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

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 H(2)O 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

A Model Analysis of Arterial Oxygen Desaturation during Apnea in Preterm Infants

Rapid arterial O2 desaturation during apnea in the preterm infant has obvious clinical implications but to date no adequate explanation for why it exists. Understanding the factors influencing the rate of arterial O2 desaturation during apnea () is complicated by the non-linear O2 dissociation curve, falling pulmonary O2 uptake, and by the fact that O2 desaturation is biphasic, exhibiting a rapid phase (stage 1) followed by a slower phase when severe desaturation develops (stage 2). Using a mathematical model incorporating pulmonary uptake dynamics, we found that elevated metabolic O2 consumption accelerates throughout the entire desaturation process. By contrast, the remaining factors have a restricted temporal influence: low pre-apneic alveolar causes an early onset of desaturation, but thereafter has little impact; reduced lung volume, hemoglobin content or cardiac output, accelerates during stage 1, and finally, total blood O2 capacity (blood volume and hemoglobin content) alone determines during stage 2. Preterm infants with elevated metabolic rate, respiratory depression, low lung volume, impaired cardiac reserve, anemia, or hypovolemia, are at risk for rapid and profound apneic hypoxemia. Our insights provide a basic physiological framework that may guide clinical interpretation and design of interventions for preventing sudden apneic hypoxemia

The action of the canine diaphragm on the lower ribs depends on activation.

Conventional wisdom maintains that the diaphragm lifts the lower ribs during isolated contraction. Recent studies in dogs have shown, however, that supramaximal, tetanic stimulation of the phrenic nerves displaces the lower ribs caudally and inward. In the present study, the hypothesis was tested that the action of the canine diaphragm on these ribs depends on the magnitude of muscle activation. Two experiments were performed. In the first, the C5 and C6 phrenic nerve roots were selectively stimulated in 6 animals with the airway occluded, and the level of diaphragm activation was altered by adjusting the stimulation frequency. In the second experiment, all the inspiratory intercostal muscles were severed in 7 spontaneously breathing animals, so that the diaphragm was the only muscle active during inspiration, and neural drive was increased by a succession of occluded breaths. The changes in airway opening pressure and the craniocaudal displacements of ribs 5 and 10 were measured in each animal. The data showed that 1) contraction of the diaphragm causes the upper ribs to move caudally; 2) during phrenic nerve stimulation, the lower ribs move cranially when the level of diaphragm activation is low, but they move caudally when the level of muscle activation is high and the entire rib cage is exposed to pleural pressure; and 3) during spontaneous diaphragm contraction, however, the lower ribs always move cranially, even when neural drive is elevated and the change in pleural pressure is large. It is concluded that the action of the diaphragm on the lower ribs depends on both the magnitude and the mode of muscle activation. These findings can reasonably explain the apparent discrepancies between previous studies. They also imply that observations made during phrenic nerve stimulation do not necessarily reflect the physiological action of the diaphragm.Journal ArticleResearch Support, Non-U.S. Gov'tSCOPUS: ar.jinfo:eu-repo/semantics/publishe