Cardiorespiratory adaptation during sleep in infants and children

Short apneas and periodic breathing in preterm infants in the neonatal intensive care unit-Effects of sleep position, sleep state, and age

This observational study investigated the effects of sleep position and sleep state on short apneas and periodic breathing in hospitalized preterm infants longitudinally, in relation to postmenstrual age. Preterm infants (25-31 weeks gestation, n = 29) were studied fortnightly after birth until discharge, in prone and supine positions, and in quiet sleep and active sleep. The percentage of time spent in each sleep state (percentage of time in quiet sleep and percentage of time in active sleep), percentage of total sleep time spent in short apneas and periodic breathing, respectively, the percentage of falls from baseline in heart rate, arterial oxygen saturation and cerebral tissue oxygenation index during short apneas and periodic breathing, and the associated percentage of total sleep time with systemic (arterial oxygen saturation < 90%) and cerebral hypoxia (cerebral tissue oxygenation index < 55%) were analysed using a linear mixed model. Results showed that the prone position decreased (improved) the percentage of falls from baseline in arterial oxygen saturation during both short apneas and periodic breathing, decreased the proportion of infants with periodic breathing and the periodic breathing-associated percentage of total sleep time with cerebral hypoxia. The percentage of time in quiet sleep was higher in the prone position. Quiet sleep decreased the percentage of total sleep time spent in short apneas, the short apneas-associated percentage of falls from baseline in heart rate, arterial oxygen saturation, and proportion of infants with systemic hypoxia. Quiet sleep also decreased the proportion of infants with periodic breathing and percentage of total sleep time with cerebral hypoxia. The effects of sleep position and sleep state were not related to postmenstrual age. In summary, when sleep state is controlled for, the prone sleeping position has some benefits during both short apneas and periodic breathing. Quiet sleep improves cardiorespiratory stability and is increased in the prone position at the expense of active sleep, which is critical for brain maturation. This evidence should be considered in positioning preterm infants.

Hyperoxic ventilatory response in infants is related to nocturnal hypoxaemia

Background

The carotid bodies primarily serve as oxaemia sensors that affect tidal breathing. Their function has not yet been studied in infants with nocturnal hypoxaemia. This cross-sectional study aimed to characterise the hyperoxic ventilatory response (HVR) in infants and its relationship to nocturnal hypoxaemia.

Methods

The HVR was analysed in term infants aged <24 months with childhood interstitial lung disease (chILD), those with severe recurrent wheezing (wheeze), and nonrespiratory controls. The HVR timing was characterised using hyperoxia response time (HRT1), and HVR magnitude was characterised by the relative change in minute volume between normoxia and 30-s hyperoxia (VE_dH30). Time spent with an arterial haemoglobin oxygen saturation (SpO2) <90% during overnight monitoring (t90) was estimated.

Results

HVR data were available for 23 infants with chILD, 24 wheeze and 14 control infants. A significant decrease in minute volume under 30 s of hyperoxia was observed in all patients. HRT1 was shorter in chILD (5.6±1.2 s) and wheeze (5.9±1.5 s) groups than in the controls (12.6±5.5 s) (ANOVA p<0.001). VE_dH30 was increased in the chILD group (24.3±8.0%) compared with that in the controls (14.7±9.2%) (p=0.003). t90 was abnormal in the wheeze (8.0±5.0%) and chILD (32.7±25.8%) groups and higher in the chILD group than in the controls (p<0.001). HRT1 negatively correlated with t90 in all groups.

Conclusion

Significant differences in HVR timing and magnitude were noted in the chILD, wheeze and control groups. A relationship between nocturnal hypoxaemia and HRT1 was proposed. HVR characterisation may help identify patients with abnormal nocturnal SpO2.

Autonomic cardiovascular control is altered by intermittent hypoxia in preterm infants

AIM

Preterm infants frequently experience short apnoeas and periodic breathing. Animal studies have shown that repetitive hypoxia associated with periodic breathing can alter autonomic control. We aimed to elucidate if apnoea and periodic breathing were associated with changes in autonomic control assessed using heart rate variability, thus exacerbating the consequences of respiratory disturbance.

METHODS

Forty very preterm infants (15 M/25 F) were studied at 34.3 weeks post-menstrual age with daytime polysomnography. Total power, low frequency (LF, sympathetic+parasympathetic activity) high frequency (HF, parasympathetic activity) and LF/HF (sympathovagal balance) were calculated.

RESULTS

Infants were divided into those with above and below the median total sleep time spent with respiratory events: Active sleep (AS) 13%, Quiet sleep (QS) 10%. In AS, including respiratory events, Total power (p < 0.05) and HF power (p < 0.05) were higher in the above median group. During AS excluding respiratory events, Total power (p < 0.05) and HF power (p = 0.061) were higher and LF power (p < 0.01) and LF/HF (p < 0.05) were lower in the above median group. There were no differences in HRV parameters in QS.

CONCLUSION

This study provides new evidence that short apnoeas, particularly periodic breathing, which is currently not detected or treated in the neonatal unit can affect autonomic cardiovascular control.

Home continuous positive airway pressure therapy in infants: a single-center experience

STUDY OBJECTIVES

There are limited data on indications and outcomes of home continuous positive airway pressure (CPAP) therapy in the first year of life. We aimed to analyze the clinical, demographic, and polysomnographic characteristics of a cohort of children initiated on home CPAP for treatment of sleep-disordered breathing and as respiratory support in the first year of life.

METHODS

Children started on CPAP in the first year of life at the Queensland Children's Hospital were retrospectively evaluated for clinical and demographic parameters, underlying diagnoses, respiratory support, airway surgical intervention, and polysomnography results at baseline and on CPAP.

RESULTS

Twenty-nine infants (median age [interquartile range] at CPAP initiation, 182 days [126-265.5 days]) were included. The underlying etiology included Trisomy 21 (n = 6), craniofacial syndromes (n = 5), hypotonia (n = 8; 5 with noncraniofacial syndrome), airway malacia (n = 5), skeletal dysplasia (n = 2), nonsyndromic upper airway obstruction (n = 2), and chronic neonatal lung disease (n = 1). The median (interquartile range) obstructive apnea-hypopnea index was 14 events/h (6.2-31 events/h) at CPAP initiation, which improved on CPAP to 3.4 events/h (1.4-6.4 events/h). The median (interquartile range) transcutaneous CO2 max remained unchanged on CPAP (56.6 mm Hg [49-66.5 mm Hg] pre-CPAP vs 54.9 mm Hg [47-62 mm Hg] on CPAP). Fifteen children needed surgical airway intervention (11 pre-CPAP and 4 post-CPAP). CPAP therapy could be successfully stopped in 9 children, 2 children needed tracheostomy, and 1 child died during the follow-up period.

