Effect of hypoxia on laryngeal reflex apnea--implications for sudden infant death

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.

Intralaryngeal application of ATP evokes apneic response mainly via acting on P2X3 (P2X2/3) receptors of the superior laryngeal nerve in postnatal rats

Aerosolized adenosine 5'-triphosphate (ATP) induces cough and bronchoconstriction by activating vagal sensory fibers' P2X3 and P2X2/3 receptors (P2X3R and P2X2/3R). The goal of this study is to determine the effect of these receptors on the superior laryngeal nerve (SLN)-mediated cardiorespiratory responses to ATP challenge. We compared the cardiorespiratory responses to intralaryngeal perfusion of either ATP or α,β-methylene ATP in rat pups before and after 1) intralaryngeal perfusion of A-317491 (a P2X3R and P2X2/3R antagonist); 2) bilateral section of the SLN; and 3) peri-SLN treatment with capsaicin (to block conduction in superior laryngeal C-fibers, SLCFs) or A-317491. The immunoreactivity (IR) of P2X3R and P2X2R was determined in laryngeal sensory neurons of the nodose/jugular ganglia. Lastly, a whole-cell patch clamp recording was used to determine ATP- or α,β-mATP-induced currents without and with A-317491 treatment. It was found that intralaryngeal perfusion of both ATP and α,β-mATP induced immediate apnea, hypertension, and bradycardia. The apnea was eliminated and the hypertension and bradycardia were blunted by intralaryngeal perfusion of A-317491 and peri-SLN treatment with either A-317491 or capsaicin, while all of the cardiorespiratory responses were abolished by bilateral section of the SLN. P2X3R- and P2X2R-IR were observed in nodose and jugular ganglionic neurons labeled by fluoro-gold (FG). ATP- and α,β-mATP-induced currents recorded in laryngeal C-neurons were reduced by 75% and 95% respectively by application of A-317491. It is concluded that in anesthetized rat pups, the cardiorespiratory responses to intralaryngeal perfusion of either ATP or α,β-mATP are largely mediated by activation of SLCFs' P2X3R-P2X2/3R.

Take a deep breath and wake up: The protean role of serotonin preventing sudden death in infancy

Recordings from infants who died suddenly and unexpectedly demonstrate the occurrence of recurring apneas, ineffective gasping, and finally, failure to restore eupnea and arouse prior to death. Immunohistochemical and autoradiographic data demonstrate a constellation of serotonergic defects in the caudal raphe nuclei in infants who died of Sudden Infant Death Syndrome (SIDS). The purpose of this review is to synthesize what is known about adaptive responses of the infant to severely hypoxic conditions, which unleash a flood of neuromodulators that inhibit cardiorespiratory function, thermogenesis, and arousal and the emerging role of serotonin, which combats this cardiorespiratory inhibition to foster autoresuscitation, eupnea, and arousal to ensure survival following an hypoxic episode. The laryngeal and carotid body chemoreflexes are potent in newborns and infants, and both reflexes can induce apnea and bradycardia, which may be adaptive initially, but must be terminated if an infant is to survive. Serotonin has a unique ability to touch on each of the processes that may be required to recover from hypoxic reflex apnea: gasping, the restoration of heart rate and blood pressure, termination of apneas and, eventually, stimulation of eupnea and arousal. Recurrent apneic events, bradycardia, ineffective gasping and a failure to terminate apneas and restore eupnea are observed in animals harboring defects in the caudal serotonergic system models - all of these phenotypes are reminiscent of and compatible with the cardiorespiratory recordings made in infants who subsequently died of SIDS. The caudal serotonergic system provides an organized, multi-pronged defense against reflex cardiorespiratory inhibition and the hypoxia that accompanies prolonged apnea, bradycardia and hypotension, and any deficiency of caudal serotonergic function will increase the propensity for sudden unexplained infant death.

Chemical receptors of the arytenoid: A comparison of human and mouse

OBJECTIVES/HYPOTHESIS

The larynx is a highly responsive organ exposed to mechanical, thermal, and chemical stimuli. Chemicals elicit responses both in intraepithelial nerve fibers and in specialized chemosensory cells, including scattered solitary cells as well as taste cells organized into taste buds. Activation of both chemosensory cells and taste buds in the larynx elicit cough, swallow, or apnea with exposure to sour or bitter substances, and even by water or sweet-tasting chemicals. In an effort to begin understanding their function, we sought to compare the distribution, density, and types of chemosensory cells and chemoresponsive nerve fibers in laryngeal epithelium of humans and mice.

STUDY DESIGN

Animal and human laboratory analysis.

METHODS

Using immunohistochemistry, we identified taste cells and polymodal nociceptive nerve fibers in the arytenoid area of the laryngeal epithelium of the following: 1) infants undergoing supraglottoplasty for laryngomalacia, and 2) a cadaveric specimen procured from a 34-year-old donor. We then compared these findings to both preweanling and mature mouse tissue.

RESULTS

Arytenoid tissue from both human and mouse contained many taste buds containing type II taste cells-bitter, sweet, or umami sensing-which were innervated by nerve fibers expressing P2X3 type adenosine triphosphate receptors. Type III cells (acid responsive) were also present, but they were fewer in human tissue than in equivalent tissue from mice. In both species, the epithelium was densely innervated by free nerve endings.

CONCLUSIONS

Our findings suggest that from a standpoint of chemosensation, human and mouse larynges are biologically similar. This suggests that a murine model can be used effectively in laryngeal chemosensory research.

LEVEL OF EVIDENCE

NA Laryngoscope, 130:423-430, 2020.

Otolaryngic Manifestations in Children Presenting with Apparent Life-Threatening Events

Apparent life-threatening event (ALTE) is a term used to characterize an event of unknown cause after an infant is found limp, cyanotic, bradycardic, and/or requiring resuscitation. Like sudden infant death syndrome (SIDS), ALTE is a general term used until a precise diagnosis can be established. The relationship between ALTE and SIDS has not been clearly defined, although 7 to 15 percent of children with ALTE die of SIDS. If children with ALTE are at greater risk for SIDS, morbidity and mortality may be prevented if the underlying pathology can be identified and corrected or closely monitored. The otolaryngologist is being consulted more frequently to evaluate children who have been through an ALTE to help elucidate any underlying pathology that may have caused the near-death experience. This retrospective chart review reports the evaluation of 30 infants with ALTE requiring consultation by the Division of Pediatric Otolaryngology at the Children's Memorial Hospital in Chicago during a 3-year period. We reviewed the literature and here compare our findings with current animal models. Of the 30 children evaluated, 53% had gastroesophageal reflux, 40% had laryngeal abnormalities, 13% had tracheal abnormalities, and 10% had pharyngeal abnormalities. Thirteen percent of the children had nonotolaryngic anomalies identified during evaluation. Surgical intervention was required in 10 patients and medical treatment was used in 18. When evaluating a child with ALTE, a complete history and physical examination, evaluation for gastroesophageal reflux, assessment for upper airway obstruction by radiographs and endoscopy, and a multidisciplinary approach are recommended. (Otolaryngol Head Neck Surg 1997;116:575–9.)

