Question

What is the mechanism by which hyperventilation may cause muscle spasm?

• A. Decreased calcium
• B. Decreased carbon dioxide
• C. Decreased potassium
• D. Decreased sodium

Hint:

Hyperventilation causes respiratory alkalosis, which lowers ionised (free) calcium.

Answer 1

The Correct Option is A

Step 1: Track what hyperventilation does to blood gases.
Fast, deep breathing blows off carbon dioxide faster than the body makes it, so the blood level of \(CO_2\) falls. This is called hypocapnia.

Step 2: Link falling \(CO_2\) to blood pH.
Less \(CO_2\) in the blood means less carbonic acid, so blood pH rises. This is respiratory alkalosis.

Step 3: See what alkalosis does to calcium.
Roughly half of the calcium in plasma travels bound to albumin, and the rest travels free (ionised). A rise in pH makes albumin bind hydrogen ions less, which frees up extra negative sites on albumin. Calcium then binds more tightly to albumin, so the free, ionised calcium fraction in the blood falls, even though the total calcium level does not change.

Step 4: Connect low ionised calcium to nerve and muscle excitability.
Ionised calcium normally stabilises nerve cell membranes by blocking sodium channels from opening too easily. When ionised calcium drops, nerve membranes become more excitable, firing action potentials with a smaller trigger. This causes tetany, muscle twitching and spasm, classically seen as carpopedal spasm in someone who hyperventilates.

Step 5: Rule out the other options.
Carbon dioxide itself falls, but it is the resulting alkalosis and fall in ionised calcium, not the CO2 drop directly, that triggers the spasm. Potassium and sodium do shift slightly with alkalosis, but neither is the main driver of the tetany seen with hyperventilation.

Final Answer:
\[ \boxed{\text{Decreased ionised calcium}} \]