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.