A 17 year old male presents to the emergency room after being stabbed in the chest, on chest x-ray his right lung was fully collapsed, and yet his SpO2 was 94% on room air - why?

What is hypoxic pulmonary vasoconstriction?

  • Hypoxic Pulmonary Vasoconstriction is a physiologic protective mechanism which prevents right to left shunting of blood.

  • Right to left shunt causes hypoxemia unresponsive to oxygen therapy

One would expect that this patient would have a 50% shunt due to perfusion but no ventilation of the right lung; this does not happen. Hypoxic pulmonary vasoconstriction (HPV) takes place. Many of the tissues in the body are capable of regulating their own blood flow the heart, the kidney, the brain and the gut all autoregulate blood flow. It appears that HVP is a similar mechanism within the lung, to prevent right to left intrapulmonary shunting, and thus the presence of deoxygenated blood in the peripheral circulation. This process is most florid in utero, when blood is diverted away from the lungs through the ductus arteriosis, due to high pulmonary arterial pressures. We know that pulmonary smooth muscle cells are extremely sensitive to alveolar oxygen tensions, but the mechanism of vasoconstriction is unknown. HPV is probably multifactorial in origin and modulated by a variety of endothelium dependent factors (nitric oxide, endothelin, prostacyclin etc).

Certain pharmacological interventions and disease processes interfere with HPV: general anesthesia with volatile agents such as isoflurane, and the use of systemic vasodilators such as sodium nitroprusside and prostacyclin, reverse HPV and may cause ventilation-perfusion mismatch. Acute lung injuries and, in particular, lung contusions, may have a similar effect. The result is ventilation-perfusion mismatch and possible right to left shunting of deoxygenated blood. The treatment is recruitment of collapsed alveoli using continuous positive airway pressure (CPAP), and positioning the patient away from the injury side (good side down, always).


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