Laryngeal chemoreflexes induced by acid, water, and saline in nonsedated newborn lambs during quiet sleep

Effects of Nasal Respiratory Support on Laryngeal and Esophageal Reflexes in Preterm Lambs

BACKGROUND

Significant cardiorespiratory events can be triggered in preterm infants as part of laryngeal chemoreflexes (LCRs) and esophageal reflexes (ERs). We previously showed that nasal continuous positive airway pressure (nCPAP) blunted the cardiorespiratory inhibition induced with LCRs. Therefore, we aimed to compare the effects of nCPAP and high-flow nasal cannulas (HFNC) on the cardiorespiratory events induced during LCRs and ERs. The hypothesis is that nCPAP but not HFNC decreases the cardiorespiratory inhibition observed during LCRs and ERs.

METHODS

Eleven preterm lambs were instrumented to record respiration, ECG, oxygenation, and states of alertness. LCRs and ERs were induced during non-rapid eye movement sleep in a random order under these conditions: nCPAP 6 cmH2O, HFNC 7 L/min, high-flow nasal cannulas 7 L/min at a tracheal pressure of 6 cmH2O, and no respiratory support.

RESULTS

nCPAP 6 cmH2O decreased the cardiorespiratory inhibition induced with LCRs, but not with ERs in preterm lambs. This blunting effect was less marked with HFNC 7 L/min, even when the tracheal pressure was maintained at 6 cmH2O.

CONCLUSIONS

nCPAP might be a treatment for cardiorespiratory events related to LCRs in newborns, either in the context of laryngopharyngeal refluxes or swallowing immaturity. Our preclinical results merit to be confirmed through clinical studies.

IMPACT

Laryngeal chemoreflexes can be responsible for significant cardiorespiratory inhibition in newborns, especially preterm. Nasal continuous positive airway pressure at 6 cmH2O significantly decreased this cardiorespiratory inhibition. High-flow nasal cannulas at 7 L/min had a lesser effect than nasal continuous positive airway pressure. Esophageal stimulation was responsible for a smaller cardiorespiratory inhibition, which was not significantly modified by nasal continuous positive airway pressure or high-flow nasal cannulas. Nasal continuous positive airway pressure should be tested for its beneficial effect on cardiorespiratory events related to laryngeal chemoreflexes in preterm newborns.

Apneic response to fentanyl in adult rats: Role of laryngeal afferents

Intravenous (systemic) bolus injection of fentanyl (FNT) reportedly induces an immediate vagal-mediated apnea; however, the precise origin of vagal afferents responsible for this apnea remains unknown. We tested whether intralaryngeal (local) application of FNT would also trigger an apnea and whether the apneic response to both local and systemic administration of FNT was laryngeal afferent-mediated. Cardiorespiratory responses to FNT were recorded in anesthetized male adult rats with and without bilateral sectioning of the superior laryngeal nerve (SLNx) or peri-SLN capsaicin treatment (SLNcap) to block local C-fiber signal conduction. Opioid mu-receptor (MOR)-immunoreactivity was detected in laryngeal C- and myelinated neurons. We found that local and systemic administration of FNT elicited an immediate apnea. SLNx, rather than SLNcap, abolished the apneic response to local FNT application though MORs were abundantly expressed in both laryngeal C- and myelinated neurons. Importantly, SLNx failed to affect the apneic response to systemic FNT administration. These results lead to the conclusion that laryngeal afferents' MORs are responsible for the apneic response to local, but not systemic, administration of FNT.

Liquiritin apioside attenuates laryngeal chemoreflex but not mechanoreflex in rat pups

