At high altitude, the primary stimulus for increased ventilation in the first few hours is mediated by which of the following?

Explanation

## Acute Hypoxic Ventilatory Response at High Altitude **Key Point:** At high altitude, hypoxemia (low PaO₂) is the dominant stimulus for increased ventilation in the acute phase, detected by peripheral chemoreceptors (carotid and aortic bodies). ### Mechanism of Acute Response When an individual ascends to high altitude: 1. Atmospheric pressure decreases → alveolar PO₂ falls → arterial PaO₂ drops 2. Peripheral chemoreceptors (carotid and aortic bodies) sense this decrease in PaO₂ 3. Afferent signals travel via glossopharyngeal (CN IX) and vagal (CN X) nerves to the respiratory centers 4. Ventilation increases to restore oxygenation **High-Yield:** The peripheral chemoreceptors are exquisitely sensitive to PaO₂ **below 60 mmHg**. At sea level (PaO₂ ~95 mmHg), they contribute minimally; at altitude, they become the dominant drive. ### Why Not the Other Options? | Stimulus | Timing | Role at Altitude | |----------|--------|------------------| | ↓ PaCO₂ (central) | Minutes to hours | Secondary; develops *after* initial hyperventilation | | ↑ H⁺ in CSF | Hours to days | Becomes important in acclimatization (metabolic compensation) | | Temperature | Static | Not a primary acute stimulus | **Clinical Pearl:** The initial hyperventilation at altitude causes respiratory alkalosis (↓ PaCO₂, ↑ pH), which paradoxically *dampens* the ventilatory drive. Over hours, the kidneys excrete bicarbonate to lower CSF pH, restoring the hypoxic drive—this is acclimatization. **Mnemonic:** **CHEMO-HYPOX** — Chemoreceptors respond to HYPOXia (not initially to CO₂ changes at altitude).