John E. Remmers

Automated analysis of digital oximetry in the diagnosis of obstructive sleep apnoea

BACKGROUND The gold standard diagnostic test for obstructive sleep apnoea (OSA) is overnight polysomnography (PSG) which is costly in terms of time and money. Consequently, a number of alternatives to PSG have been proposed. Oximetry is appealing because of its widespread availability and ease of application. The diagnostic performance of an automated analysis algorithm based on falls and recovery of digitally recorded oxygen saturation was compared with PSG. METHODS Two hundred and forty six patients with suspected OSA were randomly selected for PSG and automated off line analysis of the digitally recorded oximeter signal. RESULTS The PSG derived apnoea hypopnoea index (AHI) and oximeter derived respiratory disturbance index (RDI) were highly correlated (R = 0.97). The mean (2SD) of the differences between AHI and RDI was 2.18 (12.34)/h. The sensitivity and specificity of the algorithm depended on the AHI and RDI criteria selected for OSA case designation. Using case designation criteria of 15/h for AHI and RDI, the sensitivity and specificity were 98% and 88%, respectively. If the PSG derived AHI included EEG based arousals as part of the hypopnoea definition, the mean (2SD) of the differences between RDI and AHI was –0.12 (15.62)/h and the sensitivity and specificity profile did not change significantly. CONCLUSIONS In a population of patients suspected of having OSA, off line automated analysis of the oximetry signal provides a close estimate of AHI as well as excellent diagnostic sensitivity and specificity for OSA.

Neural and Anatomic Factors Related to Upper Airway Occlusion During Sleep

Both neural and anatomical factors play an important role in the maintenance of upper airway patency. An abnormality in one or both of these factors is felt to be the underlying cause of obstructive sleep apnea.

Evidence that ventilatory rhythmogenesis in the frog involves two distinct neuronal oscillators

In Rana catesbeiana the upper airways are used for two distinct yet highly coordinated ventilatory behaviours: buccal ventilation and lung inflation cycles. How these behaviours are generated and coordinated is unknown. The purpose of this study was to identify putative rhythmogenic brainstem loci involved in these ventilatory behaviours. We surveyed the isolated postmetamorphic brainstem to determine sites where local depolarization, produced by microinjecting the non‐NMDA glutamate receptor agonist, AMPA, augmented the ventilatory motor patterns. Two sites were identified: a caudal site, at the level of cranial nerve (CN) X, where AMPA injections caused increased buccal burst frequency but abolished lung bursts, and a rostral site, between the levels of CN VIII and IX, where injections increased the frequency of both types of ventilatory bursts. These two sites were further examined using GABA microinjections to locally inhibit cells. GABA injected into the caudal site suppressed the buccal rhythm but the lung rhythm continued, albeit at a different frequency. When GABA was injected into the rostral site the lung bursts were abolished but the buccal rhythm continued. When the two sites were physically separated by transection, both rostral and caudal brainstem sections were capable of rhythmogenesis. The results suggest the respiratory network within the amphibian brainstem is composed of at least two distinct but interacting oscillators, the buccal and lung oscillators. These putative oscillators may provide a promising experimental model for studying coupled oscillators in vertebrates.

Action of intercostal muscle afferents on the respiratory rhythm of anesthetized cats

Experiments designed to reveal an action of intercostal afferent stimulation on the rhythm of breathing were carried out on barbiturate-anesthetized, paralyzed cats, ventilated by a positive pressure respirator. Tetanic stimuli were applied to midthoracic, external intercostal nerves of intact animals and of animals displaying an apneustic pattern consequent to vagotomy and pontine lesions. In intact animals, the respiratory rhythm, as indicated by phrenic efferent activity, could be paced by intercostal nerve stimuli when they were timed to occur in a particular relationship to the lung volume cycle. Apparently, the action of the stimuli supplemented the phasic, volume-related afferent input. The response required considerable spatial summation and resulted from the action of intercostal afferents to either shorten inspiration or prolong expiration. These reflex effects could be attributed to activation of group II afferents. Although the actions of these afferents were relatively weak in the intact preparation, they proved potent in terminating an apneustic breath; brief stimuli, if they activated group II afferents, cut short phrenic discharge and initiated an expiratory phase, thereby restoring a reasonably normal respiratory rhythm.

