Impediment in upper airway stabilizing forces assessed by phrenic nerve stimulation in sleep apnea patients

Abstract

Background: The forces developed during inspiration play a key role in determining upper airway stability and the occurrence of nocturnal breathing disorders. Phrenic nerve stimulation applied during wakefulness is a unique tool to assess Upper airway dynamic properties and to measure the overall mechanical effects of the inspiratory process on UA stability.

Objectives: To compare the flow/pressure responses to inspiratory and expiratory twitches between sleep apnea subjects and normal subjects.

Methods: Inspiratory and expiratory twitches using magnetic nerve stimulation completed in eleven untreated sleep apnea subjects and ten normal subjects.

Results: In both groups, higher flow and pressure were reached during inspiratory twitches. The two groups showed no differences in expiratory twitch parameters. During inspiration, the pressure at which flow-limitation occurred was more negative in normals than in apneic subjects, but not reaching significance (p = 0.07). The relationship between pharyngeal pressure and flow adequately fitted with a polynomial regression model providing a measurement of upper airway critical pressure during twitch. This pressure significantly decreased in normals from expiratory to inspiratory twitches (-11.1 +/- 1.6 and -15.7 +/- 1.0 cm H2O respectively, 95% CI 1.6-7.6, p < 0.01), with no significant difference between the two measurements in apneic subjects. The inspiratory/expiratory difference in critical pressure was significantly correlated with the frequency of nocturnal breathing disorders.

Conclusion: Inspiratory-related upper airway dilating forces are impeded in sleep apnea patients.

Figure 1

Example of the changes in inspiratory flow and pharyngeal andesophageal pressures in response to PNS-induced diaphragm twitch applied during expiration and inspiration in a representative normal subject. Time 0 indicates the application of the twitch. I max and I min stand respectively for the maximal and minimal flow values reached during the twitch. Pphar lim indicates the pharyngeal pressure value corresponding to Imax. Peso peak indicates the driving pressure corresponding to I min. See text for abbreviations.

Figure 2

Plot of the flow/pharyngeal pressure relationships obtained with data presented in Figure 1. The flow-limited nature of the twitch-induced flow is clearly demonstrated by the flow drop associated with the pharyngeal pressure decrease once Pphar lim and the corresponding I max have been reached. A clear difference in the flow/pressure curves can be seen between the expiratory and inspiratory twitches.

Figure 3

k1/k2 ratios for expiratory and inspiratory twitches in the normal and OSA groups. There was no difference between the two groups for expiratory twitches while the ratio decreased significantly during inspiratory twitches in normals and increased in OSA subjects. Boxes identify the 25 to 75 th percentiles of the data with the median value indicated. Horizontal lines outside the boxes depict the 10 to 90 th percentiles. Closed circles represent outliers. *: significant difference between inspiratory and expiratory values.

Figure 4

Correlation between individual values of the apnea + hypopnea index and the corresponding UA aperture inspiratory efficiency index.