Liquiritin apioside (LA), a main flavonoid component of licorice, reportedly suppresses cough responses to inhalation of aerosolized capsaicin [CAP; a stimulant to transient receptor potential vanilloid 1 (TRPV1)] in conscious guinea pigs via acting on peripheral nerves. However, the evidence of LA having a direct effect on airway sensory fibers is lacking. Considering the important role laryngeal chemoreceptors and mechanoreceptors play in triggering apnea and cough, we studied whether LA suppressed the apneic responses to stimulation of these receptors via directly acting on the superior laryngeal nerve (SLN). Intralaryngeal delivery of chemical [CAP, HCl, and distilled water (DW)] and mechanical [an air-pulse (AP)] stimulations was applied in anesthetized rat pups to evoke the apnea. These stimuli were repeated after intralaryngeal LA treatment or peri-SLN LA treatment to determine the direct effect of LA on the SLN. Our results showed that all stimuli triggered an immediate apnea. Intralaryngeal LA treatment significantly attenuated the apneic response to chemical but not mechanical stimulations. The same attenuation was observed after peri-SLN LA treatment. Owing that TRPV1 receptors of laryngeal C fibers are responsible for the CAP-triggered apneas, the LA impact on the activity of laryngeal C neurons retrogradely traced by DiI was subsequently studied using a patch-clamp approach. LA pretreatment significantly altered the electrophysiological kinetics of CAP-induced currents in laryngeal C neurons by reducing their amplitudes, increasing the rise times, and prolonging the decay times. In conclusion, our results, for the first time, reveal that LA suppresses the laryngeal chemoreceptor-mediated apnea by directly acting on the SLN (TRPV1 receptors of laryngeal C fibers).

Reflex cardiorespiratory events from esophageal origin are heightened by preterm birth

The involvement of gastroesophageal refluxes in cardiorespiratory events of preterm infants remains controversial. While a few studies in full-term newborn animals have shown that stimulation of esophageal receptors leads to cardiorespiratory reflexes, the latter remain largely unknown, especially after premature birth. The present study aimed to 1) characterize the cardiorespiratory reflexes originating from esophageal receptors in newborn lambs and 2) test the hypotheses that preterm birth enhances reflex cardiorespiratory inhibition and that C-fibers are involved in these reflexes. Eight full-term lambs and 10 lambs born 14 days prematurely were studied. Following surgical instrumentation, a 6-h polysomnography was performed without sedation to record electrocardiogram, respiratory movements, arterial pressure, laryngeal constrictor muscle activity, state of alertness, and hemoglobin oxygen saturation. Five esophageal stimulations of the upper and/or lower esophagus, including rapid balloon inflation and/or HCl injection, were performed in random order. A second recording was performed in full-term lambs 24 h later, after C-fiber blockade by capsaicin. Results confirmed that esophageal stimulations induced inhibitory cardiorespiratory reflexes combined with protective mechanisms, including laryngeal closure, swallowing, coughing, increased arterial pressure, and arousal. Preterm birth heightened cardiorespiratory inhibition. The strongest cardiorespiratory inhibition was observed following simultaneous stimulation of the lower and upper esophagus. Finally, cardiorespiratory inhibition was decreased after C-fiber blockade. In conclusion, esophageal stimulation induces inhibitory cardiorespiratory reflexes, which are partly mediated by C-fibers and more pronounced in preterm lambs. Clinical relevance of these findings requires further studies, especially in conditions associated with increased cardiorespiratory events, e.g., neonatal infection.NEW & NOTEWORTHY Preterm birth heightens the cardiorespiratory events triggered by esophageal stimulation. The most extensive cardiorespiratory events are induced by simultaneous stimulation of the proximal and distal esophagus.

Moderate Hyperbilirubinemia Alters Neonatal Cardiorespiratory Control and Induces Inflammation in the Nucleus Tractus Solitarius

