Coordination between glottic adductor muscle and diaphragm EMG activity in fetal lambs in utero

Neonatal apnea: what's new?

Apnea of prematurity (AOP) remains a major clinical problem in present day neonatology that warrants frequent evaluations and imposes challenges in therapeutic strategies. Although the pathogenesis of AOP is poorly understood, it is probably a manifestation of physiologic immaturity of breathing control rather than a pathologic disorder. Immature breathing responses to hypoxia, hypercapnia and exaggerated inhibitory pulmonary reflexes in preterm infants might also contribute to the occurrence or severity of AOP. Recent data suggest a role for genetic predisposition. Although typically resolve with maturation, the role of bradycardia and desaturation episodes associated with AOP in the development of sleep disorder breathing and neurodevelopmental delay needs further clarification. Pharmacological treatment with methylxanthines and CPAP remain the mainstay for treatment of AOP. However, recent studies have implicated central inhibitory neuromodulators including prostaglandins, GABA and adenosine in its pathogenesis, the fact that might provide future specific targets for treatment. This review will summarize new insights involving these issues as well as others involving the pathogenesis, treatment strategies and consequences of apnea in premature infants.

Upper airway pressure-flow relationships and pharyngeal constrictor EMG activity during prolonged expiration in awake goats

We undertook the present investigation to establish whether narrowing/closure of the upper airway occurs during spontaneous and provoked respiratory rhythm disturbances and whether pharyngeal constrictor muscle recruitment occurs coincident with upper airway occlusion during prolonged expiratory periods. Upper airway pressure-flow relationships and middle pharyngeal constrictor (mPC) EMG activities were recorded in 11 adult female goats during spontaneous and provoked prolongations in expiratory time (Te). A total of 213 spontaneous prolongations of expiration were recorded. Additionally, 169 prolonged expiratory events preceded by an augmented breath were included in the analyses. In separate trials on different days, Te was prolonged by systemic administration of dopamine, by raising the inspired fraction of O2 from 0.10 to 1.00 during poikilocapnic conditions or by systemic administration of clonidine. Continuous tonic activation of the mPC EMG was observed during each prolonged Te period regardless of the duration or initiating cause. However, significant increases in subglottic tracheal pressure, with expiratory airflow braking indicative of upper airway narrowing or closure, was only observed during spontaneous events without a preceding augmented breath and during clonidine-induced events. Tonic mPC activation proved an unreliable indicator of airway occlusion. Furthermore, mPC muscle activation alone is not sufficient to induce pharyngeal occlusion during prolonged expiration. Our data suggest that airway closure is not a common occurrence during provoked respiratory disturbances in awake goats. We propose that airway closure, when present during prolonged Te, is more likely dependent on activation of laryngeal adductor muscles with glottic braking independent of pharyngeal narrowing.

Catathrenia (nocturnal groaning): an abnormal respiratory pattern during sleep

Catathrenia (nocturnal groaning) is a rare condition characterized by monotonous irregular groans occurring during sleep. Ten patients (five women; mean age: 27 +/- 7.4 years, range: 15-41) with sleep-related groaning persisting for years or decades and normal daytime fibreoptic laryngoscopy and respiratory function tests underwent videopolysomnographic recording (VPSG) analysing their respiratory patterns during sleep. After the VPSG, all patients were clinically followed up for a mean period of 4.9 +/- 3.5 years. On VPSG, all patients showed nocturnal groaning during NREM sleep and particularly during REM sleep stages. Groaning was associated with disproportionate prolonged expiration causing reduced breathing rate without oxygen desaturation. The breathing pattern with prolonged expiration and sound production alternated with a normal respiratory pattern without groaning. Endoesophageal pressure during groaning showed mildly positive swings at the initial phase of expiration suggesting a partial mild expiratory upper airway obstruction. At the end of the follow-up period, all patients reported persistent nocturnal groaning but no other clinical manifestations. Groaning confined to sleep alternating with normal breathing and the absence of long-term clinical consequences suggest that catathrenia is because of an abnormality of the internal respiratory drive system, possibly related to persistence of a neonatal (vestigial) type of breathing pattern.

Un modèle amphibian pour l’étude du développement du contrôle de la respiration

Recent medical advances have made it possible for babies to survive premature birth at increasingly earlier developmental stages. This population requires costly and sophisticated medical care to address the problems associated with immaturity of the respiratory system. In addition to pulmonary complications, respiratory instability and apnea reflecting immaturity of the respiratory control system are major causes of hospitalization and morbidity in this highly vulnerable population. These medical concerns, combined with the curiosity of physiologists, have contributed to the expansion of research in respiratory neurobiology. While most researchers working in this field commonly use rodents as an animal model, recent research using in vitro brainstem preparation from bullfrogs (Rana catesbeiana) have revealed the technical advantages of this animal model, and shown that the basic principles underlying respiratory control and its ontogeny are very similar between these two groups of vertebrates. The present review highlights the recent advances in the area of research with a focus on intermittent (episodic) breathing and the role of serotonergic and GABAergic modulation of respiratory activity during development.

