Inhibitory effects of hyperthermia on mechanisms involved in autoresuscitation from hypoxic apnea in mice: a model for thermal stress causing SIDS

Hyperthermia: Is it always an accidental death?

INTRODUCTION

The research aimed to determine individual variations in different core temperature measurements before the experiment, after submersion, after 20 min of exposure for heat stroke.

METHODS

Rats were divided into three groups depending on the temperature and length of exposure to water: CG, G41-20 and G41-UD. The protocol was made according to the earlier described methodology of heat shock induction.

RESULTS

A significant difference was observed in the G41-UD group; p < 0.0005. The lowest body temperature of the rats was observed, from normothermia, and the highest temperature after death, 37.87 ± 0.62 °C vs 41.20 ± 0.76 °C, the difference between all three groups is p < 0.0005.

CONCLUSION

Exposure of Wistar rats to water temperatures in the CG and G41 groups led to a significant change in core temperature. In the control group, the thermoregulatory mechanism firmly established normothermia, while hyperthermia was revealed in the G41 group during the 20-minute exposure.

Hyperthermia and Heat Stress as Risk Factors for Sudden Infant Death Syndrome: A Narrative Review

BACKGROUND AND OBJECTIVES

Heat stress and hyperthermia are common findings in sudden infant death syndrome (SIDS) victims. It has been suggested that thermal stress can increase the risk of SIDS directly via lethal hyperthermia or indirectly by altering autonomic functions. Major changes in sleep, thermoregulation, cardiovascular function, and the emergence of circadian functions occur at the age at which the risk of SIDS peaks-explaining the greater vulnerability at this stage of development. Here, we review the literature data on (i) heat stress and hyperthermia as direct risk factors for SIDS, and (ii) the indirect effects of thermal loads on vital physiological functions.

RESULTS

Various situations leading to thermal stress (i.e., outdoors temperatures, thermal insulation from clothing and bedding, the prone position, bed-sharing, and head covering) have been analyzed. Hyperthermia mainly results from excessive clothing and bedding insulation with regard to the ambient thermal conditions. The appropriate amount of clothing and bedding thermal insulation for homeothermia requires further research. The prone position and bed-sharing do not have major thermal impacts; the elevated risk of SIDS in these situations cannot be explained solely by thermal factors. Special attention should be given to brain overheating because of the head's major role in body heat losses, heat production, and autonomic functions. Thermal stress can alter cardiovascular and respiratory functions, which in turn can lead to life-threatening events (e.g., bradycardia, apnea with blood desaturation, and glottal closure). Unfortunately, thermal load impairs the responses to these challenges by reducing chemosensitivity, arousability, and autoresuscitation. As a result, thermal load (even when not lethal directly) can interact detrimentally with vital physiological functions.

CONCLUSIONS

With the exception of excessive thermal insulation (which can lead to lethal hyperthermia), the major risk factors for SIDS appears to be associated with impairments of vital physiological functions when the infant is exposed to thermal stress.

Development of behavioral responses to thermal challenges

Body temperature regulation involves the development of responses to cold and warm challenges. Matching our understanding of the development of body temperature regulation to warm challenges with that of cold challenges will enhance our understanding of the ontogeny of thermoregulation and reveal different adaptive specializations. Warm and cold thermoregulation are important processes, and they include direct thermal effects on offspring, as well as indirect effects on them, such as those imposed by thermally associated alterations of maternal behavior. The present paper is a selective review of the existing literature and a report of some new empirical data, aimed at processes of mammalian development, especially those affecting behavior. We briefly discuss the development of body temperature regulation in rats and mice, and thermal aspects of maternal behavior with emphasis on responses to high temperatures. The new data extend previous analyses of individual and group responses in developing rodents to warm and cool ambient temperatures. This literature not only reveals a variety of adaptive specializations during development, but it points to the earlier appearance in young mammals of abilities to combat heat loss, relative to protections from hyperthermia. These relative developmental delays in compensatory defenses to heating appear to render young mammals especially vulnerable to environmental warming. We describe cascading consequences of warming-effects that illustrate interactions across levels of physiological, neural, and behavioral development.

