Respiratory · Year 1 · from Respiratory
Case 2: ARDS with Refractory Hypoxemia (Shunt Physiology)
Case Presentation
A 45-year-old man is admitted to the ICU with severe community-acquired pneumonia due to Streptococcus pneumoniae. Despite intubation and mechanical ventilation with FiO2 100%, his oxygen saturation remains at 82%. Vital signs show heart rate 115 bpm, blood pressure 95/60 mmHg on vasopressors, and temperature 39.2C. Chest X-ray shows bilateral diffuse alveolar infiltrates.
Arterial blood gas on FiO2 100% shows pH 7.28, PaCO2 45 mmHg, PaO2 52 mmHg. The P/F ratio is 52 (52/1.0), indicating severe ARDS. Importantly, increasing FiO2 from 60% to 100% improved PaO2 only minimally (from 48 to 52 mmHg).
This patient demonstrates true shunt physiology. In ARDS, alveoli filled with inflammatory exudate and edema cannot participate in gas exchange. Blood flowing through these non-ventilated regions (V/Q = 0) returns to the left heart with oxygen tension equal to mixed venous blood. When this desaturated blood mixes with oxygenated blood from functional alveoli, the result is arterial hypoxemia.
The key distinction of shunt is poor response to supplemental oxygen. Even at FiO2 100%, the shunted blood never contacts ventilated alveoli, so its oxygen content cannot improve. This distinguishes shunt from V/Q mismatch, which typically responds well to supplemental oxygen because increased PAO2 can improve oxygenation of blood in low V/Q regions. The shunt equation estimates that this patient has approximately 40% shunt (based on PaO2 of 52 mmHg on 100% oxygen).
The patient is treated with lung-protective ventilation, increased PEEP to recruit collapsed alveoli and reduce shunt fraction, prone positioning to improve V/Q matching, and antimicrobial therapy. Over several days, as pneumonia resolves and alveoli are recruited, the shunt fraction decreases and oxygenation improves.
Key Learning Points
- Shunt (V/Q = 0) represents perfusion of completely non-ventilated alveoli
- Shunt causes hypoxemia that responds poorly to supplemental oxygen (the hallmark distinguishing feature)
- In ARDS, alveolar flooding creates extensive intrapulmonary shunt
- PEEP recruits collapsed alveoli, converting shunt units to functional gas-exchanging units
- Prone positioning improves V/Q matching by redistributing perfusion to better-ventilated regions