Development of oxygen sensitivity in infants of smoking and non-smoking mothers

Defining and Promoting Pediatric Pulmonary Health: Understanding Sleep and Ventilatory Health

Healthy sleep and optimal ventilatory control begin in early development and are crucial for positive child outcomes. This paper summarizes information presented at the Sleep and Ventilatory Control sessions of the National Heart, Lung, and Blood-sponsored 2021 Defining and Promoting Pediatric Pulmonary Health workshop. These sessions focused on pediatric sleep health, screening for sleep health and sleep disorders in primary care using the electronic health record, infant sleep and ventilatory control, and home sleep testing. Throughout this summary, we discuss key gaps in and barriers to promoting sleep and ventilatory health that were identified during the workshop sessions. We conclude with strategies to address these gaps and barriers and directions for future multidisciplinary research, patient care, and training.

Quantitative and Qualitative Changes in Peripheral Chemoreceptor Activity in Preterm Infants

Rationale: Preterm infants are at risk for ventilatory control instability that may be due to aberrant peripheral chemoreceptor activity. Although term infants have increasing peripheral chemoreceptor contribution to overall ventilatory drive with increasing postnatal age, how peripheral chemoreceptor contribution changes in preterm infants with increasing postmenstrual age is not known. Objectives: To evaluate peripheral chemoreceptor activity between 32 and 52 weeks postmenstrual age in preterm infants, using both quantitative and qualitative measures. Methods: Fifty-five infants born between 24 weeks, 0 days gestation and 28 weeks, 6 days gestation underwent hyperoxic testing at one to four time points between 32 and 52 weeks postmenstrual age. Quantitative [Formula: see text] decreases were calculated, and qualitative responses were categorized as apnea, continued breathing with a clear reduction in [Formula: see text], sigh breaths, and no response. Measurements and Main Results: A total of 280 hyperoxic tests were analyzed (2.2 ± 0.3 tests per infant at each time point). Mean peripheral chemoreceptor contribution to ventilatory drive was 85.2 ± 20.0% at 32 weeks and 64.1 ± 22.0% at 52 weeks. Apneic responses were more frequent at earlier postmenstrual ages. Conclusions: Among preterm infants, the peripheral chemoreceptor contribution to ventilatory drive was greater at earlier postmenstrual ages. Apnea was a frequent response to hyperoxic testing at earlier postmenstrual ages, suggesting high peripheral chemoreceptor activity. A clearer description of how peripheral chemoreceptor activity changes over time in preterm infants may help explain how ventilatory control instability contributes to apnea and sleep-disordered breathing later in childhood. Clinical trial registered with www.clinicaltrials.gov (NCT03464396).

Impact of Prenatal Nicotine Exposure on Placental Function and Respiratory Neural Network Development

Smoking during pregnancy is associated with multiple undesirable outcomes in infants, such as low birth weight, increased neonatal morbidity and mortality, and catastrophic conditions like sudden infant death syndrome (SIDS). Nicotine, the most addictive and teratogenic substance in tobacco smoke, reaches and crosses the placenta and can be accumulated in the amniotic fluid and distributed by fetal circulation, altering the cholinergic transmission by acting on the nicotinic acetylcholine receptors (nAChRs) expressed from very early gestational stages in the placenta and fetal tissue. Because nAChRs influence the establishment of feto-maternal circulation and the emergence of neuronal networks, prenatal nicotine exposure can lead to multiple alterations in newborns. In this mini-review, we discuss the undeniable effects of nicotine in the placenta and the respiratory neural network as examples of how prenatal nicotine and smoking exposition can affect brain development because dysfunction in this network is involved in SIDS etiology.

Nicotinic Receptors in the Brainstem Ascending Arousal System in SIDS With Analysis of Pre-natal Exposures to Maternal Smoking and Alcohol in High-Risk Populations of the Safe Passage Study

