Respiratory modulation of muscle sympathetic nerve activity in obstructive sleep apnoea

Obstructive sleep apnoea (OSA) is associated with elevated muscle sympathetic nerve activity (MSNA) during normoxic daytime wakefulness, leading to hypertension. We tested the hypothesis that respiratory–sympathetic coupling, postulated to be the underlying cause of neurogenic hypertension, is increased in OSA. Muscle sympathetic nerve activity, blood pressure, ECG and respiration were recorded in 21 normotensive control subjects and 21 newly diagnosed patients with OSA before and after 6 and 12 months of treatment with continuous positive airway pressure. Muscle sympathetic nerve activity was recorded via tungsten microelectrodes inserted percutaneously into the peroneal nerve. Cardiac and respiratory modulation of MSNA was quantified from the cross-correlation histograms constructed between the sympathetic spikes and either ECG or respiration. Muscle sympathetic nerve activity was significantly elevated in newly diagnosed OSA patients compared with control subjects (53 ± 2 versus 28 ± 2 bursts min−1). There was a significant fall in MSNA after 6 months of continuous positive airway pressure (37 ± 2 bursts min−1), with no further change after 12 months (37 ± 2 bursts min−1). There were no significant differences in the magnitude of respiratory modulation of MSNA between the OSA patients and control subjects (40 ± 3.1 versus 39 ± 3.4%). However, when considering the normalized temporal profile there were changes in the respiratory patterning of MSNA in OSA, with more activity occurring in postinspiration and less in inspiration and expiration. This was largely reversed following long-term continuous positive airway pressure.