A 48-year-old man with cirrhosis complains of breathlessness that worsens when he stands upright and improves on lying flat. Arterial blood gas shows PaO2 of 58 mmHg sitting and 72 mmHg supine. Contrast echocardiography shows delayed appearance of microbubbles in the left atrium. The mechanism responsible for positional worsening of his hypoxaemia is:
- A Gravitational increase in blood flow through dilated basal pulmonary vessels that fail to oxygenate it fully ✓
- B Elevation of the diaphragm on standing compressing the lower lobes and reducing ventilation
- C Standing-induced redistribution of perfusion away from the apical towards the mid zones
- D Positional worsening of portopulmonary hypertension reducing cardiac output on standing
Correct answer: A. Gravitational increase in blood flow through dilated basal pulmonary vessels that fail to oxygenate it fully
Explanation
Hepatopulmonary syndrome features intrapulmonary vascular dilatations concentrated in the lung bases, producing a low ventilation-perfusion ratio and true anatomical shunt. Upright posture increases basal perfusion through these abnormal vessels, so oxygenation falls on standing (orthodeoxia) and improves supine, matching the clinical picture of platypnea. Portopulmonary hypertension is a distinct entity with elevated pulmonary artery pressure and does not produce positional improvement lying flat. Delayed microbubble passage confirms intrapulmonary shunting.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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