CONCLUSIONS

Home CPAP as respiratory support is an effective long-term therapy in infancy, and these patients can be weaned from CPAP therapy even if it was initiated early. Prospective studies with predefined criteria for CPAP initiation and cessation would help ascertain long-term outcomes in this poorly researched group.

CITATION

Joshi SS, Sivapalan D, Leclerc M-J, Kapur N. Home continuous positive airway pressure therapy in infants: a single-center experience. J Clin Sleep Med. 2023;19(3):473-477.

The value of cardiorespiratory parameters for sleep state classification in preterm infants: A systematic review

Cardiorespiratory activity is highly associated with infants' sleep duration and quality. We performed a systematic literature search of PubMed and EMBASE databases to investigate if and how cardiorespiratory parameters can be used for sleep state classification in preterm infants and in what way maturation influences this relation. All retrieved citations were screened against predetermined inclusion and exclusion criteria. Only studies of preterm infants (<37 wk postmenstrual age during sleep state classification) admitted to a neonatal ward and of whom at least one sleep state and one cardiorespiratory parameter was measured, were included. Two researchers independently reviewed the included studies on methodological quality. Of the 1097 initially retrieved studies, 23 were included for analysis. Heart rate and respiration frequency are strongly correlated with active sleep and quiet sleep. In quiet sleep, as compared to active sleep, respiratory frequency is more stable, and the heart rate is lower and less variable. This association, however, differed across preterm birth subtypes (i.e., extremely, very or late preterm), indicating that maturation - in the form of both gestational and postnatal age - influences the cardiorespiratory characteristics of preterm sleep states. The knowledge gained from this review can help improve behavioral sleep classification and automated sleep classification algorithms for preterm infants.

Sources of variability in expiratory flow profiles during sleep in healthy young children

Standard lung function tests are not feasible in young children, but recent studies show that the variability of expiratory tidal breathing flow-volume (TBFV) curves during sleep is a potential indirect marker of lower airway obstruction. However, the neurophysiological sources of the TBFV variability in normal subjects has not been established. We investigated sleep stages and body position changes as potential sources for the TBFV curve variability. Simultaneous impedance pneumography (IP), polysomnography (PSG) and video recordings were done in 20 children aged 1.4-6.9 years without significant respiratory disorders during sleep. The early part of expiratory TBFV curves are less variable between cycles of REM than NREM sleep. However, within individual sleep cycles, TBFV curves during N3 are the least variable. The differences in TBFV curve shapes between sleep stages are the main source of overnight variability in TBFV curves and the changes in body position have a lesser impact.

When does prone sleeping improve cardiorespiratory status in preterm infants in the NICU?

Study Objectives

Preterm infants undergoing intensive care are often placed prone to improve respiratory function. Current clinical guidelines recommend preterm infants are slept supine from 32 weeks’ postmenstrual age, regardless of gestational age at birth. However, respiratory function is also related to gestational and chronological ages and is affected by sleep state. We aimed to identify the optimal timing for adopting the supine sleeping position in preterm infants, using a longitudinal design assessing the effects of sleep position and state on cardiorespiratory stability.

Methods

Twenty-three extremely (24–28 weeks’ gestation) and 33 very preterm (29–34 weeks’ gestation) infants were studied weekly from birth until discharge, in both prone and supine positions, in quiet and active sleep determined by behavioral scoring. Bradycardia (heart rate ≤100 bpm), desaturation (oxygen saturation ≤80%), and apnea (pause in respiratory rate ≥10 s) episodes were analyzed.

Results

Prone positioning in extremely preterm infants reduced the frequency of bradycardias and desaturations and duration of desaturations. In very preterm infants, prone positioning only reduced the frequency of desaturations. The position-related effects were not related to postmenstrual age. Quiet sleep in both preterm groups was associated with fewer bradycardias and desaturations, and also reduced durations of bradycardia and desaturations in the very preterm group.

Conclusions

Cardiorespiratory stability is improved by the prone sleep position, predominantly in extremely preterm infants, and the improvements are not dependent on postmenstrual age. In very preterm infants, quiet sleep has a more marked effect than the prone position. This evidence should be considered in individualizing management of preterm infant positioning.

Effects of Prone Sleeping on Cerebral Oxygenation in Preterm Infants

OBJECTIVE

To determine the effect of prone sleeping on cerebral oxygenation in preterm infants in the neonatal intensive care unit.

STUDY DESIGN

Preterm infants, divided into extremely preterm (gestational age 24-28 weeks; n = 23) and very preterm (gestational age 29-34 weeks; n = 33) groups, were studied weekly until discharge in prone and supine positions during active and quiet sleep. Cerebral tissue oxygenation index (TOI) and arterial oxygen saturation (SaO₂) were recorded. Cerebral fractional tissue extraction (CFOE) was calculated as CFOE = (SaO₂ - TOI)/SaO₂.

RESULTS

In extremely preterm infants, CFOE increased modestly in the prone position in both sleep states at age 1 week, in no change in TOI despite higher SaO₂. In contrast, the very preterm infants did not have position-related differences in CFOE until the fifth week of life. In the very preterm infants, TOI decreased and CFOE increased with active sleep compared with quiet sleep and with increasing postnatal age.

CONCLUSION

At 1 week of age, prone sleeping increased CFOE in extremely preterm infants, suggesting reduced cerebral blood flow. Our findings reveal important physiological insights in clinically stable preterm infants. Further studies are needed to verify our findings in unstable preterm infants regarding the potential risk of cerebral injury in the prone sleeping position in early postnatal life.

Intermittent Hypoxemia in Infants Born Late Preterm: A Prospective Cohort Observational Study

OBJECTIVE

To determine if late preterm infants are at increased risk of intermittent hypoxemic events compared with term infants.