Prenatal nicotinic exposure prolongs superior laryngeal C-fiber-mediated apnea and bradycardia through enhancing neuronal TRPV1 expression and excitation

Maternal cigarette smoke, including prenatal nicotinic exposure (PNE), is responsible for sudden infant death syndrome (SIDS). The fatal events of SIDS are characterized by severe bradycardia and life-threatening apneas. Although activation of transient receptor potential vanilloid 1 (TRPV1) of superior laryngeal C fibers (SLCFs) could induce bradycardia and apnea and has been implicated in SIDS pathogenesis, how PNE affects the SLCF-mediated cardiorespiratory responses remains unexplored. Here, we tested the hypothesis that PNE would aggravate the SLCF-mediated apnea and bradycardia via up-regulating TRPV1 expression and excitation of laryngeal C neurons in the nodose/jugular (N/J) ganglia. To this end, we compared the following outcomes between control and PNE rat pups at postnatal days 11-14: 1) the cardiorespiratory responses to intralaryngeal application of capsaicin (10 µg/ml, 50 µl), a selective stimulant for TRPV1 receptors, in anesthetized preparation; 2) immunoreactivity and mRNA of TRPV1 receptors of laryngeal sensory C neurons in the N/J ganglia retrogradely traced by 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate; and 3) TRPV1 currents and electrophysiological characteristics of these neurons by using whole-cell patch-clamp technique in vitro Our results showed that PNE markedly prolonged the apneic response and exacerbated the bradycardic response to intralaryngeal perfusion of capsaicin, which was associated with up-regulation of TRPV1 expression in laryngeal C neurons. In addition, PNE increased the TRPV1 currents, depressed the slow delayed rectifier potassium currents, and increased the resting membrane potential of these neurons. Our results suggest that PNE is capable of aggravating the SLCF-mediated apnea and bradycardia through TRPV1 sensitization and neuronal excitation, which may contribute to the pathogenesis of SIDS.-Gao, X., Zhao, L., Zhuang, J., Zang, N., Xu, F. Prenatal nicotinic exposure prolongs superior laryngeal C-fiber-mediated apnea and bradycardia through enhancing neuronal TRPV1 expression and excitation.

Determinants of Fatal Apnea Responses to Acid Stimulation of the Larynx in Piglets

Objectives:

This study explores the physiological determinants of laryngeal chemoreflex (LCR) response severity under hypoxic conditions.

Methods:

Thirty-four piglets underwent hypoxic laryngeal stimulation. Physiologic data were collected, and responses were graded as mild, moderate, or profound.

Results:

Prestimulation hypoxia caused respiratory depression and carbon dioxide retention in profound responders and respiratory stimulation in mild and moderate responders (p < .05). Resumption of respiration occurred in all animals when the Paco2 rose by a mean ± SD of 15.1 ± 6.5 mm Hg (p > .05). There was a significant difference between mild, moderate, and severe responders in change in arterial Pao2 and hydrogenated hemoglobin saturation during the LCR-induced response (p < .001 for both).

Conclusions:

Resumption of respiration is associated with accumulation of arterial Paco2. The respiratory response to hypoxia predicts the severity of the LCR response. The severity of the LCR-induced response is associated with changes in arterial Pao2 and hydrogenated hemoglobin saturation during the LCR-inducedapnea.

Serotonin in the solitary tract nucleus shortens the laryngeal chemoreflex in anaesthetized neonatal rats

What is the central question of this study? Failure to terminate apnoea and arouse is likely to contribute to sudden infant death syndrome (SIDS). Serotonin is deficient in the brainstems of babies who died of SIDS. Therefore, we tested the hypothesis that serotonin in the nucleus of the solitary tract (NTS) would shorten reflex apnoea. What is the main finding and its importance? Serotonin microinjected into the NTS shortened the apnoea and respiratory inhibition associated with the laryngeal chemoreflex. Moreover, this effect was achieved through a 5-HT3 receptor. This is a new insight that is likely to be relevant to the pathogenesis of SIDS. The laryngeal chemoreflex (LCR), an airway-protective reflex that causes apnoea and bradycardia, has long been suspected as an initiating event in the sudden infant death syndrome. Serotonin (5-HT) and 5-HT receptors may be deficient in the brainstems of babies who die of sudden infant death syndrome, and 5-HT seems to be important in terminating apnoeas directly or in causing arousals or as part of the process of autoresuscitation. We hypothesized that 5-HT in the brainstem would limit the duration of the LCR. We studied anaesthetized rat pups between 7 and 21 days of age and made microinjections into the cisterna magna or into the nucleus of the solitary tract (NTS). Focal, bilateral microinjections of 5-HT into the caudal NTS significantly shortened the LCR. The 5-HT1a receptor antagonist, WAY 100635, did not affect the LCR consistently, nor did a 5-HT2 receptor antagonist, ketanserin, alter the duration of the LCR. The 5-HT3 specific agonist, 1-(3-chlorophenyl)-biguanide, microinjected bilaterally into the caudal NTS significantly shortened the LCR. Thus, endogenous 5-HT released within the NTS may curtail the respiratory depression that is part of the LCR, and serotonergic shortening of the LCR may be attributed to activation of 5-HT3 receptors within the NTS. 5-HT3 receptors are expressed presynaptically on C fibre afferents of the superior laryngeal nerve, and serotonergic shortening of the LCR may be mediated presynaptically by enhanced activation of inhibitory interneurons within the NTS.

Interleukin-1β and interleukin-6 enhance thermal prolongation of the LCR in decerebrate piglets

Thermal stress and prior upper respiratory tract infection are risk factors for the Sudden Infant Death Syndrome. The adverse effects of prior infection are likely mediated by interleukin-1β (IL-1β). Therefore, we examined the single and combined effects of IL-1β and elevated body temperature on the duration of the Laryngeal Chemoreflex (LCR) in decerebrate neonatal piglets ranging in age from post-natal day (P) 3 to P7. We examined the effects of intraperitoneal (I.P.) injections of 0.3mg/Kg IL-1β with or without I.P. 10mg/Kg indomethacin pretreatment on the duration of the LCR, and in the same animals we also examined the duration of the LCR when body temperature was elevated approximately 2°C. We found that IL-1β significantly increased the duration of the LCR even when body temperature was held constant. There was a significant multiplicative effect when elevated body temperature was combined with IL-1β treatment: prolongation of the LCR was significantly greater than the sum of independent thermal and IL-1β-induced prolongations of the LCR. The effects of IL-1β, but not elevated body temperature, were blocked by pretreatment with indomethacin alone. We also tested the interaction between IL-6 given directly into the nucleus of the solitary tract (NTS) bilaterally in 100ngm microinjections of 50μL and pretreatment with indomethacin. Here again, there was a multiplicative effect of IL-6 treatment and elevated body temperature, which significantly prolonged the LCR. The effect of IL-6 on the LCR, but not elevated body temperature, was blocked by pretreatment with indomethacin. We conclude that cytokines interact with elevated body temperature, probably through direct thermal effects on TRPV1 receptors expressed pre-synaptically in the NTS and through cytokine-dependent sensitization of the TRPV1 receptor. This sensitization is likely initiated by cyclo-oxygenase-2 dependent synthesis of prostaglandin E2, which is stimulated by elevated levels of IL-1β or IL-6. Inflammatory sensitization of the LCR coupled with thermal prolongation of the LCR may increase the propensity for apnea and Sudden Infant Death Syndrome.

