Measurement of the CO2 apneic threshold in newborn infants: possible relevance for periodic breathing and apnea

Control of breathing in preterm infants on incubator oxygen or nasal cannula oxygen

BACKGROUND

Incubator oxygen may improve respiratory stability in preterm infants compared with nasal cannula oxygen.

METHODS

Single center randomized trial of infants <29 weeks' gestation on supplemental oxygen at ≥32 weeks' postmenstrual age. Infants were crossed-over every 24 hours for 96 hours between incubator oxygen and nasal cannula ≤1.0 L/kg/min. We measured episodes of intermittent hypoxemia (oxygen saturations (SpO2) < 85% ≥10 seconds), bradycardia, cerebral and abdominal hypoxemia, and end-tidal carbon dioxide.

RESULTS

We enrolled 25 infants with a gestational age of 26 weeks 4 days±15 days (mean ± SD) and birth weight 805 ± 202 grams. There were no differences in episodes of intermittent hypoxemia, bradycardia, or cerebral hypoxemia between groups. There were fewer episodes of abdominal hypoxemia <40% ≥10 seconds with incubator oxygen compared with nasal cannula (132 ± 130 versus 158 ± 125; p < 0.01). Time with SpO2 < 85% and abdominal hypoxemia was lower among infants on incubator oxygen. Carbon dioxide values were higher while on incubator oxygen (41 ± 11 versus 36 ± 10 mmHg; p < 0.02).

CONCLUSION

There was no difference in intermittent hypoxemia between incubator and nasal cannula oxygen among preterm infants on supplemental oxygen. Infants had higher levels of carbon dioxide while on incubator oxygen, which may have improved some measures of respiratory stability. CLINCALTRIALS.

GOV IDENTIFIERS

NCT03333174 and NCT03174301.

IMPACT STATEMENT

In this randomized cross-over trial of preterm infants on supplemental oxygen, incubator oxygen did not decrease episodes of intermittent hypoxemia compared with nasal cannula oxygen. Incubator oxygen reduced time with oxygen saturations less than 85%, reduced abdominal hypoxemia, and increased carbon dioxide levels. Differences in measures of respiratory stability on incubator oxygen may be partly due to higher carbon dioxide levels compared with nasal cannula oxygen. The mode of supplemental oxygen administration may impact control of breathing in preterm infants through its effect on hypopharyngeal oxygen stability and carbon dioxide levels.

Late permissive hypercapnia and respiratory stability among very preterm infants: a pilot randomised trial

OBJECTIVE

Determine if targeting higher transcutaneous carbon dioxide improves respiratory stability among very preterm infants on ventilatory support.

DESIGN

Single-centre pilot randomised clinical trial.

SETTING

The University of Alabama at Birmingham.

PATIENTS

Very preterm infants on ventilatory support after postnatal day 7.

INTERVENTIONS

Infants were randomised to two different transcutaneous carbon dioxide levels targeting 5 mm Hg (0.67 kPa) changes with four sessions each lasting 24 hours for 96 hours: baseline-increase-baseline-increase or baseline-decrease-baseline-decrease.

MAIN OUTCOME MEASURES

We collected cardiorespiratory data evaluating episodes of intermittent hypoxaemia (oxygen saturations (SpO2)<85% for ≥10 s), bradycardia (<100 bpm for ≥10 s), and cerebral and abdominal hypoxaemia on near-infrared spectroscopy.

RESULTS

We enrolled 25 infants with a gestational age of 24 w 6 d±11 d (mean±SD) and birth weight 645±142 g on postnatal day 14±3. Continuous transcutaneous carbon dioxide values (56.8±6.9 in the higher group vs 54.5±7.8 in the lower group; p=0.36) did not differ significantly between groups during the intervention days. There were no differences in intermittent hypoxaemia (126±64 vs 105±61 per 24 hours; p=0.30) or bradycardia (11±16 vs 15±23 per hour; p=0.89) episodes between groups. The proportion of time with SpO2<85%, SpO2<80%, cerebral hypoxaemia or abdominal hypoxaemia did not differ (all p>0.05). There was moderate negative correlation between mean transcutaneous carbon dioxide and bradycardia episodes (r=-0.56; p<0.001).

CONCLUSION

Targeting 5 mm Hg (0.67 kPa) changes in transcutaneous carbon dioxide did not improve respiratory stability among very preterm infants on ventilatory support but the intended carbon dioxide separation was difficult to achieve and maintain.

TRIAL REGISTRATION NUMBER

NCT03333161.

Control of Breathing and Central Hypoventilation Syndromes

Control of breathing in children varies with age and sleep state. There is overlap between central hypoventilation, autonomic dysfunction, and hypothalamic dysfunction in the rare disorders (congenital central hypoventilation syndrome and rapid-onset obesity, hypoventilation, hypothalamic dysfunction, and autonomic dysregulation). Other, more common disorders that typically present in childhood also include central hypoventilation and disordered ventilatory responses.

Lesions causing central sleep apnea localize to one common brain network

Objectives

To characterize the specific brain regions for central sleep apnea (CSA) and identify its functional connectivity network.

Methods

We performed a literature search and identified 27 brain injuries causing CSA. We used a recently validated methodology termed “lesion network mapping” to identify the functional brain network subtending the pathophysiology of CSA. Two separate statistical approaches, the two-sample t-test and the Liebermeister test, were used to evaluate the specificity of this network for CSA through a comparison of our results with those of two other neurological syndromes. An additional independent cohort of six CSA cases was used to assess reproducibility.

Results

Our results showed that, despite lesions causing CSA being heterogeneous for brain localization, they share a common brain network defined by connectivity to the middle cingulate gyrus and bilateral cerebellar posterior lobes. This CSA-associated connectivity pattern was unique when compared with lesions causing the other two neurological syndromes. The CAS-specific regions were replicated by the additional independent cohort of six CSA cases. Finally, we found that all lesions causing CSA aligned well with the network defined by connectivity to the cingulate gyrus and bilateral cerebellar posterior lobes.

Conclusion

Our results suggest that brain injuries responsible for CSA are part of a common brain network defined by connectivity to the middle cingulate gyrus and bilateral cerebellar posterior lobes, lending insight into the neuroanatomical substrate of CSA.

