Breath-holding syncope is caused by which physiologic change related to pre-immersion hyperventilation?

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Multiple Choice

Breath-holding syncope is caused by which physiologic change related to pre-immersion hyperventilation?

Explanation:
Lowering carbon dioxide through pre-immersion hyperventilation reduces the stimulus to breathe. Breathing is driven mainly by CO2 levels and brain pH, so when CO2 is swept down (hypocapnia) the respiratory centers dial back their drive and you can hold your breath longer. But hypocapnia also causes cerebral blood vessels to constrict, decreasing blood flow to the brain. As oxygen becomes scarce during the breath hold, this combination makes the brain more vulnerable to hypoxia, increasing the risk of breath-holding syncope.

Lowering carbon dioxide through pre-immersion hyperventilation reduces the stimulus to breathe. Breathing is driven mainly by CO2 levels and brain pH, so when CO2 is swept down (hypocapnia) the respiratory centers dial back their drive and you can hold your breath longer. But hypocapnia also causes cerebral blood vessels to constrict, decreasing blood flow to the brain. As oxygen becomes scarce during the breath hold, this combination makes the brain more vulnerable to hypoxia, increasing the risk of breath-holding syncope.

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