Heart rate response to transient chemoreceptor stimulation in term infants is modified by exposure to maternal smoking

Peripheral Neuroplasticity of Respiratory Chemoreflexes, Induced by Prenatal Nicotinic Exposure: Implication for SIDS

Sudden Infant Death Syndrome (SIDS) occurs during sleep in seemingly healthy infants. Maternal cigarette smoking and hypoxemia during sleep are assumed to be the major causal factors. Depressed hypoxic ventilatory response (dHVR) is observed in infants with high risk of SIDS, and apneas (lethal ventilatory arrest) appear during the fatal episode of SIDS. Disturbance of the respiratory center has been proposed to be involved, but the pathogenesis of SIDS is still not fully understood. Peripherally, the carotid body is critical to generate HVR, and bronchopulmonary and superior laryngeal C-fibers (PCFs and SLCFs) are important for triggering central apneas; however, their roles in the pathogenesis of SIDS have not been explored until recently. There are three lines of recently accumulated evidence to show the disorders of peripheral sensory afferent-mediated respiratory chemoreflexes in rat pups with prenatal nicotinic exposure (a SIDS model) in which acute severe hypoxia leads to dHVR followed by lethal apneas. (1) The carotid body-mediated HVR is suppressed with a reduction of the number and sensitivity of glomus cells. (2) PCF-mediated apneic response is largely prolonged via increased PCF density, pulmonary IL-1β and serotonin (5-hydroxytryptamine, 5-HT) release, along with the enhanced expression of TRPV1, NK₁R, IL1RI and 5-HT₃R in pulmonary C-neurons to strengthen these neural responses to capsaicin, a selective stimulant to C-fibers. (3) SLCF-mediated apnea and capsaicin-induced currents in superior laryngeal C-neurons are augmented by upregulation of TRPV1 expression in these neurons. These results, along with hypoxic sensitization/stimulation of PCFs, gain insight into the mechanisms of prenatal nicotinic exposure-induced peripheral neuroplasticity responsible for dHVR and long-lasting apnea during hypoxia in rat pups. Therefore, in addition to the disturbance in the respiratory center, the disorders of peripheral sensory afferent-mediated chemoreflexes may also be involved in respiratory failure and death denoted in SIDS victims.

Maturation of the Autonomic Nervous System in Premature Infants: Estimating Development Based on Heart-Rate Variability Analysis

This study aims at investigating the development of premature infants' autonomic nervous system (ANS) based on a quantitative analysis of the heart-rate variability (HRV) with a variety of novel features. Additionally, the role of heart-rate drops, known as bradycardias, has been studied in relation to both clinical and novel sympathovagal indices. ECG data were measured for at least 3 h in 25 preterm infants (gestational age ≤32 weeks) for a total number of 74 recordings. The post-menstrual age (PMA) of each patient was estimated from the RR interval time-series by means of multivariate linear-mixed effects regression. The tachograms were segmented based on bradycardias in periods after, between and during bradycardias. For each of those epochs, a set of temporal, spectral and fractal indices were included in the regression model. The best performing model has R 2 = 0.75 and mean absolute error MAE = 1.56 weeks. Three main novelties can be reported. First, the obtained maturation models based on HRV have comparable performance to other development models. Second, the selected features for age estimation show a predominance of power and fractal features in the very-low- and low-frequency bands in explaining the infants' sympathovagal development from 27 PMA weeks until 40 PMA weeks. Third, bradycardias might disrupt the relationship between common temporal indices of the tachogram and the age of the infant and the interpretation of sympathovagal indices. This approach might provide a novel overview of post-natal autonomic maturation and an alternative development index to other electrophysiological data analysis.

