If arterial carbon dioxide FALLS below normal, what happens to ventilation?

Correct answer: C. It decreases — breathing becomes slower and shallower

• A. Increased ventilation is the response to a RISE in CO₂ (hypercapnia), not a fall.
• B. Oxygen is the SECONDARY drive, mediated by the peripheral chemoreceptors in the carotid and aortic bodies, and it only becomes significant when PaO₂ falls below about 60 mmHg.
• C. CORRECT. CO₂ is the PRIMARY drive to respiration in health. Central chemoreceptors in the medulla sense hydrogen ion concentration in the cerebrospinal fluid, which tracks arterial CO₂ because CO₂ crosses the blood-brain barrier freely and forms carbonic acid. A FALL in CO₂ means fewer hydrogen ions, less chemoreceptor stimulation, and therefore REDUCED rate and depth of breathing — the negative feedback that corrects the hypocapnia. This is precisely what happens after hyperventilation, and why the appropriate response to a hyperventilating anxious patient is to have them rebreathe into cupped hands rather than give oxygen.
• D. That describes Cheyne-Stokes respiration, seen in heart failure and central neurological disease, not the normal response to hypocapnia.

Note: The relevance to this patient is the HYPOXIC DRIVE question. In severe long-standing COPD with chronic CO₂ retention, the central chemoreceptors adapt and hypoxaemia becomes the more important stimulus — so uncontrolled high-flow oxygen can theoretically blunt respiratory drive. In practice, never withhold oxygen from a patient in genuine distress; titrate it to a target saturation of 88–92% rather than the usual 94–98%.