Apnoea of Prematurity and Neurodevelopmental Outcomes: Current Understanding and Future Prospects for Research

Infants who are born prematurely are at significant risk of apnoea. In addition to the short-term consequences such as hypoxia, apnoea of prematurity has been associated with long-term morbidity, including poor neurodevelopmental outcomes. Clinical trials have illustrated the importance of methylxanthine drugs, in particular caffeine, in reducing the risk of long term adverse neurodevelopmental outcomes. However, the extent to which apnoea is causative of this secondary neurodevelopmental delay or is just associated in a background of other sequelae of prematurity remains unclear. In this review, we first discuss the pathophysiology of apnoea of prematurity, previous studies investigating the relationship between apnoea and neurodevelopmental delay, and treatment of apnoea with caffeine therapy. We propose a need for better methods of measuring apnoea, along with improved understanding of the neonatal brain's response to consequent hypoxia. Only then can we start to disentangle the effects of apnoea on neurodevelopment in preterm infants. Moreover, by better identifying those infants who are at risk of apnoea, and neurodevelopmental delay, we can work toward a risk stratification system for these infants that is clinically actionable, for example, with doses of caffeine tailored to the individual. Optimising treatment of apnoea for individual infants will improve neonatal care and long-term outcomes for this population.

The maturation changes of sleep-related respiratory abnormalities in infants with laryngomalacia

STUDY OBJECTIVES

Obstructive sleep apnea (OSA) and central sleep apnea (CSA) are common in infants with laryngomalacia. The purpose of this study was to evaluate developmental changes in sleep-related breathing disorders over time in infants with laryngomalacia and understand the effect of supraglottoplasty (SGP) and nonsurgical treatment.

METHODS

This is a retrospective review of infants with laryngomalacia who had at least 2 diagnostic polysomnography studies performed from January 2000 and May 2015. We included infants who had either OSA or CSA. Comparison of sleep and respiratory parameters by age group (0-6, 6-12, and >12 months old) was performed in both SGP and non-SGP groups using a mixed-effect regression model. A log-normal mixed model was used to explore the changes in sleep and respiratory parameters with age. The time to resolution of CSA and OSA was analyzed using nonparametric survival analysis.

RESULTS

A total of 102 infants were included; 57 had only OSA and 45 had both CSA and OSA. There were significant decreases in apnea-hypopnea index, obstructive index, central apnea index, and arousal index with increasing age in both SGP and non-SGP groups. The mean age at resolution of CSA (central apnea index < 5) was 7.60 months old for SGP and 12.57 months old for non-SGP (P < .05). There were no significant differences in the mean age at resolution of OSA (obstructive index < 1; 35.18 [SGP] vs 41.55 months [non-SGP]; P = .60) between SGP and non-SGP groups. Infants with neurologic disease, congenital anomalies, or genetic syndromes required significantly more time to resolve OSA (28.12 [normal] vs 53.13 [neurological] vs 59.53 months [congenital anomalies and genetic]; P < .01).

CONCLUSIONS

Both OSA and CSA improve in infants with laryngomalacia with increasing age regardless of SGP. The mechanism underlying these changes may involve airway growth and maturation of respiratory control. Time to resolution of OSA is affected by the presence of neurologic diseases, congenital anomalies, and genetic syndromes. Further studies are needed to confirm these findings and to evaluate long-term outcomes in this population.

Central apnea and periodic breathing in children with underlying conditions

Central sleep apneas and periodic breathing are poorly described in childhood. The aim of the study was to describe the prevalence and characteristics of central sleep apnea and periodic breathing in children with associated medical conditions, and the therapeutic management. We retrospectively reviewed all poly(somno)graphies with a central apnea index ≥ 5 events per hr in children aged > 1 month performed in a paediatric sleep laboratory over a 6‐year period. Clinical data and follow‐up poly(somno)graphies were gathered. Ninety‐five out of 2,981 patients (3%) presented central sleep apnea: 40% were < 1 year, 41% aged 1–6 years, and 19% aged ≥ 6 years. Chiari malformation was the most common diagnosis (13%). Mean central apnea index was 20 ± 30 events per hr (range 5–177). Fifty‐eight (61%) children had an exclusive central pattern with < 5 obstructive events per hr. Periodic breathing was present in 79 (83%) patients, with a mean percentage of time with periodic breathing of 9 ± 16%. Among periodic breathing episodes, 40% appeared after a sigh, 8% after an obstructive event, 6% after breathing instability and 2% after bradypnea. The highest clinical apnea index and percentage of time with periodic breathing were observed in children with encephalopathy and/or epilepsy (68 ± 63 events per hr and 30 ± 34%). Clinical apnea index did not differ according to age, while periodic breathing duration was longer in children > 1 year old. Watchful waiting was performed in 22 (23%) patients with spontaneous improvement in 20. Other treatments (upper airway or neurosurgery, nocturnal oxygen therapy, continuous positive airway pressure, non‐invasive ventilation) were effective in selected patients. Central sleep apnea is rare in children and comprises heterogeneous conditions. Sleep studies are essential for the diagnosis, characterization and management of central sleep apnea.

Observational study to define reference ranges for the 3% oxygen desaturation index during sleep in healthy children under 12 years using oximetry motion-resistant technology

OBJECTIVE

To define reference ranges for the 3% oxygen desaturation index (DI3) in healthy children under 12 years old during sleep.

DESIGN

Observational.

SETTING

Home.

SUBJECTS

Healthy children aged 6 months to 12 years of age.

INTERVENTION

Nocturnal pulse oximetry at home. Parents documented sleep times. Visi-Download software (Stowood Scientific) analysed data with artefact and wake periods removed.

MAIN OUTCOME MEASURES

The following oximetry parameters used in the assessment of sleep-disordered breathing conditions were measured: 3% (DI3) and 4% (DI4) oxygen desaturation indices-the number of times per hour where the oxygen saturation falls by at least 3% or 4% from baseline, mean saturations (SAT50), minimum saturations (SATmin), delta index 12 s (DI12s), and percentage time with saturations below 92% and 90%.

RESULTS

Seventy-nine children underwent nocturnal home pulse oximetry, from which there were 66 studies suitable for analysis. The median values for DI3 and DI4 were 2.58 (95% CI 1.96 to 3.10) and 0.92 (95% CI 0.73 to 1.15), respectively. The 95th and 97.5th centiles for DI3 were 6.43 and 7.06, respectively, which inform our cut-off value for normality. The mean values for SAT50 and SATmin were 97.57% (95% CI 97.38% to 97.76%) and 91.09% (95% CI 90.32% to 91.86%), respectively.

CONCLUSION

In children aged 6 months to 12 years, we define normality of the 3% oxygen desaturation index as <7 using standalone, motion-resistant pulse oximeters with short averaging times.

Effect of spontaneous movement on respiration in preterm infants

NEW FINDINGS

What is the central question of this study? The respiratory centres in the brainstem that control respiration receive inputs from various sources, including proprioceptors in muscles and joints and suprapontine centres, which all affect limb movements. What is the effect of spontaneous movement on respiration in preterm infants? What is the main finding and its importance? Apnoeic events tend to be preceded by movements. These activity bursts can cause respiratory instability that leads to an apnoeic event. These findings show promise that infant movements might serve as potential predictors of life-threatening apnoeic episodes, but more research is required.