Laryngeal reflex apnea in neonates: effects of CO2 and the complex influence of hypoxia

We have examined influence of hypocapnia, mild hypercapnia and hypoxia on the durations of fictive apnea and respiratory disruption elicited by injection of 0.1ml of water into the laryngeal lumen-the laryngeal chemoreflex (LCR)-in 20 unanesthetized, decerebrate, vagotomized piglets aged 4-10 days that were paralyzed and ventilated with a constant frequency and tidal volume. The LCR was enhanced by hypocapnia and attenuated by hypercapnia as reported by others. The responses to laryngeal stimulation during hypoxia were varied and complex: some animals showed abbreviated responses during the tachypnea of early hypoxia, followed after 10-15min by more prolonged apnea and respiratory disruption accompanying the reduction in ventilatory activity that commonly occurs during sustained hypoxia in neonates. We speculate that this later hypoxic enhancement of the LCR may be due to accumulation of adenosine in the brain stem.

Characteristics of glottic closure reflex in a canine model

PURPOSE

The most important function of the larynx is airway protection which is provided through a polysynaptic reflex closure triggered by the receptors in the glottic and supraglottic mucosa, evoking the reflex contraction of the laryngeal muscles especially by strong adduction of vocal cords. Based on the hypotheses that central facilitation is essential for this bilateral adductor reflex and that its disturbance can result in weakened laryngeal closure, we designed this study to elucidate the effect of central facilitation on this protective reflex.

MATERIALS AND METHODS

Seven adult, 20 kg mongrel dogs underwent evoked response laryngeal electromyography under 0.5 to 1.0 MAC (minimum alveolar concentration) isoflurane anesthesia. The internal branch of the superior laryngeal nerve was stimulated through bipolar platinum-iridium electrodes, and recording electrodes were positioned in the ipsilateral and contralateral thyroarytenoid muscles.

RESULTS

Ipsilateral reflex closure was consistently recorded regardless of anesthetic levels. However, contralateral reflex responses disappeared as anesthetic levels were deepened. Additionally, late responses (R2) were detected in one animal at lower level of anesthesia.

CONCLUSIONS

Deepened level of anesthesia affects central facilitation and results in the loss of the crossed adductor reflex, predisposing to a weakened glottic closure response. Precise understanding of this effect may possibly provide a way to prevent aspiration in unconscious patients.

Some aspects of clinical relevance in the maturation of respiratory control in infants

Two reflex mechanisms important for survival are discussed. Brain stem and cardiovascular mechanisms that are responsible for recovery from severe hypoxia (autoresuscitation) are important for survival in acutely hypoxic infants and adults. Failure of this mechanism may be important in sudden infant death syndrome (SIDS), because brain stem-mediated hypoxic gasping is essential for successful autoresuscitation and because SIDS infants appear to attempt to autoresuscitate just before death. A major function of another mechanism is to protect the airway from fluid aspiration. The various components of the laryngeal chemoreflex (LCR) change during maturation. The LCR is an important cause of prolonged apneic spells in infants. Consequently, it also may have a role in causing SIDS. Maturational changes and/or inadequacy of this reflex may be responsible for pulmonary aspiration and infectious pneumonia in both children and adults.

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.

The role of respiratory control disorders in SIDS

Although sudden death in infants resulting from cardiac arrhythmias are well documented these appear to account for no more than 5-10% of SIDS cases. Sudden respiratory failure currently is viewed as the most likely cause of death in the remainder. Accidental asphyxiation appears to have a causal role in less then 50% of deaths diagnosed as SIDS. The rest are most likely do to some form of acute respiratory failure. Although failure of autoresuscitation or failure to arouse from sleep likely contribute to the final sequence of events leading to at least some SIDS deaths, these cannot be regarded as causes of the primary respiratory failure initiating the fatal sequence. Past and current studies provide strong circumstantial evidence that obstructive sleep apnea and/or apnea of prematurity likely account for respiratory failure leading to SIDS in some or many deaths. In drawing conclusions it is well to recognize that mechanisms leading to death in SIDS are heterogeneous and therefore there is room for several plausible theories for respiratory or circulatory abnormalities contributing to SIDS.

Fatal Apnea in Piglets by way of Laryngeal Chemoreflex: Postmortem Findings as Anatomic Correlates of Sudden Infant Death Syndrome in the Human Infant

OBJECTIVES/HYPOTHESIS

Intrathoracic petechiae are a prominent diagnostic finding in sudden infant death syndrome (SIDS) victims. In this study, the laryngeal chemoreflex (LCR) was elicited experimentally to discover whether intrathoracic petechiae would be produced by way of the LCR. The hypothesis was that water stimulation of the larynx in piglets, leading to death by prolonged apnea, would produce postmortem findings similar to those found in SIDS victims.

STUDY DESIGN

Using the piglet as an animal model, the LCR was initiated by way of water stimulation of the larynx, resulting in death. Normoxic and hypoxic conditions were established before the stimulation. The piglets were studied postmortem to determine the relationship between the physiologic mechanisms of the LCR and characteristic pathologic findings in SIDS.

METHODS

Using protocols approved by animal care, 14 mixed-breed piglets aged 7 to 14 days were sedated with a ketamine/xylazine mixture. Respiratory and pressure-monitoring devices were affixed and light anesthesia maintained with Surital infusion. In 10 of the piglets, a small catheter was placed between the arytenoid cartilages, and 5 mL of tap water was introduced over 1 second. The LCR ensued, producing periods of central apnea bordered by gasping efforts and resulting in hypoxemia and death in all cases. Four piglets underwent this manipulation in normoxic conditions. Six breathed a hypoxic gas mixture for 1 hour to bring their Po2 down to below 50 torr before water was introduced into the larynx. Four control piglets breathed the hypoxic gas mixture for 1 hour (without water stimulation or LCR) before Surital overdose. Within 24 hours of death, all piglets underwent thoracoabdominal autopsy by a blinded evaluator experienced in SIDS pathology.

RESULTS

The autopsies revealed nothing remarkable in the abdominal viscera of any of the experimental animals. Thymus, heart, and lungs were graded 0 to 4 to indicate the degree of petechiae on external surfaces. Average cumulative scores (ACS) were applied to each animal. The control (hypoxic) piglets had no petechiae (ACS 0.0). The normoxic experimental piglets had moderate petechiae (ACS 3.5). The hypoxic experimental piglets had more prominent petechiae (ACS 6.3).

CONCLUSIONS

Stimulation of the LCR, leading to death by prolonged apnea, produces postmortem findings in piglets similar to those found in SIDS victims. Petechiae were more severe among piglets pretreated with a hypoxic mixture of gases. This study supports the hypothesis that initiation of the LCR may produce pathologic features often prominent in SIDS.

Prolongation of the laryngeal chemoreflex after inhibition of the rostral ventral medulla in piglets: a role in SIDS?

We tested the hypothesis that inhibition of neurons within the rostral ventral medulla (RVM) would prolong the laryngeal chemoreflex (LCR), a putative stimulus in the sudden infant death syndrome (SIDS). We studied the LCR in 19 piglets, age 3-16 days, by injecting 0.05 ml of saline or water into the larynx during wakefulness, non-rapid eye movement (NREM) sleep, and REM sleep, before and after 1 or 10 mM muscimol dialysis in the RVM. Muscimol prolonged the LCR (P < 0.05), and the prolongation was greater when the LCR was stimulated with water compared with saline (P < 0.02). The LCR was longer during NREM sleep than during wakefulness and longest during REM sleep (REM compared with wakefulness). Muscimol had no effect on the likelihood of arousal from sleep after LCR stimulation. We conclude that the RVM provides a tonic facilitatory drive to ventilation that limits the duration of the LCR, and loss of this drive may contribute to the SIDS when combined with stimuli that inhibit respiration.