STUDY DESIGN

Prospective, cohort, observational study of late preterm infants (34^0/7-36^6/7 weeks gestational age) and term infants (39^0/7-41^6/7 weeks gestational age). Overnight pulse oximetry recordings were performed on days 2-3 after birth, at term equivalent age, and at 45 weeks postmenstrual age. The primary outcome was the frequency of intermittent hypoxemic events per hour (desaturation ≥10% below the preceding baseline SpO₂) on the oximetry recording on days 2-3 after birth. Data were analyzed by the Student t test and general linear mixed model.

RESULTS

Eighty-five infants were enrolled (late preterm n = 43; term infants n = 42). On days 2-3 after birth, late preterm infants had more intermittent hypoxemic events than term infants (events per hour, mean ± standard error of the mean, 2.5 ± 1.2 vs 1.0 ± 1.2; P < .0001). On mixed model analysis, late preterm infants had a higher frequency of intermittent hypoxemic events at term equivalent age, which decreased to a similar frequency as in term infants by 45 weeks postmenstrual age (events per hour; term equivalent age, late preterm: least squares mean, 3.7 [95% CI, 2.7-5.1] vs term: least squares mean, 1.7 [95% CI, 1.2-2.3]; 45 weeks postmenstrual age, late preterm: least squares mean, 1.5 [95% CI, 1.1-2.1] vs term: least squares mean, 1.9 [95% CI, 1.4-2.6]; P < .0005).

CONCLUSIONS

Late preterm infants are at greater risk of intermittent hypoxemia than term infants soon after birth. We speculate that preventing intermittent hypoxemia in late preterm infants may improve neurodevelopmental outcomes.

Waking up too early - the consequences of preterm birth on sleep development

Good quality sleep of sufficient duration is vital for optimal physiological function and our health. Sleep deprivation is associated with impaired neurocognitive function and emotional control, and increases the risk for cardiometabolic diseases, obesity and cancer. Sleep develops during fetal life with the emergence of a recognisable pattern of sleep states in the preterm fetus associated with the development, maturation and connectivity within neural networks in the brain. Despite the physiological importance of sleep, surprisingly little is known about how sleep develops in individuals born preterm. Globally, an estimated 15 million babies are born preterm (<37 weeks gestation) each year, and these babies are at significant risk of neural injury and impaired brain development. This review discusses how sleep develops during fetal and neonatal life, how preterm birth impacts on sleep development to adulthood, and the factors which may contribute to impaired brain and sleep development, leading to altered neurocognitive, behavioural and motor capabilities in the infant and child. Going forward, the challenge is to identify specific risk factors for impaired sleep development in preterm babies to allow for the design of interventions that will improve the quality and quantity of sleep throughout life.

Cardiovascular autonomic dysfunction in sudden infant death syndrome

A failure of cardiorespiratory control mechanisms, together with an impaired arousal response from sleep, are believed to play an important role in the final event of sudden infant death syndrome (SIDS). The 'triple risk model' describes SIDS as an event that results from the intersection of three overlapping factors: (1) a vulnerable infant, (2) a critical developmental period in homeostatic control and (3) an exogenous stressor. In an attempt to understand how the triple risk hypothesis is related to infant cardiorespiratory physiology, many researchers have examined how the known risk and protective factors for SIDS alter infant cardiovascular control during sleep. This review discusses the association between the three components of the triple risk hypothesis and major risk factors for SIDS, such as prone sleeping, maternal smoking, together with three "protective" factors, and cardiovascular control during sleep in infants, and discusses their potential involvement in SIDS.

Active sleep unmasks apnea and delayed arousal in infant rat pups lacking central serotonin

Sudden infant death syndrome (SIDS), occurring during sleep periods, is highly associated with abnormalities within serotonin (5-HT) neurons, including reduced 5-HT. There is evidence that future SIDS cases experience more apnea and have abnormal arousal from sleep. In rodents, a loss of 5-HT neurons is associated with apnea in early life and, in adulthood, delayed arousal. As the activity of 5-HT neurons changes with vigilance state, we hypothesized that the degree of apnea and delayed arousal displayed by rat pups specifically lacking central 5-HT varies with state. Two-week-old tryptophan hydroxylase 2-deficient (TPH2-/-) and wild-type (WT) rat pups were placed in plethysmographic chambers supplied with room air. At the onset of active (AS) or quiet (QS) sleep, separate groups of rats were exposed to hypercapnia (5% CO₂) or mild hypoxia (~17% O₂) or maintained in room air. Upon arousal, rats received room air. Apnea indexes and latencies to spontaneous arousal from AS and QS were determined for pups exposed only to room air. Arousal latencies were also calculated for TPH2-/- and WT pups exposed to hypoxia or hypercapnia. Compared with WT, TPH2-/- pups hypoventilated in all states but were profoundly more apneic solely in AS. TPH2-/- pups had delayed arousal in response to increasing CO₂, and AS selectively delayed the arousal of TPH2-/- pups, irrespective of the gas they breathed. Thus infants who are deficient in CNS 5-HT may be at increased risk for SIDS in AS because of increased apnea and delayed arousal compared with QS.NEW & NOTEWORTHY Sudden infant death syndrome (SIDS) occurs during sleep and is associated with central serotonin (5-HT) deficiency. We report that rat pups deficient in central 5-HT (TPH2-/-) are profoundly more apneic in active sleep (AS) but not quiet sleep (QS). Unlike control pups, the arousal of TPH2-/- pups in air, CO₂, and hypoxia was delayed in AS compared with QS. Thus for infants deficient in central 5-HT, the risk of SIDS may be higher in AS than in QS.

Overnight Polysomnographic Characteristics and Oxygen Saturation of Healthy Infants, 1 to 18 Months of Age, Born and Residing At High Altitude (2,640 Meters)

BACKGROUND

Approximately 8% of the world population resides above 1,600 m, with about 10 million people living above 2,500 m in Colombia. However, reference values for polysomnography (PSG) and oxygen saturation (Spo2) of children < 2 years old residing at high altitude are currently unavailable.

METHODS

Healthy infants aged 1 to 18 months born and residing at high altitude (Bogotá: 2,640 m) underwent overnight PSG. Four age groups were defined: group 1, < 45 days; group 2, 3 to 4 months; group 3, 6 to 7 months; and group 4, 10 to 18 months. Of 122 children enrolled, 50 had three consecutive PSG tests and were analyzed as a longitudinal subcohort.