Hyperbilirubinemia (HB) occurs in 90% of preterm newborns. Moderate HB can induce acute neurological disorders while severe HB has been linked to a higher incidence of apneas of prematurity. The present study aimed to test the hypothesis that even moderate HB disrupts cardiorespiratory control in preterm lambs. Two groups of preterm lambs (born 14 days prior to term), namely control (n = 6) and HB (n = 5), were studied. At day 5 of life, moderate HB (150–250 μmol/L) was induced during 17 h in the HB group after which cardiorespiratory control as well as laryngeal and pulmonary chemoreflexes were assessed during baseline recordings and during hypoxia. Recordings were repeated 72 h after HB induction, just before euthanasia. In addition, neuropathological studies were performed to investigate for cerebral bilirubin deposition as well as for signs of glial reactivity in brainstem structures involved in cardiorespiratory control. Results revealed that sustained and moderate HB: (i) decreased baseline respiratory rate and increased the time spent in apnea; (ii) blunted the cardiorespiratory inhibition normally observed during both laryngeal and pulmonary chemoreflexes; and (iii) increased heart rate in response to acute hypoxia. These acute physiological changes were concurrent with an activation of Alzheimer type II astrocytes throughout the brain, including the brainstem. Concomitantly, bilirubin deposits were observed in the leptomeninges, but not in brain parenchyma. While most cardiorespiratory alterations returned to normal 72 h after HB normalization, the expression of glial fibrillary acid protein (GFAP) and ionized calcium binding adaptor molecule 1 (Iba1) was still increased within the nucleus tractus solitarius. In conclusion, moderate and sustained HB in preterm lambs induced cardiorespiratory alterations, the latter of which were associated with neurohistopathological changes. These changes are indicative of an inflammatory response in the brainstem neuroanatomical substrates involved in cardiorespiratory control.

Effects of Moderate Hyperbilirubinemia on Nutritive Swallowing and Swallowing-Breathing Coordination in Preterm Lambs

BACKGROUND

Hyperbilirubinemia (HB) occurs in 90% of preterm newborns. HB induces acute neurological disorders (somnolence, abnormal tone, feeding difficulties, auditory dysfunction) and alterations in respiratory control. These findings suggest brainstem neurotoxicity that could also affect swallowing centers.

OBJECTIVE

To test the hypothesis that HB impairs nutritive swallowing (NS) and swallowing-breathing coordination.

METHODS

Two groups of preterm lambs (born 14 days prior to term), namely control (n = 6) and HB (n = 5), were studied. On day 5 of life (D0), moderate HB (150-250 µmol/l) was induced during 17 h in the HB group. Swallowing was assessed via recording of pharyngeal pressure and respiration by respiratory inductance plethysmography and pulse oximetry. The effect of HB on NS was assessed during standardized bottle-feeding. A second recording was performed 48 h after recovery from HB (D3).

RESULTS

Swallows were less frequent (p = 0.003) and of smaller volume (p = 0.01) in HB lambs while swallowing frequency was decreased (p = 0.004). These differences disappeared after HB normalization. Swallowing-breathing coordination was impaired in HB lambs, with a decrease in percent time with NS burst-related apneas/hypopneas at D0 and D3. Simultaneously, HB lambs tended to experience more severe desaturations (<80%) during bottle-feeding. Finally, following bottle-feeding, the respiratory rate was significantly lower, along with an increased apnea duration in HB lambs.

CONCLUSIONS

Swallowing and swallowing-breathing coordination are altered by acute moderate HB in preterm lambs. Decreased efficiency at bottle-feeding is accompanied by continuation of breathing during swallow bursts, which may promote lung aspiration.

Effects of caffeine and/or nasal CPAP treatment on laryngeal chemoreflexes in preterm lambs

Current knowledge suggests that laryngeal chemoreflexes (LCR) are involved in the occurrence of certain neonatal apneas/bradycardias, especially in the preterm newborn. While caffeine and/or nasal continuous positive airway pressure (nCPAP) are the most frequent options used for treating apneas in preterm newborns, their effects on LCR-related apneas/bradycardias are virtually unknown. The aim of the present study was to test the hypothesis that caffeine and/or nCPAP decreases LCR-related cardiorespiratory inhibition in a preterm ovine model. Seven preterm lambs were born vaginally on gestational day 133 (normal gestation: 147 days) after intramuscular injections of betamethasone and mifepristone. Five days after birth, a chronic surgical instrumentation was performed to record states of alertness, electrocardiogram, systemic arterial pressure, and electromyographic activity of a laryngeal constrictor muscle, as well as to insert a transcutaneous supraglottal catheter. LCR were induced in quiet sleep under four conditions: 1) control (without caffeine or nCPAP); 2) nCPAP (5 cmH2O, without caffeine); 3) caffeine (10 mg/kg infused intravenously for 30 min, without nCPAP); and 4) nCPAP + caffeine. Our results showed that nCPAP consistently blunted LCR-related cardiorespiratory inhibition vs. control condition, contrary to caffeine whose overall effect was nonsignificant. In addition, nCPAP condition was characterized by a more consistent and rapid arousal after HCl injection. No significant differences were observed between all tested conditions with regard to swallowing and cough. It is concluded that nCPAP should be further assessed for its usefulness in treating neonatal apneas linked to LCR.