Gastroesophageal reflux and the upper airway

EER is a disorder commonly seen in otolaryngologic practice and differs from GERD in its clinical manifestations, pathophysiology, and response to treatment. Its association with numerous disorders in children should lead otolaryngologists to consider this diagnosis in all patients with voice, airway, and swallowing complaints.

Impaired cardiorespiratory recovery after laryngeal stimulation in nicotine-exposed young lambs

The hypothesis that postnatal nicotine exposure weakens cardiorespiratory recovery from reflex apnea and bradycardia was tested in eight lambs continuously infused with nicotine from the day of birth at a dose of 1 to 2 mg.kg(-1).d(-1). Eight age-matched lambs infused with saline served as controls. Apnea and bradycardia were elicited by laryngeal stimulation with 1 mL of water (laryngeal chemoreflex) both during air breathing [0.21 fraction of inspired oxygen (FiO(2))] and mild hypoxia (0.10 FiO(2)) at a mean postnatal age of 5 +/- 1, 14 +/- 1, and 28 +/- 1 d. Ventilation, heart rate, and blood pressure were similar in the two groups at rest. In response to laryngeal chemoreflex stimulation, nicotine-treated lambs had a more pronounced decrease in ventilation (p < 0.05), longer reflex apnea (p < 0.001 in 0.21 FiO(2); p < 0.01 in 0.10 FiO(2)), and greater reflex bradycardia (p < 0.01). During reflex apnea, sighs were less efficient in restoring heart rate to prestimulation level, and a greater decrease in heart rate was observed before sighs in nicotine-treated lambs. These effects were most apparent at 5 d of age, when nicotine-treated lambs also had lower ventilation during hypoxia (p < 0.05). The response to hyperoxia was comparable in the two groups at all ages. The ability to terminate laryngeal chemoreflex-induced apnea is attenuated in young lambs continuously exposed to nicotine. This attenuation is present both in normoxia and in hypoxia and is accompanied by reduced effects from sighing on cardiac autoresuscitation.

Maturation and transformation of reflexes that protect the laryngeal airway from liquid aspiration from fetal to adult life

Several reflexes are initiated in the fetus and newborn when hypochloremic or strongly acidic solutions contact the epithelium that surrounds the entrance to the laryngeal airway. These reflexes, known collectively as the laryngeal chemoreflex (LCR), include startle, rapid swallowing, apnea, laryngeal constriction, hypertension, and bradycardia. Many studies have shown that prolonged apnea associated with the LCR may be life threatening and might conceivably be a cause of sudden infant death syndrome. This certainly may be true, but the concept of a lethal LCR paradoxically contrasts with the view that these several reflexes have an important airway-protective role. As the infant matures, rapid swallowing and apnea become much less pronounced, whereas cough and possibly laryngeal constriction become more prominent. This transformation is primarily related to central neural processing rather than to changes in the airway mucosal "water receptors" that initiate the reflex. The LCR develops in the fetus, in an all-aqueous environment, during a period in which aspiration of amniotic fluid poses a serious threat to life. This and other considerations suggest that the transformation in LCR responses from fetal to adult life can be viewed as functionally appropriate to their primary role in defending the airway from aspiration. The laryngeal "water receptors" that initiate the LCR in infants and adults alike appear to be the primary sensory mechanism for defending the airway from aspiration of liquids.

Age dependence of laryngeal chemoreflex in puppies

Previously collected data have indicated that the laryngeal chemoreflex (LCR) response is exaggerated during a critical period of postnatal development in several experimental animals. These animals had fewer anatomic and physiological similarities to humans than do puppies. This investigation of the LCR in 14 anesthetized puppies was undertaken to determine age-related differences in the response to stimulation of the supraglottic laryngeal mucosa by saline solution, distilled water, cow's milk, and acid at pH 1.0. The dogs were divided into 3 age groups: group 1 consisted of 4 dogs that were 2 weeks old, and in groups 2 and 3 there were 5 puppies each, of 4 and 6 weeks of age, respectively. The LCR response (laryngospasm, apnea, respiratory depression, and bradycardia) was found in the puppies only after stimulation of the laryngeal mucosa with acid at pH 1.0, and it was more easily achieved in the 4- and 6-week age groups than in the 2-week group. These findings suggest that the LCR is an age-dependent response that appears in dogs only after 2 weeks of age. The important implication of this finding is that postnatal neural maturation may influence the laryngeal reflex in humans to some extent.

Laryngospasm and diaphragmatic arrest in immature dogs after laryngeal acid exposure: a possible model for sudden infant death syndrome

Laryngopharyngeal reflux has been proposed as a possible cause of sudden infant death syndrome (SIDS). We investigated the efferent laryngeal and diaphragmatic responses to acid exposure on the laryngeal mucosa using a neonatal canine model. Electromyographic (EMG) recordings from the thyroarytenoid muscle and the diaphragm were measured with hooked-wire electrodes. Reproducible laryngospasm responses occurred in all animals after laryngeal exposure to hydrochloric acid at pH 2.0 or less. Laryngospasm occurred in combination with tachypnea and increased diaphragmatic activity in most of the animals. Laryngospasm was associated with prolonged apnea and total cessation of diaphragmatic EMG activity in 1 animal, and in another, initial tachypnea was followed by erratic diaphragmatic activity and brief apnea. Laryngeal acid exposure (below pH 2.0) causes laryngospasm and may result in paradoxical apneic events in neonatal dogs. Acid-induced, laryngospasm-associated apnea may represent a potential cause of SIDS, and the immature dog appears to be an excellent model for further investigations.

Effects of intralaryngeal carbon dioxide and acetazolamide on the laryngeal chemoreflex

Sudden infant death syndrome is the leading cause of death in infants in the United States. The laryngeal chemoreflex (LCR) is thought to contribute to its pathogenesis. In adult animals, increasing levels of intralaryngeal CO2 result in a decrease in ventilatory activity. Intravenous acetazolamide (AZ) abolishes this response. The purpose of this study was to determine the effects of intralaryngeal CO2 and AZ on the LCR and respiratory physiology of piglets under normoxic and hypoxic conditions. We applied 0% or 10% CO2 in a randomized order to the larynx of 26 piglets. Intubation via tracheotomy prevented inhalation of the gas mixtures. Laryngeal stimulation was performed under normoxic conditions (PaO2 of >70 mm Hg) in 15 animals and under hypoxic conditions (PaO2 of 50 to 65 mm Hg) in 11 animals both with and without intravenous AZ (5 mg/kg). Respiratory and cardiovascular response data were recorded. Ten percent intralaryngeal CO2 has no significant effect on mean baseline respiratory rate, systemic PaCO2 or PaO2 levels, or apnea duration (p > .05). The use of AZ (versus no AZ) resulted in significantly higher baseline respiratory rates (64 versus 51 breaths per minute; p = .016), a decreased baseline systemic PaCO2 level (38.8 versus 45.9 mm Hg; p < .001), a higher baseline PaO2 level (97.9 versus 82.8 mm Hg; p < .001), shorter mean apnea durations (15.5 versus 24.8 seconds; p = .001), a higher lowest O2 saturation level after the stimulus (78.0% versus 68.4%; p = .003), and fewer profound apneas (10 of 90 versus 41 of 90 trials; p < .001). We conclude that 10% intralaryngeal CO2 does not decrease ventilatory activity in piglets and has no significant effect on the LCR. Acetazolamide, however, appears to have a protective effect against the LCR, resulting in shorter and less severe apneas. The protective effect of AZ against the LCR appears to be related to its ability to stimulate the respiratory drive and increase oxygenation at baseline.

