The development of stability of respiration in human infants: changes in ventilatory responses to spontaneous sighs

Infant periodic breathing and apneic threshold

A mathematical model was proposed to predict the role played by apneic threshold in periodic breathing in preterm infants. Prior models have mainly applied linear control theory which predicted instability but could not explain sustained periodic breathing. Apneic threshold to CO2 which has been postulated to play a major role in infant periodic breathing is a nonlinear effect and cannot be described by linear theory. Another previously unexplored nonlinear factor affecting instability is brain vascular volume change with CO2 which affects time delay to chemoreceptors. The current model explored the influences of apneic threshold, central and peripheral chemoreceptor gains, cardiac output, lung volume, and circulatory time delay on periodic breathing. Apneic threshold was found to play a major role in ventilatory responses to spontaneous sighs. Sighs led to apneic pauses followed by periods of periodic breathing with peripheral chemoreceptor CO2 gain, cardiac output, and lung volume were at reported normal levels. Apneic threshold when exceeded was observed to cause an asymmetry in the periodic breathing cycling and an increased periodic breathing frequency. Sighs in infants occur frequently enough to lead to repeated stimulation within the epoch duration of periodic breathing for a single sigh. Multiple sighs may then play a major role in promoting continuous periodic breathing in infants. Peripheral chemoreceptor gain estimated using endogenous CO2 led to validated predicted periodic breathing cycle duration as a function of age. Brain vascular volume increase with CO2 contributes to periodic breathing in very young (1-2 day old) preterm infants.

Noninvasive Respiratory Support Effects on Sighs in Preterm Infants by Electrical Impedance Tomography

OBJECTIVE

To evaluate differences regarding sigh frequency between noninvasive respiratory support types and to assess regional ventilation distribution, delta Z, and end-expiratory lung impedance differences before and after sighs.

METHODS

Very low-birth-weight infants with gestational ages less than 32 wk were included in the study. Participants were split into two groups: those receiving continuous positive airway pressure and infants receiving high-flow nasal cannula therapy.

RESULTS

The study enrolled 30 infants. The high-flow nasal cannula therapy group had more sighs per 10-min period than infants receiving continuous positive airway pressure (p = 0.016). Ventilation distribution was similar in the anterior and right ventilation distribution compartments pre- and post-sigh (46.30% vs. 45.68% and 54.27% vs. 55.26%, respectively). No statistically significant increase in end-expiratory lung impedance or delta Z was observed in global or separate lung regions (p > 0.05).

CONCLUSION

The study has demonstrated that sighs are more frequent in infants receiving high-flow nasal cannula respiratory support compared to continuous positive airway pressure. Spontaneously occurring sighs on noninvasive respiratory support due to respiratory distress syndrome (RDS) do not increase end-expiratory lung impedance or alter delta Z, and appear to have limited clinical significance.

TRIAL REGISTRATION

Prospectively registered at www.

CLINICALTRIALS

gov , reg. No. NCT04542096, reg. date 01/09/2020.

Ventilatory control instability as a predictor of persistent periodic breathing in preterm infants

BACKGROUND

Periodic breathing (PB) is common in preterm infants. We aimed to characterize the contribution of ventilatory control instability to the presence and persistence of PB longitudinally.

METHODS

Infants born between 28 and 32 weeks of gestation were studied using daytime polysomnography at: 32-36 weeks postmenstrual age (PMA) (N = 32), 36-40 weeks PMA (N = 20), 3 months corrected age (CA) (N = 18) and 6 months CA (N = 19). Loop gain, a measure of sensitivity of the ventilatory control system, was estimated by fitting a mathematical model to ventilatory patterns associated with spontaneous sighs.

RESULTS

The time spent in PB decreased from 32-36 weeks PMA to 6 months CA (P = 0.005). Across all studies, studies with PB (N = 62) were associated with higher loop gain compared to those without PB (N = 23) (estimated marginal mean ± SEM: 0.445 ± 0.01 vs 0.388 ± 0.02; P = 0.020). A threshold of loop gain >0.415 (measured at 32-36 weeks PMA) provided a sensitivity of 86% and a specificity of 75% to detect the presence of PB at 6 months CA.