SIDS-CDF Hypothesis Revisited: Cause vs. Contributing Factors

The sudden infant death syndrome (SIDS)–critical diaphragm failure (CDF) hypothesis was first published by Siren and Siren in 2011 (1). Since its publication, the hypothesis has continued to generate interest and several colleagues have contributed perspectives and insights to it (2–5). The basic premise of the hypothesis is that the diaphragm is a vital organ that must continuously generate adequate force to maintain ventilation, and that CDF is a terminal event and the cause of death in SIDS. I have argued in two follow-up articles that all SIDS factors either increase the workload of the respiratory muscles, the diaphragm being the primary muscle affected, or reduce its force generating capacity (6, 7). The SIDS–CDF hypothesis posits that SIDS has many contributing factors but only one cause, namely, the failure of the vital respiratory pump. There are several known SIDS factors, such as the prone sleeping position, non-lethal infections, deep sleep, gestational prematurity, low birth weight, cigarette smoke, male gender, and altitude, but of these, some such as the prone sleeping position more significantly both impact diaphragm function and correlate with SIDS. However, SIDS cases are multifactorial and as such can be caused by different combinations of factors. An infection combined with a prone sleeping position and elevated room temperature could lead to SIDS, whereas in other circumstances, low birth weight, cigarette smoke, prone sleeping position, and altitude could result in CDF and SIDS. The SIDS–CDF hypothesis also posits that SIDS does not have a congenital or genetic origin, and that efforts to identify significant genetic anomalies in SIDS victims are unlikely to be successful (8–11).

Cardiorespiratory control and cytokine profile in response to heat stress, hypoxia, and lipopolysaccharide (LPS) exposure during early neonatal period

Sudden infant death syndrome (SIDS) is one of the most common causes of postneonatal infant mortality in the developed world. An insufficient cardiorespiratory response to multiple environmental stressors (such as prone sleeping positioning, overwrapping, and infection), during a critical period of development in a vulnerable infant, may result in SIDS. However, the effect of multiple risk factors on cardiorespiratory responses has rarely been tested experimentally. Therefore, this study aimed to quantify the independent and possible interactive effects of infection, hyperthermia, and hypoxia on cardiorespiratory control in rats during the neonatal period. We hypothesized that lipopolysaccharide (LPS) administration will negatively impact cardiorespiratory responses to increased ambient temperature and hypoxia in neonatal rats. Sprague-Dawley neonatal rat pups were studied at postnatal day 6-8. Rats were examined at an ambient temperature of 33°C or 38°C. Within each group, rats were allocated to control, saline, or LPS (200 μg/kg) treatments. Cardiorespiratory and thermal responses were recorded and analyzed before, during, and after a hypoxic exposure (10% O2). Serum samples were taken at the end of each experiment to measure cytokine concentrations. LPS significantly increased cytokine concentrations (such as TNFα, IL-1β, MCP-1, and IL-10) compared to control. Our results do not support a three-way interaction between experimental factors on cardiorespiratory control. However, independently, heat stress decreased minute ventilation during normoxia and increased the hypoxic ventilatory response. Furthermore, LPS decreased hypoxia-induced tachycardia. Herein, we provide an extensive serum cytokine profile under various experimental conditions and new evidence that neonatal cardiorespiratory responses are adversely affected by dual interactions of environmental stress factors.

Potential Central Nervous System Involvement in Sudden Unexpected Infant Deaths and the Sudden Infant Death Syndrome

Sudden unexpected infant death (SUID) in infancy which includes Sudden Infant Death Syndrome (SIDS) is the commonest diagnosed cause of death in the United States for infants 1 month to 1 year of age. Central nervous system mechanisms likely contribute to many of these deaths. We discuss some of these including seizure disorders, prolonged breath holding, arousal from sleep and its habituation, laryngeal reflex apnea potentiated by upper airway infection, and failure of brainstem-mediated autoresuscitation. In the conclusions section, we speculate how lives saved through back sleeping might result in later developmental problems in certain infants who otherwise might have died while sleeping prone.