Second place-resident clinical science award 1999: laryngeal chemoreflex severity and end-apnea PaO(2) and PaCO(2)

OBJECTIVE

The laryngeal chemoreflex (LCR) is a model for investigating the sudden infant death syndrome. The severity of the LCR-induced response may vary. This study examines the conditions under which recovery from the LCR-induced apnea occurs.

METHODS

Twenty-five piglets underwent normoxic laryngeal stimulation (Pao(2) > 70 mm Hg); 11 then underwent hypoxic stimulation (Pao(2) 50-65 mm Hg). Cardiovascular and respiratory responses were recorded.

RESULTS

Recovery Pao(2) was lower during profound responses (Pao(2) = 45.9 +/- 12.8 mm Hg) than during moderate (Pao(2) = 54.9 +/- 7.5 mm Hg) and mild (Pao(2) = 60.6 +/- 10.3 mm Hg) responses (analysis of variance [ANOVA], P = 0.05). Recovery PaCO(2) did not vary (ANOVA, P > 0.05). Blood pressure and O(2) saturation declined at faster rates with increasing severity of response (ANOVA, P < 0.05 for both).

CONCLUSIONS

Resumption of respiration after LCR-induced apnea is associated with a consistent level of PaCO(2). The severity of the response is associated with recovery PaO(2) levels.

The role of gastro-oesophageal reflux in the aetiology of SIDS

Gastro-oesophageal reflux (GOR) has been identified as a possible cause of SIDS. Several features of GOR unique to infants presenting with apparent life-threatening events (ALTEs) have led to its 'pathogenic' definition. One is that the life-threatening apnoea itself is initiated by GOR, another is that the ALTE relates to prolonged reflux during sleep, in a vulnerable sleep-state, and finally that the ALTE relates to excessive quantities of GOR. The presumption of GOR 'pathology' as a cause of SIDS however, is questionable in these susceptible infants for three reasons: firstly, GOR is physiological and occurs in most infants; secondly, there is no general consensus on what constitutes normal physiological reflux, and thirdly, variation in the recording technique and methods of data analysis and interpretation may account for the differences between study groups. It seems likely therefore if GOR is implicated in SIDS, additional factors are involved. Under certain circumstances, physiological GOR may trigger life-threatening apnoea in apparently healthy infants, that leads to SIDS. One mechanism that could explain such a death is reflex apnoea by stimulation of laryngeal chemoreceptors (LCR) during sleep. The conditions under which this could be fatal are the occurrence of gastric contents refluxed to the level of the pharynx during sleep, in the young infant who has depressed swallowing and arousal. That is, the occurrence of GOR to the level of the pharynx during sleep, an infrequent event that is usually innocuous, could be converted to a fatal event if swallowing is impaired and arousal depressed, by a variety of mediating factors such as prone sleeping, prematurity, sedatives, seizures or upper respiratory tract infections. The identification of LCR responses, particularly in prone sleeping and premature infants provide further evidence that this mechanism may be implicated in the aetiology of SIDS in apparently healthy infants.

Why the prone position is a risk factor for sudden infant death syndrome

INTRODUCTION

The laryngeal chemoreflex may explain why prone sleeping increases the risk of sudden infant death syndrome (SIDS). Swallowing and arousal are crucial to prevent laryngeal chemoreflex stimulation. Our aim was to examine these reflexes and breathing responses in healthy neonates after pharyngeal infusion of water in the supine versus the prone position, controlling for sleep state.

METHODS

A total of 10 term infants were recruited after parental consent and ethics approval. Polygraphic recordings included sleep state (active and quiet sleep by electroencephalogram, eye movements, breathing, and behavior), cardiorespiratory measurements (nasal airflow, chest wall movements, heart rate, and oxygen saturation), swallowing, and esophageal activity (solid state pressure catheter). Initial sleeping position was assigned randomly. Measurements were made for 1 minute before and after 0.4 mL of water was instilled into the oropharynx. To detect a 30% decrease in swallowing, power analysis indicated that >/=10 babies were required. Analysis, blinded to position, was made using nonparametric statistics.

RESULTS

Of the 164 infusions, the most commonly evoked airway protective responses to pharyngeal infusion were swallowing (95%) and arousal (54%). After infusion in active sleep, there was a significant reduction in swallowing and breathing when the prone position was compared with the supine position (prone: 21.3 [1.0] swallows/min and -9.6 [2.1] breaths/min; and supine: 32 (2.2) and -2. 9 (1.5), respectively). However, there was no difference in the occurrence of arousal after water infusion.

CONCLUSION

These data suggest that airway protection is compromised in the prone sleeping position during active sleep, even in healthy infants exposed to minute pharyngeal fluid volumes of 0.4 mL. This is because swallowing rate is reduced significantly, and there is no compensatory increase in arousal. The reduction in airway protective reflexes when in the prone position and in active sleep may be the mechanism for the increased risk of SIDS in the prone position.

Reduced inspiratory drive following laryngeal chemoreflex apnea during hypoxia

Respiratory inhibition following laryngeal water administration was investigated by breath-by-breath analysis of inspiratory ventilation (VI) and central inspiratory drive (P0.1) in 15 unanesthetized lambs studied in 0.21 FIO2 (PaO2: 82-92 torr, PaCO2 41-43 torr) and in 0.1 FIO2 (Pao2 30-34 torr, PaCO2 32-33 torr). During the 30 sec period after stimulation, VI decreased significantly compared to prestimulation levels both in 0.21 FIO2 (-22, -21 and -18%) and in 0.1 FI(O2), (-16, -23 and -19%) at 5, 16 and 29 days, respectively. In contrast, P0.1 remained at prestimulation levels during normoxia in all age groups (1, 10 and 9%, NS), but decreased significantly during hypoxia (-11 and -13%, P < 0.05) at 16 and 29 days, respectively. Poststimulation apnea duration was significantly related to the decrease in VI (P < 0.001) but not to the change in P0.1. Laryngeal stimulation during hypoxemia/hypocapnia induces a prolonged decrease of central inspiratory drive in postneonatal lambs, a finding of potential significance for the mechanisms of sudden infant death syndrome.

Effect of sleep state on the laryngeal chemoreflex in neonatal piglets

The laryngeal chemoreflex (LCR) is a brain stem-mediated response that is a potential mechanism for sudden infant death syndrome. The vast majority of sudden infant death occurs during sleep, yet it remains to be established whether there is a particular sleep state that makes an infant animal more susceptible to apneic events via the LCR. The purpose of this study was to investigate the LCR during different sleep states in the neonatal piglet. In this study, continuous physiologic monitoring and electroencephalographic, electro-oculographic, and electromyographic techniques were utilized to study neonatal piglets during a hypnotic induced sleep model. Propofol drip anesthetic was utilized to provide an anesthetic state and was titrated for dose-dependent sedation. The LCR was initiated in 11 animals during quiet sleep, rapid eye movement sleep, and the anesthetic state. Baseline respiratory and cardiovascular responses were measured. Durations of apnea were recorded and compared. This study found that despite known physiologic differences in respiratory control during different sleep states as compared to the anesthetic state, there appears to be no increased risk of profound apnea in one state versus another in piglets 19 to 28 days old.

