Ventilatory response of the newborn infant to mild hypoxia

Ventilatory response and stability of oxygen saturation during a hypoxic challenge in very preterm infants

BACKGROUND

Preterm infants have immature control of breathing and impaired pulmonary gas exchange. We hypothesized that infants with bronchopulmonary dysplasia (BPD) have a blunted ventilatory response and peripheral oxygen saturation (SpO₂ ) instability during a hypoxic challenge.

METHODS

We evaluated the response to hypoxia in 57 very preterm infants (38 no BPD, 10 mild BPD, 9 moderate-to-severe BPD) at 36 weeks' postmenstrual age. The fraction of inspired oxygen (F_I O₂ ) was reduced stepwise at 5-min intervals to achieve peripheral SpO₂ between 86% and 95%. The lowest permissible F_I O₂ and SpO₂ were 0.14% and 86%. We recorded SpO₂ , F_I O₂ , and the respiratory signal (respiratory inductive plethysmography). We calculated respiratory rate (RR), tidal volume (V_T ), minute ventilation (V_E ), and respiratory drive (ratio between V_T and inspiratory time, V_T /T_I ). SpO₂ variability was expressed as the interquartile range (IQR).

RESULTS

F_I O₂ was reduced from a median (Q1, Q3) of 0.21 (0.21, 0.21) to 0.17 (0.17, 0.18). We observed a marked individual variability in the ventilatory response to the hypoxic challenge, regardless of BPD severity. At the lowest permissible F_I O₂ , 37 (65%) infants reduced their V_E , and 20 (35%) increased minute ventilation; 20 infants (35%) developed periodic breathing associated with increased SpO₂ IQR and lower SpO₂ minima, and 16 (28%) exhibited an oscillatory pattern in V_E and SpO₂ without end-expiratory pauses, regardless of BPD severity.

CONCLUSION

In very preterm infants, a mild hypoxic challenge reduced ventilation, increased SpO₂ variability and periodic breathing regardless of BPD severity.

Erythropoietin and caffeine exert similar protective impact against neonatal intermittent hypoxia: Apnea of prematurity and sex dimorphism

Apnea of prematurity (AoP) is associated with severe and repeated episodes of arterial oxygen desaturation (intermittent hypoxia - IH), which in turn increases the number of apneas. So far, there is no data addressing whether IH leads to sex-specific respiratory consequences, neither if drugs targeting AoP are more effective in males or females. We used rat pups for investigating whether IH-mediated increase of apneas is sex-specific. We also tested whether caffeine (treatment of choice of AoP), erythropoietin (Epo - a neuroprotective factor and potent respiratory stimulant), and combination of both (caffeine+Epo) prevent the IH-mediated formation of apneas in a sex-dependent manner. Newborn rats exposed to IH (21% - 10% FIO₂-8 h a day - 10 cycles per hour) during postnatal days (P) 3-10 were used in this work. Animals were administered drug vehicle, Epo, caffeine and Epo + caffeine (daily from P3 to P10) gavage. At P10 the frequency of apneas at rest (as an index of respiratory dysfunction induced by IH), and respiratory parameters were measured by plethysmography. Our results showed that IH significantly increases the number of apneas in male but not in female rat pups. Moreover, caffeine and Epo in males similarly prevented the increase of apneas induced by IH, and the administration of both drugs together did not provide a cumulative beneficial effect. No impact of drugs was evidenced in females. Apart from apneas, IH increased the normoxic basal ventilation (ventilation at rest) of male animals, and treatments did not prevent such alteration. Besides, no IH- nor treatment-mediated modulation of basal ventilation was found in the basal ventilation of female animals. Analysis of the activity of pro- and antioxidative molecules revealed that IH induces oxidative stress in the brainstem of male and female animals and that all tested treatments similarly prevented such oxidative imbalance in pups of both sexes. We concluded that neonatal IH and the treatments tested to prevent its respiratory consequences are sex-specific. The mechanics associated with such prevention are directly linked with the prevention of oxidative stress and the maturation of the brain. These findings are relevant to understanding better the AoP disorder and for proposing Epo as a new therapeutical tool.

