What is the most likely cause of hypoxemia in a patient with a history of respiratory infection, presenting with shortness of breath (SOB) and low oxygen saturation?

Most Likely Diagnosis: Pulmonary Edema (ARDS)

The most likely cause is pulmonary edema, specifically Acute Respiratory Distress Syndrome (ARDS), which characteristically presents with severe hypoxemia (PO2 5 kPa, SpO2 78%) with preserved ventilation (normal pH, PCO2, HCO3) and tachypnea 5 days after a respiratory infection. 1

Pathophysiological Reasoning

This clinical presentation demonstrates classic Type 1 respiratory failure with the following key features:

• Severe hypoxemia (PO2 5 kPa/38 mmHg, SpO2 78%) despite normal ventilation parameters 1
• Normal PCO2 despite severe hypoxemia occurs because CO2 is 20 times more diffusible than oxygen, allowing even diseased alveoli to eliminate CO2 effectively while failing to oxygenate blood 1
• Timeline of 5 days post-respiratory infection is consistent with ARDS development, which typically occurs 1-5 days after the initial insult 1
• Progressive dyspnea with tachypnea (RR 22) reflects the ventilation-perfusion mismatch and intrapulmonary shunting characteristic of ARDS 1, 2

Why Other Options Are Incorrect

COPD (Option A) - Unlikely

• COPD typically presents with Type 2 respiratory failure (hypercapnia with elevated PCO2) 3
• This patient has normal PCO2, which excludes COPD exacerbation as the primary diagnosis 3
• Daytime hypercapnia is unlikely unless respiratory muscle strength is reduced to 40% of predicted, which is not suggested here 3

Drug Overdose (Option B) - Unlikely

• Respiratory depressant drug overdoses cause hypoventilation with elevated PCO2 3
• This patient has normal PCO2 and normal pH, excluding significant respiratory depression 3
• The 5-day progressive course following respiratory infection does not fit an acute overdose pattern 1

Myasthenia Gravis (Option C) - Unlikely

• Neuromuscular weakness causes Type 2 respiratory failure with hypercapnia when severe enough to cause hypoxemia 3
• In chronic muscle weakness, PaO2 is usually only mildly abnormal unless complicated by atelectasis or respiratory infection 3
• The normal PCO2 in this patient excludes significant neuromuscular respiratory failure 3

Critical Management Priorities

Immediate oxygen therapy must be initiated with a target SpO2 of 94-98%, and mechanical ventilation should be prepared if oxygen therapy fails 1, 3

Key management steps:

• Start high-flow oxygen immediately - at SpO2 78%, this patient requires reservoir mask at 15 L/min or bag-valve mask 3
• Obtain arterial blood gas analysis to confirm diagnosis and guide ventilatory support 1
• Prepare for mechanical ventilation as ARDS with this degree of hypoxemia frequently requires ventilatory support 4, 2

Common Pitfalls to Avoid

• Do not withhold oxygen due to concerns about CO2 retention when PO2 is critically low (5 kPa) - the immediate risk of hypoxic brain injury outweighs theoretical hypercapnia concerns 1
• Do not delay treatment while obtaining chest X-ray, as radiographic changes in ARDS often lag behind physiological derangements by hours to days 1
• Do not rely on clinical examination alone - no examination findings were present in this case, yet severe hypoxemia exists 3, 5, 6

Mechanism of Severe Hypoxemia in ARDS

The severe hypoxemia occurs through two primary mechanisms 1:

• Ventilation-perfusion (V/Q) mismatch - areas of perfused but poorly ventilated lung 2, 7
• Intrapulmonary shunting - completely collapsed or fluid-filled alveoli without gas exchange, with persistence of pulmonary blood flow to consolidated lung due to impaired hypoxic pulmonary vasoconstriction 2, 7

Answer: D. Pulmonary edema (ARDS)