Exaggerated ventilatory drive estimates from epiglottic and esophageal pressure deflections in the presence of airway occlusion

Physiological Determinants of Snore Loudness

Rationale: The physiological factors modulating the severity of snoring have not been adequately described. Airway collapse or obstruction is generally the leading determinant of snore sound generation; however, we suspect that ventilatory drive is of equal importance. Objective: To determine the relationship between airway obstruction and ventilatory drive on snore loudness. Methods: In 40 patients with suspected or diagnosed obstructive sleep apnea (1-98 events/hr), airflow was recorded via a pneumotachometer attached to an oronasal mask, ventilatory drive was recorded using calibrated intraesophageal diaphragm electromyography, and snore loudness was recorded using a calibrated microphone attached over the trachea. "Obstruction" was taken as the ratio of ventilation to ventilatory drive and termed flow:drive, i.e., actual ventilation as a percentage of intended ventilation. Lower values reflect increased flow resistance. Using 165,063 breaths, mixed model analysis (quadratic regression) quantified snore loudness as a function of obstruction, ventilatory drive, and the presence of extreme obstruction (i.e., apneic occlusion). Results: In the presence of obstruction (flow:drive = 50%, i.e., doubled resistance), snore loudness increased markedly with increased drive (+3.4 [95% confidence interval, 3.3-3.5] dB per standard deviation [SD] change in ventilatory drive). However, the effect of drive was profoundly attenuated without obstruction (at flow:drive = 100%: +0.23 [0.08-0.39] dB per SD change in drive). Similarly, snore loudness increased with increasing obstruction exclusively in the presence of increased drive (at drive = 200% of eupnea: +2.1 [2.0-2.2] dB per SD change in obstruction; at eupneic drive: +0.14 [-0.08 to 0.28] dB per SD change). Further, snore loudness decreased substantially with extreme obstruction, defined as flow:drive <20% (-9.9 [-3.3 to -6.6] dB vs. unobstructed eupneic breathing). Conclusions: This study highlights that ventilatory drive, and not simply pharyngeal obstruction, modulates snore loudness. This new framework for characterizing the severity of snoring helps better understand the physiology of snoring and is important for the development of technologies that use snore sounds to characterize sleep-disordered breathing.

Neural ventilatory drive decline as a predominant mechanism of obstructive sleep apnoea events

BACKGROUND

In the classic model of obstructive sleep apnoea (OSA), respiratory events occur with sleep-related dilator muscle hypotonia, precipitating increased neural ventilatory 'drive'. By contrast, a drive-dependent model has been proposed, whereby falling drive promotes dilator muscle hypotonia to precipitate respiratory events. Here we determine the extent to which the classic versus drive-dependent models of OSA are best supported by direct physiological measurements.

METHODS

In 50 OSA patients (5-91 events/hour), we recorded ventilation ('flow', oronasal mask and pneumotach) and ventilatory drive (calibrated intraoesophageal diaphragm electromyography, EMG) overnight. Flow and drive during events were ensemble averaged; patients were classified as drive dependent if flow fell/rose simultaneously with drive. Overnight effects of lower drive on flow, genioglossus muscle activity (EMGgg) and event risk were quantified (mixed models).

RESULTS

On average, ventilatory drive fell (rather than rose) during events (-20 (-42 to 3)%_baseline, median (IQR)) and was strongly correlated with flow (R=0.78 (0.24 to 0.94)). Most patients (30/50, 60%) were classified as exhibiting drive-dependent event pathophysiology. Lower drive during sleep was associated with lower flow (-17 (-20 to -14)%/drive) and EMGgg (-3.5 (-3.8 to -3.3)%_max/drive) and greater event risk (OR: 2.2 (1.8 to 2.5) per drive reduction of 100%_eupnoea); associations were concentrated in patients with drive-dependent OSA (ie, flow: -37 (-40 to -34)%/drive, OR: 6.8 (5.3 to 8.7)). Oesophageal pressure-without tidal volume correction-falsely suggested rising drive during events (classic model).

CONCLUSIONS

In contrast to the prevailing view, patients with OSA predominantly exhibit drive-dependent event pathophysiology, whereby flow is lowest at nadir drive, and lower drive raises event risk. Preventing ventilatory drive decline is therefore considered a target for OSA intervention.