Pre-natal exposures to nicotine and alcohol are known risk factors for sudden infant death syndrome (SIDS), the leading cause of post-neonatal infant mortality. Here, we present data on nicotinic receptor binding, as determined by 125I-epibatidine receptor autoradiography, in the brainstems of infants dying of SIDS and of other known causes of death collected from the Safe Passage Study, a prospective, multicenter study with clinical sites in Cape Town, South Africa and 5 United States sites, including 2 American Indian Reservations. We examined 15 pons and medulla regions related to cardiovascular control and arousal in infants dying of SIDS (n = 12) and infants dying from known causes (n = 20, 10 pre-discharge from time of birth, 10 post-discharge). Overall, there was a developmental decrease in 125I-epibatidine binding with increasing postconceptional age in 5 medullary sites [raphe obscurus, gigantocellularis, paragigantocellularis, centralis, and dorsal accessory olive (p = 0.0002-0.03)], three of which are nuclei containing serotonin cells. Comparing SIDS with post-discharge known cause of death (post-KCOD) controls, we found significant decreased binding in SIDS in the nucleus pontis oralis (p = 0.02), a critical component of the cholinergic ascending arousal system of the rostral pons (post-KCOD, 12.1 ± 0.9 fmol/mg and SIDS, 9.1 ± 0.78 fmol/mg). In addition, we found an effect of maternal smoking in SIDS (n = 11) combined with post-KCOD controls (n = 8) on the raphe obscurus (p = 0.01), gigantocellularis (p = 0.02), and the paragigantocellularis (p = 0.002), three medullary sites found in this study to have decreased binding with age and found in previous studies to have abnormal indices of serotonin neurotransmission in SIDS infants. At these sites, 125I-epibatidine binding increased with increasing cigarettes per week. We found no effect of maternal drinking on 125I-epibatidine binding at any site measured. Taken together, these data support changes in nicotinic receptor binding related to development, cause of death, and exposure to maternal cigarette smoking. These data present new evidence in a prospective study supporting the roles of developmental factors, as well as adverse exposure on nicotinic receptors, in serotonergic nuclei of the rostral medulla-a finding that highlights the interwoven and complex relationship between acetylcholine (via nicotinic receptors) and serotonergic neurotransmission in the medulla.

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.

Pathogenesis of central and complex sleep apnoea

Central sleep apnoea (CSA) - the temporary absence or diminution of ventilatory effort during sleep - is seen in a variety of forms including periodic breathing in infancy and healthy adults at altitude and Cheyne-Stokes respiration in heart failure. In most circumstances, the cyclic absence of effort is paradoxically a consequence of hypersensitive ventilatory chemoreflex responses to oppose changes in airflow, that is elevated loop gain, leading to overshoot/undershoot ventilatory oscillations. Considerable evidence illustrates overlap between CSA and obstructive sleep apnoea (OSA), including elevated loop gain in patients with OSA and the presence of pharyngeal narrowing during central apnoeas. Indeed, treatment of OSA, whether via continuous positive airway pressure (CPAP), tracheostomy or oral appliances, can reveal CSA, an occurrence referred to as complex sleep apnoea. Factors influencing loop gain include increased chemosensitivity (increased controller gain), reduced damping of blood gas levels (increased plant gain) and increased lung to chemoreceptor circulatory delay. Sleep-wake transitions and pharyngeal dilator muscle responses effectively raise the controller gain and therefore also contribute to total loop gain and overall instability. In some circumstances, for example apnoea of infancy and central congenital hypoventilation syndrome, central apnoeas are the consequence of ventilatory depression and defective ventilatory responses, that is low loop gain. The efficacy of available treatments for CSA can be explained in terms of their effects on loop gain, for example CPAP improves lung volume (plant gain), stimulants reduce the alveolar-inspired PCO₂ difference and supplemental oxygen lowers chemosensitivity. Understanding the magnitude of loop gain and the mechanisms contributing to instability may facilitate personalized interventions for CSA.

Interactive effects of maternal cigarette smoke, heat stress, hypoxia, and lipopolysaccharide on neonatal cardiorespiratory and cytokine responses

Maternal cigarette smoke (CS) exposure exhibits a strong epidemiological association with Sudden Infant Death Syndrome, but other environmental stressors, including infection, hyperthermia, and hypoxia, have also been postulated as important risk factors. This study examines whether maternal CS exposure causes maladaptations within homeostatic control networks by influencing the response to lipopolysaccharide, heat stress, and/or hypoxia in neonatal rats. Pregnant dams were exposed to CS or parallel sham treatments daily for the length of gestation. Offspring were studied at postnatal days 6-8 at ambient temperatures (T_a) of 33°C or 38°C. Within each group, rats were allocated to control, saline, or LPS (200 µg/kg) treatments. Cardiorespiratory patterns were examined using head-out plethysmography and ECG surface electrodes during normoxia and hypoxia (10% O₂). Serum cytokine concentrations were quantified from samples taken at the end of each experiment. Our results suggest maternal CS exposure does not alter minute ventilation (V̇e) or heart rate (HR) response to infection or high temperature, but independently increases apnea frequency. CS also primes the inflammatory system to elicit a stronger cytokine response to bacterial insult. High T_a independently depresses V̇e but augments the hypoxia-induced increase in V̇e Moreover, higher T_a increases HR during normoxia and hypoxia, and in the presence of an immune challenge, increases HR during normoxia, and reduces the increase normally associated with hypoxia. Thus, while most environmental risk factors increase the burden on the cardiorespiratory system in early life, hyperthermia and infection blunt the normal HR response to hypoxia, and gestational CS independently destabilizes breathing by increasing apneas.

