Ventilator-Associated Barotrauma

Ventilator associated barotrauma is the escape of air from over-distended or ruptured alveoli into extra-alveolar spaces during positive pressure ventilation. Forms are pneumothorax, pneumomediastinum, subcutaneous emphysema, pneumopericardium, pneumoperitoneum, pulmonary interstitial emphysema and air embolism. It occurs in about 3 to 15% of ventilated patients, and is more frequent in acute respiratory distress syndrome, severe asthma, chronic obstructive pulmonary disease, necrotizing pneumonia, lung abscess and interstitial lung disease.

Mechanism: high alveolar pressure ruptures alveolar walls (the pressure gradient between the alveolus and the surrounding vascular sheath is the critical factor). Air dissects along the bronchovascular sheaths toward the hilum and mediastinum (the Macklin effect), then into the pleura, neck, subcutaneous tissue or pericardium. The risk rises with high peak and plateau pressures, high positive end-expiratory pressure, large tidal volumes and auto-positive end-expiratory pressure.

Pneumothorax in a ventilated patient converts to tension pneumothorax quickly because positive pressure continues to push air into the pleural space. It is a leading cause of sudden cardiovascular collapse and pulseless electrical activity in intensive care. Complications: tension pneumothorax, bronchopleural fistula with persistent air leak, hemodynamic collapse, respiratory failure and death.

• Sudden desaturation, rising peak airway pressure and ventilator pressure alarms, falling tidal volume on pressure-limited modes
• Hypotension, tachycardia and distended neck veins, progressing to pulseless electrical activity
• Reduced breath sounds and hyperresonance on one side, tracheal deviation (late) and asymmetric chest movement
• Subcutaneous emphysema with crepitus, and Hamman sign (a crunching sound synchronous with the heartbeat) in pneumomediastinum
• Continuous bubbling in a chest drain (air leak)
• Red flags: hypotension with high airway pressures, sudden cardiac arrest, and expanding subcutaneous emphysema with airway compromise

1. Immediate clinical diagnosis in an unstable patient, without waiting for imaging.
2. Bedside lung ultrasound: absent lung sliding, absent B-lines and a lung point (sensitivity above 90%).
3. Chest radiograph in a stable patient: visceral pleural line, deep sulcus sign (supine film), mediastinal air outlining the heart, the continuous diaphragm sign, subcutaneous gas and pericardial air.
4. CT chest for loculated pneumothorax, pneumomediastinum, bronchopleural fistula and underlying lung disease.
5. Arterial blood gas and ECG, and echocardiography for pericardial air or tamponade physiology.
6. Review of ventilator data: peak, plateau and driving pressures, positive end-expiratory pressure, tidal volume and intrinsic positive end-expiratory pressure.

Differentials: tension pneumothorax from other causes, endobronchial intubation, mucus plugging, pulmonary embolism, cardiac tamponade, bronchospasm and auto-positive end-expiratory pressure.

1. Tension pneumothorax:
   • Immediate needle decompression with a 14-gauge, 8 cm cannula in the fourth or fifth intercostal space at the anterior axillary line (or second space at the midclavicular line), then insertion of an intercostal drain.
   • Chest drain size 24 to 28 Fr (a 14 Fr small-bore drain is acceptable in stable patients) through the fifth intercostal space in the mid-axillary line, connected to an underwater seal, with suction of -10 to -20 cmH2O if the lung does not re-expand. Never clamp the drain while the patient is ventilated.
2. Any pneumothorax in a ventilated patient needs a chest drain, even if small, since positive pressure converts it to a tension pneumothorax.
3. Reduce airway pressures: tidal volume 6 mL/kg predicted body weight, plateau pressure 30 cmH2O or lower, lowest positive end-expiratory pressure that maintains oxygenation, shorter inspiratory time and permissive hypercapnia (pH of 7.20 or higher).
4. Treat bronchospasm: salbutamol 5 mg nebulized every 20 minutes for 3 doses with ipratropium 500 micrograms nebulized.
5. Sedation and neuromuscular blockade for severe asynchrony: propofol 1 to 3 mg/kg/h IV, and cisatracurium 0.15 mg/kg IV bolus.
6. Pneumomediastinum and subcutaneous emphysema: usually conservative, with optimization of ventilation. Subcutaneous incisions or drains only when airway compromise develops. Pneumopericardium with tamponade needs pericardial drainage.
7. Persistent air leak (beyond 5 to 7 days) or bronchopleural fistula:
   • Minimize airway pressures, spontaneous ventilation when possible, and independent lung ventilation or high-frequency ventilation in refractory cases.
   • Bronchoscopy with endobronchial valve or sealant.
   • Chemical pleurodesis with talc 4 to 5 g as slurry through the drain, or video-assisted thoracoscopic surgery with pleurodesis.
8. Chest drain complications: monitor for bleeding, infection, organ injury and re-expansion pulmonary edema.

Follow-up: daily drain output and air leak assessment, and chest radiograph after drain removal. Counsel on recovery and on the risk of recurrence.