ABM Clinical Protocol #25: Recommendations for preprocedural fasting for the breastfed infant: "NPO" Guidelines

A central goal of The Academy of Breastfeeding Medicine is the development of clinical protocols for managing common medical problems that may impact breastfeeding success. These protocols serve only as guidelines for the care of breastfeeding mothers and infants and do not delineate an exclusive course of treatment or serve as standards of medical care. Variations in treatment may be appropriate according to the needs of an individual patient. These guidelines are not intended to be all-inclusive, but to provide a basic framework for physician education regarding breastfeeding.

Effects of nasal continuous positive-airway pressure on nutritive swallowing in lambs

Current knowledge suggests that, to be successful, oral feeding in preterm infants should be initiated as soon as possible, often at an age where immature respiration still requires ventilatory support in the form of nasal continuous positive airway pressure (nCPAP). While some neonatologist teams claim great success with initiation of oral feeding in immature infants with nCPAP, others strictly wait for this ventilatory support to be no longer necessary before any attempt at oral feeding, fearing laryngeal penetration and tracheal aspiration. Therefore, the aim of the present study was to provide a first assessment of the effect of various levels of nCPAP on bottle-feeding in a neonatal ovine model, including feeding safety, feeding efficiency, and nutritive swallowing-breathing coordination. Eight lambs born at term were surgically instrumented 48 h after birth to collect recordings of electrical activity of laryngeal constrictor muscle, electrocardiography, and arterial blood gases. Two days after surgery, lambs were bottle-fed under five randomized nCPAP conditions, including without any nCPAP or nasal mask and nCPAP of 0, 4, 7, and 10 cmH(2)O. Results revealed that application of nCPAP in the full-term lamb had no deleterious effect on feeding safety and efficiency or on nutritive swallowing-breathing coordination. The present study provides a first and unique insight on the effect of nCPAP on oral feeding, demonstrating its safety in newborn lambs born at term. These results open the way for further research in preterm lambs to better mimic the problems encountered in neonatology.

Effects of simulated reflux laryngitis on laryngeal chemoreflexes in newborn lambs

It has been suggested that reflux laryngitis (RL) is involved in apneas-bradycardias of the newborn. The aim of the present study was to develop a unique RL model in newborn lambs to test the hypothesis that RL enhances the cardiorespiratory components of the laryngeal chemoreflexes (LCR) in the neonatal period. Gastric juice surrogate (2 ml of normal saline solution with HCl pH 2 + pepsin 300 U/ml) (RL group, n = 6) or normal saline (control group, n = 6) was repeatedly injected onto the posterior aspect of the larynx, 3 times a day for 6 consecutive days, via a retrograde catheter introduced into the cervical esophagus. Lambs instilled with gastric juice surrogate presented clinical signs of RL, as well as moderate laryngitis on histological observation. Laryngeal chemoreflexes were thereafter induced during sleep by injection of 0.5 ml of HCl (pH 2), ewe's milk, distilled water or saline into the laryngeal vestibule via a chronic, transcutaneous supraglottal catheter. Overall, RL led to a significantly greater respiratory inhibition compared with the control group during LCR, including longer apnea duration ( P = 0.01), lower minimal respiratory rate ( P = 0.002), and a more prominent decrease in arterial hemoglobin saturation (SpO2) ( P = 0.03). No effects were observed on cardiac variables. In conclusion, 1) our unique neonatal ovine model presents clinical and histological characteristics of RL; and 2) the presence of RL in newborn lambs increases the respiratory inhibition observed with LCR, at times leading to severe apneas and desaturations.