Il-1β and prostaglandin E2 attenuate the hypercapnic as well as the hypoxic respiratory response via prostaglandin E receptor type 3 in neonatal mice

Prostaglandin E2 (PGE2) serves as a critical mediator of hypoxia, infection, and apnea in term and preterm babies. We hypothesized that the prostaglandin E receptor type 3 (EP3R) is the receptor responsible for PGE2-induced apneas. Plethysmographic recordings revealed that IL-1β (ip) attenuated the hypercapnic response in C57BL/6J wild-type (WT) but not in neonatal (P9) EP3R−/− mice ( P < 0.05). The hypercapnic responses in brain stem spinal cord en bloc preparations also differed depending on EP3R expression whereby the response was attenuated in EP3R−/− preparations ( P < 0.05). After severe hypoxic exposure in vivo, IL-1β prolonged time to autoresuscitation in WT but not in EP3R−/− mice. Moreover, during severe hypoxic stress EP3R−/− mice had an increased gasping duration ( P < 0.01) as well as number of gasps ( P < 0.01), irrespective of intraperitoneal treatment, compared with WT mice. Furthermore, EP3R−/− mice exhibited longer hyperpneic breathing efforts when exposed to severe hypoxia ( P < 0.01). This was then followed by a longer period of secondary apnea before autoresuscitation occurred in EP3R−/− mice ( P < 0.05). In vitro, EP3R−/− brain stem spinal cord preparations had a prolonged respiratory burst activity during severe hypoxia accompanied by a prolonged neuronal arrest during recovery in oxygenated medium ( P < 0.05). In conclusion, PGE2 exerts its effects on respiration via EP3R activation that attenuates the respiratory response to hypercapnia as well as severe hypoxia. Modulation of the EP3R may serve as a potential therapeutic target for treatment of inflammatory and hypoxic-induced detrimental apneas and respiratory disorders in neonates.

Control of breathing activity in the fetus and newborn

Breathing movements have been demonstrated in the fetuses of every mammalian species investigated and are a critical component of normal fetal development. The classic sheep preparations instrumented for chronic fetal monitoring determined that fetal breathing movements (FBMs) occur in aggregates interspersed with long periods of quiescence that are strongly associated with neurophysiological state. The fetal sheep model also provided data regarding the neurochemical modulation of behavioral state and FBMs under a variety of in utero conditions. Subsequently, in vitro rodent models have been developed to advance our understanding of cellular, synaptic, network, and more detailed neuropharmacological aspects of perinatal respiratory neural control. This includes the ontogeny of the inspiratory rhythm generating center, the preBötzinger complex (preBötC), and the anatomical and functional development of phrenic motoneurons (PMNs) and diaphragm during the perinatal period. A variety of newborn animal models and studies of human infants have provided insights into age-dependent changes in state-dependent respiratory control, responses to hypoxia/hypercapnia and respiratory pathologies.

Cardiorespiratory coupling in health and disease

Cardiac and respiratory activities are intricately linked both functionally as well as anatomically through highly overlapping brainstem networks controlling these autonomic physiologies that are essential for survival. Cardiorespiratory coupling (CRC) has many potential benefits creating synergies that promote healthy physiology. However, when such coupling deteriorates autonomic dysautonomia may ensue. Unfortunately there is still an incomplete mechanistic understanding of both normal and pathophysiological interactions that respectively give rise to CRC and cardiorespiratory dysautonomia. Moreover, there is also a need for better quantitative methods to assess CRC. This review addresses the current understanding of CRC by discussing: (1) the neurobiological basis of respiratory sinus arrhythmia (RSA); (2) various disease states involving cardiorespiratory dysautonomia; and (3) methodologies measuring heart rate variability and RSA.

Perinatal nicotine/smoking exposure and carotid chemoreceptors during development

Tobacco smoking is still a common habit during pregnancy and is the most important preventable cause of many adverse perinatal outcomes. Prenatal smoking exposure can produce direct actions of nicotine in the fetus with the disruption of body and brain development, and actions on the maternal-fetal unit by causing repeated episodes of hypoxia and exposure to many toxic smoke products (such as carbon monoxide). Specifically, nicotine through binding to nicotinic acetylcholine receptors have ubiquitous effects and can affect carotid chemoreception development through structural, functional and neuroregulatory alterations of the neural circuits involved in the chemoafferent pathway, as well as by interfering with the postnatal resetting of the carotid bodies. Reduced carotid body chemosensitivity and tonic activity have thus been reported by the majority of the human and animal studies. This review focuses on the effects of perinatal exposure to tobacco smoke and nicotine on carotid chemoreceptor function during the developmental period. A description of the effects of smoking and nicotine on the control of breathing related to carotid body activity, and of the possible physiopathological mechanisms at the origin of these disturbances is presented.

