Critical Care · Year 4 · from Critical Care
Case 1: Severe ARDS Requiring Lung-Protective Ventilation
Clinical Image
Source: Wikimedia Commons - Lobar Pneumonia X-ray - CC BY-SA 4.0
Case Presentation
A 42-year-old previously healthy woman is admitted to the ICU with severe community-acquired pneumonia progressing to ARDS. She was intubated in the emergency department for hypoxemic respiratory failure with a P/F ratio of 88. Her height is 165 cm (5'5"), so her ideal body weight is calculated as 45.5 + 2.3(5) = 57 kg. Initial ventilator settings are: volume control mode, tidal volume 340 mL (6 mL/kg IBW), respiratory rate 24/min, FiO2 80%, and PEEP 14 cmH2O. Post-intubation ABG shows pH 7.28, PaCO2 52 mmHg, PaO2 72 mmHg. Plateau pressure is measured at 28 cmH2O and driving pressure (Pplat - PEEP) is 14 cmH2O. The team accepts the mild hypercapnia (permissive hypercapnia) to maintain lung-protective tidal volumes. Over the next 24 hours, her oxygenation worsens despite increasing PEEP to 16 cmH2O and FiO2 to 100%, with P/F ratio falling to 68. Given her severe ARDS (P/F less than 100), the team initiates prone positioning for 16 hours daily. After 4 hours of prone positioning, her P/F ratio improves to 145. She continues prone positioning for 3 days, then gradually tolerates supine position with stable oxygenation. By day 7, she is transitioned to pressure support ventilation and successfully extubated on day 10.
Key Learning Points
- Tidal volume in mechanical ventilation is calculated using ideal body weight (IBW) based on height, NOT actual body weight: Males: 50 + 2.3(inches over 60); Females: 45.5 + 2.3(inches over 60)
- Lung-protective ventilation targets: tidal volume 6 mL/kg IBW (can be reduced to 4 mL/kg if needed), plateau pressure less than 30 cmH2O, driving pressure less than 15 cmH2O
- Permissive hypercapnia (tolerating elevated CO2) is acceptable to maintain lung-protective ventilation, as long as pH remains above 7.20
- Prone positioning for 12-16 hours daily reduces mortality in severe ARDS (P/F less than 150) and should be initiated early