CONCLUSIONS

The course of PB in preterm infants is related to changes in loop gain. Higher loop gain at 32-36 weeks PMA was associated with a greater risk of persistent PB at 6 months CA.

IMPACT

The developmental trajectory of periodic breathing and its relationship to ventilatory control instability is currently unclear. Unstable ventilatory control is a determinant of periodic breathing in preterm infants up to 6 months corrected age. Infants who display greater ventilatory control instability at 32-36 weeks postmenstrual age may be at increased risk of persistent periodic breathing at 6 months corrected age. Assessment of ventilatory control stability may assist in the early identification of infants at risk of persistent periodic breathing and its potential adverse effects.

The sigh and related behaviors

Breathing is a critical, complex, and highly integrated behavior. Normal rhythmic breathing, also referred to as eupnea, is interspersed with different breathing related behaviors. Sighing is one of such behaviors, essential for maintaining effective gas exchange by preventing the gradual collapse of alveoli in the lungs, known as atelectasis. Critical for the generation of both sighing and eupneic breathing is a region of the medulla known as the preBötzinger Complex (preBötC). Efforts are underway to identify the cellular pathways that link sighing as well as sneezing, yawning, and hiccupping with other brain regions to better understand how they are integrated and regulated in the context of other behaviors including chemosensation, olfaction, and cognition. Unraveling these interactions may provide important insights into the diverse roles of these behaviors in the initiation of arousal, stimulation of vigilance, and the relay of certain behavioral states. This chapter focuses primarily on the function of the sigh, how it is locally generated within the preBötC, and what the functional implications are for a potential link between sighing and cognitive regulation. Furthermore, we discuss recent insights gained into the pathways and mechanisms that control yawning, sneezing, and hiccupping.

Prematurity, the diagnosis of bronchopulmonary dysplasia, and maturation of ventilatory control

Infants born before 32 weeks gestational age and receiving respiratory support at 36 weeks postmenstrual age (PMA) are diagnosed with bronchopulmonary dysplasia (BPD). This label suggests that their need for supplemental oxygen (O₂ ) is primarily due to acquired dysplasia of airways and airspaces, and that the supplemental O₂ is treating residual parenchymal lung disease. However, emerging evidence suggests that immature ventilatory control may also contribute to the need for supplemental O₂ at 36 weeks PMA. In all newborns, maturation of ventilatory control continues ex utero and is a plastic process. Among premature infants, supplemental O₂ mitigates the hypoxemic effects of delayed maturation of ventilatory control, as well as reduces the duration and frequency of periodic breathing events. Nevertheless, prematurity is associated with altered and occasionally aberrant maturation of ventilatory control. Infants born prematurely, with or without a diagnosis of BPD, are more prone to long-lasting effects of dysfunctional ventilatory control. This review addresses normal and abnormal maturation of ventilatory control and suggests how aberrant maturation complicates assigning the diagnosis of BPD. Greater awareness of the interaction between parenchymal lung disease and delayed maturation of ventilatory control is essential to understanding why a given premature infant requires and is benefitting from supplemental O₂ at 36 weeks PMA.

Morinda citrifolia (Noni) fruit juice promotes vascular endothelium function in hypertension via glucagon-like peptide-1 receptor-CaMKKβ-AMPK-eNOS pathway

Morinda citrifolia (Noni) is extensively used in herbal remedies to prevent and treat various diseases, including hypertension. The purpose of this study was to investigate the vascular effects of noni fruit juice and characterize the upstream signaling pathways. We measured the systolic blood pressure, diastolic blood pressure, 24-hr urinary nitric oxide (NO) metabolite excretion, bodyweight (BW), and urine examination in SHR.Cg-Leprcp/NDmcr (SHR/cp) rats after 6 weeks noni juice (15 ml/kg) treatment. Noni juice significantly decreased blood pressure and 24-hr urinary NO metabolite without change of BW or urine volume. Furthermore, the noni juice extract (NJE) promoted endothelial vasorelaxation in rat aorta rings and NO product through an increase in phosphorylation of endothelial nitric oxide synthase (eNOS) in human umbilical vein endothelial cells (HUVECs). NJE might act on a glucagon like peptide-1 receptor (GLP-1R) via Ca²⁺ /calmodulin-dependent protein kinase kinase β (CaMKKβ)-AMPK signaling with pretreatment of their inhibitors or antagonist in HUVECs. Deacetylasperulosidic acid (DAA) was an active compound in noni juice to improve NO release through same pathway in HUVECs. These results suggested that noni is a novel dietary plant that probably regulates GLP-1R-induced CaMKKβ-AMPK-eNOS pathway to improve endothelium-dependent relaxation, thus reduce the blood pressure probably via one of its responsible ingredient DAA.