Two regions in the isolated brainstem of the frog that modulate respiratory-related activity.

Using microinjection techniques, we have explored the isolated, complete midline sectioned brainstem of the frog (Rana catesbeiana) to identify regions that influence the endogenous respiratory-related motor activity. Ten-nanoliter injections of lidocaine (1%), GABA (100 mM) and glutamate (10 and 100 mM) into discrete regions of the rostral and the caudal brainstem produced different effects on the phasic neural discharge. In the rostral site lidocaine, GABA and glutamate injections altered neural burst frequency with little or no effect on burst amplitude. In the caudal site, responses to lidocaine and GABA injections consisted primarily of decreases in neural burst amplitude, often, but not always associated with minor decreases in burst frequency. In this same region, the response to glutamate was characterized by a temporary interruption of the rhythmic neural burst activity. The largest responses to substance injection in both regions were obtained at sites ranging between 200 and 500 μm from the ventral surface, in the ventral medullary reticular formation. The results reveal the existence of two areas in the frog brainstem that influence respiratory motor output, one related to the respiratory burst frequency and the other related to the amplitude of the motor output.

Electrophysiological properties of rostral medullary respiratory neurones in the cat: an intracellular study.

1. We recorded the membrane potentials of sixty‐three respiratory neurones in the rostral, ventral medulla of decerebrate vagotomized cats. Stable recordings were obtained in thirty‐eight expiratory and twenty‐five inspiratory neurones. Axonal projections were identified by antidromic invasion after electrical stimulation of the region of the dorsal respiratory group (DRG), spinal cord, and the cervical vagus, superior laryngeal and pharyngeal nerves. 2. Two types of expiratory neurones were encountered: those in which the membrane potential progressively depolarized (augmenting neurons, n = 22) and those in which the membrane potential repolarized (decrementing or post‐inspiratory neurones, n = 16) during the interval between phrenic bursts. Both types were hyperpolarized during inspiration by chloride‐dependent, inhibitory postsynaptic potentials (IPSPs) which decreased membrane resistance. In augmenting neurones two waves of IPSPs appeared, one early and one late in inspiration. 3. Five out of seventeen augmenting expiratory neurones tested were antidromically activated by contralateral stimulation of the spinal cord (n = 3) or the DRG (n = 2). Spinal axons were not detected in any of the sixteen decrementing expiratory neurones tested. Of thirteen expiratory neurones tested with pharyngeal nerve stimulation, one (an augmenting type) was antidromically activated. Superior laryngeal or vagal axons could not be demonstrated for any expiratory neurones. 4. Two types of inspiratory neurones were also encountered: those displaying progressive depolarization throughout inspiration (n = 5) and those which gradually repolarized after maximal depolarization at the onset of inspiration (n = 10). None of the former had identifiable spinal or medullary axons, but superior laryngeal axons were demonstrated in three and pharyngeal axons were found in three. None of the latter was antidromically activated from any of the sites stimulated. 5. Stimulation of the superior laryngeal or pharyngeal nerves evoked excitatory postsynaptic potentials (EPSPs) in all neurones except in post‐inspiratory neurones. In these, stimulation of the superior laryngeal or pharyngeal nerves evoked IPSPs in five of twelve neurones tested. 6. We conclude that a spectrum of respiratory neurones lie within or ventral to the retrofacial nucleus. These neurones may control upper‐airway muscles or may play a role in chemoreception.

Fictive respiratory rhythm in the isolated brainstem of frogs

Spontaneous rhythmically bursting activity was recorded from the trigeminal, vagal and hypoglossal nerve roots of the isolated brainstem from the frogsRana catesbeiana andRana pipiens superfused with a bicarbonate-free HEPES-buffer solution. Burst frequency, burst duration and the activity profile of the spontaneous neural discharges in vitro resembled those of a less radical preparation, the decerebrate, fictively breathing frog. After complete midsagittal section, each half of the isolated brainstem generated its own rhythmic neural activity which resembled that of the intact isolated brainstem. The spontaneous activity generated within each half of the brainstem is probably coordinated by decussating axons or by groups of neurons located along the midline of the brainstem. Our results suggest that these coordinating entities extend the length of the brainstem (in a rostro-caudal dimension) and the degree of contact rather than the location of the contact between the two halves of the brainstem determines the synchronization of the right and left halves. Burst frequency of both the intact and hemisected brainstem preparation was decreased by alkaline challenge and increased by acid challenge. We conclude that this endogeneous rhythmic activity represents the efferent motor output underlying lung ventilation in these animals.