RESULTS

A total of 281 PSG tests were performed in 122 infants (56% girls): group 1, 106 PSG tests; group 2, 89 PSG tests; group 3, 61 PSG tests; and group 4, 25 PSG tests. Active sleep diminished and quiet sleep increased with maturation. Apnea-hypopnea indexes (total, central, and obstructive) were highest in group 1 (21.4, 12.4, and 6.8/h total sleep time, respectively) and diminished with age (P < .001). Mean Spo2 during waking and sleep increased with age (P < .001). Nadir Spo2 values during respiratory events were lower in younger infants. Longitudinal assessments of 50 infants confirmed the temporal trends described for the cross-sectional dataset.

CONCLUSIONS

Healthy infants (≤ 18 months old) born and residing at high altitude show preserved sleep architecture but higher apnea-hypopnea indexes and more prominent desaturation with respiratory events than do those living at low altitude. The current study findings can be used as reference values for infants at high altitude.

Effects of intrauterine growth restriction on sleep and the cardiovascular system: The use of melatonin as a potential therapy?

Intrauterine growth restriction (IUGR) complicates 5-10% of pregnancies and is associated with increased risk of preterm birth, mortality and neurodevelopmental delay. The development of sleep and cardiovascular control are closely coupled and IUGR is known to alter this development. In the long-term, IUGR is associated with altered sleep and an increased risk of hypertension in adulthood. Melatonin plays an important role in the sleep-wake cycle. Experimental animal studies have shown that melatonin therapy has neuroprotective and cardioprotective effects in the IUGR fetus. Consequently, clinical trials are currently underway to assess the short and long term effects of antenatal melatonin therapy in IUGR pregnancies. Given melatonin's role in sleep regulation, this hormone could affect the developing infants' sleep-wake cycle and cardiovascular function after birth. In this review, we will 1) examine the role of melatonin as a therapy for IUGR pregnancies and the potential implications on sleep and the cardiovascular system; 2) examine the development of sleep-wake cycle in fetal and neonatal life; 3) discuss the development of cardiovascular control during sleep; 4) discuss the effect of IUGR on sleep and the cardiovascular system and 5) discuss the future implications of melatonin therapy in IUGR pregnancies.

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.

Maturation of heart rate and blood pressure variability during sleep in term-born infants

STUDY OBJECTIVES

Abnormal blood pressure control is implicated in the sudden infant death syndrome (SIDS). However, no data exist on normal development of blood pressure control during infancy. This study assessed maturation of autonomic control of blood pressure and heart rate during sleep within the first 6 months of life.

PARTICIPANTS

Term infants (n = 31) were studied longitudinally at 2-4 weeks, 2-3 months, and 5-6 months postnatal age.

INTERVENTIONS

Infants underwent daytime polysomnography at each age studied. Blood pressure and heart rate were recorded during quiet (QS) and active (AS) sleep in undisturbed baseline and head-up tilt conditions.

MEASUREMENTS AND RESULTS

Autonomic control was assessed using spectral indices of blood pressure and heart rate variability (BPV and HRV) in ranges of low frequency (LF, reflecting sympathetic + parasympathetic activity) and high frequency (HF, parasympathetic activity), total power (LF+HF), and LF/HF ratio (sympathovagal balance). With increasing postnatal age and predominantly during QS, HRV-LF, HRV-HF, and HRV total power increased, while HRV-LF/HF decreased. BPV-LF/HF also decreased with postnatal age. All changes were evident in both baseline and head-up tilt conditions. BPV-LF and BPV total power during tilts were markedly reduced in QS versus AS at each age.

CONCLUSIONS

In sleeping infants, sympathetic vascular modulation of the circulation decreases with age, while parasympathetic control of heart rate is strengthened. These normative data will aid in the early identification of conditions where autonomic function is impaired, such as in SIDS.

Polysomnography in preterm infants and children with chronic lung disease

OBJECTIVE

To determine the utility of overnight polysomnography (PSG) in assessing pulmonary reserve in stable preterm children with chronic lung disease (CLD).

STUDY DESIGN

A retrospective review and descriptive study of overnight PSGs and clinic visits of preterm infants/children less than 3 years of age who were diagnosed with bronchopulmonary dysplasia at discharge from the hospital and enrolled in the Johns Hopkins CLD patient registry between 2008 and 2010.

RESULTS

Sixty-two clinically stable patients underwent at least one overnight polysomnogram for clinical indications. The majority of patients were referred for oxygen titration (71%). PSGs from first studies revealed a mean respiratory disturbance index (RDI) of 8.2 ± 10.1 events/hr and a mean O(2) saturation (SaO(2) ) nadir of 86.2 ± 5.7%. In patients who underwent more than one PSG (n = 23), a significant decrease in RDI (P < 0.001) was found between the first study (mean age: 8.0 ± 3.3 months) and second study (mean age: 13.4 ± 5.2 months). Outpatient clinical measures of mean room air SaO(2) and respiratory rate were not predictive of PSG measures of RDI and SaO(2) nadir.

CONCLUSION

Mean RDI was higher in stable preterm infants/children with CLD compared to previously published controls. RDI decreased with age in stable preterm infants/children with CLD suggesting improved pulmonary reserve with age. Outpatient clinical measures (respiratory rate and room air SaO(2) ) did not correlate with RDI and SaO(2) nadir indicating that overnight PSG is more sensitive in assessing pulmonary reserve than outpatient clinical measures.

Preterm birth alters the maturation of baroreflex sensitivity in sleeping infants

OBJECTIVE

Impaired blood pressure (BP) control may underpin the increased incidence of the sudden infant death syndrome (SIDS) in preterm infants. This study aimed to examine the effects of preterm birth, postnatal age, and sleep state on BP control by measuring baroreflex sensitivity (BRS) across the first 6 months of term-corrected age (CA), when SIDS risk is greatest.

METHODS

Preterm (n = 25) and term (n = 31) infants were studied longitudinally at 2 to 4 weeks, 2 to 3 months, and 5 to 6 months CA using daytime polysomnography. BP was recorded during quiet (QS) and active (AS) sleep using a photoplethysmographic cuff placed around the infant's wrist (Finometer [FMS, Finapres Medical Systems, Amsterdam, Netherlands]). BRS (milliseconds/mm Hg) was assessed in 1- to 2-minute epochs using cross-spectral analysis.