Effects of prenatal cigarette smoke exposure on BDNF, PACAP, microglia and gliosis expression in the young male mouse brainstem

Cigarette smoke exposure during pregnancy into infancy affects brain growth and development in both short and long term (into adulthood). Using a mouse model of pre- into post- natal cigarette smoke exposure (SE), we aimed to determine the effects on brain derived neurotrophic factor (BDNF) and its receptor TrkB, neuropeptide pituitary adenylate cyclase activating polypeptide (PACAP) and its receptor PAC1, and astrocyte (GFAP) and microglia (Iba-1) immunohistochemical expression, in seven nuclei of the medulla and the facial (FAC) nucleus of the pons. Male pups of dams exposed to two cigarettes (nicotine <1.2 mg, CO <15 mg) twice daily for six weeks prior to mating, during gestation and lactation (n = 5; SE), were compared to pups exposed to air under the same condition (n = 5; SHAM) at postnatal day 20. Expression changes were only evident for BDNF, TrkB and PAC1 and included decreased BDNF in the hypoglossal (XII) nucleus and nucleus of the solitary tract (NTS), increased TrkB in XII but decreased TrkB in the FAC, and increased PAC1 in 4 nuclei of the medulla including the NTS. These results suggest that the effect of SE on the brainstem are region and marker selective, affecting regions of respiratory control (XII and NTS), and restricted to the BDNF system and PAC1, with no effect on activation states of astrocytes or microglia.

Cigarette smoke exposure effects on the brainstem expression of nicotinic acetylcholine receptors (nAChRs), and on cardiac, respiratory and sleep physiologies

Cigarette smoking during pregnancy is the largest modifiable risk factor for adverse outcomes in the infant. Investigations have focused on the psychoactive component of cigarettes, nicotine. One proposed mechanism leading to adverse effects is the interaction between nicotine and its nicotinic acetylcholine receptors (nAChRs). Much data has been generated over the past three decades on the effects of cigarette smoke exposure (CSE) on the expression of the nAChRs in the brainstem and physiological parameters related to cardiac, respiration and sleep, in the offspring of smoking mothers and animal models of nicotine exposure. This review summarises this data and discusses the main findings, highlighting that findings in animal models closely correlate with those from human studies, and that the major brainstem sites where the expression level for the nAChRs are consistently affected include those that play vital roles in cardiorespiration (hypoglossal nucleus, dorsal motor nucleus of the vagus, nucleus of the solitary tract), chemosensation (nucleus of the solitary tract, arcuate nucleus) and arousal (rostral mesopontine sites such as the locus coeruleus and nucleus pontis oralis). These findings provide evidence for the adverse effects of CSE during and after pregnancy to the infant and the need to continue with the health campaign advising against CSE.

Antenatal smoking and substance-misuse, infant and newborn response to hypoxia

OBJECTIVES

To determine at the peak age for sudden infant death syndrome (SIDS) the ventilatory response to hypoxia of infants whose mothers substance misused in pregnancy (SM infants), or smoked during pregnancy (S mothers) and controls whose mothers neither substance misused or smoked. In addition, we compared the ventilatory response to hypoxia during the neonatal period and peak age of SIDS.

WORKING HYPOTHESIS

Infants of S or SM mothers compared to control infants would have a poorer ventilatory response to hypoxia at the peak age of SIDS.

STUDY DESIGN

Prospective, observational study.

PATIENT-SUBJECT SELECTION

Twelve S; 12 SM and 11 control infants were assessed at 6-12 weeks of age and in the neonatal period.

METHODOLOGY

Changes in minute volume, oxygen saturation, heart rate, and end tidal carbon dioxide levels on switching from breathing room air to 15% oxygen were assessed. Maternal and infant urine samples were tested for cotinine, cannabinoids, opiates, amphetamines, methadone, cocaine, and benzodiazepines.

RESULTS

The S and SM infants had a greater decline in minute volume (P = 0.037, P = 0.016, respectively) and oxygen saturation (P = 0.031) compared to controls. In all groups, the magnitude of decline in minute volume in response to hypoxia was higher in the neonatal period compared to at 6-12 weeks (P < 0.001).

CONCLUSIONS

Both maternal substance misuse and smoking were associated with an impaired response to a hypoxic challenge at the peak age for SIDS. The hypoxic ventilatory decline was more marked in the neonatal period compared to the peak age for SIDS indicating a maturational effect. Pediatr Pulmonol. 2017;52:650-655. © 2016 Wiley Periodicals, Inc.