Evolution of gustatory reflex systems in the brainstems of fishes

The great number of species of teleosts permits highly specialized forms to evolve to occupy particular niches. This diversity allows for extreme variations in brain structure according to particular sensory or motor adaptations. In the case of the taste system, goldfish (Carassius auratus L., 1758) and some carps have evolved a specialized intraoral food-sorting apparatus along with corresponding specializations of gustatory centers in the brainstem. A comparison of circuitry within the complex vagal lobe of goldfish, and of the simpler gustatory lobes in catfish (Ictalurus punctatus Rafinesque, 1818) shows numerous similarities in organization and neurotransmitters. Double labeling studies using horseradish peroxidase and biotinylated dextran amine in catfish shows a direct projection from the vagal lobe to the motoneurons of nucleus ambiguous which innervate oropharyngeal musculature. Therefore, a three neuron reflex arc connects gustatory input to motor output. In the vagal lobe of goldfish, a similar three neuron arc can be identified: from primary gustatory afferent, to vagal lobe interneuron, thence to dendrites of the vagal motoneurons that innervate the pharyngeal muscles. Therefore, despite large differences in the gross appearance of the vagal gustatory systems in the brains of catfish and goldfish, the essential connectivity and circuitry is similar. This suggests that evolutionary change in the central nervous system largely proceeds by rearrangement and elaboration of existing systems, rather than by addition of new structures or circuits.

Upper airway reflexes in response to gastric reflux

Gastric reflux, and especially laryngopharyngeal reflux, occur several times a day in every infant. Most often, this does not pose any problem. However, in certain conditions, the contact between the refluxate and the upper airway mucosa can trigger several reflexes leading to cardiorespiratory inhibition. This is especially true for the laryngeal chemoreflexes, which are triggered by laryngeal penetration of gastric refluxate. The laryngeal chemoreflexes are held responsible for a subset of apnoeas of prematurity, many apparent life-threatening events, and probably some cases of sudden infant death syndrome. Although a number of experiments in newborn animals, as well as a few clinical studies in human infants, have been performed in the last 40 years to evaluate laryngeal chemoreflexes, their true role in neonatal cardiorespiratory events is still highly debated. In addition, many uncertainties persist with regard to treatment and prevention of their potentially dramatic consequences.

Effects of postnatal smoke exposure on laryngeal chemoreflexes in newborn lambs

Laryngeal chemoreflexes (LCR), which are elicited by the contact of liquids such as gastric refluxate with laryngeal mucosa, may trigger some cases of sudden infant death syndrome. Indeed, while LCR in mature mammals consist of protective responses, previous animal data have shown that LCR in immature newborns can include laryngospasm, apnea, bradycardia, and desaturation. The present study was aimed at testing the hypothesis that postnatal exposure to cigarette smoke is responsible for enhancing cardiorespiratory inhibition observed with LCR. Eight lambs were exposed to cigarette smoke (20 cigarettes/day) over 16 days and compared with seven control lambs. Urinary cotinine/creatinine ratio was measured at a level relevant to previously published levels in infants. On days 15 and 16, 0.5 ml of HCl (pH 2), milk, distilled water, or saline was injected onto the larynx via a chronic supraglottal catheter during sleep. Results showed that exposure to cigarette smoke enhanced respiratory inhibition (P < 0.05) and tended to enhance cardiac inhibition and decrease swallowing and arousal during LCR (P < 0.1). Overall, these results were observed independently of the state of alertness and the experimental solution tested. In conclusion, 16-day postnatal exposure to cigarette smoke increases cardiorespiratory inhibition and decreases protective mechanisms during LCR in nonsedated full-term lambs.

Postnatal autonomic activity in the preterm lamb

BACKGROUND

Early postnatal abnormalities of autonomic nervous system (ANS) activity, including at baseline and following cardiorespiratory challenge, are involved in apneas-bradycardias of prematurity, apparent life-threatening events of infancy and sudden infant death syndrome. Literature data suggest that baseline ANS activity does not mature normally after premature birth.

OBJECTIVES

This study performed in preterm lambs was aimed at assessing ANS maturation at baseline and following laryngeal chemoreflexes (LCR), a group of reflexes triggered by the contact of liquids with the laryngeal mucosa.