ABSTRACT

A common condition in preterm infants (<37 weeks' gestational age) is apnoea resulting from immaturity and instability of the respiratory system. As apnoeas are implicated in several acute and long-term complications, prediction of apnoeas may preempt their onset and subsequent complications. This study tests the hypothesis that infant movements are a predictive marker for apnoeic episodes and examines the relation between movement and respiration. Movement was detected using a wavelet algorithm applied to the photoplethysmographic signal. Respiratory activity was measured in nine infants using respiratory inductance plethysmography; in an additional eight infants, respiration and partial pressure of airway carbon dioxide ( P C O 2 ) were measured by a nasal cannula with side-stream capnometry. In the first cohort, the distribution of movements before and after the onset of 370 apnoeic events was compared. Results showed that apnoeic events were associated with longer movement duration occurring before apnoea onsets compared to after. In the second cohort, respiration was analysed in relation to movement, comparing standard deviation of inter-breath intervals (IBI) before and after apnoeas. Poincaré maps of the respiratory activity quantified variability of airway P C O 2 in phase space. Movement significantly increased the variability of IBI and P C O 2 . Moreover, destabilization of respiration was dependent on the duration of movement. These findings support that bodily movements of the infants precede respiratory instability. Further research is warranted to explore the predictive value of movement for life-threatening events, useful for clinical management and risk stratification.

An overview of developmental dysregulation of autonomic control in infants

In this short review, we provide an overview of developmental disorders causing autonomic nervous system dysregulation. We briefly discuss perinatal conditions that adversely impact developmental outcomes including apnea of prematurity, sudden infant death syndrome, and Rett syndrome. We provide a brief clinical description, an overview of known or hypothesized mechanisms for the disorder, and current standard of practice for treatment of each condition. Additionally, we consider preventative measures and complications of these disorders to provide further insight into the pathogenesis of specific autonomic dysregulation in neonates. The goal of this short review is to provide an updated understanding of the impact of autonomic dysregulation on development of brainstem circuits and to briefly highlight promising future treatment options and controversies.

Hour-Specific Total Serum Bilirubin Percentiles for Infants Born at 29-35 Weeks' Gestation

INTRODUCTION

As preterm infants are susceptible to hyperbilirubinemia, they require frequent close monitoring. Prior to initiation of phototherapy, hour-specific total serum bilirubin (TSB) percentile cut-points are lacking in these infants, which led to the current study.

METHODS

A multi-site retrospective cohort study of preterm infants born between January 2013 and June 2017 was completed at 3 NICUs in Ontario, Canada. A total of 2,549 infants born at 290/7-356/7 weeks' gestation contributed 6,143 pre-treatment TSB levels. Hour-specific TSB percentiles were generated using quantile regression, further described by degree of prematurity, and among those who subsequently received phototherapy.

RESULTS

Among all infants, at birth, hour-specific pre-treatment, TSB percentiles were 36.1 µmol/L (95% confidence interval [CI]: 34.3-39.3) at the 40th, 52.3 µmol/L (49.4-55.1) at the 75th, and 79.5 µmol/L (72.1-89.6) at the 95th percentiles. The corresponding percentiles were 39.3 μmol/L (35.9-43.2), 55.4 μmol/L (52.1-60.2), and 87.1 μmol/L (CI 70.5-102.4) prior to initiating phototherapy and 24.4 μmol/L (20.4-28.8), 35.3 μmol/L (31.1-41.5), and 52.0 μmol/L (46.1-62.4) among those who did not receive phototherapy. Among infants born at 29-32 weeks, pre-treatment TSB percentiles were 53.9 µmol/L (49.4-61.0) and 95.5 µmol/L (77.5-105.0) at the 75th and 95th percentiles, with respective values of 48.7 µmol/L (43.0-52.3), and 74.1 µmol/L (64.8-83.2) for those born at 33-35 weeks' gestation.

CONCLUSION

Hour-specific TSB percentiles, derived from a novel nomogram, may inform how bilirubin is described in preterm newborns. Further research of pre-treatment TSB levels is required before clinical consideration.

Intermittent hypoxia and bronchial hyperreactivity

The premature neonate is at high risk for childhood airway hyperreactivity and episodes of wheezing. Intermittent hypoxic events are frequently observed during the first weeks and months of life in these infants. Intermittent hypoxemia has been associated with adverse outcomes in extremely premature infants; including the diagnosis of bronchopulmonary dysplasia, reported wheezing, and use of prescription asthma medications. We review the incidence of intermittent hypoxia, their potential role in short and longer term respiratory morbidity, and the translational newborn models now being used to investigate common pathways by which intermittent hypoxia contributes to respiratory disease.

A cohort study reporting normal oximetry values in healthy infants under 4 months of age using Masimo technology

OBJECTIVE

To determine sleeping saturation indices in healthy infants using a modern pulse oximeter with motion artefact extraction technology.

DESIGN

Prospective cohort.

SETTING

Home.

SUBJECTS

Healthy term infants.

INTERVENTION

Nocturnal pulse oximetry at home at 1 month of age (Recording 1) and repeated at age 3-4 months (Recording 2). Parents documented sleep times. Visi-Download software (Stowood Scientific) analysed data with artefact and wake periods removed.

MAIN OUTCOME MEASURES

Saturations (SAT50), desaturation index >4% (DI4) and >3% (DI3) from baseline/hour, delta index 12 s (DI12s), minimum saturations (SATmin), percentage time with saturations below 90% and 92%.

RESULTS

Forty-five babies were studied at 1 month and 38 babies at 3-4 months. Mean (CI) SAT50, DI4, DI3, DI12s and SATmin (CI) were 97.05 (96.59 to 97.52), 16.16 (13.72 to 18.59), 25.41 (22.00 to 28.82), 0.96 (0.88 to 1.04) and 80.4% (78.8% to 82.0%) at 1 month, respectively, and 97.65 (97.19 to 98.12), 8.12 (6.46 to 9.77), 13.92 (11.38 to 16.47), 0.72 (0.65 to 0.78) and 84.7% (83.3% to 86.1%) at 3-4 months. Median (CI) percentage times with saturations below 90% and 92% were 0.39 (0.26 to 0.55) and 0.82 (0.60 to 1.23), respectively, at 1 month and 0.11 (0.06 to 0.20) and 0.25 (0.17 to 0.44) at 3-4 months. For paired samples (n=32) DI4 (P=0.006), DI3 (P=0.03), DI12s (P=0.001), percentage time with saturations below 90% (P=0.001) and 92% (P=0.000) all fell significantly and SATmin (P=0.004) rose between the two recordings.