Postnatal maturation of breathing stability and loop gain: the role of carotid chemoreceptor development

Any general model of respiratory control must explain a puzzling array of breathing patterns that are observed during the course of a lifetime. Particular challenges are to understand why periodic breathing is rarely seen in the first few days after birth, reaches a peak at 2-4 weeks postnatal age, and disappears by 6 months, why it is prevalent in preterm infants, and why it reappears in adults at altitude or with heart failure. In this review we use the concept of loop gain to obtain quantitative insight into the genesis of unstable breathing patterns with a particular focus on how changes in carotid body function could underlie the age-related dependence of periodic breathing.

The carotid body and arousal in the fetus and neonate

Arousal from sleep is a major defense mechanism in infants against hypoxia and/or hypercapnia. Arousal failure may be an important contributor to SIDS. Areas of the brainstem that have been found to be abnormal in a majority of SIDS infants are involved in the arousal process. Arousal is sleep state dependent, being depressed during AS in most mammals, but depressed during QS in human infants. Repeated exposure to hypoxia causes a progressive blunting of arousal that may involve medullary raphe GABAergic mechanisms. Whereas CB chemoreceptors contribute heavily to arousal in response to hypoxia, serotonergic central chemoreceptors have been implicated in the arousal response to CO(2). Pulmonary or chest wall mechanoreceptors also contribute to arousal in proportion to the ventilatory response and decreases in their input may contribute to depressed arousal during AS. Little is known about specific arousal pathways beyond the NTS. Whether CB chemoreceptor stimulation directly stimulates arousal centers or whether this is done indirectly through respiratory networks remains unknown. This review will focus on arousal in response to hypoxia and CO(2) in the fetus and newborn and will outline what we know (and do not know) about the involvement of the carotid body in this process.

Development of ventilatory control in infants

Most abnormalities of ventilatory control in infants are due to immaturity or abnormal development of ventilatory control. This includes a broad range, from rare disorders like congenital central hypoventilation syndrome to common problems such as apnoea of prematurity. Development of the ventilatory control system, including central respiratory rhythmogenesis and central and peripheral chemoreception, begins early in gestation and continues for weeks or months after birth. Development of the neural components of central rhythmogenesis and their highly complex interconnectivity results from complex, timing-sensitive interactions between patterning and other genes, transcription factors and neurotrophic factors. At birth, nearly all aspects of ventilatory control remain immature, especially in preterm infants; and postnatal maturation can be altered by hypoxia, toxins and other stressors. Clinical care may be greatly enhanced by increased awareness of ventilatory control maturation and related disorders.

Influence of prenatal nicotine exposure on development of the ventilatory response to hypoxia and hypercapnia in neonatal rats

In a recent study (Huang YH et al. Respir Physiol Neurobiol 143: 1-8, 2004), we showed that prenatal nicotine exposure (PNE) increased the frequency of spontaneous apneic events on the first 2 days of life in unanesthetized neonatal rats. Here we test the hypothesis that PNE blunts chemoreceptor reflexes. Ventilatory responses to three levels each of hypoxia (inspired O(2) fraction: 16, 12, and 10%) and hypercapnia (3, 6, and 9% inspired CO(2) fraction, all in 50% O(2), balance N(2)), and one level each of combined hypoxia-hypercapnia (H/H; 12% inspired O(2) fraction/5% inspired CO(2) fraction) and hyperoxia (50% O(2), 50% N(2)) were recorded with head-out plethysmography in neonatal rats exposed to either nicotine (N = 12) or physiological saline (N = 12) in the prenatal period. Recordings were made on postnatal day 1 (P1), P3, P6, P9, P12, and P18, in each animal. The change in ventilation in response to hypoxia was blunted in PNE animals on P1 and P3, but there were no other treatment effects. Hyperoxia significantly depressed ventilation in both groups from P3-P18, but there were no significant treatment effects. The ventilatory response to 3, 6, and 9% inspired CO(2) was significantly blunted in PNE animals at all ages studied, due exclusively to a blunted tidal volume response. PNE also blunted the ventilatory response to H/H at all ages, due primarily to blunting of the tidal volume response. PNE had no significant effect on body mass or metabolic rate, except that PNE animals had a slightly higher mass on P18 and a lower metabolic rate on P1. As shown by others, PNE has small and inconsistent effects on hypoxic ventilatory responses, but here we show that responses to hypercapnia and H/H are consistently blunted by PNE due to a diminished tidal volume response. The combination of reduced hypoxic and hypercapnic sensitivity over the first 3 days of life may define an especially vulnerable developmental period.

