Effects of cooling on laryngeal reflexes in the dog

Expiratory braking defines the breathing patterns of asphyxiated neonates during therapeutic hypothermia

Introduction

Although neonatal breathing patterns vary after perinatal asphyxia, whether they change during therapeutic hypothermia (TH) remains unclear. We characterized breathing patterns in infants during TH for hypoxic-ischemic encephalopathy (HIE) and normothermia after rewarming.

Methods

In seventeen spontaneously breathing infants receiving TH for HIE and in three who did not receive TH, we analyzed respiratory flow and esophageal pressure tracings for respiratory timing variables, pulmonary mechanics and respiratory effort. Breaths were classified as braked (inspiratory:expiratory ratio ≥1.5) and unbraked (<1.5).

Results

According to the expiratory flow shape braked breaths were chategorized into early peak expiratory flow, late peak expiratory flow, slow flow, and post-inspiratory hold flow (PiHF). The most braked breaths had lower rates, larger tidal volume but lower minute ventilation, inspiratory airway resistance and respiratory effort, except for the PiHF, which had higher resistance and respiratory effort. The braked pattern predominated during TH, but not during normothermia or in the uncooled infants.

Conclusions

We speculate that during TH for HIE low respiratory rates favor neonatal braked breathing to preserve lung volume. Given the generally low respiratory effort, it seems reasonable to leave spontaneous breathing unassisted. However, if the PiHF pattern predominates, ventilatory support may be required.

Outdoor Cold Air Versus Room Temperature Exposure for Croup Symptoms: A Randomized Controlled Trial

OBJECTIVES

Croup is the most common cause of acute upper airway obstruction in children. The benefits of treating croup with steroids are well established, with an onset of effect 30 minutes after administration. We investigated whether a 30-minute exposure to outdoor cold air might improve mild to moderate croup symptoms before the onset of action of steroids.

METHODS

This open-label, single-center, randomized controlled trial, enrolled children aged 3 months to 10 years with croup and a Westley Croup Score (WCS) ≥2 attending a tertiary pediatric emergency department. Participants were randomized (1:1) to either a 30-minute exposure to outdoor cold (<10°C) atmospheric air or to indoor ambient room air immediately after triage and administration of a single-dose oral dexamethasone. The primary endpoint was a decrease in WCS ≥2 points from baseline at 30 minutes. Analyses were intention to treat.

RESULTS

A total of 118 participants were randomly assigned to be exposed to outdoor cold air (n = 59) or indoor room temperature (n = 59). Twenty-nine of 59 children (49.2%) in the outdoor group and 14 of 59 (23.7%) in the indoor group showed a decrease in WCS ≥2 points from baseline at 30 minutes after triage (risk difference 25.4% [95% confidence interval 7.0-43.9], P = .007). Patients with moderate croup benefited the most from the intervention at 30 minutes (risk difference 46.1% [20.6-71.5], P < .001).

CONCLUSIONS

A 30-minute exposure to outdoor cold air (<10°C), as an adjunct to oral dexamethasone, is beneficial for reducing the intensity of clinical symptoms in children with croup, especially when moderate.

[Stridor in neonates with hypoxic-ischaemic encephalopathy subject to selective cerebral or whole body hypothermia]

INTRODUCTION

Hypothermia treatment has improved the prognosis of asphyxiated neonates. Widely used, it has demonstrated to be safe without severe side effects. The aim of this article is to review the incidence of stridor amongst asphyxiated newborns treated with hypothermia in our unit.

METHODS

Retrospective chart review of our patients.

RESULTS

Stridor was presented in 7/75 (9.3%) of patients during hypothermia. Three received whole body hypothermia, 3 selective cerebral, and in one case both techniques were used. All cases required increased respiratory support.

CONCLUSIONS

Different mechanisms may be responsible for the appearance of stridor in patients with hypoxic-ischaemic encephalopathy (HIE). In our series the incidence of stridor was similar for the two hypothermia devices. To better understand these possible side effects of hypothermia, further prospective studies (which should include laryngoscopy) are needed.

