronchiectasis or bronchiolectasis, and 6 indicates crazy-paving appearance. For each of the six zones, we estimates the percentage of the zone affected by the predominant pattern in increments of 10%, Zone Score = (Weight Score of the Pattern) × (Percentage of Zone Involved), Total Ichikado Score = (Sum of Six Zone Scores) / 6. A higher score indicating more extensive and severe lung involvement.

The proportion of participants experiencing any predefined MetaNeb-related adverse events during the intervention period.

Adverse events include, but are not limited to: Oxygen desaturation (SpO₂ decrease ≥5% from baseline or SpO₂ <90%); Hemodynamic instability (heart rate change ≥20% from baseline or systolic blood pressure change ≥20% from baseline); New-onset arrhythmia; Barotrauma (including pneumothorax confirmed by imaging); Treatment intolerance leading to premature discontinuation.

Adverse events will be assessed during each treatment session and recorded according to predefined criteria.