RESULTS

In preterm infants, postnatal age had no significant effect on BRS within either QS or AS. This was in contrast to the maturational increase in QS observed in term infants. Compared with term infants, BRS of preterm infants was 38% higher at 2 to 4 weeks CA and 29% lower at 5 to 6 months CA during QS (P <.05). Comparing sleep states, BRS of preterm infants was 26% lower in QS compared with AS at 2 to 3 months CA (P <.05).

CONCLUSIONS

Preterm birth impairs the normal maturational increase in BRS, resulting in a substantial reduction in BRS at 5 to 6 months CA during QS. Lower BRS during QS compared with AS at 2 to 3 months CA may place preterm infants at an increased risk for cardiovascular instability at this age of peak incidence of 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.

Awake and asleep oxygen saturations in infants with chronic neonatal lung disease

AIM

There is uncertainty about the best method of withdrawing supplemental oxygen in babies with chronic neonatal lung disease (CNLD). Some authors advocate withdrawal of oxygen in the day, but continuing supplementation during sleep, based on early work suggesting that oxygen saturations are lower during sleep, which did not accord with our clinical impression. We re-examined the hypothesis that babies have lower saturations while asleep.

METHODS

We studied infants with CNLD during the day, while awake and asleep. We recorded video with simultaneous real-time capture of oxygen saturation (SpO2), heart rate and plethysmographic waveform from pulse oximetry. Behavioural state was scored using observation and video and classified as awake (feeding, active or quiet) or sleep.

RESULTS

Thirteen infants had analysable data, although one had strikingly lower SpO2 values while awake and was excluded from analysis. The infants had a median gestation of 26 weeks and were studied at a median (range) postmenstrual age of 66 (37-130) weeks, for 229 (89-330) min. Mean SpO2 was 97.6% during sleep and 97.0% awake (p=0.011).

CONCLUSION

Babies with CNLD have lower oxygen saturation while awake. There is no physiological justification for increasing oxygen during sleep, or withdrawing selectively during the daytime, although larger studies are needed to confirm this finding.

Cardiovascular control during sleep in infants: Implications for Sudden Infant Death Syndrome

In infants the cardiorespiratory system undergoes significant functional maturation after birth and these changes are sleep-state dependent. Given the immaturity of these systems it is not surprising that infants are at risk of cardiorespiratory instability, especially during sleep. A failure of cardiovascular control mechanisms in particular is believed to play a role in the final event of Sudden Infant Death Syndrome (SIDS). The "triple risk model" describes SIDS as an event that results from the intersection of three overlapping factors: (1) a vulnerable infant, (2) a critical development period in homeostatic control, and (3) an exogenous stressor. This review summarises normal development of cardiovascular control during sleep in infants and describes the association of impaired cardiovascular control with the three overlapping factors proposed to be involved in SIDS pathogenesis.

The lateral paragigantocellular nucleus modulates parasympathetic cardiac neurons: a mechanism for rapid eye movement sleep-dependent changes in heart rate

Rapid eye movement (REM) sleep is generally associated with a withdrawal of parasympathetic activity and heart rate increases; however, episodic vagally mediated heart rate decelerations also occur during REM sleep. This alternating pattern of autonomic activation provides a physiological basis for REM sleep-induced cardiac arrhythmias. Medullary neurons within the lateral paragigantocellular nucleus (LPGi) are thought to be active after REM sleep recovery and play a role in REM sleep control. In proximity to the LPGi are parasympathetic cardiac vagal neurons (CVNs) within the nucleus ambiguus (NA), which are critical for controlling heart rate. This study examined brain stem pathways that may mediate REM sleep-related reductions in parasympathetic cardiac activity. Electrical stimulation of the LPGi evoked inhibitory GABAergic postsynaptic currents in CVNs in an in vitro brain stem slice preparation in rats. Because brain stem cholinergic mechanisms are involved in REM sleep regulation, we also studied the role of nicotinic neurotransmission in modulation of GABAergic pathway from the LGPi to CVNs. Application of nicotine diminished the GABAergic responses evoked by electrical stimulation. This inhibitory effect of nicotine was prevented by the alpha7 nicotinic receptor antagonist alpha-bungarotoxin. Moreover, hypoxia/hypercapnia (H/H) diminished LPGi-evoked GABAergic current in CVNs, and this inhibitory effect was also prevented by alpha-bungarotoxin. In conclusion, stimulation of the LPGi evokes an inhibitory pathway to CVNs, which may constitute a mechanism for the reduced parasympathetic cardiac activity and increase in heart rate during REM sleep. Inhibition of this pathway by nicotinic receptor activation and H/H may play a role in REM sleep-related and apnea-associated bradyarrhythmias.

Sleep in infants with congenital heart disease

OBJECTIVES

To investigate hypoxia and sleep disordered breathing in infants with congenital heart disease.

METHODS

Prospective study. In-hospital full polysomnography was performed on 14 infants with congenital heart disease, age 7 +/-1 months, and in 7 normal infants, age 10 +/-2 months. Congenital heart disease infants were classified as acyanotic (n=7) or cyanotic (n=7).

RESULTS

Nutritional status, assessed by the Gomez classification and expressed as % weight for age, was 70 +/-7, 59 +/-11 and 94 +/-16 in the acyanotic, cyanotic congenital heart disease and control infants, respectively (p<0.001). The respiratory disturbance index (AHI, events per hour) was [median (25-75%)]: 2.5 (1.0-3.4), 2.4 (1.5-3.1) and 0.7 (0.7-0.9) in acyanotic, cyanotic CHD infants and controls, respectively (p=0.013). Almost all congenital heart disease infants (11 out of 14) and only one control infant had an AHI >1 event/hour. The minimum oxygen saturation was 79% (74-82), 73% (57-74) and 90% (90-91) in the acyanotic, cyanotic congenital heart disease infants and controls, respectively (p <0.001). The arousal index (events/hour) was similar among the three groups at 8.4 +/-2.4, 10.3 +/-8.7 and 6.5 +/-3, respectively (p=0.451).

CONCLUSIONS

Infants with congenital heart disease frequently present with sleep-disordered breathing associated with oxygen desaturations but not arousals. Therefore, sleep may represent a significant burden to infants with congenital heart disease.