In utero exposure to smoking and peripheral chemoreceptor function in preterm neonates

OBJECTIVE

We aimed to assess the involvement of peripheral chemoreceptor tonic activity in the ventilatory pattern during sleep in preterm neonates exposed in utero to maternal smoking.

PATIENTS AND METHODS

Peripheral chemoreceptor activity was measured at thermoneutrality in neonates (postmenstrual age: 36.1 +/- 1.2 weeks) born to nonsmoking (n = 21) or smoking (n = 16) mothers by performing a 30-second hyperoxic test during active and quiet sleep. Blood oxygen saturation, baseline ventilatory parameters, and central apnea were monitored.

RESULTS

Prenatal smoking exposure did not modify baseline ventilation. It was interesting to note that prenatal smoking exposure decreased the peripheral chemoreceptor tonic activity during active sleep and increased the response time during quiet sleep. These changes could explain the increase in the time spent in apnea (both with and without blood oxygen desaturation) and in the mean duration of apneic episodes with desaturation found in neonates exposed to smoking in utero. The involvement of a change in the chemoreceptor function is supported by the fact that the peripheral chemoreceptor tonic activity was negatively correlated with the mean duration of apneic episodes with desaturation in the control group only.

CONCLUSIONS

To our knowledge, this is the first study to reveal that prenatal smoking exposure does not directly modify baseline ventilatory parameters in the neonate but has a negative impact on peripheral chemoreceptor tonic activity. These alterations may increase the risk of sleep respiratory disorders, especially via apnea with desaturation.

Maturation of respiratory control and the propensity for breathing instability in a sheep model

Limited evidence suggests that the ventilatory interaction between O2 and CO2 is additive after birth and becomes multiplicative with postnatal development. Such a switch may be linked to the propensity for periodic breathing (PB) in infancy. To test this idea, we characterized the maturation of the respiratory controller and its effect on breathing stability in ∼10-day-old lambs and 6-mo-old sheep. We measured 1) carotid body sensitivity via dynamic ventilatory responses to step changes in O2 and CO2, 2) steady-state ventilatory sensitivity to CO2 under hypoxic and hyperoxic conditions, 3) the dependence of the apneic threshold on arterial Po2, and 4) the effect of hypoxic or hypercapnic gas inhalation during induced PB. Stability of the system was assessed using surrogate measures of loop gain. Peripheral sensitivity to O2 was higher in newborn than in older animals ( P < 0.05), but peripheral CO2 sensitivity was unchanged. Central CO2 sensitivity was reduced with age, but the slopes of the ventilatory responses to CO2 were the same in hypoxia and hyperoxia. Reduced arterial Po2 caused a leftward shift in the apneic threshold at both ages. Inspiration of hypoxic gas during PB immediately halted PB, whereas hypercapnia stopped PB only after one or two further PB cycles. We conclude that the controller in the sheep remains additive over the first 6 mo of life. Our results also show that the loop gain of the respiratory control system is reduced with age, possibly as a result of a reduction of peripheral O2 sensitivity.

Minimizing the risks of sudden infant death syndrome: to swaddle or not to swaddle?

OBJECTIVE

To evaluate the effects of swaddling on infant arousability, particularly the progression of subcortical activation (SCA) to full cortical arousal (CA), because impaired arousal may contribute to sudden infant death syndrome.

STUDY DESIGN

Healthy term infants, who were routinely swaddled (n = 15) or unswaddled (n = 12) at home, were studied with daytime polysomnography at 3 to 4 weeks and 3 months after birth. When both swaddled and unswaddled, arousability was assessed with a pulsatile jet of air at the nostrils.

RESULTS

Larger increases in overall arousal thresholds (SCA plus CA) with swaddling were observed in infants who were easiest to arouse when unswaddled. Swaddling did not alter SCA or CA frequencies of routinely swaddled infants at either age. In infants who were naïve to swaddling, arousal thresholds were increased and CA frequency decreased during swaddled quiet sleep at 3 months.

CONCLUSIONS

This study provides a scientific basis for assessing the safety of swaddling in infant care practice. The decreased cortical arousals observed in infants unfamiliar with swaddling may correspond to the increased risk of sudden infant death syndrome for inexperienced prone sleepers.