Postmenstrual age at discharge in premature infants with and without ventilatory pattern instability

RATIONALE

To determine if ventilatory pattern instability, manifested as periodic breathing (PB) during physiologic challenge testing, affects postmenstrual age (PMA) at discharge.

METHODS

Eighty infants underwent challenge testing at 36 weeks PMA. Infants breathing supplemental O₂ received a room air challenge (RAC, N = 51); those breathing ambient air underwent a hypoxic challenge test (HCT, N = 29). Infants were assigned one of four ventilatory control phenotypes based on the presence or absence of PB during their test, and if they passed or failed because of hypoxemia during the challenge test.

RESULTS

There were no clinical or demographic differences between groups. Infants who passed their challenge testing were, on average, discharged 1.6 weeks sooner than those who failed. The groups of ventilatory control phenotypes differed in PMA at discharge (p = 0.0020), but those with PB were younger by PMA at discharge.

CONCLUSIONS

Ventilatory pattern instability did not prolong time to discharge. Passing either challenge was associated with earlier discharge, suggesting these tests might identify infants who can have nasal cannula support removed and be safely discharged sooner. Most of the infants who failed their challenge tests with PB were receiving nasal cannula support. Nasal cannula support may be not only treating hypoxemia due to bronchopulmonary dysplasia (BPD), but also mitigating their ventilatory pattern instability.

Assessing ventilatory instability using the response to spontaneous sighs during sleep in preterm infants

Study Objectives

Periodic breathing (PB) is common in newborns and is an obvious manifestation of ventilatory control instability. However, many infants without PB may still have important underlying ventilatory control instabilities that go unnoticed using standard clinical monitoring. Methods to detect infants with "subclinical" ventilatory control instability are therefore required. The current study aimed to assess the degree of ventilatory control instability using simple bedside recordings in preterm infants.

Methods

Respiratory inductance plethysmography (RIP) recordings were analyzed from ~20 minutes of quiet sleep in 20 preterm infants at 36 weeks post-menstrual age (median [range]: 36 [34-40]). The percentage time spent in PB was also calculated for each infant (%PB). Spontaneous sighs were identified and breath-by-breath measurements of (uncalibrated) ventilation were derived from RIP traces. Loop gain (LG, a measure of ventilatory control instability) was calculated by fitting a simple ventilatory control model (gain, time-constant, delay) to the post-sigh ventilatory pattern. For comparison, periodic inter-breath variability was also quantified using power spectral analysis (ventilatory oscillation magnitude index [VOMI]).

Results

%PB was strongly associated with LG (r2 = 0.77, p < 0.001) and moderately with the VOMI (r2 = 0.21, p = 0.047). LG (0.52 ± 0.05 vs. 0.30 ± 0.03; p = 0.0025) and the VOMI (-8.2 ± 1.1 dB vs. -11.8 ± 0.9 dB; p = 0.026) were both significantly higher in infants that displayed PB vs. those without.

Conclusions

LG and VOMI determined from the ventilatory responses to spontaneous sighs can provide a practical approach to assessing ventilatory control instability in preterm infants. Such simple techniques may help identify infants at particular risk for ventilatory instabilities with concomitant hypoxemia and its associated consequences.

Cardioventilatory Control in Preterm-born Children and the Risk of Obstructive Sleep Apnea

RATIONALE

The contribution of ventilatory control to the pathogenesis of obstructive sleep apnea (OSA) in preterm-born children is unknown.

OBJECTIVES

To characterize phenotypes of ventilatory control that are associated with the presence of OSA in preterm-born children during early childhood.