Blood pressure and heart rate patterns during sleep are altered in preterm-born infants: implications for sudden infant death syndrome

OBJECTIVE

Preterm infants are at an increased risk of sudden infant death syndrome, which may result from immature autonomic control of heart rate and blood pressure. Previous studies have demonstrated that preterm infants have altered heart rate and blood pressure control at term-equivalent age; however, little information is available beyond this age. The aim of this study was to determine the effect of preterm birth on heart rate and blood pressure control over the first 6 months of life after reaching term-equivalent age, including the age at which sudden infant death syndrome risk is increased, to understand the pathogenesis of sudden infant death syndrome.

METHODS

Preterm (n=25) and term (n=20) infants were studied longitudinally at 2 to 4 weeks', 2 to 3 months', and 5 to 6 months' term-corrected age by using daytime polysomnography. A photoplethysmographic cuff (Finometer) around the infant's wrist measured blood pressure during quiet and active sleep.

RESULTS

Blood pressure was lower in the preterm group during both quiet and active sleep at all ages studied. In contrast, there were no differences between groups in heart rate. Within the infants in the preterm group, blood pressure averaged lower at 2 to 3 months' corrected age compared with both 2 to 4 weeks' and 5 to 6 months' corrected age and was lower in quiet sleep compared with active sleep at all ages studied. Heart rate decreased with increasing age and was lower in quiet sleep compared with active sleep at 5 to 6 months' corrected age.

CONCLUSIONS

Sleep state and age affect heart rate and blood pressure patterns in prematurely born infants over the first 6 months of term-corrected age. It is notable that preterm infants had persistently lower blood pressure compared with age-matched term infants, signifying long-term alterations in cardiovascular control in infants born prematurely.

Sleep fragmentation and evidence for sleep debt in alcohol-exposed infants

BACKGROUND

Infants exposed prenatally to alcohol are at increased risk for poor neurodevelopmental outcome including Sudden Infant Death Syndrome.

AIM

To examine the relationship between prenatal alcohol exposure, sleep, arousal and sleep-related spontaneous motor movements in early infancy.

STUDY DESIGN

Low-income women (N=13) were interviewed regarding pre- and pregnancy rates of alcohol, cigarette smoking and other substance use in the perinatal period. Infants were examined in a laboratory nap study using EEG, videography and actigraphy at 6-8 weeks of age. Estimates of maternal pre- and pregnancy alcohol use were used to divide infants into high vs. low maternal alcohol use groups.

SUBJECTS

Mother-infant dyads recruited from a family practice clinic.

OUTCOME MEASURES

Sleep-related spontaneous movements, behavioral state, and maternal assessments of infant alertness and irritability.

RESULTS

Pre-pregnancy rates of alcohol consumption including binge drinking correlated with maternal report of poor infant alertness, and increased irritability. High maternal exposure groups showed increased sleep fragmentation, e.g., frequency and duration of wakefulness following sleep onset and decreased active sleep. Bout analysis of the temporal structure of sleep-related spontaneous movements showed significantly reduced bout duration associated with high maternal alcohol use.

CONCLUSION

These results present evidence that prenatal alcohol exposure disrupts postnatal sleep organization and suppresses spontaneous movements during sleep, and increased sleep fragmentation promotes sleep deprivation. Results are consistent with the SIDS model of chronic sleep debt and suggest that attenuated sleep-related movements should be examined as an important modulator of cardiorespiratory functions during sleep in high-risk groups.

Pneumocystis: a novel pathogen in chronic obstructive pulmonary disease?

Chronic obstructive pulmonary disease (COPD) results in significant morbidity and mortality. Smoking has long been recognized as the primary risk factor for development of COPD, but factors determining the severity or pattern of disease in smokers are largely unknown. Recent interest has focused on the potential role of infectious agents and the associated host response in accelerating progression of airway obstruction or in perpetuating its progression following discontinuation of tobacco exposure. Pneumocystis jirovecii is a fungal pathogen that causes pneumonia in immunocompromised individuals. Recent evidence has linked this organism with COPD. Using sensitive molecular techniques, low levels of Pneumocystis have been detected in the respiratory tract of certain individuals and termed colonization. Several findings support the theory that colonization with Pneumocystis is involved in the "vicious circle" hypothesis of COPD in which colonization with organisms perpetuates an inflammatory and lung remodeling response. Pneumocystis colonization is more prevalent in smokers and in those with severe COPD. The presence of Pneumocystis in the lungs, even at low levels, produces inflammatory changes similar to those seen in COPD, with increases in numbers of neutrophils and CD8(+) lymphocytes. HIV-infected subjects who have had PCP develop permanent airway obstruction, and HIV-infected patients have a high prevalence of both emphysema and Pneumocystis colonization. In addition, a non-human primate model of colonization shows development of airway obstruction and radiographic emphysema. Additional studies are needed to confirm the role of Pneumocystis in the pathogenesis of COPD, given that this agent might be a treatable co-factor in disease progression.

Ontogeny of EEG-sleep from neonatal through infancy periods

Serial neonatal and infant electroencephalographic (EEG)-polysomnographic studies document the ontogeny of cerebral and noncerebral physiologic behaviors based on visual inspection or computer analyses. EEG patterns and their relationship to other physiologic signals serve as templates for normal brain organization and maturation, subserving multiple interconnected neuronal networks. Interpretation of serial EEG-sleep patterns also helps track the continuity of brain functions from intrauterine to extrauterine time periods. Recognition of the ontogeny of behavioral and electrographic patterns provides insight into the developmental neurophysiological expression of neural plasticity. Sleep ontogenesis from neonatal and infancy periods documents expected patterns of postnatal brain maturation, which allows for alterations from genetically programmed neuronal processes under stressful and/or pathological conditions. Automated analyses of cerebral and noncerebral signals provide time- and frequency-dependent computational phenotypes of brain organization and maturation in healthy or diseased states. Research pertaining to the developmental origins of health and disease can use these computational phenotypes to design longitudinal studies for the assessment of gene-environment interactions. Computational strategies may ultimately improve our diagnostic skills to identify special-needs children and to track the neurorehabilitative care of the high-risk fetus, neonate, and infant.