The effects of sleeping position, maternal smoking and substance misuse on the ventilatory response to hypoxia in the newborn period

BACKGROUND

Maternal smoking, substance misuse in pregnancy and prone sleeping increase the risk of sudden infant death syndrome (SIDS). We examined the effect of maternal smoking, substance misuse and sleeping position on the newborn response to hypoxia.

METHODS

Infants born between 36 and 42 weeks of gestational age underwent respiratory monitoring in the prone and supine sleeping position before and during a hypoxic challenge. Minute ventilation (MV) and end-tidal carbon dioxide (ETCO₂) levels were assessed.

RESULTS

Sixty-three infants were studied: 22 controls, 23 whose mothers smoked and 18 whose mothers substance-misused and smoked. In the supine position, baseline MV was higher and ETCO₂ levels were lower in infants of substance-misusing mothers compared to controls (p = 0.015, p = 0.017, respectively). Infants of substance-misusing mothers had a lower baseline MV and higher ETCO₂ levels in the prone position (p = 0.005, p = 0.004, respectively). When prone, the rate of decline in minute ventilation in response to hypoxia was greater in infants whose mothers substance-misused and smoked compared to controls (p = 0.002) and infants of smoking mothers (p = 0.016).

CONCLUSION

The altered response to hypoxia in the prone position of infants whose mothers substance-misused and smoked in pregnancy may explain their increased vulnerability to SIDS.

The hypoxic test in preterm neonates reinvestigated

AIM

We currently lack a suitable gold-standard method for implementation on modern equipment to assess peripheral chemoreceptor sensitivity. The aim of the present study was to develop an accurate and reproducible method for assessing peripheral chemoreceptors sensitivity in sleeping preterm neonates.

METHODS

A poïkilocapnic hypoxic test was performed twice during rapid eye movement sleep (REM sleep) and non-rapid eye movement sleep (nonREM sleep). The infant breathed hypoxic gas (15% O₂ ) for 60 s. The ventilatory response to hypoxia was assessed by comparing minute ventilation during the control period (21% O₂ ) with successive 4-cycles sequences during hypoxia. We detected the first statistically significant increase in minute ventilation and recorded the corresponding response time.

RESULTS

During normoxia, minute ventilation was higher during REM sleep than in nonREM sleep (428.1 mL · min^-1  · kg^-1 [307.7-633.6]; 388.8 mL · min^-1  · kg^-1 [264.7-608.0], respectively; P = 0.001). After hypoxia, minute ventilation increased in both REM and nonREM sleep. The response was significantly higher in REM than in nonREM (25.3% [10.8-80.0] and 16.8% [7.5-33.2], respectively; P = 0.005). The intraclass correlation coefficients for all respiratory parameters were above 0.90.

CONCLUSION

We have developed a highly reliable method for assessing peripheral chemoreceptors sensitivity at the response time to hypoxia. In the future, researchers could use this method to assess the involvement of peripheral chemoreceptors in infants who experience chronic hypoxia (e.g. in bronchopulmonary dysplasia and recurrent apnea).

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.

Efficient breathing at neonatal ages: A sex and Epo-dependent issue

During postnatal life, the respiratory control system undergoes intense development and is highly responsive to stimuli emerging from the environment. In fact, interruption of breathing prevents gas exchange and results in systemic hypoxia that, if prolonged, can lead to cardio-respiratory failure or sudden infant death. Moreover, in newborns and infants, respiratory disorders related to neural control dysfunction show significant sexual dimorphism with a higher prevalence in males. To this day, the therapeutic tools available to alleviate these respiratory disorders remain limited. Furthermore, the factors explaining the sexual dimorphism in newborns and during infancy remain unknown. Erythropoietin (Epo) was originally discovered as a cytokine able to increase the production of red blood cells upon conditions of reduced oxygen availability. We now know that Epo is a cytokine also secreted by neurons and astrocytes that protects the brain during trauma or hypoxic stress in a sex dependent manner. In this novel line of research, our previous studies demonstrated at adult ages that cerebral Epo acts as a respiratory stimulant in rodents and humans. These results provided a strong rationale for exploring the role of cerebral Epo in neuronal respiratory control during postnatal development. The objective of this review is to summarize our recent findings showing that cerebral Epo is a potent sex-specific respiratory stimulant at neonatal ages. Keeping in mind that Epo is routinely and safely administrated in newborn humans for anemia and neonatal asphyxia, we predict that our research provides the basis necessary to promote the clinical use of Epo against neonatal respiratory disorders related to neural control dysfunction.

