Reflex effect of esophageal distension on respiratory muscle activity and pressure

The Real Relevance of Nonacid Reflux in Pediatric Patients

Gastroesophageal reflux (GER) is one of the most common presenting complaints to pediatric gastroenterologists, and a wide variety of symptoms in children are potentially attributable to typical or atypical GER. While reflux diagnosis and treatment paradigms classically have focused on targeting acid, there is a growing recognition of the prevalence and significance of nonacid GER in both children and adults. This review explores the role of nonacid reflux in pediatric patients, including definitions, associations with symptoms, pathophysiologic mechanisms, and treatment implications.

Characterization and mechanisms of the supragastric belch in the cat

A response in which a belch occurs without gastric involvement, i.e., the supragastric belch (SGB), has been characterized in humans. The aims of this study were to determine whether animals have an SGB and, if so, to determine its mechanisms. Studies were conducted in decerebrate cats (n = 30) with electromyographic electrodes on hyoid, pharyngeal, esophageal, and diaphragm muscles. The effects of distending different regions of the esophagus in different manners using a balloon were quantified to determine the most appropriate stimulus for activating the cat SGB. The effects of esophageal perfusion of lidocaine (n = 3), vagus nerve transection (n = 3), or esophageal acidification (n = 5) on activation of the SGB were determined. Rapid large distensions of the thoracic esophagus best activated responses similar to the human SGB, i.e., rapid inhalation followed by a belch. The rapid inhalation was associated with activation of hiatal fibers and the belch with activation of dome fibers of the diaphragm. The rapid inhalation response was independent of the belch response. Lidocaine perfusion of the esophagus blocked the belch response without blocking the rapid inhalation, HCl perfusion sensitized the esophagus to activation of both the rapid inhalation and the belch response, and vagotomy blocked both responses. We conclude that the cat has an SGB that is composed of two independent reflex responses, i.e., rapid inhalation and belch, that are mediated by the vagus nerves and tension/mucosal receptors of the esophagus and sensitized by esophageal acid exposure. We hypothesize that the SGB is a learned voluntarily activated reflex response.NEW & NOTEWORTHY Rapid strong distension of the thoracic esophagus activates rapid inhalation followed by a belch, which is the sequence of responses that compose the human supragastric belch (SGB). The rapid inhalation and belch phases of the cat SGB are activated by hiatal and dome fibers of the diaphragm, respectively, and are mediated by the vagus nerves and tension/mucosal receptors of the esophagus and sensitized by esophageal acid exposure. There are many similarities between the cat and human SGB.

The caudal solitary complex is a site of central CO(2) chemoreception and integration of multiple systems that regulate expired CO(2)

The solitary complex is comprised of the nucleus tractus solitarius (NTS, sensory) and dorsal motor nucleus of the vagus (DMV, motor), which functions as an integrative center for neural control of multiple systems including the respiratory, cardiovascular and gastroesophageal systems. The caudal NTS-DMV is one of the several sites of central CO(2) chemoreception in the brain stem. CO(2) chemosensitive neurons are fully responsive to CO(2) at birth and their responsiveness seems to depend on pH-sensitive K(+) channels. In addition, chemosensitive neurons are highly sensitive to conditions such as hypoxia (e.g., neural plasticity) and hyperoxia (e.g., stimulation), suggesting they employ redox and nitrosative signaling mechanisms. Here we review the cellular and systems physiological evidence supporting our hypothesis that the caudal NTS-DMV is a site for integration of respiratory, cardiovascular and gastroesophageal systems that work together to eliminate CO(2) during acute and chronic respiratory acidosis to restore pH homeostasis.

Motor control of the diaphragm in anesthetized rabbits

Diaphragmatic regions are recruited in a specialized manner either as part of a central motor program during non-respiratory maneuvers, e.g. vomiting, or because of reflex responses, e.g. esophageal distension. Some studies in cats and dogs suggest that crural and costal diaphragm may be differentially activated also in response to respiratory stimuli from chemoreceptors or lung and chest wall mechanoreceptors. To verify whether this could occur also in other species, the EMG activity from the sternal, costoventral, costodorsal, and crural diaphragm was recorded in 42 anesthetized rabbits in response to various respiratory maneuvers, such as chemical stimulation, mechanical loading, lung volume and postural changes before and after vagotomy, or a non-respiratory maneuver such as esophageal distension. Regional activity was evaluated from timing of the raw EMG signal, and amplitude and shape of the moving average EMG. In all animals esophageal distension caused greater inhibition of the crural than sternal and costal diaphragm, whereas under all the other conditions differential diaphragmatic activation never occurred. These results indicate that in response to respiratory stimuli the rabbit diaphragm behaves as a single unit under the command of the central respiratory control system.

Comparison of the motor discharge to the crural and costal diaphragm in the rat

We compared the efferent innervation of the crural and costal regions of the mammalian diaphragm with regard to axonal motor discharge patterns and conduction speeds. Recordings were obtained from single crural (233) and costal (133) phrenic motoneurones. Median conduction speeds, calculated by spike triggered averaging (13.7 ms(-1) crural and 11.8 ms(-1) costal), and frequency histograms of conduction speed were not statistically significantly different between the two populations (p=0.27: Mann-Whitney test and p=0.9: Kolmogorov-Smirnov test, respectively). There was no difference in the proportions of inspiratory, post-inspiratory or non-respiratory units encountered in the crural and costal phrenic branches. Units that lacked respiratory rhythm did not express cardiac rhythm and were insensitive to ganglion blockade. In conclusion, there were few differences noted between the two motor pools and this may be related to the fact that the rat does not differentially regulate its diaphragm during swallowing and is not an emetic species.

Esophageal balloon dilation in children: prospective analysis of hemodynamic changes and complications during general anesthesia

STUDY OBJECTIVE

To investigate hemodynamic changes and complications in children during balloon dilation of esophageal strictures.

DESIGN

Prospective, controlled study.

SETTING

University teaching hospital.

PATIENTS

5 ASA physical status I and II pediatric patients with benign esophageal stricture related to ingestion of caustic substances.

INTERVENTIONS

Anesthesia was induced with intravenous propofol two mg/kg and cisatracurium 0.2 mg/kg and maintained with 66% nitrous oxide and one minimum alveolar concentration of sevoflurane in oxygen. In each session, balloon size was increased until the stricture was opened.

MEASUREMENTS AND RESULTS

A total of 18 sessions and 99 dilations in 5 children performed over a one-year period were included in the study. In 8 of 18 sessions, esophageal stricture was located in the middle one third of the esophagus; and in the others, in the upper one third. Four cases experienced bleeding; two cases, inability to ventilate due to obstruction of the endotracheal tube tip by the inflated balloon; and two cases, postextubation bronchospasm. In 95 of the 99 dilations, while the balloon was inflated, heart rate was faster and blood pressure increased significantly.

CONCLUSION

Anesthesiologists should keep in mind the possibility of hemodynamic instability and possible endotracheal tube tip obstruction by the inflated balloon and safeguard the airway against bleeding, secretions, and radio-opaque fluid during esophageal balloon dilation.

Acid and non-acid gastro-esophageal refluxes in children with chronic pulmonary diseases

OBJECTIVES

Acid gastro-esophageal reflux has been shown associated with chronic pulmonary diseases. The role of non-acid refluxes in these children is still unknown. Therefore we investigated the prevalence of acid and non-acid refluxes, and their association with desaturations, in children with chronic pulmonary symptoms.

METHODS

In 25 children aged 6 months to 15 years with unexplained chronic cough, wheeze or sputum production, refluxes were assessed by 24h-multiple intra-esophageal impedance measurements, simultaneous pH metry and continuous recording of oxygen saturation.

RESULTS

pH in the proximal and distal esophagus as well as six impedance channels were evaluated in all subjects. A mean of 129.4 refluxes per day per patient was detected. Complete and technically usable readings of oxygen saturation were obtained in 14 children. In this group the subjects had a mean of 112.6 refluxes and 92.6 desaturations per day per patient. The symptom index and symptom sensitivity index for acid refluxes were 34.7% and 24.6%, respectively, for non-acid refluxes 3.0% and 66.7%, respectively.

CONCLUSIONS

We found a high prevalence of acid reflux and a very low number of non-acid refluxes in this population. The symptom index was negative for all types of reflux, whereas the symptom sensitivity index was positive for both acid and non-acid reflux. Our data support a relation between acid gastro-esophageal refluxes and chronic pulmonary symptoms; however, this study does not support a role of non-acid reflux in children with respiratory symptoms, which are not on antacid medication.

The importance of multichannel intraluminal impedance in the evaluation of children with persistent respiratory symptoms

OBJECTIVES

Previous evidence suggests an association between gastroesophageal reflux disease and chronic respiratory disease in children. Despite antisecretory antacid therapy, respiratory symptoms often persist supporting a role for nonacid reflux. The aim of this study was to determine whether nonacid reflux occurs in children with chronic respiratory disease.

METHODS

Twenty-eight children (mean age: 6.5 +/- 5.6 yr) with persistent respiratory symptoms on antacid medications underwent 24 h pH/multichannel intraluminal impedance (pH/MII) recording. The symptom index (SI) and the symptom sensitivity index (SSI) were calculated for each patient. Logistic regression was performed to determine which reflux characteristics were associated with a high degree of symptom correlation present during the occurrence of symptoms.

RESULTS

A total of 1,822 reflux episodes were detected by pH/MII, 45% of which were nonacidic. The mean SI increased using pH/MII (35.7 +/- 28.5) compared to pH probe alone (14.6 +/- 18.9; p= 0.002); no differences in the mean SSI using pH/MII compared to pH probe alone were identified. Significantly more patients had a positive SI using pH/MII than pH probe alone (p= 0.035); there was no difference in the number of patients with a positive SSI using pH/MII compared to pH probe alone. Multivariate analysis revealed that symptoms occurred more frequently when the reflux was nonacidic, mixed, and full column. Also, younger children were more likely to have the simultaneous occurrence of symptoms and reflux.

CONCLUSIONS

Nonacid reflux may be an important predictor of respiratory symptoms. pH/MII provided important information in the evaluation of children with intractable respiratory symptoms.

Expiratory pharyngeal airway obstruction during sleep: a multiple element model

OBJECTIVE/HYPOTHESIS

In patients with obstructive sleep apnea and snoring, airway obstruction during sleep is not limited to inspiration but may also occur with expiration. The aim of this study was to assess the segmental mechanics of expiratory obstruction.

DESIGN

Experimental study of a convenience sample of 20 patients with snoring and mild obstructive sleep apnea.

METHODS

During sedated sleep, airflow, airway pressure measurements (supraglottic, oropharyngeal, nasopharyngeal, and nasal mask), and either supraglottic/retroglossal or retropalatal areas were simultaneously measured. Nasal continuous positive airway pressures were experimentally adjusted during single breath tests (SBTs) to modify upper airway size. Airway mechanics were evaluated during pressure drops on expiration.

RESULTS

The predominant level of expiratory obstruction was supraglottic/retroglossal level alone (65%) or combined supraglottic/retroglossal and retropalatal (17.6%). In nonobstructed SBTs, compliance curves derived from supraglottic/retroglossal and retropalatal pressures were similar but diverged in obstructed breaths. Compliance during expiration was greater in the supraglottic/retroglossal segment compared to the retropalatal segment. Retropalatal cross-sectional size was smaller during early and late expiration on obstructed than on nonobstructed breaths independent of airway pressure measures. The rate of expiratory collapse was increased at all time points measured (P <.005) in the retropalatal segment on obstructed as compared with nonobstructed breaths.

CONCLUSIONS

During expiration, the supraglottic/retroglossal level is obstructed more frequently and has greater compliance than the retropalatal segment. Failure of upstream pressures to describe pharyngeal obstruction supports a multi-element model of collapse. Segments interact during expiration, with increased retropalatal collapse on obstructed as compared with nonobstructed breaths. Increased collapse on expiration provides a mechanism for increased obstruction on subsequent inspiratory breaths.

Convergent pathways for cardiac- and esophageal-somatic motor reflexes in rats

Chest pain of esophageal and cardiac origin is often difficult to distinguish due to similar sensations and localization. We have shown that spasm-like contractions of the spinotrapezius muscles evoked by noxious cardiac stimulation could potentially sensitize muscle afferent fibers and produce angina-like referred pain. In this study, we proposed that a similar type of spinotrapezius contraction evoked by esophageal stimulation could produce nociceptive responses with similar quality and localization as evoked by cardiac stimulation. An objective of this study was to show convergence of pathways to the spinotrapezius muscles by measuring electromyographic (EMG) activity between the cardiac- and esophageal-motor reflexes. We also investigated afferent pathways of esophageal-motor reflexes by disrupting or activating the left sympathetic chain and vagus nerves; these pathways form the afferent limbs of the cardiac-motor reflexes. Results showed that more than 95% of animals responding to noxious cardiac stimulation also responded to esophageal distension. Transection of the left sympathetic chain to reduce upper thoracic visceral afferent innervation significantly decreased cardiac-evoked EMG activity or total motor unit potentials (t-MUP). In contrast, however, the transection did not significantly decrease t-MUP evoked by esophageal distension. Bilateral vagotomy and vagal afferent stimulation increased and decreased the cardiac-evoked t-MUP, respectively. However, the same vagal manipulations did not influence t-MUP evoked by esophageal distension. This study demonstrated that the spinotrapezius muscle could be activated by noxious stimulation of two different visceral organs. The spinotrapezius muscle contractions evoked by esophageal distension are produced in part by activation of esophageal afferent fibers found in upper thoracic sympathetic nerves, but not by activation of the vagus nerves.

Upper thoracic respiratory interneurons integrate noxious somatic and visceral information in rats

The aim of this study was to determine if thoracic respiratory interneurons (TRINs) might receive peripheral noxious somatic and visceral inputs. Extracellular potentials of 78 respiration-related T(3) neurons, whose activity was driven by central respiratory output, were recorded from the intermediate zone in pentobarbital anesthetized, paralyzed, and ventilated male rats. These neurons were identified as interneurons by their locations and by the absence of antidromic activation from the cervical sympathetic trunk and cerebellum. Thoracic esophageal distension (ED) was produced by water inflation of a latex balloon (0.1-0.5 ml, 20 s). A catheter was placed in the pericardial sac to administer 0.2 ml bradykinin (10(-5) M) for noxious cardiac stimulation. Of 78 TRINs examined for ED, activity of 24 TRINs increased and activity of 8 TRINs decreased. Intrapericardial bradykinin increased activity in 26/65 TRINs tested and decreased activity in 5 TRINs. Seventy-four TRINs were tested for effects of brush, pressure, and pinch of the chest and upper back areas. No TRINs responded to brushing hair. Low-threshold responses to pressure were observed in 27 TRINs. Fourteen TRINs were wide dynamic range and 4 TRINs had high-threshold responses. Peripheral stimuli affected all types of TRINs, including inspiratory, expiratory, and biphasic neurons. Simultaneous phrenic recordings showed that effects of various somatic and visceral stimuli on TRINs were independent of central respiratory drive. Various somatovisceral and viscerovisceral patterns of input were observed in TRINs. The results suggested that TRINs participate in intraspinal processing and integration of nociceptive information from somatic fields and visceral organs.

The diaphragm: two physiological muscles in one

To the respiratory physiologist or anatomist the diaphragm muscle is of course the prime mover of tidal air. However, gastrointestinal physiologists are becoming increasingly aware of the value of this muscle in helping to stop gastric contents from refluxing into the oesophagus. The diaphragm should be viewed as two distinct muscles, crural and costal, which act in synchrony throughout respiration. However, the activities of these two muscular regions can diverge during certain events such as swallowing and emesis. In addition, transient crural muscle relaxations herald the onset of spontaneous acid reflux episodes. Studying the motor control of this muscular barrier may help elucidate the mechanism of these episodes. In the rat, the phrenic nerve divides into three branches before entering the diaphragm, and it is possible to sample single neuronal activity from the crural and costal branches. This review will discuss our recent findings with regard to the type of motor axons running in the phrenic nerve of the rat. In addition, we will outline our ongoing search for homologous structures in basal vertebrate groups. In particular, the pipid frogs (e.g. the African clawed frog, Xenopus laevis) possess a muscular band around the oesophagus that appears to be homologous to the mammalian crural diaphragm. This structure does not appear to interact directly with the respiratory apparatus, and could suggest a role for this region of the diaphragm, which was not originally respiratory.

Evidence for a peripheral mechanism of esophagocrural diaphragm inhibitory reflex in cats

The esophagogastric junction (EGJ) is guarded by two sphincters, a smooth muscle lower esophageal sphincter (LES) and a skeletal muscle crural diaphragm. These two sphincters relax simultaneously under certain physiological conditions, i.e., swallowing, belching, vomiting, transient LES relaxation, and esophageal distension. Esophageal distension-induced crural diaphragm relaxation is mediated through vagal afferents that are thought to exert inhibitory influence on the central mechanism (brain stem) of crural diaphragm contraction. We conducted studies in 10 cats to determine whether a mechanism of crural diaphragm relaxation was located at the level of the neuromuscular junction and/or muscle. Stimulation of the crural diaphragm neuromuscular junction through 1) the electrodes implanted in the muscle and 2) the bilateral phrenic nerve resulted in an increase in EGJ pressure. Nicotinic receptor blockade (pancuronium, 0.2 mg/kg) abolished the EGJ pressure increase caused by electrical stimulation of the neuromuscular junction. Esophageal distension and bolus-induced secondary esophageal peristalsis caused relaxation of the EGJ during the stimulation of the neuromuscular junction. Bilateral phrenicotomy and vagotomy had no influence on this relaxation. These data suggest the existence of a peripheral mechanism of crural diaphragm inhibition. This peripheral inhibitory mechanism may reside at the level of either the neuromuscular junction or the skeletal muscle.

Vagovagal reflex motility patterns of the rat esophagus

Esophageal reflex motility and its neural correlates were investigated in 94 urethan-anesthetized adult male albino rats. When distended by means of a stationary balloon, the cervical and thoracic esophageal portion responded with a single pressure wave (type I response), whereas the diaphragmatic (intercrural) segment exhibited rhythmic contractions (type II response). Balloon deflation resulted in an off response aboral to the balloon. Bilateral cervical vagotomy or systemic D-tubocurarine abolished all types of reflex responses. Both type I and type II responses were associated with multiunit discharges in the central subnucleus of the solitary tract complex (NTSC) and the compact formation of the nucleus ambiguus (AMBC). Type I discharges, consisting of single bursts, and type II discharges, consisting of rhythmic 0.6-Hz bursts, preceded intraesophageal pressure waves in a fixed phase relationship, persisted after contralateral vagotomy, and were eliminated by ipsilateral vagotomy. During neuromuscular paralysis, peak intraburst discharge rates were reduced in both the NTSC and AMBC, with a concomitant decrease in rhythmicity. It is concluded that bolusevoked peristalsis of the rat esophagus is 1) segmentally organized; 2) effected by a bilateral uncrossed reflex arc consisting of vagal viscerosensory, NTSC premotor, and AMBC motoneurons innervating the striated muscle tunic and 3) strongly facilitated by reafferent feedback.

Southwestern Internal Medicine Conference: preoperative pulmonary evaluation

The purpose of preoperative pulmonary assessment is to predict which patients are at greater risk of pulmonary complications, under which circumstances such complications may occur, and whether surgery should be denied based on that risk. In this article, the author addresses the following major issues in preoperative pulmonary assessment: 1) the risk of pulmonary complications in relation to the type of surgical procedure; 2) the value of preoperative pulmonary function testing, including when such testing should be performed and how the results should be used; and 3) guidelines for assessment of those patients about to undergo resectional surgery of the lung.

Vagal afferent dysfunction in naturally occurring canine esophageal motility disorder

Few studies have examined the vagal afferent innervation of the esophagus in naturally occurring esophageal motility disorders. The present study assessed the integrity of distension-sensitive vagal afferents innervating the esophagus in naturally occurring canine megaesophagus. In the dog, esophageal distension induces reflex inhibition of crural diaphragm electromyographic activity that is mediated by vagal afferents innervating esophageal mechanoreceptors. This reflex was measured during stepwise esophageal distension in six dogs with congenital idiopathic megaesophagus, two dogs with megaesophagus secondary to esophageal striated muscle disease, and eight matched controls. In contrast to control dogs, inhibition of crural electromyographic activity was not observed in megaesophagus dogs with esophageal distension within the control volume range. With esophageal distensions far in excess of the control volume range, inhibition of crural electromyographic activity was not observed in five of six dogs with congenital idiopathic megaesophagus, while crural inhibition was observed in the two dogs with secondary megaesophagus. These findings indicate that a defect is present in the vagal afferent innervation to the esophagus in a majority of dogs with congenital idiopathic megaesophagus.

Distribution of motor activity to expiratory muscles during sciatic nerve stimulation in the dog

The present study examined the effect of increasing sensory input from the lower limbs (assessed from the response to electrical stimulation of the sciatic nerve) on the distribution of electrical activity to the expiratory muscles. Expiratory muscle response to sciatic nerve stimulation (SNS) was compared to the response of the inspiratory muscles, and to their response to hypercapnia. In 16 anesthetized dogs afferent SNS increased ventilation and augmented the integrated EMG of all six expiratory muscles studied. Increases in abdominal muscle electrical activity were not uniform, being greater in the transversus abdominis and external oblique as compared to the internal oblique and the rectus abdominis. Increases in thoracic expiratory and inspiratory muscle activity during SNS were similar in magnitude. SNS performed while dogs were breathing 7% CO2, produced increased neural activity similar to those observed during O2 breathing. During CO2 rebreathing, at equal levels of minute ventilation, expiratory muscle responses to SNS and to CO2 were similar. In contrast, the rate of rise of the inspiratory muscle EMGs was greater during SNS. The present study indicates that the abdominal muscles participate in the respiratory response to afferent neural drive from skeletal muscles. The magnitude of their response is independent of their pre-stimulation level of activity and is similar to that observed during CO2 stimulated breathing.