Intermittent hypercapnic hypoxia effects on the nicotinic acetylcholine receptors in the developing piglet hippocampus and brainstem

This study investigated the effects of acute (1 day) vs repeated (4 days) exposure to intermittent hypercapnic hypoxia (IHH) on the immunohistochemical expression of α2, α3, α5, α7, α9 and β2 nicotinic acetylcholine receptor (nAChR) subunits in the developing piglet hippocampus and brainstem medulla, and how prior nicotine exposure alters the response to acute IHH. Five piglet groups included: 1day IHH (1D IHH, n=9), 4days IHH (4D IHH, n=8), controls exposed only to air cycles for 1day (1D Air, n=6) or 4days (4D Air, n=5), and pre-exposed to nicotine for 13days prior to 1day IHH (Nic+1D IHH, n=7). The exposure period alternated 6min of HH (8%O₂, 7%CO₂, balance N₂) and 6min of air over 48min, while controls were switched from air-to-air. Results showed that: 1. repeated IHH induces more changes in nAChR subunit expression than acute IHH in both the hippocampus and brainstem medulla, 2. In the hippocampus, α2 and β2 changed the most (increased) following IHH and the CA3, CA2 and DG were mostly affected. In the brainstem medulla, α2, α5, α9 and β2 were changed (decreased) in most nuclei with the hypoglossal and nucleus of the solitary tract being mostly affected. 3. Pre-exposure to nicotine enhanced the changes in the hippocampus but dampened those in the brainstem medulla. These findings indicate that the nAChRs (predominantly with the α2/β2 complex) are affected by IHH in critical hippocampal and brainstem nuclei during early brain development, and that pre-exposure to nicotine alters the pattern of susceptibility to IHH.

Camera-Based, Non-Contact, Vital-Signs Monitoring Technology May Provide a Way for the Early Prevention of SIDS in Infants

Sudden infant death syndrome (SIDS) is the unexplained death, usually during sleep, of a baby younger than 1-year-old. Even though researchers have discovered some factors that may put babies at extra risk, SIDS remains unpredictable up until now. One hypothesis is that impaired cardiovascular control may play a role in the underlying mechanism of SIDS. A reduction of heart rate variability (HRV) and progressive decrease in heart rate (HR) have been observed in infants who have later succumbed to SIDS. Many clues indicated the heart could be the final weakness in SIDS. Therefore, continuous monitoring of the dynamic changes within the heart may provide a possible preventive strategy of SIDS. Camera-based photoplethysmography was recently demonstrated as a contactless method to determine HR and HRV. This perspective presents a hypothesis that a camera-based, non-contact, vital-sign monitoring technology, which can indicate abnormal changes or a sudden loss of vital signs in a timely manner, may enable a crucial and low-cost means for the early prevention of SIDS in newborn infants.

Lower hypoxic ventilatory response in smokers compared to non-smokers during abstinence from cigarettes

BACKGROUND

Carotid body O₂-chemosensitivity determines the hypoxic ventilatory response (HVR) as part of crucial regulatory reflex within oxygen homeostasis. Nicotine has been suggested to attenuate HVR in neonates of smoking mothers. However, whether smoking affects HVR in adulthood has remained unclear and probably blurred by acute ventilatory stimulation through cigarette smoke. We hypothesized that HVR is substantially reduced in smokers when studied after an overnight abstinence from cigarettes i.e. after nicotine elimination.

METHODS

We therefore determined the isocapnic HVR of 23 healthy male smokers (age 33.9 ± 2.0 years, BMI 24.2 ± 0.5 kg m^-2, mean ± SEM) with a smoking history of >8 years after 12 h of abstinence and compared it to that of 23 healthy male non-smokers matched for age and BMI.

RESULTS

Smokers and non-smokers were comparable with regard to factors known to affect isocapnic HVR such as plasma levels of glucose and thiols as well as intracellular levels of glutathione in blood mononuclear cells. As a new finding, abstinent smokers had a significantly lower isocapnic HVR (0.024 ± 0.002 vs. 0.037 ± 0.003 l min^-1 %^-1BMI^-1, P = 0.002) compared to non-smokers. However, upon re-exposure to cigarettes the smokers' HVR increased immediately to the non-smokers' level.

CONCLUSIONS

This is the first report of a substantial HVR reduction in abstinent adult smokers which appears to be masked by daily smoking routine and may therefore have been previously overlooked. A low HVR may be suggested as a novel link between smoking and aggravated hypoxemia during sleep especially in relevant clinical conditions such as COPD.