A radiotelemetry system for polysomnographic recordings in lambs

The use of telemetry in biomedical research is rapidly increasing and is soon likely to be standard in research fields where prolonged recordings of multiple physiologic variables are necessary in freely moving animals. We describe our experience with a radiotelemetry system designed in our laboratory for polysomnographic recordings in newborn lambs. The system comprises eight channels: four channels for electromyographic activity of respiratory muscles, and one channel each for electrocorticogram, eye movement, electrocardiogram, and nasal flow. Using this system, we report our experience on the study of respiration and spontaneous apneas in the various states of consciousness, based on more than 500 h of recordings. Addition of radiotelemetry to our armamentarium has proven to be of invaluable help for the study of neonatal apneas and has now become routine in our laboratory.

The Larynx and Neonatal Apneas

OBJECTIVE: We sought to review the current literature on the role of the larynx in neonatal apneas. STUDY DESIGN: We conducted a literature search and analysis of the published and other available information on the topic.

RESULTS: Publications on this topic in otolaryngology journals are rare, although more basic sciences-oriented journals have published interesting results from animal studies during the past 20 years. The scarce data on human neonates are also presented.

CONCLUSIONS: The authors follow the phylogenetic and oncogenetic pathways to explain the active role of the larynx in neonatal apneas. Although most of these results are derived from animal studies, they certainly improve our understanding of human neonatal apneas.

Central GABAergic mechanisms are involved in apnea induced by SLN stimulation in piglets

Stimulation of the superior laryngeal nerve (SLN) results in apnea in animals of different species, the mechanism of which is not known. We studied the effect of the GABA(A) receptor blocker bicuculline, given intravenously and intracisternally, on apnea induced by SLN stimulation. Eighteen 5- to 10-day-old piglets were studied: bicuculline was administered intravenously to nine animals and intracisternally to nine animals. The animals were anesthetized and then decerebrated, vagotomized, ventilated, and paralyzed. The phrenic nerve responses to four levels of electrical SLN stimulation were measured before and after bicuculline. SLN stimulation caused a significant decrease in phrenic nerve amplitude, phrenic nerve frequency, minute phrenic activity, and inspiratory time (P < 0.01) that was proportional to the level of electrical stimulation. Increased levels of stimulation were more likely to induce apnea during stimulation that often persisted beyond cessation of the stimulus. Bicuculline, administered intravenously or intracisternally, decreased the SLN stimulation-induced decrease in phrenic nerve amplitude, minute phrenic activity, and phrenic nerve frequency (P < 0.05). Bicuculline also reduced SLN-induced apnea and duration of poststimulation apnea (P < 0.05). We conclude that centrally mediated GABAergic pathways are involved in laryngeal stimulation-induced apnea.

Clonidine induces upper airway closure in awake goats

We examined the effects of the alpha(2)-adrenoceptor (alpha(2)-AR) agonist clonidine on pressure-flow relationships in the upper airway. Inspired and expired airflows, subglottic tracheal pressure (PTR), mask pressure and middle pharyngeal constrictor (MPC) and diaphragm electromyogram (EMG) activities were recorded in awake standing goats. Clonidine-induced central apneas were always associated with continuous tonic activation of the MPC. Subglottic PTR during expiration increased progressively in a dose-dependent manner after clonidine administration. In all cases, positive subglottic PTR was maintained throughout the duration of clonidine-induced apneas and was sufficient to retard or prevent expiratory flow during early and mid-expiration indicating complete airway closure. The effects of clonidine were reversed by selective alpha(2)-AR blockade with SKF-86466. Central apneas after spontaneous augmented breaths (sighs) were associated with continuous tonic activation of the MPC throughout the duration of the prolonged TE intervals. However, subglottic PTR during expiration was not significantly different from control breaths and there was no evidence of increased expiratory airway resistance or delayed expiratory flow. We conclude that continuous tonic activation of pharyngeal adductor muscles appears to be a constant feature of central apnea in the awake goat independent of the initiating cause of the apnea. However, our data suggest that MPC activation alone may not be sufficient to cause complete closure of the upper airway during central apnea.