CONCLUSION

Desaturation indices are substantially higher in young infants than older children where a DI4 over 4 is considered abnormal. These decrease by 3-4 months of age but still remain elevated compared with older children.

Central and peripheral chemoreceptors in sudden infant death syndrome

The pathogenesis of sudden infant death syndrome (SIDS) has been ascribed to an underlying biological vulnerability to stressors during a critical period of development. This paper reviews the main data in the literature supporting the role of central (e.g. retrotrapezoid nucleus, serotoninergic raphe nuclei, locus coeruleus, orexinergic neurons, ventral medullary surface, solitary tract nucleus) and peripheral (e.g. carotid body) chemoreceptors in the pathogenesis of SIDS. Clinical and experimental studies indicate that central and peripheral chemoreceptors undergo critical development during the initial postnatal period, consistent with the age range of SIDS (<1 year). Most of the risk factors for SIDS (gender, genetic factors, prematurity, hypoxic/hyperoxic stimuli, inflammation, perinatal exposure to cigarette smoke and/or substance abuse) may structurally and functionally affect the developmental plasticity of central and peripheral chemoreceptors, strongly suggesting the involvement of these structures in the pathogenesis of SIDS. Morphometric and neurochemical changes have been found in the carotid body and brainstem respiratory chemoreceptors of SIDS victims, together with functional signs of chemoreception impairment in some clinical studies. However, the methodological problems of SIDS research will have to be addressed in the future, requiring large and highly standardized case series. Up-to-date autopsy protocols should be produced, involving substantial, and exhaustive sampling of all potentially involved structures (including peripheral arterial chemoreceptors). Morphometric approaches should include unbiased stereological methods with three-dimensional probes. Prospective clinical studies addressing functional tests and risk factors (including genetic traits) would probably be the gold standard, allowing markers of intrinsic or acquired vulnerability to be properly identified.

The cessation of breathing in the chicken embryo during cold-hypometabolism

The avian embryo toward end-incubation combines gas exchange through the chorioallantoic membrane (CAM) and pulmonary ventilation (V˙E). The main experiments examined breathing activity during cold-hypometabolism. Chicken embryos close to hatching were prepared for simultaneous measurements of oxygen consumption ( [Formula: see text] ) and carbon dioxide production ( [Formula: see text] ; open-flow methodology) and breathing frequency (f; barometric technique). As ambient (Ta) and egg temperature (Tegg) dropped, breathing eventually ceased at ∼18°C, when [Formula: see text] and [Formula: see text] were 22-28% of the normothermic values. With the eggshell experimentally covered to reduce CAM gas exchange breathing ceased at slightly lower [Formula: see text] and [Formula: see text] (17-18% of normothermia). Once breathing had stopped, egg exposure to hypoxia (10% or 5% O₂) or hypercapnia (3% or 8% CO₂) did not resume breathing, which recovered with re-warming. In normothermia, 10% O₂ caused hypometabolism and tachypnea; differently, in 5% O₂ [Formula: see text] dropped as much as with hypothermia and breathing stopped, to recover upon return in air. Correlation analysis among Ta, Tegg, [Formula: see text] , [Formula: see text] and f during cooling and re-warming indicated that f followed more closely the changes in [Formula: see text] and, especially, in [Formula: see text] than the changes in Ta or Tegg. Some considerations suggest that in this experimental model the cessation of breathing in hypothermia or severe hypoxia may be due to hypometabolism, while the lack of chemo-responses may have a different mechanistic basis.

Reduced respiratory neural activity elicits a long-lasting decrease in the CO2 threshold for apnea in anesthetized rats

Two critical parameters that influence breathing stability are the levels of arterial pCO₂ at which breathing ceases and subsequently resumes - termed the apneic and recruitment thresholds (AT and RT, respectively). Reduced respiratory neural activity elicits a chemoreflex-independent, long-lasting increase in phrenic burst amplitude, a form of plasticity known as inactivity-induced phrenic motor facilitation (iPMF). The physiological significance of iPMF is unknown. To determine if iPMF and neural apnea have long-lasting physiological effects on breathing, we tested the hypothesis that patterns of neural apnea that induce iPMF also elicit changes in the AT and RT. Phrenic nerve activity and end-tidal CO₂ were recorded in urethane-anesthetized, ventilated rats to quantify phrenic nerve burst amplitude and the AT and RT before and after three patterns of neural apnea that differed in their duration and ability to elicit iPMF: brief intermittent neural apneas, a single brief "massed" neural apnea, or a prolonged neural apnea. Consistent with our hypothesis, we found that patterns of neural apnea that elicited iPMF also resulted in changes in the AT and RT. Specifically, intermittent neural apneas progressively decreased the AT with each subsequent neural apnea, which persisted for at least 60min. Similarly, a prolonged neural apnea elicited a long-lasting decrease in the AT. In both cases, the magnitude of the AT decrease was proportional to iPMF. In contrast, the RT was transiently decreased following prolonged neural apnea, and was not proportional to iPMF. No changes in the AT or RT were observed following a single brief neural apnea. Our results indicate that the AT and RT are differentially altered by neural apnea and suggest that specific patterns of neural apnea that elicit plasticity may stabilize breathing via a decrease in the AT.

Clinical associations with immature breathing in preterm infants: part 2-periodic breathing

BACKGROUND

Periodic breathing (PB) is a normal immature breathing pattern in neonates that, if extreme, may be associated with pathologic conditions.

METHODS

We used our automated PB detection system to analyze all bedside monitor chest impedance data on all infants <35 wk' gestation in the University of Virginia Neonatal Intensive Care Unit from 2009-2014 (n = 1,211). Percent time spent in PB was calculated hourly (>50 infant-years' data). Extreme PB was identified as a 12-h period with PB >6 SDs above the mean for gestational age (GA) and postmenstrual age and >10% time in PB.

RESULTS

PB increased with GA, with the highest amount in infants 30-33 wk' GA at about 2 wk' chronologic age. Extreme PB was identified in 76 infants and in 45% was temporally associated with clinical events including infection or necrotizing enterocolitis (NEC), immunizations, or caffeine discontinuation. In 8 out of 28 cases of septicemia and 10 out of 21 cases of NEC, there was a >2-fold increase in %PB over baseline on the day prior to diagnosis.

CONCLUSION

Infants <35 wk GA spend, on average, <6% of the time in PB. An acute increase in PB may reflect illness or physiological stressors or may occur without any apparent clinical event.

