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Sympathetic Chemoreflex Responses in Obstructive Sleep Apnea and Effects of Continuous Positive Airway Pressure Therapy^*

^* From the Division of Pulmonary, Allergy and Critical Care (Dr. Imadojemu), Penn State Heart & Vascular Institute (Drs. Mawji and Leuenberger, Ms. Gray, and Ms. Hogeman), Department of Health Evaluation Sciences (Mr. Kunselman), The Milton S. Hershey Medical Center, The Pennsylvania State University College of Medicine, Hershey, PA.

Correspondence to: Urs A. Leuenberger, MD, Penn State Heart & Vascular Institute, Mail Code H047, 500 University Dr, Hershey, PA 17033; e-mail: uleuenberger{at}psu.edu

Abstract

Background: Sympathetic nerve activity is increased in awake and regularly breathing patients with obstructive sleep apnea (OSA). Over time, repetitive hypoxic stress could alter sympathetic chemoreflex function in OSA.

Methods: We determined the responses to acute hypoxia (fraction of inspired oxygen of 0.1, for 5 min), static handgrip exercise, and the cold pressor test (CPT) in 24 patients with OSA (age, 50 ± 3 years [mean ± SEM]; apnea-hypopnea index, 47 ± 6 events per hour) and in 14 age- and weight-matched nonapneic control subjects. Muscle sympathetic nerve activity (MSNA) [peroneal microneurography], BP, and ventilation were monitored.

Results: Basal MSNA was higher in OSA patients compared to control subjects (45 ± 4 bursts per minute vs 33 ± 4 bursts per minute, respectively; p < 0.05). Furthermore, compared to control subjects, the MSNA responses to hypoxia were markedly enhanced in OSA (p < 0.001). Whereas the ventilatory responses to hypoxia tended to be increased in OSA (p = 0.06), the BP responses did not differ between the groups (p = 0.45). The neurocirculatory reflex responses to handgrip exercise and to the CPT were similar in the two groups (p = not significant). In OSA patients who were retested after 1 to 24 months of continuous positive airway pressure (CPAP) therapy (n = 11), basal MSNA (p < 0.01) and the responses of MSNA to hypoxia (p < 0.01) decreased significantly, whereas the ventilatory responses remained unchanged (p = 0.82).

Conclusion: These data suggest that the sympathetic responses to hypoxic chemoreflex stimulation are enhanced in OSA and may normalize in part following CPAP therapy.

Key Words: autonomic nervous • hypoxemia • sleep apnea