Effect of cold air inhalation and isometric exercise on coronary blood flow and myocardial function in humans

The effects of cold air inhalation and isometric exercise on coronary blood flow are currently unknown, despite the fact that both cold air and acute exertion trigger angina in clinical populations. In this study, we used transthoracic Doppler echocardiography to measure coronary blood flow velocity (CBV; left anterior descending coronary artery) and myocardial function during cold air inhalation and handgrip exercise. Ten young healthy subjects underwent the following protocols: 5 min of inhaling cold air (cold air protocol), 5 min of inhaling thermoneutral air (sham protocol), 2 min of isometric handgrip at 30% of maximal voluntary contraction (grip protocol), and 5 min of isometric handgrip at 30% maximal voluntary contraction while breathing cold air (cold + grip protocol). Heart rate, blood pressure, inspired air temperature, CBV, myocardial function (tissue Doppler imaging), O(2) saturation, and pulmonary function were measured. The rate-pressure product (RPP) was used as an index of myocardial O(2) demand, whereas CBV was used as an index of myocardial O(2) supply. Compared with the sham protocol, the cold air protocol caused a significantly higher RPP, but there was a significant reduction in CBV. The cold + grip protocol caused a significantly greater increase in RPP compared with the grip protocol (P = 0.045), but the increase in CBV was significantly less (P = 0.039). However, myocardial function was not impaired during the cold + grip protocol relative to the grip protocol alone. Collectively, these data indicate that there is a supply-demand mismatch in the coronary vascular bed when cold ambient air is breathed during acute exertion but myocardial function is preserved, suggesting an adequate redistribution of blood flow.

Effects of C fiber blockade on cardiorespiratory responses to laryngeal stimulation in concious lambs

The primary aim of the study was to explore cardiorespiratory reflexes originating from laryngeal C fiber endings in the neonatal period. Seventeen lambs were instrumented for recording glottal adductor and diaphragm EMG, heart rate, systemic arterial pressure and respiratory movements. C fiber blockade was induced in eight lambs by 30 mg/kg capsaicin, the remaining nine lambs serving as controls. Cardiorespiratory reflexes were induced in non-sedated lambs by flowing air, menthol or 13% CO2, or by injecting water or 50 microg capsaicin in the laryngeal inlet through an endoscope. Responses to all stimuli but capsaicin were similar between the two groups. While cardiorespiratory responses were induced by capsaicin in control lambs, the responses were significantly inhibited in lambs with C fiber blockade. We conclude that laryngeal C fiber endings are functional and responsible for laryngeal chemoreflexes in newborn lambs.

Effect of recurrent laryngeal nerve paralysis on superior laryngeal nerve afferents during evoked vocalization

We tested the hypothesis that vocal fold paresis leads to a substantial reduction in activity from the internal branch of the superior laryngeal nerve (iSLN) during respiration and evoked vocalization. The iSLN afferent activity was measured before and after recurrent laryngeal nerve paresis by cold block in decerebrate cats during spontaneous respiration and electrically evoked vocalization. Response rate patterns of 33 iSLN single units from 11 cats were categorized into 5 groups based on responses to vocalization. Only 24% of single units during spontaneous respiration and 18% during evoked phonation displayed activity pattern changes as a result of paresis. Those fibers affected were heterogeneous in discharge pattern, but none of the units that followed voice frequency lost this characteristic when the motor nerve was cooled. The relative insensitivity of iSLN activity to motor paralysis suggests that the receptors studied are coupled to tissue such that passive interaction rather than active muscular contraction is the major stimulus.