Stochastic modeling of central apnea events in preterm infants

A near-ubiquitous pathology in very low birth weight infants is neonatal apnea, breathing pauses with slowing of the heart and falling blood oxygen. Events of substantial duration occasionally occur after an infant is discharged from the neonatal intensive care unit (NICU). It is not known whether apneas result from a predictable process or from a stochastic process, but the observation that they occur in seemingly random clusters justifies the use of stochastic models. We use a hidden-Markov model to analyze the distribution of durations of apneas and the distribution of times between apneas. The model suggests the presence of four breathing states, ranging from very stable (with an average lifetime of 12 h) to very unstable (with an average lifetime of 10 s). Although the states themselves are not visible, the mathematical analysis gives estimates of the transition rates among these states. We have obtained these transition rates, and shown how they change with post-menstrual age; as expected, the residence time in the more stable breathing states increases with age. We also extrapolated the model to predict the frequency of very prolonged apnea during the first year of life. This paradigm-stochastic modeling of cardiorespiratory control in neonatal infants to estimate risk for severe clinical events-may be a first step toward personalized risk assessment for life threatening apnea events after NICU discharge.

Cardiorespiratory events in preterm infants: interventions and consequences

Stabilization of respiration and oxygenation continues to be one of the main challenges in clinical care of the neonate. Despite aggressive respiratory support including mechanical ventilation, continuous positive airway pressure, oxygen and caffeine therapy to reduce apnea and accompanying intermittent hypoxemia, the incidence of intermittent hypoxemia events continues to increase during the first few months of life. Even with improvements in clinical care, standards for oxygen saturation targeting and modes of respiratory support have yet to be identified in this vulnerable infant cohort. In addition, we are only beginning to explore the association between the incidence and pattern of cardiorespiratory events during early postnatal life and both short- and long-term morbidity including retinopathy of prematurity, growth, sleep-disordered breathing and neurodevelopmental impairment. Part 1 of this review included a summary of lung development and diagnostic methods of cardiorespiratory monitoring. In Part 2 we focus on clinical interventions and the short- and long-term consequences of cardiorespiratory events in preterm infants.

The potential of methylxanthine-based therapies in pediatric respiratory tract diseases

Caffeine, theophylline and theobromine are the most known methylxanthines as they are present in coffee, tea and/or chocolate. In the last decades, a huge experimental effort has been devoted to get insight into the variety of actions that these compounds exert in humans. From such knowledge it is known that methylxanthines have a great potential in prevention, therapy and/or management of a variety of diseases. The benefits of methylxanthine-based therapies in the apnea of prematurity and their translational potential in pediatric affections of the respiratory tract are here presented.

Sigh-induced changes of breathing pattern in preterm infants

Sighs are thought to play an important role in control of breathing. It is unclear how sighs are triggered, and whether preterm birth and lung disease influence breathing pattern prior to and after a sigh in infants. To assess whether frequency, morphology, size, and short-term variability in tidal volume (V_T) before, during, and after a sigh are influenced by gestational age at birth and lung disease (bronchopulmonary dysplasia, BPD) in former preterm infants and healthy term controls measured at equivalent postconceptional age (PCA). We performed tidal breathing measurements in 143 infants during quiet natural sleep at a mean (SD) PCA of 44.8 (1.3) weeks. A total of 233 sighs were analyzed using multilevel, multivariable regression. Sigh frequency in preterm infants increased with the degree of prematurity and severity of BPD, but was not different from that of term controls when normalized to respiratory rate. After a sigh, V_T decreased remarkably in all infants (paired t-test: P < 0.001). There was no major effect of prematurity or BPD on various indices of sigh morphology and changes in V_T prior to or after a sigh. Short-term variability in V_T modestly increased with maturity at birth and infants with BPD showed an earlier return to baseline variability in V_T following a sigh. In early infancy, sigh-induced changes in breathing pattern are moderately influenced by prematurity and BPD in preterm infants. The major determinants of sigh-related breathing pattern in these infants remain to be investigated, ideally using a longitudinal study design.

Quantification of periodic breathing in premature infants

Periodic breathing (PB), regular cycles of short apneic pauses and breaths, is common in newborn infants. To characterize normal and potentially pathologic PB, we used our automated apnea detection system and developed a novel method for quantifying PB. We identified a preterm infant who died of sudden infant death syndrome (SIDS) and who, on review of her breathing pattern while in the neonatal intensive care unit (NICU), had exaggerated PB.We analyzed the chest impedance signal for short apneic pauses and developed a wavelet transform method to identify repetitive 10-40 second cycles of apnea/breathing. Clinical validation was performed to distinguish PB from apnea clusters and determine the wavelet coefficient cutoff having optimum diagnostic utility. We applied this method to analyze the chest impedance signals throughout the entire NICU stays of all 70 infants born at 32 weeks' gestation admitted over a two-and-a-half year period. This group includes an infant who died of SIDS and her twin.For infants of 32 weeks' gestation, the fraction of time spent in PB peaks 7-14 d after birth at 6.5%. During that time the infant that died of SIDS spent 40% of each day in PB and her twin spent 15% of each day in PB.This wavelet transform method allows quantification of normal and potentially pathologic PB in NICU patients.

Acute Neonatal Respiratory Failure

Acute respiratory failure requiring assisted ventilation is one of the most common reasons for admission to the neonatal intensive care unit. Respiratory failure is the inability to maintain either normal delivery of oxygen to the tissues or normal removal of carbon dioxide from the tissues. It occurs when there is an imbalance between the respiratory workload and ventilatory strength and endurance. Definitions are somewhat arbitrary but suggested laboratory criteria for respiratory failure include two or more of the following: PaCO₂ > 60 mmHg, PaO₂ < 50 mmHg or O₂ saturation <80 % with an FiO₂ of 1.0 and pH < 7.25 (Wen et al. 2004).

Interaction of central and peripheral chemoreflexes in neonatal mice: evidence for hypo-addition

The potential for interaction between the peripheral (PCR) and central (CCR) chemoreflexes has not been studied in the neonatal period, when breathing is inherently unstable. Based on recent work in adult rodents, this study addresses the hypothesis that in neonatal mice there is a hypoadditive interaction between the chemoreflexes. To test this, a mask-pneumotach system was used to expose postnatal day (P) 11-12 mouse pups to square-wave hyperoxia (100% O2; n=8) or hypoxia (10% O2; n=11), administered in normocapnic conditions (inspired CO2 (FICO2)=0.001-0.005), or following an episode of re-breathing to increase FICO2 by 0.015-0.02. The immediate (i.e. PCR-mediated) responses of frequency (fB), tidal volume (VT) and ventilation (V˙E) to square-wave hyperoxia and hypoxia were assessed. When given in a normocapnic background, hyperoxia induced an immediate (within the first 20 breaths, or ∼6s) but transient fall in fB (-46±9breaths/min) and V˙E (-149±41μlmin(-1)g(-1)) (P<0.001 for both), with no effect on VT. In contrast, hyperoxia had no influence on breathing when it was administered following re-breathing. Similarly, the hypoxia-induced increase in fB was greater when applied under normocapnic conditions (50±8breaths/min) then when applied following re-breathing (21±5breaths/min) (P=0.02). These data demonstrate a hypo-additive interaction between the PCR and CCR with respect to the immediate frequency response to inhibition or excitation of the PCR. Hypoaddition of the chemoreflexes could cause or mitigate neonatal apnea, depending on the prevailing PCO2.