Neurorespiratory pattern of gill and lung ventilation in the decerebrate spontaneously breathing tadpole

A decerebrate, spontaneously breathing tadpole preparation (Taylor-Kollros stages 16-19) was used to test the general hypothesis that the efferent bursting activities of cranial nerves (CN) V, VII and spinal nerve (SN) II are respiratory in nature, and, in particular, to identify separate and specific neural correlates of gill and lung ventilation. Oropharyngeal pressure (POP), intrapulmonary pressure (PIP), electromyogram (EMG) of the buccal levator muscle (interhyoideus), and efferent neural activities of CN V, CN VII and SN II were recorded while the animal was exposed to hyperoxia (100% inspired O2), normoxia (21% inspired O2), and hypoxia (10, 5 and 0% inspired O2). Gill ventilation, indicated by fluctuations in POP at constant PIP, was characterized by high-frequency, low-amplitude bursts of action potentials in CN V and VII and interhyoideus EMG without phasic activity in SN II. Lung breaths, indicated by oscillations in POP and PIP were characterized by large bursts in EMG, CN V and VII together with a large burst in SN II. The amplitude of the moving average of nerve activities associated with lung ventilation was significantly larger than those associated with gill ventilation. During gill ventilation, the burst in CN V led that in CN VII, and both preceded the rise in POP. By contrast, a more synchronous neural burst onset pattern was observed during lung ventilation. The results document the neural, muscular, and mechanical characteristics of gill and lung ventilation in the tadpole, and establish bursting activity in SN II as a specific marker for lung ventilation in the metamorphic tadpole.

Depth profiles of pH and PO2 in the in vitro brainstem preparation of the tadpole Rana catesbeiana.

Extracellular pH and PO2 was recorded in the isolated in vitro brainstem of the metamorphic tadpole, Rana catesbeiana while the brainstem preparation was superfused with oxygenated mock cerebrospinal fluid of pH = 7.8, PCO2 = 17 Torr, PO2 = 600 Torr at 23 degrees C. Using pH and PO2 microelectrodes, the ventral medullary surface was penetrated at midline and lateral sites between cranial nerves V and X. Mean pH and PO2 gradients of 0.07 pH units/100 microns and 60 Torr/100 microns were detected in the superfusate, 100-200 microns above the ventral surface of the brainstem. These gradients remained virtually constant for the first 100-200 microns below the medullary surface. Beyond this level, pH and PO2 gradients decreased in a curvilinear fashion. For midline tracts, minimum values of pH and PO2 (7.58 +/- 0.05 and 323 +/- 31 Torr) were reached at a depth of 500-750 microns, whereas for lateral tracts, mean minimum values of pH and PO2 (7.34 +/- 0.12 and 240 +/- 68 Torr), were recorded at 850-900 microns. With further electrode advancement, pH and PO2 gradients in both midline and lateral tracts reversed as levels began to increase. Between CN V and X, lateral width was 4.34 +/- 0.57 mm, while dorsal-ventral thickness in midline and lateral regions was 0.92 +/- 0.21 and 1.31 +/- 0.22 mm, respectively. Overall, the in vitro tadpole brainstem provides a robust neural preparation which, although moderately acidic, is well oxygenated throughout all tissue layers.

Developmental disinhibition: Turning off inhibition turns on breathing in vertebrates

Development requires age-dependent changes in essential behaviors. While the mechanisms determining the developmental expression of such behavior in vertebrates remain largely unknown, a few studies have identified permissive mechanisms in which the appearance of promoting signals activates pre-established networks. Here we report a different developmental process. Specifically, we show that the neuronal substrate that produces putative lung breathing in tadpoles is formed early in development, but remains more or less inactive until metamorphosis because of suppression mediated by a GABA(B) receptor-dependent mechanism. Blocking this suppression using 2-hydroxy-saclofen, a GABA(B) receptor antagonist, results in the precocious production of the putative lung breathing motor pattern. This blocker failed to augment putative lung breaths after metamorphosis. Thus, our results suggest that loss of an inhibitory signal during development (i.e., developmental disinhibition) is responsible for the developmental expression of air breathing.