Cyclooxygenases 1 and 2 differentially regulate blood pressure and cerebrovascular responses to acute and chronic intermittent hypoxia: implications for sleep apnea

Background

Obstructive sleep apnea (OSA) is associated with increased risk of cardiovascular and cerebrovascular disease resulting from intermittent hypoxia (IH)‐induced inflammation. Cyclooxygenase (COX)‐formed prostanoids mediate the inflammatory response, and regulate blood pressure and cerebral blood flow (CBF), but their role in blood pressure and CBF responses to IH is unknown. Therefore, this study's objective was to determine the role of prostanoids in cardiovascular and cerebrovascular responses to IH.

Methods and Results

Twelve healthy, male participants underwent three, 6‐hour IH exposures. For 4 days before each IH exposure, participants ingested a placebo, indomethacin (nonselective COX inhibitor), or Celebrex^® (selective COX‐2 inhibitor) in a double‐blind, randomized, crossover study design. Pre‐ and post‐IH blood pressure, CBF, and urinary prostanoids were assessed. Additionally, blood pressure and urinary prostanoids were assessed in newly diagnosed, untreated OSA patients (n=33). Nonselective COX inhibition increased pre‐IH blood pressure (P≤0.04) and decreased pre‐IH CBF (P=0.04) while neither physiological variable was affected by COX‐2 inhibition (P≥0.90). Post‐IH, MAP was elevated (P≤0.05) and CBF was unchanged with placebo and nonselective COX inhibition. Selective COX‐2 inhibition abrogated the IH‐induced MAP increase (P=0.19), but resulted in lower post‐IH CBF (P=0.01). Prostanoids were unaffected by IH, except prostaglandin E₂ was elevated with the placebo (P=0.02). Finally, OSA patients had elevated blood pressure (P≤0.4) and COX‐1 formed thromboxane A₂ concentrations (P=0.02).

Conclusions

COX‐2 and COX‐1 have divergent roles in modulating vascular responses to acute and chronic IH. Moreover, COX‐1 inhibition may mitigate cardiovascular and cerebrovascular morbidity in OSA.

Clinical Trial Registration

URL: www.clinicaltrials.gov. Unique identifier: NCT01280006

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.

Respiratory control in neonatal rats exposed to prenatal cigarette smoke

RATIONALE

Prenatal cigarette smoke (CS) exposure, increased environmental temperature, and hypoxic episodes have been postulated as major risk factors for sudden infant death syndrome.

OBJECTIVES

To test the hypothesis that maternal CS exposure disrupts eupneic breathing and depresses breathing responses of neonatal rats to thermal and hypoxic challenges.

METHODS

Experiments were performed on 1-week-old rat pups exposed prenatally to CS (n = 39) or room air (sham; n = 30). Breathing patterns were recorded by whole-body plethysmography during thermoneutral or hyperthermic states under normoxic and hypoxic conditions.

MEASUREMENTS AND MAIN RESULTS

Mean pup weight, breaths per minute, and gasping respiratory patterns were measured for both smoke- and sham-exposed groups during thermoneutral and hyperthermic states under normoxic and hypoxic conditions. Under thermoneutral conditions, hypoxia caused gasping in CS-exposed animals but not in sham-exposed animals. Furthermore, under hyperthermic conditions, whereas hypoxia induced gasping in both groups, only CS-exposed animals exhibited a pronounced and longer lasting respiratory depression after the termination of hypoxia.

CONCLUSIONS

We show that prenatal CS exposure increases the likelihood of gasplike respiration and provide the first experimental evidence that the combined effects of prenatal CS exposure and hyperthermia dramatically prolong the time required for neonates to return to eupneic breathing after hypoxia. These observations provide important evidence of how prenatal CS exposure, hypoxic episodes, and hyperthermia might place infants at higher risk for sudden infant death syndrome.