Mechanisms regulating hypoxic respiratory depression during fetal and postnatal life

Selected topics in the respiratory response to acute hypoxia in the fetus and newborn are reviewed. Peripheral chemoreceptors acting through ionotrophic glutamate receptors play an important role in affecting the initial augmentation phase. Whether fall off in peripheral chemoreceptor activity contributes to the secondary depressive phase remains controversial. A number of approaches including permanent electrolytic and reversible cooling lesions, Fos protein activation, and double-labeling immunohistochemistry has converged to show that an area in and around the locus ceruleus in the rostral pons affects the central depression. There is evidence that this is mediated by catecholamines acting at alpha(2)-adrenergic receptors. Tonic activity in early expiratory (postinspiratory) neurons may contribute to hypoxia-induced apneic episodes in the fetus and newborn. Desensitization of alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid receptors has been demonstrated in respiratory-related neurons both in vivo and in vitro. The role that this process might play in the depressive phase of the hypoxic ventilatory response has not been established. In vitro experiments with isolated brain stem-spinal cord preparations or transverse brain stem slices usually involve anoxia, whereas whole animal experiments use 8-15% O(2). Therefore, caution must be exercised in attempting to construct a unifying framework from these two approaches.

Differential effects of clonidine on upper airway abductor and adductor muscle activity in awake goats

The purpose of this study was to determine the extent to which alpha(2)-adrenoceptor (alpha(2)-AR) pathways affect the central motor output to upper airway muscles that regulate airflow. Electromyogram (EMG) measurements were made from posterior cricoarytenoid (PCA), cricothyroid (CT), thyroarytenoid (TA), and middle (MPC) and inferior (IPC) pharyngeal constrictor muscles in awake standing goats. Systemic administration of the alpha(2)-AR agonist clonidine induced a highly dysrhythmic pattern of ventilation in all animals that was characterized by alternating episodes of tachypnea and slow irregular breathing patterns, including prolonged and variable expiratory time intervals. Periods of apnea were commonly observed. Dysrhythmic ventilatory patterns induced by clonidine were associated with differential recruitment of upper airway muscles. alpha(2)-AR stimulation preferentially decreased the activity of the PCA, CT, and IPC muscles while increasing TA and MPC EMG activities. Clonidine-induced apneas were associated with continuous tonic activation of laryngeal (TA) and pharyngeal (MPC) adductors, leading to airway closure and arterial oxygen desaturation. Tonic activation of the TA and MPC muscles was interrupted only during the first inspiratory efforts after central apnea. Laryngeal abductor, diaphragm, and transversus abdominis EMG activities were completely silenced during apneic events. Ventilatory and EMG effects were reversed by selective alpha(2)-AR blockade with SKF-86466. The results demonstrate that alpha(2)-AR pathways are important modulators of central respiratory motor outputs to the upper airway muscles.

Radiotelemetry system for apnea study in lambs

Neonatal apneas are being studied in the laboratory using polysomnographic recordings in lambs. Standard equipment, requiring animal restraint, disrupts sleep and prevents development of spontaneous apneas. The aim of the current work was to develop and validate a wireless recording equipment to study freely moving lambs. Firstly, a radiotelemetry equipment composed of a multichannel FM transmitter and a receiver was developed. Secondly, to test the telemetry equipment, each biopotential - [electroencephalogram (EEG), electrooculogram (EOG), electrocardiogram (ECG), electromyograms (EMGs), nasal airflow] - was recorded simultaneously by standard equipment and by telemetry (5 lambs). The results indicated an excellent concordance between signals obtained by both systems. Finally, the 8-channel telemetry prototype was tested for polysomnographic recordings (16 lambs). Results obtained confirmed the possibility of recording frequent REM sleep periods and spontaneous apneas. In conclusion, this radiotelemetry polysomnographic equipment brings new possibilities for research on neonatal apneas.

Thyroarytenoid muscle electrical activity during spontaneous apneas in preterm lambs

Laryngeal dynamics plays a major role during perinatal life, a period of respiratory control immaturity. Continuous electromyographic (EMG) activity of a laryngeal adductor muscle (thyroarytenoid [TA] muscle), was recently observed throughout provoked central apneas, either isolated or during induced periodic breathing, in full-term lambs. The aim of the present study was to test if continuous TA EMG activity was also present during spontaneous apneas in nonsedated preterm lambs. We studied 7 premature lambs (term 131 +/- 1 d of postconceptional age). Premature birth was induced after acceleration of fetal lung maturation. Electrodes for diaphragm, inferior pharyngeal constrictor (IPC), and TA electromyograms, electrocardiogram, electroencephalogram, eye movement, and airflow recordings were implanted. Radiotelemetry recordings were repeated from 135 to 149 +/- 8 d of postconceptional age. A total of 2,088 apneas (2,020 central and 68 mixed) >/= 3 s were recorded in the lambs, including 57 epochs of periodic breathing. Continuous TA EMG activity was present throughout 88.4% of all apneas and 98.4% of apneas during periodic breathing, regardless of the sleep stage. These results suggest that active glottic closure is frequent during spontaneous central apneas in this model of prematurity. This unique model will allow us to study controlling mechanisms and consequences of glottic closure during neonatal apneas.