Respiratory dysfunctions induced by prenatal nicotine exposure

1. Maternal tobacco smoking is the principal risk factor associated with sudden infant death syndrome (SIDS), a leading cause of death of infants under 1 year of age. Victims of SIDS show a higher incidence of respiratory control abnormalities, including central apnoeas, delayed arousal responses and diminished ventilatory chemoreflexes. 2. Nicotine is likely the link between maternal tobacco smoking and SIDS. Prenatal nicotine exposure can alter the breathing pattern and can reduce hypoxia- and hypercarbia-induced ventilatory chemoreflexes. In vitro approaches have revealed that prenatal nicotine exposure impairs central chemosensitivity, switching the cholinergic contribution from a muscarinic to a nicotinic receptor-based drive. In addition, serotonergic, noradrenergic, GABAergic, glycinergic and glutamatergic, among others, are affected by prenatal nicotine. 3. Here we propose that prenatal nicotine affects the respiratory network through two main processes: (i) reorganization of neurotransmitter systems; and (ii) remodelling of neural circuits. These changes make breathing more vulnerable to fail in early postnatal life, which could be related to the pathogenesis of SIDS.

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.

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.

Prenatal nicotine exposure transiently alters the lung mechanical response to hypoxia in young lambs

To test the hypothesis that fetal nicotine exposure alters the lung mechanical response to hypoxia (10% O(2)) 10 lambs were exposed during the last fetal trimester to a low dose nicotine (LN) and 10 to a moderate dose (MN) (maternal dose 0.5 and 1.5mg/(kgday) free base, respectively). There were 10 controls (C). At 12 days, minute ventilation increased significantly less in MN compared with LN but not with C. In contrast to C and LN, MN did not show anticipated increases in dynamic compliance, specific compliance and FRC or decrease in lung resistance but had signs of airway hyperreactivity during hypoxia. Nicotine exposure did not alter the cardiovascular response. These adverse effects decreased with advancing age. In summary, prenatal nicotine exposure alters the lung mechanical response to hypoxia. We speculate that prenatal nicotine-induced alterations of lung mechanics during hypoxia may contribute to an increased vulnerability to hypoxic stress during infancy.

Biomarkers of exposure to environmental tobacco smoke in infants

Non-invasive biomonitoring of exposure to environmental tobacco smoke (ETS) by means of hair is attractive in children, although systematic evaluation is required in infants. The objective was to compare nicotine and cotinine concentrations in hair and plasma and parentally reported exposure to ETS in a birth cohort of 411 infants. Plasma was collected from 356 six-month-old infants and hair samples were collected from 368 one-year-old infants. Concentrations of nicotine and cotinine were measured by an optimized gas chromatography-mass spectrometry (GC/MS)-based method requiring 4 mg hair or 200 microl plasma. Information was obtained on the number of days with ETS exposure during the first year of life, the smoking habits of the parents, and the number of cigarettes smoked per day in the home. All three parentally reported indices of ETS exposure were significantly associated with the biomarkers, with clear dose response relationships. There was a significant association between days with ETS exposure and nicotine in hair at relatively low exposure levels (10-99 days per year), whereas the other biomarkers only showed significant increases at higher exposure levels. In conclusion, nicotine in hair appears to be the biomarker most strongly associated with parental reports on exposure to ETS in infants.

Lung-function tests in neonates and infants with chronic lung disease: tidal breathing and respiratory control

This paper is the fourth in a series of reviews that will summarize available data and critically discuss the potential role of lung-function testing in infants with acute neonatal respiratory disorders and chronic lung disease of infancy. The current paper addresses information derived from tidal breathing measurements within the framework outlined in the introductory paper of this series, with particular reference to how these measurements inform on control of breathing. Infants with acute and chronic respiratory illness demonstrate differences in tidal breathing and its control that are of clinical consequence and can be measured objectively. The increased incidence of significant apnea in preterm infants and infants with chronic lung disease, together with the reportedly increased risk of sudden unexplained death within the latter group, suggests that control of breathing is affected by both maturation and disease. Clinical observations are supported by formal comparison of tidal breathing parameters and control of breathing indices in the research setting.

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.

Respiratory control and arousal in sleeping infants

Control of the cardiovascular and respiratory systems undergoes rapid maturation during infancy. Sleep is at a lifetime maximum during this period and has a marked influence on cardiorespiratory function. The mechanisms leading to sudden infant death syndrome (SIDS) may include a failure in the neural integration of the cardiovascular and respiratory systems, with a concomitant failure to arouse from sleep. Studies have shown that sleep states exert a marked influence on respiratory control and arousability. Infants are more arousable in active sleep compared with quiet sleep from both somatosensory and respiratory stimuli. Post-natal and gestational age at birth also have a marked influence on arousability. Arousability is depressed by the major risk factors for SIDS (prone sleeping, maternal smoking, prematurity and recent infection) and is increased by factors that decrease the risk for SIDS (e.g. use of dummies, breastfeeding).