Correlation of laryngeal chemoreflex severity with laryngeal muscle response

OBJECTIVES

To examine the relationship between the severity of the laryngeal chemoreflex (LCR) and the pattern of laryngeal muscle activity during an LCR-induced apnea.

METHODS

The laryngeal mucosa of 20 piglets aged 17 to 20 days was stimulated under both hypoxic and normoxic conditions. Respiration, blood pressure, and activity of the thyroarytenoid (TA) and posterior cricoarytenoid (PCA) muscles were monitored during the LCR-induced apnea.

RESULTS

Hypoxemia resulted in a shorter average apnea duration but a greater degree of hypotension. All piglets recovered spontaneously following normoxic LCR stimulation. Hypoxic stimulation resulted in two divergent apneic responses: transient with a spontaneous recovery (17 piglets) or profound requiring resuscitation (three piglets). An increase in TA muscle activity and a decrease in PCA muscle activity was the most common response to LCR stimulation. The response of the TA and PCA muscles was maintained in piglets destined for spontaneous recovery. Decreasing TA activity and increasing PCA activity correlated with the development of a profound response.

CONCLUSIONS

The piglet demonstrates two distinct responses to hypoxic laryngeal chemostimulation that correlate with the activity of the intrinsic laryngeal muscles. Failure to maintain the activity of the TA and PCA muscles during a profound response is associated with the development of severe cardiovascular instability. This study suggests that the critical event involving the LCR is the development of complications secondary to hypoxia.

Effect of antagonism at central nervous system M3 muscarinic receptors on laryngeal chemoresponse

The laryngeal chemoresponse (LCR), comprising laryngeal adductor spasm, central apnea, and subsequent cardiovascular instability, is thought to be a factor in sudden infant death syndrome. A muscarinic subtype receptor, M3, appears to be involved in central respiratory drive and control. Both the duration of the LCR apnea and levels of M3 receptor messenger RNA in the brain stem change according to postnatal age. This study examined the effect of central nervous system antagonism at M3 receptors on the LCR with respect to animal age and dose of antagonist. Ten piglets in each of three age groups (group 1, 5 to 8 days; group 2, 18 to 21 days; and group 3, 40 to 43 days) received a series of four increasing doses of an M3 antagonist (p-fluoro-hexahydro-sila-diphenidol) by intracerebral ventricle injection. The LCR was evoked at baseline and after each dose of antagonist. An effect on susceptible animals (groups 1 and 2) was evident by the second antagonist dose, and persisted for the remainder of the experiment (2 hours). At completion of the experiment, mean apnea duration had decreased in group 1 (61%, p < .05), and group 2 (57%, p < .05), but was unchanged in group 3 (<10%, p not significant). Length of mean baseline apneas correlated directly with degree of apnea shortening. The reduction is not attributable to changes in arterial PO2 or PCO2 or baseline respiratory rate. These results support an age-related influence on the LCR by M3 receptors in younger animals that decreases with maturation.

Superior laryngeal nerve brain stem evoked response in the cat

The generator sources of each wave of the laryngeal brain stem evoked response (LBR) have yet to be precisely demonstrated, although this has been studied in several animal species. This study was carried out to record the near-field brain stem activity as well as the far-field brain stem activity in the cat under the same experimental setup, and to search for the generator sources of the waves. Under general anesthesia, the LBR tracings were recorded adjacent to and within the brain stem following direct electrical stimulation of the internal branch of the superior laryngeal nerve. Reproducible positive and negative waves were detected by the far- and near-field techniques. Mean latencies, configurations, and reproducibility of each wave were demonstrated. From these results, we speculate on the generator sources of each wave.

Ovine Fetal Laryngeal Chemoreflex Thresholds and Respiratory Effects

In newborn infants, laryngeal contact with solutions of low chloride concentration or pH evokes swallowing, laryngeal adduction, and respiratory inhibition (laryngeal chemoreflex). To determine whether the laryngeal chemoreflex is present during fetal life and its effect on fetal respiratory activity, eight time-bred ewes (128 ± 2 days) were prepared with fetal electrocortical diaphragm and esophageal electrodes and a nasopharyngeal catheter. After a 60-minute control period, increasing volumes (0.1 to 1.0 ml/kg) of 0.15 mol/L NaCl or distilled water (0.05 to 1.0 ml/kg) and decreasing concentrations of NaCl (0.15 to 0.02 mol/L) at a fixed volume (0.3 ml/kg) were sequentially administered through the nasopharyngeal catheter (38° C). The minimum water volume that stimulated swallowing was significantly less than the minimum 0.15 mol/L NaCl volume (0.10 ± 0.02 vs. 0.70 ± 0.05 ml/kg). The maximum NaCl concentration that stimulated swallowing was 0.04 ± 0.01 mol/L. During the control period, respiratory activity averaged 14.6 ± 0.7 breaths/minute and did not change during absent swallow responses or isotonic saline-induced swallows. However, respiratory activity significantly decreased during water (4.7 ± 0.6 breaths/minute) and hypotonic saline-induced swallow responses (3.7 ± 0.7 breaths/minute). Fetal electrocortical activity did not change during absent or stimulated swallows. We conclude that laryngeal water or hypotonic saline solution may stimulate fetal swallowing and suppress fetal respiratory activity, similar to the newborn laryngeal chemoreflex. We speculate that an exaggeration of the laryngeal chemoreflex apnea response in the newborn may predispose to sudden infant death syndrome.

Effect of a Second Laryngeal Stimulation during Recovery from the Laryngeal Chemoreflex

The laryngeal chemoreflex is a potential mechanism for sudden infant death. In experimental protocols in which a full recovery is allowed between stimuli, no laryngeal chemoreflex responses result in a fatal outcome. In the clinical situation there are no controls to prevent repeated laryngeal stimulation before a full recovery. The effect of a laryngeal stimulus applied during or soon after a laryngeal chemoreflex-induced apnea was investigated. Eighteen piglets were divided into groups aged 10 to 12 days, 17 to 21 days, and 32 to 36 days. Laryngeal stimulation was performed under normoxic conditions with water applied to the mucosa. Baseline respiratory and cardiovascular response data were measured. After recovery an initial stimulation was applied, followed by a second stimulation during the apnea or 5, 30, 60, or 120 seconds after restoration of breathing. No profound apneas occurred with baseline laryngeal stimulation. In piglets aged 32 to 36 and 17 to 21 days, a second laryngeal stimulus resulted in a shortened apnea duration. The response varied in piglets aged 10 to 12 days with profound apneas observed In 2 of 6 subjects and 4 of 30 trials. Piglets aged 17 to 36 days are less susceptible to the laryngeal chemoreflex during the immediate recovery period. In piglets aged 10 to 12 days, the laryngeal chemoreflex response may be more severe after a second Stimulus.