Ventilatory response to a hyperoxic test is related to the frequency of short apneic episodes in late preterm neonates

Chemoreception is frequently involved in the processes underlying apnea in premature infants. Apnea could result from a decrease in carotid body effectiveness. However, increased carotid body activity could also initiate apnea through hypocapnia following hyperventilation when the receptors are stimulated. The aim of this study was to analyze the relationship between carotid body effectiveness and short apneic episodes in older preterm neonates. Carotid body effectiveness was assessed at thermoneutrality in 36 premature neonates (2.07 +/- 0.26 kg) by performing a 30-s hyperoxic test during sleep, the oxygen inhalation involving a ventilation decrease. Blood O(2) saturation (Sp(o2)) and ventilatory parameters were monitored before and during the hyperoxic test. Short episodes of apnea (frequency and mean duration) were recorded during the morning's 3-h interfeeding interval. Pretest Sp(o2) was not related to any of the measured respiratory parameters. A higher frequency of short apneic episodes was linked to a greater ventilation decrease in response to the hyperoxic test (rho = -0.32; p = 0.01). Increased carotid body response is correlated with greater apneic episodes frequency, even in the absence of concomitant oxygen desaturation. Fetal or early postnatal hypoxemia could have increased peripheral chemoreceptor activity, which could initiate a "overshoot/undershoot" situation, which in turn could induce a critical P(o2)/P(co2) combination and apnea.

Comparison of heart rate responses during cortical and subcortical arousals in term and preterm infants

The aim of this study was to determine whether prematurity affects heart rate responses during spontaneous arousals. Polygraphic recordings were performed during undisturbed daytime naps in 35 preterm infants (gestational age at birth 32+/-2 weeks) and 35 term infants. Arousals were scored according to the recommendations of the International Paediatric Work Group on Arousals and categorized either as cortical arousals (CA) or subcortical arousals (SCA). Heart rate (HR) and respiratory frequency (RF) were measured during arousal and during the 10-s and 30-s period before and after arousal. Changes in HR and RF were expressed as the percentage of modification normalized for the 30-s period preceding arousal. Altogether, 122 arousals in preterm infants (66 CA, 56 SCA) and 105 arousals in term infants (57 CA, 48 SCA) were scored. Mean duration of the arousal period was 9+/-4 s and 8+/-3 s, respectively. In term infants, a significant increase in HR during arousal could be shown (11.3+/-8.2%; p<0.001), whereas this increase was significantly greater during CA compared to SCA (13.7+/-6.2% versus 8.4+/-9.4%; p<0.001). In contrast, HR decreased during arousal in preterm neonates (-3.9+/-19.3%; p<0.05). These findings suggest that cardiovascular control seems to be maturationally delayed in preterm infants, which may contribute to their increased risk for Sudden Infant Death Syndrome (SIDS).

Effects of prematurity on heart rate control: implications for sudden infant death syndrome

In Western countries, 5-11% of all infants are born before 37 weeks of gestation, and with improvements in modern intensive care techniques the number of these preterm infants that survive continues to increase. Preterm birth is one of the leading causes of neonatal morbidity and mortality in developed countries, accounting for 60-80% of infant deaths in those without congenital anomalies. Furthermore, in the post-neonatal period, preterm infants are at four-times greater risk of sudden infant death syndrome. It has been suggested that this increased risk is due to immature autonomic control. This article provides an overview of studies assessing autonomic control of the cardiovascular system in preterm infants.

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.

Polysomnographic findings in 320 infants evaluated for apneic events

Apnea is a common problem that causes significant parental anxiety. The aim of this study was to describe polysomnographic findings in infants who were referred over a 4-year period for an apnea and/or cyanotic event. Our hypothesis was that most infants with apnea or cyanosis events will have normal polysomnography (PSG). In total, 320 patients younger than 2 years old were recruited sequentially and prospectively. Patients underwent a day or overnight PSG by computerized polysomnograph; 78% of studies were performed with pH probe in situ. Subjects' ages ranged from 10 days to 21 months (55% male and 84% full-term babies); 55% and 74% were younger than 3 and 6 months, respectively. The average total sleep time was 473.4 min (SD, 52.3), with mean sleep efficiency of 83.5%. The distribution of sleep stages was 56.5% active, 38.5%, quiet and 5.1% indeterminate sleep. Sixty-nine percent (n = 220) of PSG studies were done overnight. There were significant differences in average sleep efficiency (78.1% vs. 83.3%) and REM sleep time (63.6% vs. 59.1%) between PSGs done during the day and overnight (P < 0.05). In total, 34 patients with apneas were studied. The median apnea index for the whole study population was 1.01 (range, 0.1-9.1). In conclusion, our study showed a high prevalence of normal polysomnographic findings in infants referred because of apnea and/or cyanotic events. New clinical prospective studies should be conducted to evaluate whether there is a correlation between PSG findings and outcome at follow-up of infants with a history of apnea and cyanosis.

Bladder voiding in sleeping infants is consistently accompanied by a cortical arousal

The aim of the study was to find out whether bladder voiding in healthy sleeping infants was accompanied by any arousal reaction. Polygraphic recordings were performed in 21 healthy infants (11 female) born at term. The infants' age at study entry was 42 +/- 4 days and actual body weight was 4852 +/- 689 g (mean +/- SD). Bladder voiding was recorded by an adapted enuresis detector which was connected to the polygraphic computer unit. Arousals were defined as suggested by the 'International Paediatric Work Group on Arousals'. Awakenings were excluded from the study. Bladder voiding was recorded at a mean time of 68 +/- 7 min after the infant had fallen asleep and occurred during quiet sleep (QS). Electroencephalogram frequency (P < 0.01) and heart rate (P < 0.05) were higher during the 5-s period before and after bladder voiding when compared with a 30-s interval before voiding. Furthermore, bladder voiding was accompanied by body movements in all infants. Respiratory frequency did not change significantly. We could demonstrate for the first time in sleeping infants, that bladder voiding during QS was accompanied by a cortical arousal.

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.

Serum testosterone and estradiol in sudden infant death

OBJECTIVE

To test the hypothesis that among infants who die unexpectedly, testosterone and/or estradiol levels are elevated in those diagnosed with SIDS versus those with known causes of death (controls).

STUDY DESIGN

Postmortem blood was collected and coded from infant autopsies, and serum was prepared and frozen until assayed for total testosterone and estradiol by fluoroimmunoassay. Subject information was then collected from the medical examiner's report.