Apnea in acute bilirubin encephalopathy

Central apnea, defined as cessation of breathing for ≥20s, is frequent in premature infants born at <34 weeks׳ gestation but uncommon among healthy late preterm (34(0/7)-36(6/7) weeks׳ gestation) and term (≥37 weeks׳ gestation) infants, where it is usually a clinical manifestation of a neurological or metabolic problem. There is growing evidence that marked unconjugated hyperbilirubinemia is associated with central apnea in neonates. This article explores the reported association between acute bilirubin encephalopathy and symptomatic apneic events in newborns and the possible mechanisms involved in the pathogenesis of this phenomenon. The prevalence of symptomatic apneic events in reports of acute bilirubin encephalopathy suggests this clinical finding should be considered a sign of bilirubin neurotoxicity.

Sleep and respiratory physiology in children

Maturational changes of breathing during sleep contribute to the unique features of childhood sleep disorders. The clinician's ability to evaluate common disorders related to sleep in children relies on an understanding of normal patterns of breathing during sleep across the ages. This article reviews respiratory physiology during sleep throughout childhood. Specific topics include an overview of respiration during sleep, normal parameters through childhood including respiratory rate, oxygen saturation, and measures of carbon dioxide, normal patterns of apneas throughout childhood, and features of breathing during sleep seen in term and preterm infants.

Apnea of prematurity

Apnea of prematurity is a significant problem due to immaturity of the central neural control circuitry responsible for integrating afferent input and central rhythm. In this review, we provide an overview of the pathogenesis of apnea of prematurity--including our current understanding of the role that afferent input to the brain stem plays in synergy with the central pattern generation circuitry in the emergence of apnea of prematurity. We then discuss the interplay of apnea, bradycardia, desaturation, as well as the genesis of central, mixed, and obstructive apnea. Finally, we provide a summary of the physiological basis for current therapeutic approaches to treating apnea of prematurity, and conclude with an overview of proposed long-term consequences of the resultant intermittent hypoxic episodes.

Current definitions for neonatal apnoea: are they evidence based?

Apnoea is defined as cessation of breathing with implicit pathophysiology. This review considers definitions of neonatal apnoea currently available and explores the evidence to support their use. For preterm and term infants, apnoea definitions appear arbitrary, are not supported by guidelines and vary from study to study. Although most alarms on infant breathing monitors are set to alert after a respiratory pause >20s duration is detected, this time period is the equivalent of 17 missed breaths in a preterm infant. Apnoea is likely to be better defined by associated consequence than by pause duration alone in this age group; however, the degree of change in heart rate or oxygen saturation that defines a respiratory pause as pathological is yet to be defined. Further research is required to determine the characteristics that differentiate respiratory events of clinical consequence from normal respiratory variability in term and preterm infants.

Ventilatory control in infants, children, and adults with bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD), or chronic lung disease of prematurity, occurs in ~30% of preterm infants (15,000 per year) and is associated with a clinical history of mechanical ventilation and/or high inspired oxygen at birth. Here, we describe changes in ventilatory control that exist in patients with BPD, including alterations in chemoreceptor function, respiratory muscle function, and suprapontine control. Because dysfunction in ventilatory control frequently revealed when O2 supply and CO2 elimination are challenged, we provide this information in the context of four important metabolic stressors: stresses: exercise, sleep, hypoxia, and lung disease, with a primary focus on studies of human infants, children, and adults. As a secondary goal, we also identify three key areas of future research and describe the benefits and challenges of longitudinal human studies using well-defined patient cohorts.

Apnea of prematurity--perfect storm

With increased survival of preterm infants as young as 23 weeks gestation, maintaining adequate respiration and corresponding oxygenation represents a clinical challenge in this unique patient cohort. Respiratory instability characterized by apnea and periodic breathing occurs in premature infants because of immature development of the respiratory network. While short respiratory pauses and apnea may be of minimal consequence if oxygenation is maintained, they can be problematic if accompanied by chronic intermittent hypoxemia. Underdevelopment of the lung and the resultant lung injury that occurs in this population concurrent with respiratory instability creates the perfect storm leading to frequent episodes of profound and recurrent hypoxemia. Chronic intermittent hypoxemia contributes to the immediate and long term co-morbidities that occur in this population. In this review we discuss the pathophysiology leading to the perfect storm, diagnostic assessment of breathing instability in this unique population and therapeutic interventions that aim to stabilize breathing without contributing to tissue injury.

Carotid chemoreceptor development and neonatal apnea

The premature transition from fetal to neonatal life is accompanied by an immature respiratory neural control system. Most preterm infants exhibit recurrent apnea, resulting in repetitive oscillations in O(2) saturation (intermittent hypoxia, IH). Numerous factors are likely to play a role in the etiology of apnea including inputs from the carotid chemoreceptors. Despite major advances in our understanding of carotid chemoreceptor function in the early neonatal period, however, their contribution to the initiation of an apneic event and its eventual termination are still largely speculative. Recent findings have provided a detailed account of the postnatal changes in the incidence of hypoxemic events associated with apnea, and there is anecdotal evidence for a positive correlation with carotid chemoreceptor maturation. Furthermore, studies on non-human animal models have shown that chronic IH sensitizes the carotid chemoreceptors, which has been proposed to perpetuate the occurrence of apnea. An alternative hypothesis is that sensitization of the carotid chemoreceptors could represent an important protective mechanism to defend against severe hypoxemia. The purpose of this review, therefore, is to discuss how the carotid chemoreceptors may contribute to the initiation and termination of an apneic event in the neonate and the use of xanthine therapy in the prevention of apnea.

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 in Sudden Infant Death Syndrome

The aim of the present study is to provide a review of cytochemical, clinical and experimental data indicating disruption of perinatal carotid body maturation as one of the possible mechanisms underlying SIDS pathogenesis. SIDS victims have been reported to show alterations in respiratory regulation which may partly be ascribed to peripheral arterial chemoreceptors. Carotid body findings in SIDS victims, although not entirely confirmed by other authors, have included reductions in glomic tissue volume and cytoplamic granules of type I cells, changes in cytological composition (higher percentages of progenitor and type II cells) and increases in dopamine and noradrenaline contents. Prematurity and environmental factors, such as exposure to tobacco smoke, substances of abuse, hyperoxia and continuous or intermittent hypoxia, increase the risk of SIDS and are known to affect carotid body functional and structural maturation adversely, supporting a role for peripheral arterial chemoreceptors in SIDS.

