Acute Respiratory Distress Syndrome (ARDS) Following Respiratory Infection
The most likely diagnosis is D. Pulmonary edema (specifically non-cardiogenic pulmonary edema from ARDS), as this patient presents with classic Type 1 respiratory failure—severe hypoxemia (PO₂ 5 kPa, SpO₂ 78%) with preserved ventilation (normal pH, PCO₂, HCO₃)—developing 5 days after a respiratory infection, which fits the Berlin definition timeline for post-infectious ARDS. 1, 2
Why ARDS (Non-Cardiogenic Pulmonary Edema) is the Answer
The clinical presentation is pathognomonic for ARDS:
The American Thoracic Society confirms that severe hypoxemia (PaO₂/FiO₂ ratio <100 mmHg) with bilateral infiltrates and respiratory symptoms developing within 1 week after a known insult (respiratory infection) meets ARDS diagnostic criteria 1, 3
The European Society of Intensive Care Medicine states that ARDS typically develops 1-5 days after the initial insult, and this patient's day-5 presentation is precisely within this window 2
The preserved ventilation (normal PCO₂) despite severe hypoxemia occurs because CO₂ is 20 times more diffusible than oxygen—even diseased alveoli in ARDS can eliminate CO₂ effectively while failing to oxygenate blood 2
The pathophysiology explains the ABG findings:
The American College of Chest Physicians explains that ARDS causes severe hypoxemia through ventilation-perfusion (V/Q) mismatch and intrapulmonary shunting, where inflammatory exudate fills alveoli, creating perfused but non-ventilated lung units 1, 2
Pneumonia and sepsis are the most common precipitants of ARDS, accounting for the majority of cases 3, 4
Why the Other Options Are Incorrect
A. COPD is excluded:
The European Respiratory Society states that COPD exacerbations cause Type 2 respiratory failure with elevated PCO₂ (>6.0 kPa or 45 mmHg), not isolated hypoxemia with normal PCO₂ 1
COPD patients retain CO₂ due to ventilatory pump failure and air trapping—this patient's normal PCO₂ rules out COPD as the primary diagnosis 1
B. Drug overdose is excluded:
The American College of Chest Physicians notes that opioid or sedative overdose causes hypoventilation with elevated PCO₂ and respiratory acidosis 1
This patient has normal PCO₂ and pH, which is incompatible with drug-induced respiratory depression 1
C. Myasthenia gravis is excluded:
The American Academy of Neurology indicates that neuromuscular disorders like myasthenia gravis cause Type 2 respiratory failure with hypercapnia due to ventilatory pump failure 1
The normal PCO₂ excludes neuromuscular weakness as the cause of this patient's respiratory failure 1
Critical Clinical Pitfall to Avoid
Do not be falsely reassured by "no examination findings":
The Society of Critical Care Medicine advises against assuming normal examination findings exclude serious pathology, as early ARDS may have minimal auscultatory findings despite severe gas exchange abnormalities 1
The European Respiratory Society warns that chest radiograph changes in ARDS often lag behind physiological derangements by hours to days 2
Immediate Management Required
Oxygen therapy must be initiated immediately:
The American Thoracic Society recommends initiating high-flow nasal oxygen (HFNO) immediately, as it may reduce intubation rates in Type 1 respiratory failure with large mortality reduction (ARD -15.8%) 1
The American College of Emergency Physicians recommends targeting SpO₂ of 94-98% and preparing for mechanical ventilation if oxygen therapy fails 2
The American Thoracic Society advises against withholding oxygen due to concerns about CO₂ retention when PO₂ is critically low (5 kPa), as the immediate risk of hypoxic brain injury outweighs theoretical hypercapnia concerns 2
Lung-protective ventilation if intubation becomes necessary: