The effect of body position on esophageal reflexes in cats: a possible mechanism of SIDS?

Abstract

BackgroundIt has been hypothesized that life-threatening events are caused by supraesophageal reflux (SER) of gastric contents that activates laryngeal chemoreflex-stimulated apnea. Placing infants supine decreases the risk of sudden infant death syndrome (SIDS). The aim of this study was to determine whether body position affects esophageal reflexes that control SER.MethodsWe instrumented the pharyngeal and esophageal muscles of decerebrate cats (N=14) to record EMG or manometry, and investigated the effects of body position on the esophago-upper esophageal sphincter (UES) contractile reflex (EUCR), esophago-UES relaxation reflex (EURR), esophagus-stimulated pharyngeal swallow response (EPSR), secondary peristalsis (SP), and pharyngeal swallow (PS). EPSR, EUCR, and SP were activated by balloon distension, EURR by air pulse, and PS by nasopharyngeal water injection. The esophagus was stimulated in the cervical, proximal thoracic, and distal thoracic regions. The threshold stimulus for activation of EUCR, EURR, and PS, and the chance of activation of EPSR and SP were quantified.ResultsWe found that only EPSR was significantly more sensitive in the supine vs. prone position regardless of the stimulus or the position of the stimulus in the esophagus.ConclusionWe hypothesize that the EPSR may contribute to the protection of infants from SIDS by placement in the supine position.

Figure 1. Types of stimuli applied and possible responses

The possible responses to balloon and air stimulation of the esophagus and water stimulation of the nasopharynx are illustrated. The balloon stimulations were in the form of a ramp, slow square wave, or rapid square wave. The ramp stimulus activated esophago-UES contractile reflex and EPSR. The slow square wave stimulus activated upper esophageal sphincter and secondary peristalsis. The rapid square wave stimulus activated esophago-UES relaxation reflex and EPSR. The air pulse stimulation activated esophago-UES relaxation reflex and EPSR. The nasopharyngeal stimulation activated PS. TH, thyrohyoideus; TP, thyropharyngeus; CP, cricopharyngeus; EMG, electromyography; E#, esophagus #cm from the CP; Mano, manometry; prox, proximal; dist, distal; Stim, stimulus artifact; EPSR, esophagus stimulated pharyngeal swallow response; PS, pharyngeal swallow; SP, secondary peristalsis; EUCR, esophago-UES contractile response; EURR, esophago–UES relaxation response; UES, upper esophageal sphincter.

Figure 2. Effects of body position on threshold for activation of esophago-upper esophageal sphincter contractile reflex at different esophageal locations using ramp (a), slow square wave (b) and rapid square wave (c) balloon distension

The thresholds for activation of upper esophageal sphincter by ramp or rapid square wave stimulation applied across the esophagus did not significantly differ between prone and supine body position. However, placement in the supine vs prone position significantly reduced (P = 0.044) the threshold stimuli needed to activate the upper esophageal sphincter by slow square wave stimulation across the esophagus. Cerv, cervical esophagus; prox; proximal; thor, thoracic.

Figure 3. Effects of body position on threshold for activation of esophago-upper esophageal sphincter relaxation reflex at different esophageal locations using air pulse (a) and proportion of esophago-UES relaxation reflex activated using rapid square wave (b) balloon stimulation

Activation of the esophago-UES relaxation reflex was not significantly affected by body position regardless of the stimulus or esophageal location. See Fig 2 for definition of symbols.

Figure 4. Effects of body position on the activation of the EPSR using slow square wave balloon distension

Placing the animal in the supine position resulted in slow square wave balloon distension of the cervical esophagus to activate EPSR as well as upper esophageal sphincter, whereas the same stimuli in the prone position only activated upper esophageal sphincter. See Figure 1 for definition of symbols.

Figure 5. Effects of body position on the proportion of stimuli activating the EPSR at different esophageal locations using ramp (a), slow square wave (b) and rapid square wave (c) balloon distension, and air pulse (d) stimulation

Activation of EPSR by all stimuli and at all regions of the esophagus was significantly (P < 0.05) affected by body position. The supine position caused over a 100% increase in the proportion of stimuli that activated EPSR. EPSR, esophagus-stimulated pharyngeal swallow response. See Figure 1 for definition of symbols.

Figure 6. Effects of body position on the proportion of stimuli activating the secondary peristalsis at different esophageal locations using ramp (a) , slow square wave (b) and rapid square wave (c) balloon distension, and air pulse (d) stimulation

Body position had no significant effect on activation of secondary peristalsis regardless of stimulus or esophageal location. . See Figure 1 for definition of symbols