Blood pressure variability in obstructive sleep apnea: role of sympathetic nervous activity and effect of continuous positive airway pressure

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Abstract
Many studies support a link between obstructive sleep apnea (OSA), increased blood pressure (BP) and/or BP variability, and sympathetic nervous system (SNS) activity. We assessed the relationship between SNS activity and 24-h BP variability in patients with OSA, and the effect of continuous positive airway pressure (CPAP) on BP variability. Forty-one patients with a respiratory disturbance index (RDI) > 15 were randomized into CPAP or CPAP placebo groups for a 1-week trial. Ambulatory BP, 24-h urine norepinephrine (NE) and polysomnography were measured prior to treatment and after 1 and 7 days of treatment. Neither RDI nor 24-h urine NE levels were related to 24-h mean BP levels. While RDI was associated only with night-time BP variability, daytime urine NE levels were associated with both night-time and daytime BP variability. After treatment, the BP variability decreased significantly but equally in both active and placebo CPAP groups. Obstructive sleep apnea is more related to BP variability than BP. Sympathetic nervous activity, as inferred from daytime urine NE, is related to changes in BP variability in OSA patients. BP variability is not specifically affected by CPAP.