Prenatal nicotinic exposure augments cardiorespiratory responses to activation of bronchopulmonary C-fibers

Rat pups prenatally exposed to nicotine (PNE) present apneic (lethal ventilatory arrest) responses during severe hypoxia. To clarify whether these responses are of central origin, we tested PNE effects on ventilation and diaphragm electromyography (EMGdi) during hypoxia in conscious rat pups. PNE produced apnea (lethal ventilatory arrest) identical to EMGdi silencing during hypoxia, indicating a central origin of this apneic response. We further asked whether PNE would sensitize bronchopulmonary C-fibers (PCFs), a key player in generating central apnea, with increase of the density and transient receptor potential cation channel subfamily V member 1 (TRPV1) expression of C-fibers/neurons in the nodose/jugular (N/J) ganglia and neurotrophic factors in the airways and lungs. We compared 1) ventilatory and pulmonary C-neural responses to right atrial bolus injection of capsaicin (CAP, 0.5 μg/kg), 2) bronchial substance P-immunoreactive (SP-IR) fiber density, 3) gene and protein expressions of TRPV1 in the ganglia, and 4) nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) protein in bronchoalveolar lavage fluid (BALF) and TrkA and TrkB genes in the ganglia between control and PNE pups. PNE markedly strengthened the PCF-mediated apneic response to CAP via increasing pulmonary C-neural sensitivity. PNE also enhanced bronchial SP-IR fiber density and N/J ganglia neural TRPV1 expression associated with increased gene expression of TrkA in the N/G ganglia and decreased NGF and BDNF in BALF. Our results suggest that PNE enhances PCF sensitivity likely through increasing PCF density and TRPV1 expression via upregulation of neural TrkA and downregulation of pulmonary BDNF, which may contribute to the PNE-promoted central apnea (lethal ventilatory arrest) during hypoxia.

Sudden unexpected death in infancy: biological mechanisms

Sudden unexpected death in infancy (SUDI) covers both explained and unexplained deaths. Unexplained cases or SIDS are likely to have multiple neural mechanisms contributing to the final event. The evidence ranges from subtle physiological signs related to autonomic control, to findings at autopsy of altered neurotransmitter systems, including the serotonergic system, a network that has an extensive homeostatic role in cardio-respiratory and thermoregulatory control. Processes may be altered by the vulnerability of the infant due to age, poor motor ability, or a genetic predisposition. The fatal event may occur in response to an environmental stress. A single final physiological route to death seems unlikely. An understanding of the reasons for explained SUDI also reminds us that a thorough investigation is required after each death occurs.

A key circulatory defence against asphyxia in infancy--the heart of the matter!

A resumption of, and escalation in, breathing efforts (hyperpnoea) reflexively accelerates heart rate (HR) and may facilitate cardiac and circulatory recovery from apnoea. We analysed whether this mechanism can produce a sustained rise in HR (tachycardia) when a sleeping infant is confronted by mild, rapidly worsening asphyxia, simulating apnoea. Twenty-seven healthy term-born infants aged 1-8 days rebreathed the expired gas for 90 s during quiet sleep to stimulate breathing and heart rate. To discriminate cardio-excitatory effects of central respiratory drive, lung inflation, hypoxia, hypercapnia and asphyxia, we varied the inspired O(2) level and compared temporal changes in response profiles as respiratory sensitivity to hypoxia and asphyxia 'reset' after birth. We demonstrate that asphyxia-induced hyperpnoea and tachycardia strengthen dramatically over the first week with different time courses and via separate mechanisms. Cardiac excitation by hypercapnia improves first, followed by a slower improvement in respiratory hypoxic drive. A rise in CO(2) consequently elicits stronger, longer lasting tachycardia than moderate increases in respiratory drive or lung expansion. We suggest that without a strong facilitating action of CO(2) on the immature heart, respiratory manoeuvres may be unable to reflexively counteract strong vagal bradycardia. This may increase the vulnerability of some infants to apnoea-asphyxia.