Beta-endorphin immunoreactivity levels in CSF after laryngeal chemoreflex activation correlate with apnoea duration in piglets

The activation of the laryngeal chemoreflex may be a pathogenic mechanism in apnoea, apparent life threatening events, and SIDS. Infants with apnoea and increased levels of beta-endorphin immunoreactivity in CSF have been successfully treated with naloxone. Beta-endorphin may induce respiratory depression, and naloxone is a beta-endorphin antagonist. We therefore wanted to measure beta-endorphin levels in CSF before and after the chemoreflex induced apnoea. This study includes 13 piglets, 5-10 days of age, treated with and without naloxone. Respiration, blood pressure, and heart rate were monitored. CSF was sampled before and after the laryngeal chemoreflex induced apnoea. We found a shorter duration of apnoea in the piglets which had received naloxone than in those which did not (p = 0.02). The beta-endorphin immunoreactivity levels in CSF increased after apnoea, and the increased levels correlated positively with the duration of the apnoea in the piglets which had not received naloxone (r = 0.94, p = 0.02), but not in those pretreated with naloxone (r = 0.1, p = 0.8). The median amount of beta-endorphin immunoreactivity in CSF after apnoea in the naloxone-treated piglets was not significantly different from that in the non-treated piglets: 615 +/- 589 (n = 7) fmol/ml CSF and 984 +/- 851 (n = 6) fmol/ml CSF, respectively. The beta-endorphin immunoreactivity levels measured before the apnoea were less than 4.3 fmol/ml CSF.

CONCLUSION

The laryngeal chemoreflex induced apnoea may possible be partly mediated by beta-endorphin.

Functional and developmental studies of the peripheral arterial chemoreceptors in rat: effects of nicotine and possible relation to sudden infant death syndrome

The drive on respiration mediated by the peripheral arterial chemoreceptors was assessed by the hyperoxic test in 3-day-old rat pups. They accounted for 22.5 +/- 8.8% during control conditions, but only for 6.9 +/- 10.0% after nicotine exposure, an effect counteracted by blockade of peripheral dopamine type 2 receptors (DA2Rs). Furthermore, nicotine reduced dopamine (DA) content and increased the expression of tyrosine hydroxylase (TH) in the carotid bodies, further suggesting that DA mediates the acute effect of nicotine on arterial chemoreceptor function. During postnatal development TH and DA2R mRNA levels in the carotid bodies decreased. Thus, nicotine from smoking may also interfere with the postnatal resetting of the oxygen sensitivity of the peripheral arterial chemoreceptors by increasing carotid body TH mRNA, as well as DA release in this period. Collectively these effects of nicotine on the peripheral arterial chemoreceptors may increase the vulnerability to hypoxic episodes and attenuate the protective chemoreflex response. These mechanisms may underlie the well-known relation between maternal smoking and sudden infant death syndrome.

Physiological studies of gastro-oesophageal reflux and airway protective responses in the young animal and human infant

1. The mechanisms that underlie the Sudden Infant Death Syndrome (SIDS) must explain its two unique features; age at death and death during apparent sleep. 2. The occurrence of gastro-oesophageal reflux (GOR) during active sleep in infants presenting with apparent life threatening episodes (ALTE) and their similar age distribution to SIDS infants, suggested that reflux could be a cause of asphyxia. 3. Sleep related GOR was found to be a physiological and not a pathological event in normal, healthy term infants. 4. In healthy term infants, those infants that were formula-fed (who have a higher incidence of SIDS) had significantly longer oesophageal clearance times for acid reflux and significantly more active sleep compared with breast fed infants. 5. In very preterm infants (who are at increased risk for SIDS), both the frequency and duration of reflux during active sleep was significantly less at term equivalent age compared with healthy term infants, suggesting additional factors must operate to promote an ALTE. 6. One mechanism which may explain the pathogenesis of GOR could be that the reflux reaches the level of the pharynx and this, in turn, stimulates laryngeal receptors to produce apnoea. 7. Simulated reflux to the level of the pharynx in the sleeping piglet evoked airway protective responses, namely swallow, arousal and occasionally expectoration, but neither apnoea nor oxygen desaturation. 8. In the same piglets treated with pentobarbitone sodium, swallowing was impaired and arousal depressed. Simulated reflux to the pharynx produced significant apnoea and oxygen desaturation and death in two of five piglets.(ABSTRACT TRUNCATED AT 250 WORDS)

Superior laryngeal nerve evoked potentials: an experimental study in the rabbit

An experimental model is described for recording laryngeal evoked potentials at the brainstem level through electrical stimulation of the superior laryngeal nerve. Rabbits were divided into two groups according to age: Group 1 consisted of 20 adults (over 6 months old), while group 2 was comprised of 1-month-old rabbits. Under general anesthesia, the superior laryngeal nerve was electrically stimulated and the corresponding evoked potentials were recorded by needle electrodes placed close to the dorsal and ventral aspects of the brain stem. Clear age-related differences were observed for latency, amplitude, and morphology of the potentials. This fact suggests postnatal maturation of the nerve structures, both peripheral and central. Sectioning the superior laryngeal nerve abolished all responses. Adduction of both vocal cords was observed in conjunction with the use of stimulation.

Influence of hypoxia on ventilatory responses to intralaryngeal CO2 in cats

In decerebrate, vagotomized cats, introduction of CO2 into the isolated laryngeal airway while systemic PCO2 is held constant evokes dose-related reflex changes in ventilatory activity. Because systemic hypoxia is known to exaggerate ventilatory responses to other types of laryngeal chemostimulation in neonates, we have compared the responses of phrenic and hypoglossal nerve activities to ventilation of the larynx with 10% CO2 during systemic hyperoxia (FIO2 = 1.00) to those during hypoxia (FIO2 = 0.12). Compared with the hyperoxic baseline condition, hypoxia stimulated phrenic activity but attenuated the reduction in phrenic activity evoked by intralaryngeal CO2. Hypoglossal activity was increased by intralaryngeal CO2 and this response appeared to be reduced by hypoxia, but neither of these findings was statistically significant. The response of phrenic activity to intralaryngeal CO2 during systemic hypercapnia was similar to that during hypoxia. The increase of phrenic activity in response to hypoxia was prevented by carotid body resection. Similarly, the hypoxic attenuation of the phrenic response to intralaryngeal CO2 appeared to be absent after carotid body resection, although this finding was not established statistically. These results differ from previous reports of exaggerated laryngeal chemoreflex responses during hypoxia. The difference may reflect differences in the receptors and synaptic mechanisms of the reflexes, the severity and time course of hypoxia or the presence or depth of general anesthesia or sleep.

Correlation between the laryngeal brain stem evoked response and the laryngeal chemoreflex in the porcine model

The laryngeal brain stem evoked response (LBR) represents the neural activity involved in laryngeal reflex pathways. The laryngeal chemoreflex (LCR) is a centrally mediated response consisting of apnea and hemodynamic changes that result from laryngeal stimulation. The purpose of this study is to determine the characteristics of the LBR that are predictive of LCR severity in the porcine model. The duration of apnea resulting from stimulation of the supraglottic larynx defined LCR severity. The LBR tracings were recorded from electrodes flanking the brain stem following direct electrical stimulation of the superior laryngeal nerve. The LBR peak latencies from piglets demonstrating prolonged LCR apnea were compared to those without an exaggerated LCR response. Two LBR peak latencies demonstrated a statistically significant difference between the two piglet groups. These peak latencies appear to be indicators of susceptibility to exaggerated laryngeal reflex sensitivity. Thus, the LBR may prove useful in identifying and evaluating subjects predisposed to conditions associated with dysfunctional laryngeal reflex activity.