RESULTS

Testosterone, but not estradiol, was significantly higher in 127 SIDS cases versus 42 controls for both males (4.8 +/- 0.4 vs 2.2 +/- 0.4 nmol, respectively; P < .005) and females (2.4 +/- 0.2 vs 1.6 +/- 0.2 nmol, respectively; P < 0.03).

CONCLUSIONS

Higher testosterone levels in infant victims of unexpected, unexplained death may indicate a role for testosterone or related steroids in SIDS. Further research is needed to understand the potential utility of testosterone as an indicator of SIDS risk.

Comparison of postnatal development of heart rate responses to trigeminal stimulation in sleeping preterm and term infants

Autonomic dysfunction has been regarded as a possible cause of the sudden infant death syndrome (SIDS) and it has been suggested that preterm infants, who are at a greater risk of SIDS than term infants, may have immature autonomic control. Our aim was to compare the maturation of cardiac autonomic control during sleep in preterm and term infants by examining heart rate responses to arousing and non-arousing trigeminal stimuli. Preterm infants (n = 15) and term infants (n = 24) were studied longitudinally with daytime polysomnography. Air-jet stimulation of the nares was delivered in both active sleep (AS) and quiet sleep (QS), and heart rate (HR) changes recorded for both arousal and non-arousal responses. Changes in HR (DeltaHR%) were calculated as the relative differences between baseline HR (BHR) and either MaxHR (arousal) or MinHR (non-arousal). Comparisons of HR changes between sleep states and postnatal ages were made with two-way anova for repeated measures and between groups with two-way anova. The increase in HR (DeltaHR%) was greater in term than preterm infants (P < 0.05), but only at 2-3 weeks corrected postnatal age (CPA). In preterm infants, there were no differences in BHR between sleep states, whereas in term infants, BHR was higher in AS than in QS at 2-3 weeks and 2-3 months of age. The smaller DeltaHR% to arousing stimuli in preterm infants compared with term infants at 2-3 weeks suggests that cardiac sympathetic activity in preterm infants may be lower than in term infants. This mechanism may account for the increased risk for SIDS of preterm infants.

Sleep-dependent changes in the coupling between heart period and arterial pressure in newborn lambs

This study assessed whether sleep-dependent changes in the relationship between heart period (HP) and mean arterial pressure (MAP) occur in newborn life. Electrodes for electrocorticographic, electromyographic, and electrooculographic monitoring and an arterial catheter for blood pressure recordings were implanted in 11 newborn lambs. HP and MAP beat-to-beat values were computed from 120-s blood pressure recordings during quiet wakefulness, active sleep, and quiet sleep. For each recording, the time shift at which the maximum of the HP versus MAP cross-correlation function was attained was identified. For each lamb and wake-sleep state, an average correlation coefficient was then computed corresponding to the median value of such time shifts. The maximum of the cross-correlation function was attained with HP lagging behind MAP. The corresponding mean correlation coefficient was significantly higher in quiet sleep (0.51 +/- 0.05) than either in quiet wakefulness (0.31 +/- 0.05) or in active sleep (0.29 +/- 0.03). Sleep-related differences in the correlation between HP and MAP were maintained after HP and MAP data were low-pass filtered at 0.3 Hz to remove their fast ventilatory oscillations. In conclusion, data indicate that the relationship between spontaneous fluctuations in HP and those in MAP is sleep-state dependent in newborn lambs. A positive HP versus MAP correlation with HP lagging behind MAP is consistent with baroreflex control of HP. Heart rhythm thus may be more tightly controlled by the baroreceptor reflex and less dependent on central autonomic commands in quiet sleep than either in quiet wakefulness or in active sleep.

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).

Effects of warm and cool thermal conditions on ventilatory responses to hyperoxic test in neonates

Body temperature interacts with respiratory control, but it is unclear what sites or mechanisms mediate those interactions. We hypothesized that warm and cool thermal conditions affect the decrease in ventilation (VE) seen during the hyperoxic test (HT), a breathing response believed to reflect the strength of the peripheral chemoreceptor drive. A breath-by-breath analysis during a 30 s HT was performed in eight premature neonates (postconceptional age: 36 +/- 1 weeks) under neutral, warm, and cool thermal conditions. Quiet sleep (QS) and active sleep (AS) were scored by neurophysiological criteria. The VE fall was higher in AS than in QS, and warm and cool conditions significantly enhanced the response only in AS (-24.2 +/- 6.0, -39.1 +/- 9.1, and -37.5 +/- 14.1% in neutral, warm, and cool conditions, respectively). Central control mechanisms of the respiratory chemoreflex may explain the increase in peripheral chemoreceptor drive during AS in response to thermal challenges, which may produce increased breathing instability leading to apnea in early life.

Ventilatory drive and the apnea-hypopnea index in six-to-twelve year old children

Background

We tested the hypothesis that ventilatory drive in hypoxia and hypercapnia is inversely correlated with the number of hypopneas and obstructive apneas per hour of sleep (obstructive apnea hypopnea index, OAHI) in children.

Methods

Fifty children, 6 to 12 years of age were studied. Participants had an in-home unattended polysomnogram to compute the OAHI. We subsequently estimated ventilatory drive in normoxia, at two levels of isocapnic hypoxia, and at three levels of hyperoxic hypercapnia in each subject. Experiments were done during wakefulness, and the mouth occlusion pressure measured 0.1 seconds after inspiratory onset (P_0.1) was measured in all conditions. The slope of the relation between P_0.1 and the partial pressure of end-tidal O₂ or CO₂ (P_ETO₂ and P_ETCO₂) served as the index of hypoxic or hypercapnic ventilatory drive.

Results

Hypoxic ventilatory drive correlated inversely with OAHI (r = -0.31, P = 0.041), but the hypercapnic ventilatory drive did not (r = -0.19, P = 0.27). We also found that the resting P_ETCO₂ was significantly and positively correlated with the OAHI, suggesting that high OAHI values were associated with resting CO₂ retention.

Conclusions

In awake children the OAHI correlates inversely with the hypoxic ventilatory drive and positively with the resting P_ETCO₂. Whether or not diminished hypoxic drive or resting CO₂ retention while awake can explain the severity of sleep-disordered breathing in this population is uncertain, but a reduced hypoxic ventilatory drive and resting CO₂ retention are associated with sleep-disordered breathing in 6–12 year old children.