Comparison of poractant alfa and lyophilized lucinactant in a preterm lamb model of acute respiratory distress

INTRODUCTION

A lyophilized formulation of lucinactant has been developed to simplify preparation and dosing. Endotracheal administration of surfactant can be associated with potentially harmful transient hemodynamic changes including decreases in cerebral blood flow and delivery of O2 to the brain. Efficacy and peri-dosing effects of poractant alfa and a lyophilized form of lucinactant were compared in this study.

METHODS

Premature lambs (126-129 d gestation) were delivered by c-section, tracheostomized, ventilated, and instrumented with cerebral laser Doppler flowmetry and tissue PO2 probes. Pulmonary compliance and tidal volumes were monitored continuously and surfactant lung distribution was assessed. Lambs received either poractant alfa or lyophilized lucinactant and were monitored for 3 h after treatment.

RESULTS

Both groups showed significant improvements in arterial pCO2, pH, pulmonary compliance, and tidal volume (all P < 0.01), a similar intra-pulmonary distribution profile, and no significant changes in arterial blood pressure or cerebral blood flow. Administration of poractant alfa was associated with higher mean airway pressures from 75 min post-dosing and transiently decreased heart rate and increased brain tissue PO2 during the first 30 min after treatment.

DISCUSSION

In this newborn lamb model of respiratory distress, lyophilized lucinactant results in improved lung function as compared with poractant alfa.

Effects of nicotine during pregnancy: human and experimental evidence

Prenatal exposure to tobacco smoke is a major risk factor for the newborn, increasing morbidity and even mortality in the neonatal period but also beyond. As nicotine addiction is the factor preventing many women from smoking cessation during pregnancy, nicotine replacement therapy (NRT) has been suggested as a better alternative for the fetus. However, the safety of NRT has not been well documented, and animal studies have in fact pointed to nicotine per se as being responsible for a multitude of these detrimental effects. Nicotine interacts with endogenous acetylcholine receptors in the brain and lung, and exposure during development interferes with normal neurotransmitter function, thus evoking neurodevelopmental abnormalities by disrupting the timing of neurotrophic actions. As exposure to pure nicotine is quite uncommon in pregnant women, very little human data exist aside from the vast literature on prenatal exposure to tobacco smoke.

The current review discusses recent findings in humans on effects on the newborn of prenatal exposure to pure nicotine and non-smoke tobacco. It also reviews the neuropharmacological properties of nicotine during gestation and findings in animal experiments that offer explanations on a cellular level for the pathogenesis of such prenatal drug exposure.

It is concluded that as findings indicate that functional nAChRs are present very early in neuronal development, and that activation at this stage leads to apoptosis and mitotic abnormalities, a total abstinence from all forms of nicotine should be advised to pregnant women for the entirety of gestation.

Prenatal cigarette smoke exposure attenuates recovery from hypoxemic challenge in preterm infants

RATIONALE

The effects of prenatal cigarette smoke (CS) exposure and hypoxemia on cardiorespiratory control have been investigated in full-term infants. However, few data are available in preterm infants, who form a particularly vulnerable population, with developmentally immature cardiorespiratory control.

OBJECTIVES

To investigate the effects of prenatal CS exposure on the duration and recovery of breathing pauses and oxygen saturation levels under baseline and hypoxemic conditions in preterm infants.

METHODS

The study was performed on 22 (12 born to smoking and 10 to nonsmoking mothers) spontaneously breathing preterm infants between 28 and 36 weeks' gestation. Cardiorespiratory variables were recorded under baseline normoxemic and hypoxemic conditions.

MEASUREMENTS AND MAIN RESULTS

Breathing pauses, pause indices, time to recovery, percent pause recovery, oxygen saturation (Sp(O2)), periods of wakefulness, and cardiorespiratory rates were compared between the two groups. Spontaneous recovery of breathing pauses (P = 0.03) and Sp(O(2)) levels (P = 0.017) were attenuated in CS-exposed infants as compared with the control group during the hypoxemic and posthypoxemic periods, respectively. The episodes of wakefulness during the hypoxemic challenge were similar between the two groups. Furthermore, CS-exposed infants showed a greater increase in heart rate (P < 0.001) during the hypoxemic challenge when compared with control infants.

CONCLUSIONS

We provide evidence of how prenatal CS exposure and hypoxemic episodes affect the duration and recovery of breathing pauses in preterm infants. These observations could help explain why these infants are at a particularly high risk for sudden infant death syndrome.