Antenatal substance misuse and smoking and newborn hypoxic challenge response

OBJECTIVES

Infants of smoking (S) and substance misusing (SM) mothers have an increased risk of sudden infant death syndrome. The aim of this study was to test the hypothesis that infants of SM or S mothers compared with infants of non-SM, non-smoking mothers (controls) would have a poorer ventilatory response to hypoxia, which was particularly marked in the SM infants.

DESIGN

Physiological study.

SETTING

Tertiary perinatal centre.

PATIENTS

21 SM; 21 S and 19 control infants. Infants were assessed before maternity/neonatal unit discharge.

INTERVENTIONS

Maternal and infant urine samples were tested for cotinine, cannabinoids, opiates, amphetamines, methadone, cocaine and benzodiazepines.

MAIN OUTCOME MEASURES

During quiet sleep, the infants were switched from breathing room air to 15% oxygen and changes in minute volume were assessed.

RESULTS

The SM infants had a greater mean increase (p=0.028, p=0.034, respectively) and a greater magnitude of decline (p<0.001, p=0.018, respectively) in minute volume than the S infants and the controls. The rate of decline in minute volume was greater in the SM infants (p=0.008) and the S infants (p=0.011) compared with the controls.

CONCLUSIONS

Antenatal substance misuse and smoking affect the infant's ventilatory response to a hypoxic challenge.

The role of physiological studies and apnoea monitoring in infants

There is evidence that failure of cardio-respiratory control mechanisms plays a role in the final event of the Sudden Infant Death Syndrome (SIDS). Physiological studies during sleep in both healthy term born infants and those at increased risk for SIDS have been widely used to investigate how the major risk and protective factors for SIDS identified from epidemiological studies might alter infant physiology. Clinical polysomnography (PSG) in infants who eventually succumbed to SIDS however demonstrated abnormalities that were neither sufficiently distinctive nor predictive to support routine use of PSG for infants at risk for SIDS. PSG findings have also been shown to be not predictive of recurrence of Apparent Life Threatening Events (ALTE) and thus international guidelines state that PSG is not indicated for routine evaluation in infants with an uncomplicated ALTE, although PSG may be indicated when there is clinical evidence of a sleep related breathing disorder. A decision to undertake home apnoea monitoring should consider the potential advantages and disadvantages of monitoring for that individual, in the knowledge that there is no evidence of the efficacy of such devices in preventing SIDS.

Cardio-respiratory control during sleep in infancy

During the first year of life and particularly the first 6 months autonomic control of the cardio-respiratory system is still undergoing maturation and infants are at risk of cardio-respiratory instability. These instabilities are most marked during sleep, which is important as infants spend the majority of each 24 hours in sleep. Sleep state has a marked effect on the cardio-respiratory system with instabilities being more common in active sleep compared to quiet sleep. Responses to hypoxia are also immature during infancy and may make young infants more vulnerable to cardio-respiratory instability. It has been proposed that an inability to respond appropriately to a life threatening event underpins the Sudden Infant Death Syndrome (SIDS). The major risk factors for SIDS, prone sleeping and maternal smoking, both impair cardio-respiratory control in normal healthy term infants.

Assessing ventilatory control in infants at high risk of sleep disordered breathing: a study of infants with cleft lip and/or palate

Neonatal exposure to intermittent hypoxia results in altered ventilatory response to subsequent hypoxia in animal models. The effect of similar exposure in human infants is unknown. Our objective was to determine the impact of sleep disordered breathing (SDB) in early infancy on ventilatory response in infants. We recruited consecutive infants with cleft lip and/or palate (CL/P) to undergo ventilatory response testing using exposure to a hypoxic (15% O(2) ) gas mixture during sleep. This population is at high risk of SDB because of smaller airway caliber and abnormal palatal muscle attachments predisposing them to airway obstruction of ranging severity from birth. Ventilatory responses were compared between infants with a low apnea-hypopnea index (AHI; AHI < 15 events/hr) and a high AHI (AHI ≥ 15 events/hr). Testing was successfully completed in 22 of 23 infants who underwent testing at 4.4 ± 4.8 months. Infants with high AHI had lower weight z-scores, higher number of oxygen desaturation events during sleep, but similar oxygen saturation (S(p) O(2) ) nadir compared to infants with low AHI. The pattern of ventilatory response to hypoxia differed between the two groups; infants with high AHI had an earlier ventilatory decline and a blunted maximal ventilatory response to hypoxia. Infants with a high AHI use a different strategy to augment ventilation in response to hypoxia; while infants with a low AHI initially increased respiratory rate, tidal volume was the first parameter to increase in infants with high AHI. These results demonstrate that SDB in infancy is associated with altered ventilatory response to hypoxia.

