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Retired NBME 21 Answers

nbme21/Block 1/Question#13 (reveal difficulty score)
A 25-year-old man develops shortness of ...
Decreased alveolar Po2 ๐Ÿ” / ๐Ÿ“บ / ๐ŸŒณ / ๐Ÿ“–
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submitted by โˆ—alexb(53)
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Hypoxic pulmonary vasoconstriction (HPV), also known as the Euler-Liljestrand mechanism, is a physiological phenomenon in which small pulmonary arteries constrict in the presence of alveolar hypoxia (low oxygen levels).

While the maintenance of ventilation/perfusion ratio during regional obstruction of airflow is beneficial, HPV can be detrimental during global alveolar hypoxia which occurs with exposure to high altitude, where HPV causes a significant increase in total pulmonary vascular resistance, and pulmonary arterial pressure, potentially leading to pulmonary hypertension and pulmonary edema.

https://en.wikipedia.org/wiki/Hypoxic_pulmonary_vasoconstriction

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lovebug  2019 FA, Pg 665. PAH +1
drdoom  ^ broken link +1
skonys  (Lemme know if I'm wrong and dumb af) But I believe we evolved this is that the body doesn't want to perfuse areas that arn't ventilated well. here's a pic https://rk.md/wp-content/uploads/2018/01/hypoxic-pulmonary-vasoconstriction-diagram.jpg who do you have in a bar brawl? Dr. Ryan or Dr. Sattar? +1
skonys  Edits: ...we evolved this so that the... +1



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submitted by โˆ—haliburton(224)
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FA 2017: Chronic hypoxic pulmonary vasoconstriction results in pulmonary hypertension and RVH.

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yotsubato  Yeah but in a chronic case this guy would produce more RBC and not be hypoxic anymore. +27



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submitted by โˆ—abhishek021196(119)
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Hypoxic pulmonary vasoconstriction (HPV), also known as the Euler-Liljestrand mechanism, is a physiological phenomenon in which small pulmonary arteries constrict in the presence of alveolar hypoxia (low oxygen levels). By redirecting blood flow from poorly-ventilated lung regions to well-ventilated lung regions, HPV is thought to be the primary mechanism underlying ventilation/perfusion matching.[1][2] The process might initially seem counterintuitive, as low oxygen levels might theoretically stimulate increased blood flow to the lungs to increase gas exchange. However, the purpose of HPV is to distribute bloodflow regionally to increase the overall efficiency of gas exchange between air and blood. While the maintenance of ventilation/perfusion ratio during regional obstruction of airflow is beneficial, HPV can be detrimental during global alveolar hypoxia which occurs with exposure to high altitude, where HPV causes a significant increase in total pulmonary vascular resistance, and pulmonary arterial pressure, potentially leading to pulmonary hypertension and pulmonary edema. Several factors inhibit HPV including increased cardiac output, hypocapnia, hypothermia, acidosis/alkalosis, increased pulmonary vascular resistance, inhaled anesthetics, calcium channel blockers, positive end-expiratory pressure (PEEP), high-frequency ventilation (HFV), isoproterenol, nitric oxide, and vasodilators.

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