Mechanisms of pathogenesis in the Sudden Infant Death Syndrome

The likely processes of the Sudden Infant Death Syndrome (SIDS) were identified many years ago (apnea, failed arousal, failed autoresuscitation, etc.). The neurophysiological basis of these processes and the neurophysiological reasons some infants die of SIDS and others do not are, however, only emerging now. We reviewed recent studies that have shed light on the way in which epidemiological risk factors, genetics, neurotransmitter receptor defects and neonatal cardiorespiratory reflex responses interact to lead to sudden death during sleep in a small number of normal appearing infants. As a result of this review and analysis, we hypothesize that the neurophysiological basis of SIDS resides in a persistence of fetal reflex responses into the neonatal period, amplification of inhibitory cardiorespiratory reflex responses and reduced excitatory cardiorespiratory reflex responses. The hypothesis we developed explores the ways in which multiple subtle abnormalities interact to lead to sudden death and emphasizes the difficulty of ante-mortem identification of infants at risk for SIDS, although identification of infants at risk remains an essential goal of SIDS research.

Lack of histamine type-1 receptors impairs the thermal response of respiration during hypoxia in mice (Mus musculus)

Thermoregulation and the hypoxic ventilatory response are modulated by histamine type-1 (H1) receptors in the brain. In this study, we tested the hypothesis that activation of H1 receptors is required for the thermal control of ventilation during normoxia and hypoxia, using conscious male wild-type and H1 receptor-knockout (H1RKO) mice (Mus musculus). Under normoxic conditions, hyperthermia (39 degrees C) decreased minute ventilation (V (E)) and oxygen consumption [Formula: see text] in both genotypes, suggesting that H1 receptors are not involved in thermal ventilatory control during normoxia. Pa(CO2) was unchanged in both hyperthermia and normothermia, suggesting that the thermal decrease in V (E) is optimized by metabolic demand. Acute hypoxic gas exposure (7% O(2)+3% CO(2) in N(2)) increased, and then decreased, V (E) in wild-type mice; this increase was augmented and sustained by hyperthermia. Hypoxic gas exposure reduced [Formula: see text] and [Formula: see text] in wild-type mice at both body temperatures; the reduced [Formula: see text] during combined hyperthermia and hypoxia was higher than during normothermia and hypoxia. In H1RKO mice, hyperthermia did not augment the V (E) response to hypoxia, and did not affect [Formula: see text] and [Formula: see text] during hypoxia. In conclusion, histamine participates in the thermal increase of ventilation during hypoxia by activating H1 receptors.

Determinants of inspiratory activity

In vitro and in vivo studies have identified the pre-Bötzinger complex as an important kernel for the generation of inspiratory activity. The mechanisms underlying inspiratory rhythm generation involve pacemaker as well as synaptic mechanisms. In slice preparations, blockade of pacemaker properties with blockers for the persistent Na+ current, and the Ca2+-activated inward cationic current, abolishes respiratory activity. Here we show that blockade of the persistent Na+ current alone is sufficient to abolish respiratory activity in the in situ preparation. Although pacemaker neurons may be critical for establishing the basic respiratory rhythm, their rhythmic output is modulated by many elements of the respiratory network. For example, levels of synaptic inhibition control whether they burst or not, and endogenously released neuromodulators, such as serotonin and substance P modulate their intrinsic membrane currents. We hypothesize that the balance between synaptic and intrinsic pacemaker properties in the respiratory network is plastic, and that alterations of this balance may lead to dynamic reconfigurations of the respiratory network, which ultimately give rise to different activity patterns.

Elevated body temperature enhances the laryngeal chemoreflex in decerebrate piglets

Hyperthermia and reflex apnea may both contribute to sudden infant death syndrome (SIDS). Therefore, we investigated the effect of increased body temperature on the inhibition of breathing produced by water injected into the larynx, which elicits the laryngeal chemoreflex (LCR). We studied decerebrated, vagotomized, neonatal piglets aged 3-15 days. Blood pressure, end-tidal CO(2), body temperature, and phrenic nerve activity were recorded. To elicit the LCR, we infused 0.1 ml of distilled water through a polyethylene tube passed through the nose and positioned just rostral to the larynx. Three to five LCR trials were performed with the piglet at normal body temperature. The animal's core body temperature was raised by approximately 2.5 degrees C, and three to five LCR trials were performed before the animal was cooled, and three to five LCR trials were repeated. The respiratory inhibition associated with the LCR was substantially prolonged when body temperature was elevated. Thus elevated body temperature may contribute to the pathogenesis of SIDS by increasing the inhibitory effects of the LCR.