Respiratory · Year 1 · from Respiratory
Case 1: Acute Respiratory Distress Syndrome (ARDS)
Clinical Image
Source: Wikimedia Commons - ARDS bilateral infiltrates - CC BY-SA 3.0
Patient Presentation
A 52-year-old man is transferred from a community hospital with progressive respiratory failure. He was admitted 4 days ago with severe community-acquired pneumonia and initially improved on antibiotics. However, over the past 48 hours, his oxygen requirements have escalated dramatically despite treatment. He is now on high-flow nasal cannula at 60 L/min and 100% FiO2 with an SpO2 of 86%.
Demographics
- Age: 52 years
- Sex: Male
- Past Medical History: Hypertension, type 2 diabetes
- Medications: Lisinopril, metformin
- Social History: Former smoker (20 pack-years, quit 10 years ago), no alcohol
Chief Complaint
Progressive hypoxemic respiratory failure despite treatment
Physical Examination
- Blood pressure: 88/54 mmHg (on norepinephrine infusion)
- Heart rate: 118 bpm
- Respiratory rate: 34/min
- Oxygen saturation: 86% on HFNC 60L/100% FiO2
- Temperature: 38.4°C
- General: Severe respiratory distress, using accessory muscles
- Respiratory: Bilateral crackles throughout, decreased breath sounds at bases
- Cardiovascular: Tachycardic, no murmurs
Workup
- ABG (on HFNC 60L/100%): pH 7.28, PaCO2 38, PaO2 58, P/F ratio: 58
- WBC: 18,500/μL
- Procalcitonin: 12.4 ng/mL
- Lactate: 3.8 mmol/L
- BNP: 120 pg/mL (mildly elevated, not consistent with cardiogenic edema)
- Chest X-ray: Bilateral diffuse alveolar infiltrates, no pleural effusion, normal cardiac silhouette
- CT chest (non-contrast): Bilateral ground-glass opacities with dependent consolidation, no PE
- Echocardiography: EF 55%, no significant valvular disease, normal filling pressures
Diagnosis
Severe ARDS (P/F ratio <100) secondary to Bacterial Pneumonia/Sepsis
Treatment
- Endotracheal intubation for mechanical ventilation
- Lung-protective ventilation (ARDSNet protocol):
- Tidal volume: 6 mL/kg ideal body weight
- Plateau pressure: Target ≤30 cmH2O
- PEEP: Titrate using PEEP/FiO2 table (started at 14 cmH2O)
- Permissive hypercapnia acceptable
- Target SpO2: 88-95%
- Prone positioning: 16+ hours daily (for severe ARDS with P/F <150)
- Conservative fluid management (target even to negative balance)
- Broad-spectrum antibiotics (escalated coverage)
- Deep sedation initially for ventilator synchrony
- Neuromuscular blockade if persistent dyssynchrony
- Stress ulcer and DVT prophylaxis
- Consider ECMO evaluation if refractory
Physiological Principles Demonstrated
- Berlin definition criteria: Acute onset (within 1 week), bilateral opacities on imaging, respiratory failure not fully explained by cardiac failure, P/F ≤300 with PEEP ≥5 cmH2O.
- Lung-protective rationale: Low tidal volumes prevent volutrauma (alveolar overdistension); limiting plateau pressure prevents barotrauma. The injured lung has reduced functional volume ("baby lung").
- PEEP mechanism: Recruits collapsed alveoli, maintains end-expiratory lung volume, improves V/Q matching, and reduces shunt fraction.
- Prone positioning benefits: Redistributes ventilation to dorsal lung (larger in cross-section), improves V/Q matching, reduces shunt, decreases compression of dorsal lung by heart and abdomen.