Effect of continuous positive airway pressure and placebo treatment on sympathetic nervous activity in patients with obstructive sleep apnea

Study objectives: We studied the effect of continuous positive airway press ure (CPAP) treatment on sympathetic nervous activity in 38 patients with ob structive sleep apnea. Design: Randomized, placebo-controlled trial. Setting: Patients underwent polysomnography on three occasions in a clinica l research center, and had BP monitored over 24 h at home. All of the patie nts had sleep apnea with a respiratory disturbance index (RDI) > 15. Interventions: The patients were randomized blindly to CPAP or placebo (CPA P at ineffective pressure) treatment. Measurements and results: Prior to therapy, the number of apneas and the se verity of nocturnal hypoxia correlated significantly with daytime urinary n orepinephrine (NE) levels, but not nighttime urinary NE levels. CPAP treatm ent 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 lymphocyte (beta (2)-adrenergic receptor sensitivity (all p < 0.05). The effect of CPAP tre atment on nighttime urine NE levels and heart rate did not differ from plac ebo treatment. There was a suggestion of an effect of placebo CPAP treatmen t on nighttime measures, but not on daytime measures. Conclusion: We conclude that daytime sympathetic nervous activation is grea ter with more severe sleep apnea. CPAP treatment diminished the daytime sym pathetic activation; the potential nighttime effect of CPAP treatment was o bscured by a small placebo effect.