Periodic breathing, a variant of Cheyne-Stokes respiration, is characterized by regular, recurrent cycles of changing tidal volumes in which the lowest tidal volume is less than half the maximal tidal volume in that cycle. It is the most frequent abnormal respiratory pattern directly related to stroke rather than underlying systemic disease, occurring in approximately 25 percent of patients. Periodic breathing may be more common among patients with subarachnoid hemorrhage.