METHODS

Heart rate variability (HRV) and baroreflex sensitivity (BRs) were measured at baseline and after LCR during polysomnographic recordings performed in five non-sedated lambs born 15 days prematurely. Laryngeal chemoreflexes were induced by distilled water or acid (pH 2) during sleep and wakefulness on postnatal days 7 (D7) and 14 (D14, full-term equivalence).

RESULTS

While the life-threatening cardiorespiratory events of the LCR observed at D7 were no longer present at D14, baseline and post-LCR HRV and BRs indices were significantly lower at D14 compared to D7 (up to p<0.001). These results suggest that an initial autonomic overactivity was present at D7 and normalized at D14.

CONCLUSION

The autonomic cardiac and baroreflex control appears to follow a specific evolution in the preterm compared to the full-term newborn lamb, with an important initial autonomic stress, which normalizes at an age equivalent to full-term. Potential relevance of these findings in relation to anomalies of cardiac control in the early postnatal period, such as apneas-bradycardias of prematurity, apparent life-threatening events of infancy and sudden infant death syndrome, awaits further studies.

Apnea, bradycardia and desaturation in preterm infants before and after feeding

OBJECTIVE

A common clinical impression is that both gastroesophageal reflux (GER) and cardiorespiratory events increase after feeding in preterm infants. We aimed to measure objectively the effects of feeding on GER, apnea, bradycardia and desaturations.

STUDY DESIGN

We conducted a retrospective review of premature infants with a gestational age of 23 to 37 weeks at birth and a post-conceptional age of 34 to 48 weeks, who were referred for multichannel intraluminal impedance (MII), pH probe and 12-h apnea evaluation. Cardiorespiratory and GER event rates during pre- and post-feeding intervals were compared.

RESULT

Thirty-six infants met the inclusion criteria. More GER events occurred after a feed than before (P=0.012). After feeds, reflux was less acidic and higher in the esophagus (P<0.05). In contrast, the rates of apnea, bradycardia and desaturations were not altered by infant feeding. Apnea of >5 s occurred at a median frequency of 0 (range 0 to 3) events per hour before a feed and 0 (0 to 2) events per hour after a feed (P=0.61).

CONCLUSION

The frequency, height and pH of GER are significantly altered by feedings in preterm infants. However, the common clinical impression that apnea, bradycardia and desaturations are more prevalent after feeding is not supported.

Origins of the inhibiting effects of nasal CPAP on nonnutritive swallowing in newborn lambs

The present study investigated the mechanism by which continuous positive airway pressure (CPAP) suppresses nonnutritive swallowing (NNS) during quiet sleep (QS) in newborn lambs. Eighteen full-term lambs were chronically instrumented and evenly distributed into three separate groups to determine the extent to which modulation of NNS may be attributed to stimulation of upper airway and/or bronchopulmonary mechanoreceptors. Six lambs were tracheotomized, six other lambs underwent a two-step bilateral intrathoracic vagotomy, and the remaining six lambs underwent chronic laryngotracheal separation (isolated upper airway group). Forty-eight hours after surgery, each nonsedated lamb underwent polysomnographic recordings on three consecutive days. States of alertness, NNS and respiratory movements were recorded. Results demonstrate that a CPAP of 6 cmH2O inhibited NNS during QS while administered directly on the lower airways and that bivagotomy prevented this inhibition. However, application of CPAP on the upper airways only also inhibited NNS during QS. Finally, the application of a CPAP of 6 cmH2O had no systematic effect on NNS-breathing coordination (assessed by the respiratory phase preceding and following NNS). In conclusion, our results suggest that bronchopulmonary receptors are implicated in the inhibiting effects of nasal CPAP of 6 cmH2O on NNS in all our experimental conditions, whereas upper airway receptors are only implicated in certain conditions.