Laryngeal reflex apnea in neonates: effects of CO2 and the complex influence of hypoxia

We have examined influence of hypocapnia, mild hypercapnia and hypoxia on the durations of fictive apnea and respiratory disruption elicited by injection of 0.1ml of water into the laryngeal lumen-the laryngeal chemoreflex (LCR)-in 20 unanesthetized, decerebrate, vagotomized piglets aged 4-10 days that were paralyzed and ventilated with a constant frequency and tidal volume. The LCR was enhanced by hypocapnia and attenuated by hypercapnia as reported by others. The responses to laryngeal stimulation during hypoxia were varied and complex: some animals showed abbreviated responses during the tachypnea of early hypoxia, followed after 10-15min by more prolonged apnea and respiratory disruption accompanying the reduction in ventilatory activity that commonly occurs during sustained hypoxia in neonates. We speculate that this later hypoxic enhancement of the LCR may be due to accumulation of adenosine in the brain stem.

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.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Neurochemical and physiological correlates of a critical period of respiratory development in the rat

Despite its vital importance to life, respiration is not mature at birth in mammals, but rather, it undergoes a great deal of growth, refinement, and adjustments postnatally. Many adjustments do not follow smooth paths, but assume abrupt changes during certain postnatal periods that may render the animal less capable of responding to respiratory stressors. The present review focuses on neurochemical and physiological correlates of a critical period of respiratory development in the rat. In addition to an imbalanced expression of reduced excitatory and enhanced inhibitory neurotransmitters, a switch in the expressions of gamma-aminobutyric acid (GABA)A receptor subunits from alpha3 to alpha1 occurs around postnatal day (P)12 in the pre-Bötzinger nucleus and the ventrolateral subnucleus of the solitary tract nucleus. Possible subunit switches in a number of other neurotransmitter receptors are discussed. These neurochemical changes are paralleled by ventilatory adjustments at the end of the second postnatal week. At P13 and under normoxia, respiratory frequency reaches its peak before assuming a gradual fall, and both tidal volume and minute ventilation exhibit a significant rise prior to a plateau or a gradual decline until P21. The response to acute hypoxia is markedly reduced between P12 and P16, being lowest at P13. Thus, the end of the second postnatal week can be considered as a critical period of respiratory development, during which multiple neurochemical and physiological adjustments and switches are orchestrated at the same time, rendering the system extremely dynamic but, at the same time, vulnerable to externally imposed perturbations and insults. The critical period embodies a time of multi-system, multifaceted growth and adjustments. It is a plastic, transitional period that is also a part of the normal development of the respiratory system.

Dyspnoea in children. Does development alter the perception of breathlessness?

Dyspnoea, the perception of an unpleasant and/or uncomfortable sensation of breathlessness, offers several physiological, anatomical and teleological analogies with pain. Pain perception has been shown to exist in the newborn, suggesting that dyspnoea may also occur from birth onwards. The perception of breathlessness will be subservient to developmental changes in the behaviour of sensors and lung and muscular receptors implicated in dyspnoea, some of which are known to be active at time of birth. For example, perinatal resetting of the arterial chemoreceptor could lead to transient depression of the dyspnoeic response to hypoxia. However, though early evoked ventilatory responses and peripheral receptor maturation do exist, dyspnoea will only occur if the corresponding central neural circuitry undergoes parallel maturation. Our knowledge of dyspnoea in later childhood is based on a small number of clinical or psychophysical studies, predominantly dealing with asthma and exercise. There is a thus a clear need for systematic assessment of the existence and severity of dyspnoea sensing in younger children that takes into account its role as an alarm mechanism for triggering adaptive and/or protective responses.

