Diane E. Davison

Nocturnal Continuous Positive Airway Pressure Decreases Daytime Sympathetic Traffic in Obstructive Sleep Apnea

BACKGROUND Patients with obstructive sleep apnea (OSA) have high levels of muscle sympathetic nerve activity (MSNA). We tested the hypothesis that long-term continuous positive airway pressure (CPAP) treatment will decrease MSNA in OSA patients. METHODS AND RESULTS We measured blood pressure, heart rate, and MSNA in 11 normotensive, otherwise healthy patients with OSA who were treated with CPAP. The measurements were obtained at baseline and after 1 month, 6 months, and 1 year of CPAP treatment. These measurements were compared with those recorded in 9 otherwise healthy OSA patients who were not treated with CPAP for 1 year. In both untreated and treated patients, blood pressure and heart rate did not change over time. MSNA was similar during repeated measurements in the untreated group. By contrast, MSNA decreased significantly over time in patients treated with CPAP. This decrease was evident after both 6 months and 1 year of CPAP treatment (P=0.02 for both). CONCLUSIONS CPAP treatment decreases muscle sympathetic traffic in patients with OSA. This effect of CPAP is evident only after an extended duration of therapy.

Cigarette Smoking Increases Sympathetic Outflow in Humans

Background—It is generally accepted that smoking increases blood pressure and inhibits muscle sympathetic nerve activity (SNA). The decrease in muscle SNA with cigarette smoking might be secondary to baroreflex responses to the pressor effect of smoking, thus obscuring a sympathetic excitatory effect of smoking. We tested the hypothesis that smoking increases sympathetic outflow. Methods and Results—We examined the effects of sham smoking, cigarette smoking, and cigarette smoking in combination with nitroprusside on muscle (baroreflex-dependent) SNA in 10 healthy habitual smokers. The 3 sessions were performed in random order, each study on a separate day. In an additional study, we also investigated the effects of sham smoking and cigarette smoking on skin (baroreflex-independent) SNA in 9 subjects. Compared with sham smoking, cigarette smoking alone increased blood pressure and decreased muscle SNA. When the blood pressure increase in response to smoking was blunted by nitroprusside infusion, there was ...

Baroreflex Control of Sympathetic Nerve Activity and Heart Rate in Obstructive Sleep Apnea

-Patients with obstructive sleep apnea are at increased risk for hypertension. The mechanisms underlying this increased risk are not known. We tested the hypothesis that obstructive sleep apnea, independent of factors such as hypertension, obesity, and age, is characterized by impairment of baroreflex sensitivity. We measured muscle sympathetic nerve activity (MSNA) and heart rate responses to activation and deactivation of baroreceptors in newly diagnosed, never treated, normotensive patients with obstructive sleep apnea. These responses were compared with those obtained in healthy control subjects closely matched for age, body mass index, and blood pressure. Heart rate and MSNA changes during infusion of phenylephrine (baroreceptor activation) were similar in the control subjects and patients with sleep apnea. Infusion of nitroprusside (baroreceptor deactivation) elicited similar decreases in mean arterial pressure (MAP) but lesser MSNA increases in patients with sleep apnea than in control subjects. Calculation of DeltaMSNA/DeltaMAP ratio revealed that baroreflex regulation of sympathetic activity for similar blood pressure changes was diminished in patients with sleep apnea in comparison to normal control subjects (P=0.01). However, increases in heart rate during nitroprusside infusion were comparable in both groups. Sympathetic, blood pressure and heart rate responses to the cold pressor test were also similar in the 2 groups. Our results indicate that normotensive patients with sleep apnea have a selective impairment of the sympathetic response to baroreceptor deactivation but not to baroreceptor activation or to the cold pressor test. The impairment of baroreflex sympathetic modulation in patients with sleep apnea is not accompanied by any impairment of baroreflex control of heart rate.