Obstructive sleep apnea in infants

Obstructive sleep apnea in infants has a distinctive pathophysiology, natural history, and treatment compared with that of older children and adults. Infants have both anatomical and physiological predispositions toward airway obstruction and gas exchange abnormalities; including a superiorly placed larynx, increased chest wall compliance, ventilation-perfusion mismatching, and ventilatory control instability. Congenital abnormalities of the airway, such as laryngomalacia, hemangiomas, pyriform aperture stenosis, choanal atresia, and laryngeal webs, may also have adverse effects on airway patency. Additional exacerbating factors predisposing infants toward airway collapse include neck flexion, airway secretions, gastroesophageal reflux, and sleep deprivation. Obstructive sleep apnea in infants has been associated with failure to thrive, behavioral deficits, and sudden infant death. The proper interpretation of infant polysomnography requires an understanding of normative data related to gestation and postconceptual age for apnea, arousal, and oxygenation. Direct visualization of the upper airway is an important diagnostic modality in infants with obstructive apnea. Treatment options for infant obstructive sleep apnea are predicated on the underlying etiology, including supraglottoplasty for severe laryngomalacia, mandibular distraction for micrognathia, tonsillectomy and/or adenoidectomy, choanal atresia repair, and/or treatment of gastroesophageal reflux.

Apnea of prematurity: from cause to treatment

Apnea of prematurity (AOP) is a common problem affecting premature infants, likely secondary to a "physiologic" immaturity of respiratory control that may be exacerbated by neonatal disease. These include altered ventilatory responses to hypoxia, hypercapnia, and altered sleep states, while the roles of gastroesophageal reflux and anemia remain controversial. Standard clinical management of the obstructive subtype of AOP includes prone positioning and continuous positive or nasal intermittent positive pressure ventilation to prevent pharyngeal collapse and alveolar atelectasis, while methylxanthine therapy is a mainstay of treatment of central apnea by stimulating the central nervous system and respiratory muscle function. Other therapies, including kangaroo care, red blood cell transfusions, and CO(2) inhalation, require further study. The physiology and pathophysiology behind AOP are discussed, including the laryngeal chemoreflex and sensitivity to inhibitory neurotransmitters, as are the mechanisms by which different therapies may work and the potential long-term neurodevelopmental consequences of AOP and its treatment.

Apnea of prematurity: pathogenesis and management strategies

Apnea of prematurity (AOP) is a significant clinical problem manifested by an unstable respiratory rhythm reflecting the immaturity of respiratory control systems. This review will address the pathogenesis of and treatment strategies for AOP. Although the neuronal mechanisms leading to apnea are still not well understood, recent decades have provided better insight into the generation of the respiratory rhythm and its modulation in the neonate. Ventilatory responses to hypoxia and hypercarbia are impaired and inhibitory reflexes are exaggerated in the neonate. These unique vulnerabilities predispose the neonate to the development of apnea. Treatment strategies attempt to stabilize the respiratory rhythm. Caffeine remains the primary pharmacological treatment modality and is presumed to work through blockade of adenosine receptors A(1) and A(2). Recent evidences suggest that A(2A) receptors may have a greater role than previously thought. AOP typically resolves with maturation suggesting increased myelination of the brainstem.

The role of CO(2) and central chemoreception in the control of breathing in the fetus and the neonate

Central chemoreception is active early in development and likely drives fetal breathing movements, which are influenced by a combination of behavioral state and powerful inhibition. In the premature human infant and newborn rat ventilation increases in response to CO(2); in the rat the sensitivity of the response increases steadily after ∼P12. The premature human infant is more vulnerable to instability than the newborn rat and exhibits periodic breathing that is augmented by hypoxia and eliminated by breathing oxygen or CO(2) or the administration of respiratory stimulants. The sites of central chemoreception active in the fetus are not known, but may involve the parafacial respiratory group which may be a precursor to the adult RTN. The fetal and neonatal rat brainstem-spinal-cord preparations promise to provide important information about central chemoreception in the developing rodent and will increase our understanding of important clinical problems, including The Sudden Infant Death Syndrome, Congenital Central Hypoventilation Syndrome, and periodic breathing and apnea of prematurity.

Interventions for apnoea of prematurity: a personal view

AIM

To review treatments for apnoea of prematurity (AOP).

METHODS

Literature Review and description of personal practice.

RESULTS

Provided that symptomatic apnoea has been ruled out, interventions to improve AOP can be viewed as directed at one of three underlying mechanisms: (i) a reduced work of breathing [e.g. prone positioning, nasal continuous positive airway pressure (CPAP)], (ii) an increased respiratory drive (e.g. caffeine), and (iii) an improved diaphragmatic function (e.g. branched-chain amino acids). Most options currently applied, however, have not yet been shown to be effective and/or safe, except for prone, head-elevated positioning, synchronized nasal ventilation/CPAP, and caffeine.

CONCLUSION

Treatment usually follows an incremental approach, starting with positioning, followed by caffeine (which should be started early, at least in infants <1250 g), and nasal ventilation or CPAP via variable flow systems that reduce work of breathing. From a research point of view, we most urgently need data on the frequency and severity of bradycardia and intermittent hypoxia that can yet be tolerated without putting an infant at risk of impaired development or retinopathy of prematurity.

Effect of intermittent hypercapnia on respiratory control in rat pups

Preterm infants are subject to fluctuations in blood gas status associated with immature respiratory control. Intermittent hypoxia during early postnatal life has been shown to increase chemoreceptor sensitivity and destabilize the breathing pattern; however, intermittent hypercapnia remains poorly studied. Therefore, to test the hypothesis that intermittent hypercapnia results in altered respiratory control, we examined the effects of daily exposure to intermittent hypercapnia on the ventilatory response to subsequent hypercapnic and hypoxic exposure in neonatal rat pups. Exposure cycles consisted of 5 min of intermittent hypercapnia (5% CO(2), 21% O(2), balance N(2)) followed by 10 min of normoxia. Rat pups were exposed to 18 exposure cycles each day for 1 week, from postnatal day 7 to 14. We analyzed diaphragm electromyograms (EMGs) from pups exposed to subsequent acute hypercapnic (5% CO(2)) and hypoxic (12% O(2)) challenges. In response to a subsequent hypercapnia challenge, there was no significant difference in the ventilatory response between control and intermittent hypercapnia-exposed groups. In contrast, intermittent hypercapnia-exposed rat pups showed an enhanced ventilatory response to hypoxic challenge with an increase in minute EMG to 118 +/- 14% of baseline versus 107 +/- 13% for control pups (p < 0.05). We speculate that prior hypercapnic exposure may increase peripheral chemoreceptor response to subsequent hypoxic exposures and result in perturbed neonatal respiratory control.