Postnatal cardiopulmonary adaptations to high altitude

Postnatal cardiopulmonary adaptations to high altitude constitute a key component of any set of responses developed to face high altitude hypoxia. Such responses are required ultimately to meet the energy demands necessary for adequate functioning at cell and organism level. After a brief insight on general and cardiopulmonary comparative studies in growing and adult organisms, differences and possible explanations for varying cardiopulmonary pathology, pulmonary artery hypertension, persistent right ventricular predominance and subacute high altitude pulmonary hypertension in different populations of children living at high altitude are discussed. Potential long-term implications of early chronic hypoxic exposure on later diseases are also presented. It is hoped that this review will help the practicing physician working at high altitude to make informed decisions concerning individual pediatric patients, specifically with regard to diagnosis and management of altitude-related cardiopulmonary pathology. Finally, plausibility and the knowledge-base of public health interventions to reduce the risks posed by suboptimal or inadequate postnatal cardiopulmonary responses to high altitude are discussed.

Maturation of the initial ventilatory response to hypoxia in sleeping infants

In infants most previous studies of the hypoxic ventilatory response (HVR) have been conducted only during quiet sleep (QS) and arousal responses have not been considered. Our aim was to quantify the maturation of the HVR in term infants during both active sleep (AS) and QS over the first 6 months of life. Daytime polysomnography was performed on 15 healthy term infants at 2-5 weeks, 2-3 and 5-6 months after birth and infants were challenged with hypoxia (15% O2, balance N2). Tests in AS always resulted in arousal; in QS tests infants either aroused or did not arouse. A biphasic HVR was observed in non arousing tests at all three ages studied. The fall in SpO2 was more rapid in arousal tests at all three ages. At 2-5 weeks, in non-arousing QS tests, there was a greater fall in respiratory frequency (f) despite a smaller fall in SpO2 compared with 2-3 and 5-6 months. When infants aroused there was no difference in the HVR between sleep states or with postnatal age. However, when infants failed to arouse from QS, arterial desaturation was less in the younger infants despite a poorer HVR. We suggest that arousal in response to hypoxia, particularly in AS, is a vital survival mechanism throughout the first 6 months of life.

Postnatal changes in ventilation during normoxia and acute hypoxia in the rat: implication for a sensitive period

Previously, we found heightened expression of inhibitory neurochemicals and depressed expression of excitatory neurochemicals with a sudden drop in metabolic activity around postnatal day (P) 12 in rat brainstem respiratory nuclei, suggesting that this period is a critical window during which respiratory control or regulation may be distinctly different. To test this hypothesis, the hypoxic ventilatory responses (HVR) to 10% oxygen were tested in rats every day from P0 to P21. Our data indicate that (1) during normoxia (N), breathing frequency (f) increased with age, peaking at P13, followed by a gradual decline, whereas both tidal volume (V(T)) and minute ventilation (.V(E) ) significantly increased in the second postnatal week, followed by a progressive increase in V(T) and a relative plateau in .V(E); (2) during 5 min of hypoxia (H), .V(E) exhibited a biphasic response from P3 onward. Significantly, the ratio of .V(E)(H) to .V(E)(N) was generally > 1 during development, except for P13-16, when it was < 1 after the first 1-2 min, with the lowest value at P13; (3) the H : N ratio for f, V(T) and .V(E) during the first 30 s and the last minute of hypoxia all showed a distinct dip at P13, after which the V(T) and .V(E) values rose again, while the f values declined through P21; and (4) the H : N ratios for f, V(T) and .V(E) averaged over 5 min of hypoxia all exhibited a sudden fall at P13. The f ratio remained low thereafter, while those for V(T) and .V(E) increased again with age until P21. Thus, hypoxic ventilatory response is influenced by both f and V(T) before P13, but predominantly by V(T) after P13. The striking changes in normoxic ventilation as well as HVR at or around P13, together with our previous neurochemical and metabolic data, strongly suggests that the end of the second postnatal week is a critical period of development for brainstem respiratory nuclei in the rat.

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.