Sudden infant death syndrome

Despite declines in prevalence during the past two decades, sudden infant death syndrome (SIDS) continues to be the leading cause of death for infants aged between 1 month and 1 year in developed countries. Behavioural risk factors identified in epidemiological studies include prone and side positions for infant sleep, smoke exposure, soft bedding and sleep surfaces, and overheating. Evidence also suggests that pacifier use at sleep time and room sharing without bed sharing are associated with decreased risk of SIDS. Although the cause of SIDS is unknown, immature cardiorespiratory autonomic control and failure of arousal responsiveness from sleep are important factors. Gene polymorphisms relating to serotonin transport and autonomic nervous system development might make affected infants more vulnerable to SIDS. Campaigns for risk reduction have helped to reduce SIDS incidence by 50-90%. However, to reduce the incidence even further, greater strides must be made in reducing prenatal smoke exposure and implementing other recommended infant care practices. Continued research is needed to identify the pathophysiological basis of SIDS.

Differential control of central cardiorespiratory interactions by hypercapnia and the effect of prenatal nicotine

Hypercapnia evokes a strong cardiorespiratory response including gasping and a pronounced bradycardia; however, the mechanism responsible for these survival responses initiated in the brainstem is unknown. To examine the effects of hypercapnia on the central cardiorespiratory network, we used an in vitro medullary slice that allows simultaneous examination of rhythmic respiratory-related activity and inhibitory synaptic neurotransmission to cardioinhibitory vagal neurons (CVNs). Hypercapnia differentially modulated inhibitory neurotransmission to CVNs; whereas hypercapnia selectively depressed spontaneous glycinergic IPSCs in CVNs without altering respiratory-related increases in glycinergic neurotransmission, it decreased both spontaneous and inspiratory-associated GABAergic IPSCs. Because maternal smoking is the highest risk factor for sudden infant death syndrome (SIDS) and prenatal nicotine exposure is proposed to be the link between maternal smoking and SIDS, we examined the cardiorespiratory responses to hypercapnia in animals exposed to nicotine in the prenatal and perinatal period. In animals exposed to prenatal nicotine, hypercapnia evoked an exaggerated depression of GABAergic IPSCs in CVNs with no significant change in glycinergic neurotransmission. Hypercapnia altered inhibitory neurotransmission to CVNs at both presynaptic and postsynaptic sites. Although the results obtained in this study in vitro cannot be extrapolated with certainty to in vivo responses, the results of this study provide a likely neurochemical mechanism for hypercapnia-evoked bradycardia and the dysregulation of this response with exposure to prenatal nicotine, creating a higher risk for SIDS.

Heart rate variability and cardiac reflexes in small for gestational age infants

To assess the influence of intrauterine growth retardation and postnatal development on heart rate variability (HRV) and cardiac reflexes, we studied 27 healthy small for gestational age (SGA) and 23 appropriate for gestational age (AGA) infants during a nap study. Resting HRV was assessed by point dispersion of Poincaré plots for overall (SDRR) and instantaneous beat-to-beat variability (SDΔRR) and the ratio (SDRR/SDΔRR). Heart rate reflex and arousal responses to a 60° head-up tilt were determined. All tests/measures were repeated twice in quiet and active sleep and in prone and supine sleep positions at 1 and 3 mo of age. SGA infants exhibited higher resting sympathetic tone [SDRR/SDΔRR: 1.9 (95% confidence interval: 1.7, 2.0) and 1.7 (95% confidence interval: 1.5, 1.8) in SGA and AGA, respectively; P = 0.046] and a tendency for a smaller tachycardic reflex response to the tilt [Δheart rate: 24 beats/min (95% confidence interval: 20, 28) and 30 (95% confidence interval: 25, 34)] in SGA and AGA, respectively; P = 0.06]. HRV indexes were reduced in the prone compared with supine position ( P < 0.0001), but reflex tilt responses were unchanged with position. SGA/AGA differences were independent of sleep position. Gestational age weight status did not influence the likelihood of arousal, but prone sleeping per se reduced the odds 2.5-fold. The findings suggest reduced autonomic activity and cardiac reflexes in SGA infants. The finding that the sympathetic component of the control of HRV was higher in SGA infants could link with findings in adulthood of an association between being born SGA and a higher risk of cardiovascular disease.