Laryngeal apnea in rat pups: effects of age and body temperature

In neonatal mammals of many species, including human infants, apnea and other reflex responses frequently arise from stimulation of laryngeal receptors by ingested or regurgitated liquids. These reflexes, mediated by afferents in the superior laryngeal nerves (SLNs), are collectively known as the laryngeal chemoreflex (LCR) and are suspected to be responsible for some cases of the sudden infant death syndrome (SIDS). The LCR is strongly enhanced by mild increases in body temperature in decerebrate piglets, a finding that is of interest because SIDS victims are often found in overheated environments. Because of the experimental advantages of studying reflex development and mechanisms in neonatal rodents, we have developed methods for eliciting laryngeal apnea in anesthetized rat pups and have examined the influence of mild hyperthermia in animals ranging in age from 3 to 21 days. We found that apnea and respiratory disruption, elicited either by intralaryngeal water or by electrical stimulation of the SLN, occurred at all ages studied. Raising body temperature by 2–3°C prolonged the respiratory disturbance in response to either stimulus. This effect of hyperthermia was prominent in the youngest animals and diminished with age. We conclude that many studies of the LCR restricted to larger neonatal animals in the past can be performed in infant rodents using appropriate methods. Moreover, the developmental changes in the LCR and in the thermal modulation of the LCR seem to follow different temporal profiles, implying that distinct neurophysiological processes may mediate the LCR and thermal prolongation of the LCR.

Infant apnea and gastroesophageal reflux: a critical review and framework for further investigation

Gastroesophageal reflux and apnea of prematurity are both common occurrences in premature infants. However, a causal relationship between the two remains controversial. Strong physiologic evidence indicates that a variety of protective reflex responses may elicit laryngeal adduction and apnea. Although a potential link between gastroesophageal reflux and apnea may exist through this pathway, clinical studies can be cited to either support or refute such a link in premature infants. The majority of gastroesophageal reflux episodes do not appear to be related to apnea. In a specific subset of events, a causal relationship may exist. Whether this is related to the character of the reflux episode or to a predisposition in a subpopulation of infants is unclear. This review presents the evidence for and against an association between gastroesophageal reflux and apnea, discusses techniques used in their evaluation, and identifies approaches for future investigation.

Laryngeal sensitivity in the neonatal period: from bench to bedside

Laryngeal sensitivity in the newborn has been a subject of great interest for both researchers and clinicians for a number of years. From a clinical standpoint, laryngeal sensitivity is essential for both preventing foreign substances from entering into the lower airway and for finely tuning upper airway resistance. However, heightened reflexes originating from the laryngeal receptors in newborns and infants, due to neural immaturity, can lead to potentially dangerous cardiorespiratory events. The latter have been linked to apneas of prematurity, apparent life-threatening events, and sudden infant death syndrome (SIDS). From a physiological standpoint, many mechanisms pertaining to reflexes originating from laryngeal receptors are yet to be fully understood. This short review is an attempt to summarize current knowledge on laryngeal sensitivity and its potential consequences upon control of breathing abnormalities encountered within the first weeks of life.

Cricothyroid muscle electrical activity during respiration and apneas in lambs

Respiratory function of the cricothyroid muscle (CT) is virtually unknown in the neonatal period. This study was aimed at assessing CT electrical activity (EMG) during respiration and central apneas in non-sedated lambs. Seven full-term and four preterm lambs were instrumented for polysomnographic recording, including EMG of the diaphragm, thyroarytenoid (TA, a glottal constrictor), posterior cricoarytenoid (PCA, the primary glottal dilator) and CT. Phasic CT EMG was usually observed during inspiration and late expiration, whereas phasic TA EMG was observed during early expiration. While TA EMG virtually disappeared in REM sleep, both inspiratory and expiratory CT EMG increased. Overall, while CT EMG was not frequently observed during central apneas in either full-term (10% of apneas, but never simultaneously with TA EMG) or preterm lambs (30% of apneas), it was associated with decreased lung volume and subglottal pressure when present alone or with PCA EMG. Our results concur with the assumption that CT behaves as a laryngeal dilator in the neonatal period.