Laryngeal brain stem evoked response in the porcine model

Exaggeration of normally protective laryngeal reflexes is thought to play a role in several disorders, including the sudden infant death syndrome. An analysis of brain stem neural activity following laryngeal stimulation may provide insight into the pathophysiology of pathologic laryngeal reflexes and help to identify individuals at risk for these disorders. The purpose of this study was to define the far-field brain stem activity following laryngeal stimulation in the porcine model. This activity has been termed the laryngeal brain stem evoked response and may represent a potentially useful and objective measure of the neuronal activity in the laryngeal reflex pathway. Electrical stimulation of the superior laryngeal nerve was performed in 14 mixed-breed piglets under a variety of physiologic conditions. A total of six positive and six negative discrete waves were detected, with mean latencies ranging from 1.24 to 7.16 milliseconds. Stimulations performed during hypoxic, hypercapneic, or hypocapneic conditions resulted in no significant differences in waveform latencies. There appears to be a reproducible, but somewhat variable, brain stem response elicited by superior laryngeal nerve stimulation that can be recorded via a far-field technique in the porcine model.

Arterial chemoreceptor influences on the laryngeal chemoreflex

Prolonged apnea and cardiovascular changes have been elicited in infant animals by the application of water to the laryngeal mucosa. Previous reports have produced conflicting evidence in regard to the possible role of arterial chemoreceptors in modulating this reflex. The present study was designed to determine the effect of carotid body stimulation or suppression on the duration of apnea and severity of cardiovascular changes in response to water in the larynx of piglets. The role of swallowing in terminating the apnea was also investigated. Hypoxia and isoproterenol, both carotid body stimuli, caused decreased apnea duration. Hyperoxia was associated with prolonged apnea duration; however, dopamine, which inhibits carotid body chemoreceptors, produced no significant change. Hypotension and bradycardia were only observed after prolonged apnea or chemoreceptor stimulation, supporting the concept that the cardiovascular component of the laryngeal chemoreflex is a result of changes in blood gas concentration rather than a direct response to laryngeal chemostimulation. The interval between water application and initiation of swallowing was not significantly affected by hypoxia or carotid body stimulation and swallowing did not always occur before resumption of breathing.

Lidocaine effects on the laryngeal chemoreflex, mechanoreflex, and afferent electrical stimulation reflex

The use of lidocaine hydrochloride as either a topical or intravenous agent has become a common practice for minimizing laryngospasm and the reflex cardiovascular effects resulting from upper airway manipulation. The efficacy and mechanism of action of lidocaine for this purpose remain unclear. We evaluated the effect of lidocaine on the laryngeal chemoreflex (LCR), mechanoreflex (LMR), and superior laryngeal nerve electrical stimulation adductor reflex (SLN-ESAR) in piglets. Cardiopulmonary responses were used to assess LCR and LMR. Latency following SLN stimulation was used to assess SLN-ESAR. Intravenous lidocaine hydrochloride at 3 mg/kg produced no suppression of the LCR, LMR, or latency (SLN-ESAR onset latency before lidocaine 11.7 +/- 0.7 milliseconds, after lidocaine 12.2 +/- 0.5 milliseconds; peak latency before lidocaine 13.2 +/- 0.2 milliseconds, after lidocaine 13.4 +/- 0.4 milliseconds). Topically applied lidocaine at the same dose eliminated both LCR and LMR responses in all animals, with return of reflex responses 15 minutes after application. No effect on the SLN-ESAR was seen with application of topical lidocaine. This study supports topical lidocaine as a suppressant of laryngeal mucosal neuroreceptors without central neural reflex effects. Intravenous lidocaine did not affect peripheral neuroreceptors, nor did it significantly affect the latency of the SLN-ESAR neural reflex arc. Intravenous and topical lidocaine differ in mechanism of action and efficacy with regard to modulation of reflex effects induced by laryngeal stimulation.

Laryngeal brain stem evoked response

Sensory stimuli to the larynx evoke a laryngeal adductor reflex mediated by the brain stem via superior and recurrent laryngeal nerves. Aberrant laryngeal reflexes have been proposed to explain a number of poorly understood disorders, including "reflex apnea," idiopathic laryngospasm, and sudden infant death syndrome. The purpose of the present study was to evaluate far field brain stem recordings following stimulation of the superior laryngeal nerve to determine whether laryngeal brain stem response is a valid measure of laryngeal activity at the brain stem level. The nerve was stimulated electrically in adult cats, and the resultant laryngeal adductor response as well as far field brain stem activity was recorded. For the latter, six reproducible positive and five reproducible negative waves were obtained via posterior pharyngeal (+) and posterior cervical (-) recording electrodes. Response threshold and latencies were measured and evaluated as a function of stimulus parameters. Wave latencies corresponded closely to those reported in prior near and far field evoked response recordings.

Hypoxia reinforces laryngeal reflex bradycardia in infants

The laryngeal chemoreflex involves bradycardia, apnea, swallowing and peripheral vasoconstriction. This reflex was studied in twelve infants, aged 5 days-28 weeks, who had sustained an apparent life-threatening event or were siblings of infants who had died of the sudden infant death syndrome. The bradycardic and apneic components of the reflex were found to be significantly, and sometimes powerfully, reinforced when elicited by pharyngeal water instillation during acute, mild hypoxia (transcutaneous PO2 4.6-8.3 kPa). Apnea duration during normoxia was 0.7-15 sec, and during hypoxia 2-30 sec. Heart rate change ranged from +26% to -21% during normoxia, as compared with -4% to -63% during hypoxia. The percentage change in heart rate was found to inversely correlate with the transcutaneous PO2-level prevailing when the reflex was elicited. The conclusion is that there is a significant reinforcement of the cardiorespiratory adjustments when the laryngeal reflex is activated during simultaneous excitation of the peripheral arterial chemoreceptors. One infant, showing a particularly strong increase of the cardiorespiratory response to laryngeal receptor stimulation during hypoxia, later died of sudden infant death syndrome.

Laryngeal chemoreflex: anatomic and physiologic study by use of the superior laryngeal nerve in the piglet

The laryngeal chemoreflex (LCR) was investigated in 21 piglets (ages 6 to 80 days old) with the use of physiologic and histologic techniques. The central projection of the superior laryngeal nerve (SLN) was determined in 14 animals by use of horseradish peroxidase-wheat germ agglutinin. Ipsilateral labeling of the solitary tract nucleus was seen. The caudal extent of the labeling varied with age. Sensory labeling of the nucleus ambiguus was present bilaterally in three younger animals and unilaterally in older piglets. Bilateral labeling of the nucleus dorsomedialis was seen in all ages. Apneic and cardiovascular response to water stimulation of laryngeal mucosa and the laryngeal adductor reflex (LAR) were examined in 16 piglets. Blunting of the apneic and cardiovascular response was seen after sacrifice of a single SLN. Hypoxia did not significantly affect the LAR or apnea duration in animals with only one intact SLN. A contralateral LAR was found in younger animals. Relevance to the LCR is also discussed.

Developments in pediatric neurolaryngology

Advances in diagnosis through the use of video and still photographs have made the recognition of pediatric laryngeal disorders much more accurate and precise. Open laryngeal surgery in children was felt to possibly impair growth and development. New procedures to correct structural congenital and acquired lesions in children have freed us from many of these reservations. One of the frontiers now being explored is neurogenic abnormalities within the pediatric larynx. At present, no uniform method of evaluation is being used other than observation of laryngeal motion. The use of electromyography in combination with still and motion photography should allow proper evaluation of the results of surgery or observation.