Study objectives: We studied the effect of continuous
positive airway pressure (CPAP) treatment on sympathetic nervous
activity in 38 patients with obstructive sleep apnea.
Design: Randomized, placebo-controlled trial.
Setting: Patients underwent polysomnography on three
occasions in a clinical research center, and had BP monitored over
24 h at home. All of the patients had sleep apnea with a
respiratory disturbance index (RDI) > 15.
Interventions: The patients were randomized blindly to CPAP
or placebo (CPAP at ineffective pressure) treatment.
Measurements and results: Prior to therapy, the number of
apneas and the severity of nocturnal hypoxia correlated significantly
with daytime urinary norepinephrine (NE) levels, but not nighttime
urinary NE levels. CPAP treatment lowered daytime BP from 99 ± 2 mm
Hg to 95 ± 3 mm Hg (mean ± SEM) and nighttime BP from 93 ± 3
mm Hg to 88 ± 3 mm Hg. Placebo CPAP treatment decreased both day and
night mean BP only 2 mm Hg. CPAP, but not placebo, treatment lowered
daytime plasma NE levels by 23%, daytime urine NE levels by 36%,
daytime heart rate by 2.6 beats/min, and increased lymphocyteβ
2-adrenergic receptor sensitivity (all p < 0.05). The
effect of CPAP treatment on nighttime urine NE levels and heart rate
did not differ from placebo treatment. There was a suggestion of an
effect of placebo CPAP treatment on nighttime measures, but not on
Conclusion: We conclude that daytime
sympathetic nervous activation is greater with more severe sleep apnea.
CPAP treatment diminished the daytime sympathetic activation; the
potential nighttime effect of CPAP treatment was obscured by a small