Variability of the initial phase of the ventilatory response to hypoxia in sleeping infants

Most of the available data on the hypoxic ventilatory response (HVR) in infants has been obtained in quiet sleep (QS), and only one study has made repeated tests in the same infant. We aimed to gain a more complete knowledge of the maturation and consistency of the initial phase of the HVR by performing multiple tests in both QS and active sleep (AS) over the first 6 mo of life in term infants. Fifteen healthy term infants were studied with daytime polysomnography longitudinally at 2-5 wk, 2-3 mo, and 5-6 mo after birth. Each infant received multiple hypoxic (15% O2, balance N2) challenges (three or more) in both AS and QS. In AS, infants consistently aroused to hypoxia; however, in QS, infants both aroused and failed to arouse. The initial phase of the HVR varied considerably between infants with the changes in ventilation/kg [SD of inspired minute ventilation per kilogram of body weight (V(I)/kg)] being more variable during AS than QS at all three ages and overall decreasing with postnatal age in both sleep states. The variability between replicate V(I)/kg measurements was also significantly greater in AS compared with QS at 2-5 wk postnatal age. There was no evidence of habituation to repeated hypoxic tests in either sleep state. Our study has demonstrated that the initial phase of the HVR is variable both between and within term infants in both AS and QS, with responses being markedly more variable during AS, and becoming more consistent with increasing postnatal age. By performing only one test or by failing to account for arousal responses, previous studies may not have detected the natural variation of the infant HVR.

Postnatal development of ventilatory and arousal responses to hypoxia in human infants

During the first year of life there is significant maturation of the hypoxic ventilatory response (HVR) in human infants. Compared with adults, healthy term infants have an immature HVR until at least 6 months of age. There are few studies in infants on the effects of sleep state on the HVR but these suggest that at early postnatal ages there is initially no sleep-state related difference; this is followed by a developmental trend towards the adult situation in which the response is depressed in REM sleep compared with NREM. Maternal cigarette smoking is a major risk factor for SIDS and the mechanism for this may involve a depressed HVR in the exposed infant; however studies are limited and the wide variation in cigarette consumption makes interpretation of results difficult. Arousal responses to hypoxia are of vital importance and a failure to arouse has been implicated in SIDS. Sleeping infants frequently fail to arouse in response to hypoxia in QS, whereas in AS they invariably arouse; furthermore arousal latency is longer in QS compared with AS. The oxygen saturation at which infants arouse is not different between sleep states, suggesting that desaturation is more rapid in AS. In QS younger infants arouse more readily than at older ages and arousal is depressed by maternal smoking. These findings suggest that depression of the arousal response to hypoxia in AS may have life-threatening consequences. Infants at increased risk for SIDS have been shown to have both depressed ventilatory and arousal responses to hypoxia, thus they may be at even greater risk.

Development of chemoreceptor responses in infants

This paper is devoted to the field of chemoreception and its role in the control of breathing in infants. We use "chemoreception" to refer to the capacity to sense and process changes in P(O2) and P(CO2), and also to react to these changes by adjusting ventilation in order to maintain homeostasis. Functional chemoreceptors are not essential to commence or even to sustain breathing efforts immediately at or after birth; the intense brain activation, which occurs at birth, is sufficient. Over subsequent days to weeks, however, this "neurogenic" drive weakens and drive from the chemoreceptors becomes critical for generating and maintaining a normal breathing rhythm. Failure of the chemoreceptors to develop normally, consequently, becomes an important underlying cause of breathing dysfunction, particularly during sleep. The paper deals with the methods available to study chemoreception in newborn infants and provide an overview of the early postnatal changes and interactions, which influence breathing at rest and under stress. The latter may be described in terms of the threshold and strength as well as the delay/speed with which ventilation changes in response to chemical stimulation. We conclude with a survey of disorders associated with chemoreceptor deficits in infancy.

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).