Response of breathing pattern to flow and pressure in the upper airway of rats

The effects of upper airway (UAW) flows and pressures on breathing pattern and respiratory muscle activities were studied in anesthetized rats breathing through a tracheostomy. A steady flow (approximately 1000 ml/kg/min) of cold dry air, or cold wet air, or warm wet air was passed through the UAW, in the expiratory direction for approximately 20 sec (20-40 sec). In other trials positive or negative pressure was applied to the isolated UAW for a similar duration. There was a marked prolongation of the expiratory duration and decreases in peak inspiratory flow, tidal volume, and peak diaphragm electromyogram (EMG) activity in response to cold dry airflow. The responses to cold wet air were reduced but still significant. Warm wet air had no effect on breathing. These responses show that UAW cooling and drying depress breathing in the rat and that cooling itself could cause the inhibition of breathing. Negative pressure induced substantial increases in genioglossus and laryngeal inspiratory activity while positive pressure caused a decrease in genioglossus activity. Positive pressure also increased expiratory time while negative pressure increased inspiratory time. These results confirm the functional role of the UAW dilating muscles in preventing UAW from collapse in rats.

Responses of neurons in the lamb nucleus tractus solitarius to stimulation of the caudal oral cavity and epiglottis with different stimulus modalities

Receptors located in the posterior oral cavity and on the epiglottis play an important role in the initiation of upper airway reflexes such as swallowing, gagging, coughing and apnea. Peripheral nerves which innervate these receptor areas terminate in the nucleus tractus solitarius (NTS). We have recorded the responses of 61 neurons in the lamb NTS to stimulation of the caudal tongue, palate and epiglottis with mechanical, chemical and thermal stimuli and mapped receptive field location. Although there was some overlap in the areas of the NTS from which neurons with oral cavity and epiglottal receptive fields could be recorded, a significant difference was observed in the mean recording sites of the two groups of neurons. Neurons with oral cavity receptive fields were located more rostral, lateral and ventral in the NTS than neurons with receptive fields on the epiglottis. Little convergence of sensory input onto single cells in the NTS was observed between the oral cavity and the epiglottis. Only one NTS neuron had a receptive field in both of these receptor areas. In contrast, a large number of neurons with oral cavity receptive fields received input from two receptor areas. These neurons had a receptive field on the tongue which was located directly beneath the receptive field on the palate. Mechanical stimuli were the most effective for neurons with either oral cavity or epiglottal receptive fields and thermal stimuli were the least effective. Neurons which responded to mechanical stimuli responded better to a moving stimulus than to a punctate one, and large increases in the strength of a punctate stimulus were required to elicit significant increases in response frequency. Most NTS neurons responded to more than one of the stimulus modalities. However, a significant difference in the mean number of stimulus modalities which elicited responses was observed between neurons with oral cavity and epiglottal receptive fields. The number of multimodal neurons with epiglottal receptive fields was higher than those with oral cavity receptive fields. The multimodal nature of neurons which responded to epiglottal or oral cavity stimulation combined with their location in reflexogenic areas of the NTS suggests that these neurons could be important in the integration of afferent input from the oral cavity and upper airway. If these NTS neurons are involved in the control of oral and upper airway reflexes it would be important for them to respond to as many of the stimulus cues as possible and the majority of these neurons do just that.

Laryngeal paralysis on receptor and reflex responses to negative pressure in the upper airway

Mechanoreceptors affected by changes in transmural pressure and/or contraction of intrinsic muscles are present in the larynx. This study is designed to test the hypothesis that laryngeal paralysis alters laryngeal mechanoreceptor and reflex responses to collapsing pressure. Experiments were carried out on anesthetized, spontaneously breathing dogs. The activity of 65 mechanoreceptors was recorded from the superior laryngeal nerve during upper airway occlusion before and during laryngeal paralysis induced by cold block of both recurrent laryngeal nerves (RLN). Esophageal and upper airway pressures were also recorded. Thirty-three laryngeal mechanoreceptors stimulated by negative pressure decreased their inspiratory activity during upper airway occlusion from 48.1 to 30.4 imp/sec when the RLNs were blocked. In contrast, 21 inspiratory modulated mechanoreceptors inhibited by negative pressure and 11 responding only to negative pressure did not change their activity during RLN block. The effect of laryngeal paralysis on the cricothyroid muscle response to negative pressure was assessed in 7 dogs and found to be minimal. These results suggest that 'drive' receptors stimulated by negative pressure do not play a significant role in the reflex activation of upper airway dilating muscles.