Cholinergic systems in brain development and disruption by neurotoxicants: nicotine, environmental tobacco smoke, organophosphates

Acetylcholine and other neurotransmitters play unique trophic roles in brain development. Accordingly, drugs and environmental toxicants that promote or interfere with neurotransmitter function evoke neurodevelopmental abnormalities by disrupting the timing or intensity of neurotrophic actions. The current review discusses three exposure scenarios involving acetylcholine systems: nicotine from maternal smoking during pregnancy, exposure to environmental tobacco smoke (ETS), and exposure to the organophosphate insecticide, chlorpyrifos (CPF). All three have long-term, adverse effects on specific processes involved in brain cell replication and differentiation, synaptic development and function, and ultimately behavioral performance. Many of these effects can be traced to the sequence of cellular events surrounding the trophic role of acetylcholine acting on its specific cellular receptors and associated signaling cascades. However, for chlorpyrifos, additional noncholinergic mechanisms appear to be critical in establishing the period of developmental vulnerability, the sites and type of neural damage, and the eventual outcome. New findings indicate that developmental neurotoxicity extends to late phases of brain maturation including adolescence. Novel in vitro and in vivo exposure models are being developed to uncover heretofore unsuspected mechanisms and targets for developmental neurotoxicants.

The effect of parental smoking on lung function and development during infancy

While the adverse effects of parental smoking on respiratory health during childhood are well recognized, its potential impact on early lung development is less clear. This review summarizes current evidence on the effect of parental smoking on lung function during infancy. It is difficult to separate the effects of pre- and postnatal exposure, since the majority of mothers who smoke in pregnancy (currently around 30% worldwide) continue to do so thereafter. Nevertheless, measurements undertaken prior to any postnatal exposure have consistently demonstrated significant changes in tidal flow patterns in infants whose mothers smoked in pregnancy. While there is, as yet, no convincing evidence from studies in human infants that smoking during pregnancy is associated with increased airway responsiveness at birth, many studies have demonstrated a reduction in forced expiratory flows (on average by 20%) in infants exposed to parental smoking. While maternal smoking during pregnancy remains the most significant source of such exposure and is likely to be responsible for diminished airway function in early life, continuing postnatal tobacco smoke exposure will increase the risk of respiratory infections, the combination of both being responsible for the two- to fourfold increased risk of wheezing illnesses observed during the first year of life in infants whose parents smoke. These findings emphasize the need to keep infants in a smoke-free environment both before and after birth, not least because of growing awareness that airway function in later life is largely determined by that during foetal development and early infancy.

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.

Prenatal nicotine exposure blunts the cardiorespiratory response to hypoxia in lambs

Because smoking during pregnancy is a major risk factor for late fetal death and the sudden infant death syndrome, we investigated cardiorespiratory defense mechanisms to hypoxia in 7 prenatally nicotine-exposed (N) lambs (approximate maternal dose: 0.5 mg/kg/day) and 11 control (C) lambs all at an average age of 5 days. The ventilatory response to 10% oxygen (hyperpnea) was significantly attenuated during quiet sleep in N lambs compared with C lambs and in N lambs aroused from sleep later compared with C lambs (161 +/- 90 versus 75 +/- 66 seconds, p < 0.05). The ventilatory response to hypoxia was similar in the two groups during wakefulness (W), whereas the heart rate response (tachycardia) was significantly lower in N lambs compared with C lambs during both activity states. The ventilatory response to hyperoxia was significantly lower in N lambs compared with C lambs during both activity states. Transition from W to quiet sleep was associated with a significant decrease in ventilation in C lambs but not in N lambs. In conclusion, prenatal nicotine exposure, at a dose comparable with moderate smoking, blunts major elements of the cardiorespiratory defense to hypoxia, i.e., the heart rate and ventilatory and arousal responses, and abolishes the normal decrease in ventilation during sleep compared with W.

Hair as a biomarker for exposure to tobacco smoke

This article provides an overview of the hair nicotine biomarker for assessment of exposure to tobacco smoke, with emphasis on environmental tobacco smoke (ETS). Measurement of nicotine in hair can be an informative tool for research looking at ETS and related illnesses. There are still unresolved issues in relation to this biomarker such as influence of hair treatment, hair colour, and growth rate on nicotine levels in hair, which need to be addressed in order to further refine this biomarker for exposure assessment. Nevertheless, hair nicotine promises to be a valid and reliable measure of longer term exposure that can be readily applied in epidemiological studies of exposure to tobacco smoke, and more specifically ETS, and its risk to health.

