Obstructive versus central: a key fork
In obstructive sleep apnea the brain keeps sending the order to breathe — the chest and belly strain — but the airway is blocked. In central sleep apnea the opposite happens: for a few seconds the respiratory center in the brainstem simply stops sending the signal. The airway is open, but no effort is made at all. Airflow and breathing effort both go quiet together.
Why the signal wavers
The respiratory center tunes breathing to carbon dioxide, sensed by the central chemoreceptors. During sleep this control loop can become unstable, especially in heart failure. Breathing then swells and fades in a slow crescendo–decrescendo with pauses in between — a pattern called Cheyne–Stokes respiration. It is a classic form of central apnea, driven by an over-sensitive feedback system that keeps overshooting and undershooting.
Central apnea also turns up with some long-acting opioid medicines, after a stroke, and at high altitude. The common thread is a breathing controller that has lost its steady rhythm rather than a throat that has closed.
Obesity hypoventilation: under-breathing around the clock
Obesity hypoventilation syndrome is a different problem again. Here the weight of the chest and abdomen, plus a blunted drive, leads to chronic hypoventilation — shallow, insufficient breathing — so that carbon dioxide builds up not just at night but during the day too. The defining sign is a daytime PaCO2 that stays high, a state of hypercapnia, in a person with obesity once other causes are excluded.
Many people with obesity hypoventilation also have obstructive sleep apnea on top, so the two overlap. The distinction still matters: obstructive apnea is about a blocked airway at night, while obesity hypoventilation is about not moving enough air overall — and managing the daytime carbon dioxide becomes part of the goal.