Postnatal maturation of laryngeal chemoreflexes in the preterm lamb

Laryngeal chemoreflexes (LCR) are triggered by the contact of liquids with the laryngeal mucosa. In the mature organism, LCR trigger lower airway protective responses (coughing, effective swallowing, and arousal) to prevent aspiration. General belief holds that LCR are responsible for apnea and bradycardia in the newborn mammal, including humans. Our laboratory has recently shown that LCR in full-term lambs are consistently analogous to the mature LCR reported in adult mammals, without significant apneas and bradycardias (St-Hilaire M, Nsegbe E, Gagnon-Gervais K, Samson N, Moreau-Bussiere F, Fortier PH, and Praud J-P. J Appl Physiol 98: 2197-2203, 2005). The aim of the present study was to assess LCR in nonsedated, newborn preterm lambs born at 132 days of gestation (term = 147 days). The preterm lambs were instrumented for recording glottal adductor electromyogram, electroencephalogram, eye movements, heart rate, respiration, and oximetry. A chronic supraglottal catheter was used for injecting 0.5 ml of saline, distilled water, and HCl (pH 2) during quiet sleep, active sleep, and wakefulness on postnatal days 7 (D7) and 14 (D14). Laryngeal stimulation by water or HCl on D7 induced significant apneas, bradycardia, and desaturation, which, at times, appeared potentially life-threatening. No significant apneas, bradycardias, or desaturation were observed on D14. No consistent effects of sleep state could be shown in the present study. In conclusion, laryngeal stimulation by liquids triggers potentially dangerous LCR in preterm lambs on D7, but not on D14. It is proposed that maturation of the LCR between D7 and D14 is partly involved in the disappearance of apneas/bradycardias of prematurity with postnatal age.

Upper airways and neonatal respiration

The upper airways exert an important influence on breathing from the fetal period onward. This review focuses on recent results obtained in the newborn, particularly on laryngeal function in the lamb. Cumulated data can be summarized as follows. Firstly, upper airway closure, either at the pharyngeal or laryngeal level, is now known to occur during central apneas. By maintaining a high apneic lung volume throughout central apneas, active laryngeal closure decreases the magnitude of post-apneic desaturation. Secondly, reflexes originating from laryngeal mucosal receptors, such as laryngeal chemoreflexes and non-nutritive swallowing, are of crucial importance within the context of preterm birth, postnatal maturation, neonatal apneas and apparent life-threatening events/sudden infant death syndrome. While laryngeal chemoreflexes appear to be mature and confer an efficient protection against aspiration in the full-term healthy newborn, they can be responsible for prolonged apneas and bradycardias in the immature preterm newborn. In regard to non-nutritive swallowing, the absence of swallowing activity during apneas in periodic breathing during quiet sleep as well as the presence of bursts of swallows with apneas in active sleep remain to be explained. Forthcoming studies will have to further delineate the impact of common clinical conditions, such as cigarette smoke exposure and/or viral respiratory infection on laryngeal chemoreflexes and non-nutritive swallowing. Better knowledge on the importance of the upper airways in neonatal respiration will ultimately aid in designing clinical applications for the understanding and treatment of severe, pathological respiratory conditions of the newborn.

C-fiber blockade influence on non-nutritive swallowing in full-term lambs

Systemic C-fiber blockade (CFB) has been reported to inhibit induced swallowing in adult guinea pigs. The aim of this study was to investigate the effects of CFB on spontaneous, non-nutritive swallowing (NNS) frequency and NNS-respiration coordination in the neonatal period. Seven CFB lambs and seven control lambs aged 2+/-1 days were chronically instrumented for recording electroencephalogram, eye movements, diaphragm EMG, thyroarytenoid muscle EMG, nasal airflow and electrocardiogram. Polysomnographic recordings were performed in non-sedated lambs, using radiotelemetry transmission. CFB lambs spent more time in active sleep than controls (p=0.02). Frequency of non-nutritive swallowing was not different in CFB and control lambs, whatever the state of alertness. In addition, CFB did not disrupt the overall respiratory-swallowing coordination, inspiratory-related NNS being the most frequent and expiratory-related NNS the least in both CFB and control lambs. Further analyses revealed that CFB had no effect on baseline respiratory and heart rate, and apnea and sigh frequency, whatever the state of alertness. Our results suggest that, in the neonatal period, C-fibers are not involved in NNS frequency and have no influence on the overall respiratory-swallowing coordination.