Hypoxia recruits a respiratory-related excitatory pathway to brainstem premotor cardiac vagal neurons in animals exposed to prenatal nicotine

The most ubiquitous form of arrhythmia is respiratory sinus arrhythmia in which the heart beat slows during expiration and heart rate increases during inspiration. Whereas respiratory sinus arrhythmia benefits pulmonary gas exchange respiratory dysfunction presents a major challenge to the cardiorespiratory system. Hypoxia evokes a pronounced bradycardia mediated by increases in parasympathetic cardiac activity. It has been hypothesized that the fatal events in sudden infant death syndrome (SIDS) are exaggerated cardiorespiratory responses to hypoxia. This study tests whether premotor cardiac vagal neurons receive rhythmic respiratory-related excitatory synaptic inputs during normoxia and hypoxia, and if animals exposed to nicotine in the prenatal period have exaggerated responses to hypoxia. Premotor cardiac vagal neurons in the nucleus ambiguus were identified in rats by the presence of a fluorescent tracer in medullary slices that generate rhythmic inspiratory-related motor discharge. Respiratory activity was recorded from the hypoglossal nerve and excitatory synaptic events in cardiac vagal neurons were isolated using patch clamp techniques. Adult female rats were implanted with osmotic minipumps that delivered nicotine at a level approximately equivalent to those that occur in moderate to heavy smokers. During normal eupneic respiration, as well as during hypoxia, premotor cardiac vagal neurons from control animals did not receive any rhythmic respiratory-related excitatory inputs. However in animals exposed to nicotine throughout the prenatal period respiratory bursts during hypoxia dramatically increased the frequency of excitatory synaptic events in cardiac vagal neurons. In summary, in animals exposed to nicotine throughout the prenatal period, but not in unexposed animals, respiratory bursts that occur during hypoxia dramatically increase the frequency of excitatory synaptic events in cardiac vagal neurons. This study establishes a likely neurochemical mechanism for the heart rate responses to hypoxia and a link between prenatal nicotine exposure and exaggerated bradycardia responses during hypoxia that may contribute to sudden infant death syndrome.

Dampened ventilatory response to added dead space in newborns of smoking mothers

BACKGROUND

Term newborns can compensate fully for an imposed dead space (tube breathing) by increasing their minute ventilation.

OBJECTIVE

To test the hypothesis that infants of smoking mothers would have an impaired response to tube breathing.

DESIGN

Prospective study.

SETTING

Perinatal service.

PATIENTS

Fourteen infants of smoking and 24 infants of non-smoking mothers (median postnatal age 37 (11-85) hours and 26 (10-120) hours respectively) were studied.

INTERVENTIONS

Breath by breath minute volume was measured at baseline and when a dead space of 4.4 ml/kg was incorporated into the breathing circuit.

MAIN OUTCOME MEASURES

The maximum minute ventilation during tube breathing was determined and the time constant of the response calculated.

RESULTS

The time constant of the infants of smoking mothers was longer than that of the infants of non-smoking mothers (median (range) 37.3 (22.2-70.2) v 26.2 (13.8-51.0) seconds, p = 0.016). Regression analysis showed that maternal smoking status was related to the time constant independently of birth weight, gestational or postnatal age, or sex (p = 0.018).

CONCLUSIONS

Intrauterine exposure to smoking is associated with a dampened response to tube breathing.

Cardiorespiratory effects of nicotine exposure during development

Exposure to tobacco smoke is a major risk factor for the sudden infant death syndrome. Nicotine is thought to be the ingredient in tobacco smoke that is responsible for a multitude of cardiorespiratory effects during development, and pre- rather than postnatal exposure is considered to be most detrimental. Nicotine interacts with endogenous acetylcholine receptors in the brain and lung, and developmental exposure produces structural changes as well as alterations in neuroregulation. Abnormalities have been described in sympathicovagal balance, arousal threshold and latency, breathing pattern at rest and apnea frequency, ventilatory response to hyperoxia or hypoxia, heart rate regulation and ability to autoresuscitate during severe hypoxia. This review discusses studies performed on infants of smoking mothers and nicotine-exposed animals yielding varying and sometimes inconsistent results that may be due to differences in experimental design, species and the dose of exposure. Taken together however, developmental nicotine exposure appears to induce vulnerability during hypoxia and a potential inability to survive severe asphyxia.