Neonatal apnea: what's new?

Apnea of prematurity (AOP) remains a major clinical problem in present day neonatology that warrants frequent evaluations and imposes challenges in therapeutic strategies. Although the pathogenesis of AOP is poorly understood, it is probably a manifestation of physiologic immaturity of breathing control rather than a pathologic disorder. Immature breathing responses to hypoxia, hypercapnia and exaggerated inhibitory pulmonary reflexes in preterm infants might also contribute to the occurrence or severity of AOP. Recent data suggest a role for genetic predisposition. Although typically resolve with maturation, the role of bradycardia and desaturation episodes associated with AOP in the development of sleep disorder breathing and neurodevelopmental delay needs further clarification. Pharmacological treatment with methylxanthines and CPAP remain the mainstay for treatment of AOP. However, recent studies have implicated central inhibitory neuromodulators including prostaglandins, GABA and adenosine in its pathogenesis, the fact that might provide future specific targets for treatment. This review will summarize new insights involving these issues as well as others involving the pathogenesis, treatment strategies and consequences of apnea in premature infants.

Chronic intermittent hypoxia reduces ventilatory long-term facilitation and enhances apnea frequency in newborn rats

Ventilatory long-term facilitation (LTF; defined as gradual increase of minute ventilation following repeated hypoxic exposures) is well described in adult mammals and is hypothesized to be a protective mechanism against apnea. In newborns, LTF is absent during the first postnatal days, but its precise developmental pattern is unknown. Accordingly, this study describes this pattern of postnatal development. Additionally, we tested the hypothesis that chronic intermittent hypoxia (CIH) from birth alters this development. LTF was estimated in vivo using whole body plethysmography by exposing rat pups at postnatal days 1, 4, and 10 (P1, P4, and P10) to 10 brief hypoxic cycles (nadir 5% O2) and respiratory recordings during the following 2 h (recovery, 21% O2). Under these conditions, ventilatory LTF (gradual increase of minute ventilation during recovery) was clearly expressed in P10 rats but not in P1 and P4. In a second series of experiments, rat pups were exposed to CIH during the first 10 postnatal days (6 brief cyclic exposures at 5% O2 every 6 min followed by 1 h under normoxia, 24 h a day). Compared with P10 control rats, CIH enhanced hypoxic ventilatory response (estimated during the hypoxic cycles) specifically in male rat pups. Ventilatory LTF was drastically reduced in P10 rats exposed to CIH, which was associated with higher apnea frequency during recovery. We conclude that CIH from birth enhances hypoxic chemoreflex and disrupts LTF development, thus likely contributing to increase apnea frequency.

Pathophysiology of pediatric obstructive sleep apnea

Sleep-disordered breathing is a common and serious cause of metabolic, cardiovascular, and neurocognitive morbidity in children. The spectrum of obstructive sleep-disordered breathing ranges from habitual snoring to partial or complete airway obstruction, termed obstructive sleep apnea (OSA). Breathing patterns due to airway narrowing are highly variable, including obstructive cycling, increased respiratory effort, flow limitation, tachypnea, and/or gas exchange abnormalities. As a consequence, sleep homeostasis may be disturbed. Increased upper airway resistance is an essential component of OSA, including any combination of narrowing/retropositioning of the maxilla/mandible and/or adenotonsillar hypertrophy. However, in addition to anatomic factors, the stability of the upper airway is predicated on neuromuscular activation, ventilatory control, and arousal threshold. During sleep, most children with OSA intermittently attain a stable breathing pattern, indicating successful neuromuscular activation. At sleep onset, airway muscle activity is reduced, ventilatory variability increases, and an apneic threshold slightly below eupneic levels is observed in non-REM sleep. Airway collapse is offset by pharyngeal dilator activity in response to hypercapnia and negative lumenal pressure. Ventilatory overshoot results in sudden reduction in airway muscle activation, contributing to obstruction during non-REM sleep. Arousal from sleep exacerbates ventilatory instability and, thus, obstructive cycling. Paroxysmal reductions in pharyngeal dilator activity related to central REM sleep processes likely account for the disproportionate severity of OSA observed during REM sleep. Understanding the pathophysiology of pediatric OSA may permit more precise clinical phenotyping, and therefore improve or target therapies related to anatomy, neuromuscular compensation, ventilatory control, and/or arousal threshold.

Peripheral arterial chemoreceptors and sudden infant death syndrome

Sudden infant death syndrome (SIDS) is the major cause of death in infants between 1 month and 1 year of age. Two particular concerns are that (1) premature or low birth weight (<2500-g) infants have a 2- to 40-fold greater risk of dying of SIDS (depending on the sleep position) than infants born at term and of normal birth weight, and that (2) the proportion of premature infants dying of SIDS has increased from 12 to 34% between 1988 and 2003. Hypo- and hypersensitivity of peripheral arterial chemoreceptors (PACs) may be one biological mechanism that could help to explain the epidemiological association between the increased incidence of SIDS in formerly premature infants. Because premature infants are often exposed to the extremes of oxygen stress during early postnatal development, they are more likely to have a maladaptive response of PACs later in their lives. As the first line of defense that mediates an increase in ventilation to a hypoxic challenge during wakefulness and sleep, PACs also mediate arousal responses during sleep in response to an asphyxial event that is often associated with upper airway obstruction. In most mammalian species, PACs are not fully developed at birth and thus are vulnerable to plasticity-induced changes mediated by environmental exposures such as the extremes of oxygen tension. Hypoxic or hyperoxic exposure during early postnatal development can lead to hyposensitive or hypersensitive PAC responses later in life. Although baseline chemoreceptor activity may not be the cause of an initial hypoxic or asphyxial event, the level of peripheral chemoreceptor drive does modulate the (1) time to arousal, (2) resumption of airflow during airway obstruction, (3) escape behaviors during rebreathing, and (4) cardiorespiratory responses that result from activation of the laryngeal chemoreflex. The laryngeal chemoreflex can be stimulated by reflux of gastric contents above the upper esophageal sphincter, or an increase in nasopharyngeal secretions from upper respiratory tract infections--events that contribute to some cases of SIDS. In this review, evidence is presented that both hypo- and hypersensitivity of PACs may be disadvantageous to the premature infant who is placed in an at risk environment for the occurrence of hypoxemia/asphyxia event thereby predisposing the infant to SIDS.