Characteristics of laryngeal cold receptors

This study was designed to further characterize the properties of previously described laryngeal cold receptors (Respir. Physiol. 59:35, 1985). Single unit action potentials were recorded from the internal branch of the superior laryngeal nerve (SLN) in anesthetized, spontaneously breathing dogs. The nervous conduction of fibers originating from 12 laryngeal cold receptors was blocked at a mean (+/- SE) temperature of 18.8 +/- 0.7 degrees C. Twelve receptors were localized on the edge of the vocal folds in correspondence of the vocal process of the arytenoid cartilage. Topical anesthesia (2% lidocaine) blocked their activity within 4-18 sec, suggesting a superficial location. Paralysis of the vocal folds during spontaneous breathing through the upper airway did not alter the activity of 9 of 13 cold receptors. On the other hand, 7 of 12 cold receptors tested with constant flow showed respiratory modulation and laryngeal paralysis abolished the modulation of 3 of these tested with a constant flow of air. During progressive cooling in a stepwise fashion, as in frigid air breathing, laryngeal cold receptors maintained a phasic discharge. Our results indicate that these endings are particularly suited for detecting changes in temperature.

Breathing pattern in rats with chronic section of the superior laryngeal nerves

In a first set of experiments we registered the integrated afferent activity of the superior laryngeal nerve (SLN) in adult anaesthetized rats. The activity increased with positive upper airway pressure (Pua); with progressively more negative Pua, the SLN activity at first declined then increased again. A second set of adult rats underwent bilateral section of the SLN (SLN denervated) or a sham operation (controls). Both groups appeared to recover promptly from the operation and 6 days later their resting breathing pattern was recorded by the barometric method. SLN denervated rats had a shorter inspiratory time (80%), hence higher frequency and mean inspiratory flow, than controls. During hypoxia (10 min at 10% O2) both groups hyperventilated with an almost identical pattern. The rats were then again anaesthetized and the right vagus cut in an attempt to reduce the afferent component from the lower airways, which may have masked the SLN regulatory contribution. One week after this second operation both SLN denervated and controls breathed more deeply and slowly than before vagotomy, but the pattern was not significantly different between the two groups, either in normoxia or hypoxia. Finally, the rats were anaesthetized and integrated diaphragm activity recorded during spontaneous breathing and the first effort against closure of the nostrils. With both vagi cut, the duration of the occluded effort was slightly longer in SLN denervated than in controls. These results suggest that in adult awake rats laryngeal afferent activity tends to decrease mean inspiratory flow. However, this regulatory contribution is small during eupnea and insignificant during hypoxic hyperventilation.

Effects of airway cooling on tracheal stretch receptors

We studied the effect of cold air on tracheal slowly adapting stretch receptors (SAR) in 6 anesthetized, spontaneously breathing dogs. Air at constant flow and two different temperatures was passed through an isolated segment of the extrathoracic trachea. We recorded SAR action potentials, esophageal pressure, tracheal pressure and temperature. With a reduction in tracheal temperature of approximately 10 degrees C the steady state response of 30 SARs to a distending pressure of 1.0 kPa decreased to 75% of control (P less than 0.001). At lower distending pressure the inhibitory effect of tracheal cooling decreased: 87% of control at 0.5 kPa (P less than 0.05, n = 8) and 96% of control at 0.2 kPa (P greater than 0.05, n = 8). The response of 13 tracheal SARs to sinusoidal pressure oscillations (0.15 kPa) superimposed on a bias pressure (0.5 kPa) was reduced (P less than 0.01) by local cooling to the same extent at the 2 pressure extremes ('peak' value = 71% of control; 'valley' = 67% of control), resulting in a similar change in receptor discharge within the oscillatory cycle. The inhibitory effect of airway cooling on stretch receptors may play a role in cold-induced bronchoconstriction.