Periodic breathing consists of clusters of breaths separated by intervals of apnea (no breathing) or near-apnea. As opposed to normal breathing which is usually regular.
Periodic breathing was originally thought to arise from serious neurological or cardiovascular disease and therefore to carry a poor outlook. It is now known that periodic breathing also tends to occur during sleep, it can occur in healthy persons, and the apnea in periodic breathing is usually central (without respiratory movements) rather than obstructive (caused by upper-airway blockage).
Periodic breathing during sleep occurs typically in patients with congestive heart failure.
See also: Cheyne-Stokes respiration