ing these problems has not been studied. 
Specifiers
An increase in the central apnea index (i.e., number of central apneas per hour of sleep) re-
flects an increase in severity of central sleep apnea. Sleep continuity and quality may be
markedly impaired with reductions in restor ative stages of non–rapid eye movement (REM)
sleep (i.e., decreased slow-wav e sleep [stage N3]). In indi viduals with severe Cheyne-
Stokes breathing, the pattern can also be ob served during resting wakefulness, a finding
that is thought to be a poor prognostic marker for mortality. 
Diagnostic Features
Central sleep apnea disorders ar e characterized by repeated episodes of apneas and hy-
popneas during sleep caused by variability in respiratory effort. These are disorders of
ventilatory control in which resp iratory events occur in a peri odic or intermittent pattern.
Idiopathic central sleep apnea  is characterized by sleepiness, insomnia, and awakenings due
to dyspnea in association with five or more  central apneas per hour of sleep. Central sleep
apnea occurring in individuals with heart failu re, stroke, or renal failure typically have a
breathing pattern called Cheyne-Stok es breathing,  which is characterized by a pattern of
periodic crescendo-decrescendo variation in tidal volume that results in central apneas
and hypopneas occurring at a frequency of at least five events per hour that are accompa-
nied by frequent arousals. Central and obstru ctive sleep apneas may coexist; the ratio of
central to obstructive apneas/hypopneas may be  used to identify which condition is pre-
dominant.
Alterations in neuromuscular control of br eathing can occur in association with med-