Respiratory modulation of muscle sympathetic nerve activity in obstructive sleep apnoea

Abstract

New Findings r What is the central question of this study?Muscle sympathetic nerve activity (MSNA) is increased in obstructive sleep apnoea (OSA), leading to hypertension. Is this due to an increase in respiratory-sympathetic coupling, as has been demonstrated in the spontaneously hypertensive rat? r What is the main finding and its importance?Using direct microelectrode recordings of MSNA in hypertensive OSA patients and normotensive control subjects, we show that the magnitude of respiratory modulation is not increased in OSA, arguing against an amplified respiratory-sympathetic coupling as the underlying cause of the neurogenic hypertension, although the temporal coupling of MSNA to respiration was stronger in OSA.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.

Finding inspiration in high blood pressure

“…Interestingly, in support of an earlier study, in which we examined respiratory modulation of MSNA in patients with essential hypertension or chronic obstructive pulmonary disease, we did not find any difference in the magnitude of respiratory modulation of MSNA in OSA patients compared with age‐matched healthy control subjects (Fatouleh et al . ). We quantified respiratory modulation by constructing cross‐correlation histograms between the times of occurrence of sympathetic spikes and the peaks of inspiration.…”

“…() and the hypertensive (OSA patients) and normotensive (age‐matched control subjects) in the study by Fatouleh et al . (). In this way, the conclusions reached in both studies stand on their own.…”

Getting jittery about the mechanism of hypertension in sleep apnoea

“…The relevance of this finding lies in the recognized failure of CPAP to lower blood pressure in OSA patients, including those with confirmed compliant CPAP usage, such as the patients in the study by Fatouleh et al . ().…”

Obstructive Sleep Apnoea and Hypertension: the Role of the Central Nervous System

“…One consistent finding is that individuals with OSA have elevated sympathetic drive during both night and day as indicated indirectly by increased urinary catecholamine levels and directly by intraneural microelectrode recordings (microneurography) of increased muscle sympathetic nerve activity (MSNA) [9][10][11][12]. Remarkably, in OSA subjects, nocturnal apneic events can increase MSNA by more than 300 % [10], and during daytime, resting MSNA is increased by approximately 50-100 % [13•, 14-16].…”