Altered breathing pattern after prenatal nicotine exposure in the young lamb

Maternal smoking during pregnancy is a risk factor for sudden fetal and infant death as well as obstructive airway disease in childhood. Fetal nicotine exposure affects organ development. The aim of the present study was to investigate effects of fetal nicotine exposure on lung function in young lambs. Nine unanesthetized, awake, prenatally nicotine-exposed lambs (N) (approximate maternal dose: 0.5 mg/kg) and 12 nonexposed control lambs (C) were studied repeatedly for 5 weeks after birth using a pneumotachograph and a computerized method for breath-by-breath determinations. N and C lambs had similar minute ventilation but a markedly different breathing pattern. At both 5 and 21 days, average age, N lambs had significantly lower tidal volumes and higher respiratory rates than C lambs. Inspiratory drive (P(0.1)) and effective impedance were significantly higher in N lambs compared with C lambs only at 5 days. Prenatal nicotine exposure appears to have long-term effects on the postnatal breathing pattern, suggesting altered lung function, e.g., increased airway resistance, decreased lung compliance, or both. The increased inspiratory drive is most likely secondary to increased impedance of the respiratory system. These changes are most marked close to birth but persist during the initial postnatal period.

Perinatal exposure to environmental tobacco smoke upregulates nicotinic cholinergic receptors in monkey brain

In humans, perinatal exposure to environmental tobacco smoke (ETS) is associated with neurobehavioral deficits. In the current study, we exposed Rhesus monkeys to ETS in late gestation and in the early neonatal period, and examined changes in neurotransmitter receptors in the brainstem and caudal portion of the cerebral cortex. Nicotinic acetylcholine receptors were markedly upregulated and the effect was selective in that there were no changes in m(2)-muscarinic acetylcholine receptors or in beta-adrenergic receptors. Nicotinic receptor upregulation is indicative of chronic cell stimulation by nicotine, and is a hallmark of nicotine-induced neuroteratogenesis. These results indicate that perinatal ETS exposes the fetus and neonate to quantities of nicotine that are sufficient to alter brain development.

Perinatal exposure to environmental tobacco smoke induces adenylyl cyclase and alters receptor-mediated cell signaling in brain and heart of neonatal rats

Perinatal exposure to environmental tobacco smoke (ETS) has adverse effects on neurobehavioral development. In the current study, rats were exposed to ETS during gestation, during the early neonatal period, or both. Brains and hearts were examined for alterations in adenylyl cyclase (AC) activity and for changes in beta-adrenergic and m2-muscarinic cholinergic receptors and their linkage to AC. ETS exposure elicited induction of total AC activity as monitored with the direct enzymatic stimulant, forskolin. In the brain, the specific coupling of beta-adrenergic receptors to AC was inhibited in the ETS groups, despite a normal complement of beta-receptor binding sites. In the heart, ETS evoked a decrease in m2-receptor expression. In both tissues, the effects of postnatal ETS, mimicking passive smoking, were equivalent to (AC) or greater than (m2-receptors) those seen with prenatal ETS mimicking active smoking; the effects of combined prenatal and postnatal exposure were equivalent to those seen with postnatal exposure alone. These data indicate that ETS exposure evokes changes in cell signaling that recapitulate those caused by developmental nicotine treatment. Since alterations in AC signaling are known to affect cardiorespiratory function, the present results provide a mechanistic link reinforcing the participation of ETS exposure, including postnatal ETS, in disturbances culminating in events like Sudden Infant Death Syndrome.

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

Modulation of heart rate (HR) during transient hyperoxia, hypoxia, and hypercapnia was studied in 46 healthy term infants on 103 occasions (postnatal d 2 to 82). Twenty-three infants had smoking mothers (median, 11 cigarettes/d). Transient chemoreceptor stimuli (100% O(2), 15% O(2), or 3% CO(2)) were presented repeatedly during quiet sleep. Beat-by-beat HR and breath-by-breath ventilation were recorded continuously. The coherently averaged HR and ventilation responses to each stimulus were calculated for each infant at each age. Outcome variables (HR change from baseline to end of stimulation, maximum HR change, and time to half-maximum) were analyzed by ANOVA. Overall, HR declined during hyperoxia (median change, 4.2 beats/min) and rose during hypoxia (median change, 4.2 beats/min) and hypercapnia (median change, 4.6 beats/min). The percentage change in HR was positively correlated with the percentage change in ventilation (p < 0.001). Increasing number of cigarettes smoked by the mother was correlated with deeper HR declines and smaller HR rises (p = 0.02). For the population as a whole, the HR response lagged 3.8 s behind the ventilatory response during hyperoxia and hypoxia (p < 0.001), whereas during hypercapnia there was no significant lag. The lag in HR response in the smoke-exposed group was 2.5 s greater than that in the control group for all three stimuli (p = 0.001), and the difference increased with the number of cigarettes smoked by the mother (p < 0.01). Both pulmonary reflexes and the type of the chemoreceptor stimulus seemed to influence HR. Maternal smoking affected the magnitude and time-course of the HR response in a dose-dependent manner.