<|im_start|>system
You are a highly knowledgeable medical expert and researcher. Provide accurate, evidence-based medical information, differential diagnoses, and detailed explanations. Always include a disclaimer that your information is not a substitute for professional medical advice.<|im_end|>
<|im_start|>user
Question: All of the following cause hyperventilation except
Options:
A. Decreased pH in CSF
B. Decreased plasma HCO3
C. CO poisoning
D. Increased adrenergic levels
Which is the correct option?<|im_end|>
<|im_start|>assistant
The correct option is C: CO poisoning. Explanation: Arterial hypoxemia is defined as an arterial P02 (partial pressure of oxygen) less than 80 mmHg, who is breathing room air at sea level. And because the partial pressure of a gas is determined by the soluble fraction of gas and not by the amount carried chemically bound to hemoglobin - the Hb does not determine P02 and hypoxemia does not depend on Hb level.
In CO poisoning effective hemoglobin that can carry O2 decreases (because CO occupies Hb binding site) since P02 or arterial blood O2 remains normal, peripheral chemoreceptors are not stimulated and hyperventilation is not seen.
 In hypoxia, with the reduction of PO2 cerebral blood flow increases (d/t vaso dilatation)to maintain O2 delivery in the brain.
The cerebral cortex can tolerate acute hypoxia for 5-10 min at 28°C, 20 min at 20°C, and 50 min at 50°C.
Hypoxic ischemia almost invariably involve hippocampus. The hippocampal CA1 neurons are vulnerable to even brief episodes of hypoxic ischemia.<|im_end|>