Responses to hypoxia during early development

Sustained hypoxia evokes a predictable cascade of ventilatory, neurochemical, and metabolic responses. Responses in immature animals are characterized by earlier and more marked depression of ventilation than fully mature animals. Ventilation during hypoxia reflects a collective system output, incorporating a number of compensatory mechanisms (stimulation or depression) from multiple systems. The time course of these responses is clearly developmentally regulated. When hypercapnia interacts with hypoxia, the ventilatory responses are enhanced but other responses are apparently unchanged. We propose a model in which responses to intermittent stimuli vary according to the point within the sequence of a single response where the stimulus interruption occurs. An intermittent stimulus may be seen as 'continuous' if the recurrence frequency exceeds a certain threshold, whereas application of slower cycles below such threshold may elicit discordant recruitment of the compensatory responses. Indeed, experimental observations on intermittent (hypercapnic or poikylocapnic) hypoxia show excitatory or depressant effects that are dictated by the cycle duration. Subject to further testing, this model may help explain how detrimental effects of hypoxic events in infancy only affect selected groups.

Ventilatory responses preceding hypoxia-induced arousal in infants: effects of sleep-state

Augmented ventilation and/or arousal in response to hypoxia are important protective mechanisms during sleep. We aimed to quantify ventilatory responses preceding hypoxia-induced arousal in infants and determine the effects of sleep-state. Fifteen term infants were studied at 2-4 weeks, 2-3 and 5-6 months of age. Ventilatory responses to 15% oxygen inhalation were expressed as breath-by-breath changes from normoxic levels and averaged over 5, 10 and 15 breaths preceding arousal. Minute ventilation preceding arousal significantly increased above normoxic levels only in AS at 5-6 months. There were no sleep-state related differences in minute ventilation, oxygen saturation or carbon dioxide levels (expressed as changes from normoxic values) at 5, 10 or 15 breaths preceding arousal. However, the rate of oxygen desaturation during hypoxia in AS was two to four times faster than in QS at each age. We conclude that the ventilatory responses preceding hypoxia-induced arousal do not differ between sleep-states and that arousal occurs at similar levels of desaturation in both states.

Pain management in the critically ill child

Children frequently received no treatment, or inadequate treatment, for pain and for painful procedures. The newborn and critically ill children are especially vulnerable to no treatment or under-treatment. Nerve pathways essential for the transmission and perception of pain are present and functioning by 24 weeks of gestation. The failure to provide analgesia for pain results in rewiring the nerve pathways responsible for pain transmission in the dorsal horn of the spinal cord and results in increased pain perception for future painful results. Many children would withdraw or deny their pain in an attempt to avoid yet another terrifying and painful experiences, such as the intramuscular injections. Societal fears of opioid addiction and lack of advocacy are also causal factors in the under-treatment of pediatric pain. False beliefs about addictions and proper use of acetaminophen and other analgesics resulted in the failure to provide analgesia to children. All children even the newborn and critically ill require analgesia for pain and painful procedures. Unbelieved pain interferes with sleep, leads to fatigue and a sense of helplessness, and may result in increased morbidity or mortality.

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

AIMS

To assess the effect of prenatal cigarette smoke exposure on the postnatal resetting of oxygen sensitivity in term infants.

METHODS

15 healthy term infants of smoking mothers (median 10 cigarettes/day) and 16 controls were studied during quiet sleep 1, 3, and 10 days and 10 weeks postnatally. Strain-gauge respiratory trace was continuously recorded. Repeated 15-s challenges with 100% O2 and 15% O2 were presented in randomised order through a face mask. A median of six hyperoxic and six hypoxic challenges per recording were obtained. Breath-by-breath ventilation in a time-window from 20 s before onset of stimulus to 60 s after was extracted. For each infant at each age, the normalised coherently averaged response to hyperoxia and hypoxia was calculated. Mean ventilation at end of the 15-s stimulus was analysed with ANOVA, as were parameters describing a function fitted to each averaged response.

RESULTS

During air breathing, smoke-exposed infants had higher respiratory rates and lower tidal volumes than controls. Nicotine concentration in infant hair, measured by gas chromatography, was positively correlated with maternal level of smoking. A long-term development in oxygen sensitivity was demonstrated in both groups. However, neither the time-course nor the magnitude of O2 responses was affected by maternal smoking. Overall, hyperoxia reduced ventilation by 6.3% at day 1, 13.2% at day 3, 29.6% at day 10, and 40.0% at week 10. Transient hypoxia increased ventilation by 3.5%, 3.2%, 6.4%, and 8.8%, respectively, at the four ages studied.