Influences of maternal cigarette smoking on infant arousability

Since the reduction in the incidence of the prone sleeping position, maternal cigarette smoking has become the strongest modifiable risk factor for Sudden Infant Death Syndrome (SIDS). This risk is dose dependent. Various mechanisms have been postulated to explain the increased risk of SIDS associated with maternal smoking, among these, impairment of arousal from sleep. This paper reviews the effects of maternal smoking on infant arousability from sleep, cardiorespiratory controls and sleep architecture. Infants exposed to maternal smoking have been shown to have both decreased spontaneous and evoked arousability from sleep. Such impairment of arousal has been demonstrated to be associated with changes in control of autonomic cardiac function. Sleep architecture appears not to be altered by smoking. During arousal, heart rate, blood pressure and breathing movements increase, while gross body movements occur to avoid the stimulus. Any impairment in arousability from sleep could occur when infants are exposed to maternal cigarette smoking, and could possibly contribute to the final pathway to SIDS.

Arousal and ventilatory responses to hypoxia in sleeping infants: effects of maternal smoking

Our aim was to determine whether maternal cigarette smoking affects arousal and ventilatory responses to hypoxia in infants. Infants born to non-smoking (NS, n = 15) and smoking mothers (SM, n= 9) were studied at 2-5 weeks, 2-3 and 5-6 months. Ventilatory responses to 15% O(2) were determined preceding arousal. At each age and in both groups, infants aroused more frequently and earlier to hypoxia in active sleep (AS) than quiet sleep (QS). Arousal latency was longer in SM infants (in QS) at 5-6 months (P < 0.05). Baseline respiratory parameters were not different between groups, except that, at 2-3 months, SM infants had higher SP(O2) during AS than NS infants. Maternal smoking did not affect ventilatory responses preceding hypoxia-induced arousal in either sleep-state at any age. We conclude that mild hypoxia stimulates ventilation and arousal in infants up to 6 months and that arousability is depressed in SM infants at 5-6 months; however, ventilatory responses preceding arousal are not adversely affected by smoking.

Development of ventilatory response to transient hypercapnia and hypercapnic hypoxia in term infants

Whereas peripheral chemoreceptor oxygen sensitivity increases markedly after birth, previous studies of ventilatory responses to CO(2) in term infants have shown no postnatal development. However, the hypercapnic challenges applied have usually been long-term, which meant that the effect of central chemoreceptors dominated. Oscillatory breathing, apneas, and sighs cause transient Pco(2) changes, probably primarily stimulating peripheral chemoreceptors. We wanted to assess whether the immediate ventilatory responses to step changes in inspired CO(2) and O(2) in term infants undergo postnatal developmental changes. Twenty-six healthy term infants were studied during natural sleep 2 d and 8 wk postnatally. Ventilatory responses to a randomized sequence of 15 s hypercapnia (3% CO(2)), hypoxia (15% O(2)), and hypercapnic hypoxia (3% CO(2) + 15% O(2)) were recorded breath-by-breath using a pneumotachometer. Response rate, stimulus-response time, and response magnitude were analyzed with ANOVA after coherent averaging. Response rate increased with age by 30% (hypercapnia), 318% (hypoxia), and 302% (hypercapnic hypoxia). Response rate during hypercapnic hypoxia exceeded rate during hypercapnia plus rate during hypoxia in wk 8, but not on d 2. Time to half-maximum response decreased by 3.4 s with age for the two hypercapnic stimuli but was unchanged for hypoxia. Response magnitude was unchanged for hypercapnia, but increased for the two hypoxic stimuli. In conclusion, an interaction between the effects of hypercapnia and hypoxia on ventilatory response rate emerged between postnatal d 2 and wk 8 in term infants. Concomitantly, stimulus-response time to hypercapnic stimuli declined markedly. The development of a prompt response to transient hypercapnia may be important for infant respiratory stability.