METHODS

Preterm- and term-born children without comorbid conditions were enrolled. They were categorized into an OSA group and a non-OSA group on the basis of polysomnography.

MEASUREMENTS AND MAIN RESULTS

Loop gain, controller gain, and plant gain, reflecting ventilatory instability, chemoreceptor sensitivity, and blood gas response to a change in ventilation, respectively, were estimated from spontaneous sighs identified during polysomnography. Cardiorespiratory coupling, a measure of brainstem maturation, was estimated by measuring the interval between inspiration and the preceding electrocardiogram R-wave. Cluster analysis was performed to develop phenotypes based on controller gain, plant gain, cardiorespiratory coupling, and gestational age. The study included 92 children, 63 of whom were born preterm (41% OSA) and 29 of whom were born at term (48% OSA). Three phenotypes of ventilatory control were derived with risks for OSA being 8%, 47%, and 77% in clusters 1, 2, and 3, respectively. There was a stepwise decrease in controller gain and an increase in plant gain from clusters 1 to 3. Children in cluster 1 had significantly higher cardiorespiratory coupling and gestational age than clusters 2 and 3. No difference in loop gain was found between clusters.

CONCLUSIONS

The risk for OSA could be stratified according to controller gain, plant gain, cardiorespiratory coupling, and gestational age. These findings could guide personalized care for children at risk for OSA.

Resonance as the Mechanism of Daytime Periodic Breathing in Patients with Heart Failure

RATIONALE

In patients with chronic heart failure, daytime oscillatory breathing at rest is associated with a high risk of mortality. Experimental evidence, including exaggerated ventilatory responses to CO₂ and prolonged circulation time, implicates the ventilatory control system and suggests feedback instability (loop gain > 1) is responsible. However, daytime oscillatory patterns often appear remarkably irregular versus classic instability (Cheyne-Stokes respiration), suggesting our mechanistic understanding is limited.

OBJECTIVES

We propose that daytime ventilatory oscillations generally result from a chemoreflex resonance, in which spontaneous biological variations in ventilatory drive repeatedly induce temporary and irregular ringing effects. Importantly, the ease with which spontaneous biological variations induce irregular oscillations (resonance "strength") rises profoundly as loop gain rises toward 1. We tested this hypothesis through a comparison of mathematical predictions against actual measurements in patients with heart failure and healthy control subjects.

METHODS

In 25 patients with chronic heart failure and 25 control subjects, we examined spontaneous oscillations in ventilation and separately quantified loop gain using dynamic inspired CO₂ stimulation.

MEASUREMENTS AND MAIN RESULTS

Resonance was detected in 24 of 25 patients with heart failure and 18 of 25 control subjects. With increased loop gain-consequent to increased chemosensitivity and delay-the strength of spontaneous oscillations increased precipitously as predicted (r = 0.88), yielding larger (r = 0.78) and more regular (interpeak interval SD, r = -0.68) oscillations (P < 0.001 for all, both groups combined).

CONCLUSIONS

Our study elucidates the mechanism underlying daytime ventilatory oscillations in heart failure and provides a means to measure and interpret these oscillations to reveal the underlying chemoreflex hypersensitivity and reduced stability that foretells mortality in this population.

The relationship between partial upper-airway obstruction and inter-breath transition period during sleep

Short pauses or "transition-periods" at the end of expiration and prior to subsequent inspiration are commonly observed during sleep in humans. However, the role of transition periods in regulating ventilation during physiological challenges such as partial airway obstruction (PAO) has not been investigated. Twenty-nine obstructive sleep apnea patients and eight controls underwent overnight polysomnography with an epiglottic catheter. Sustained-PAO segments (increased epiglottic pressure over ≥5 breaths without increased peak inspiratory flow) and unobstructed reference segments were manually scored during apnea-free non-REM sleep. Nasal pressure data was computationally segmented into inspiratory (T_I, shortest period achieving 95% inspiratory volume), expiratory (T_E, shortest period achieving 95% expiratory volume), and inter-breath transition period (T_Trans, period between T_E and subsequent T_I). Compared with reference segments, sustained-PAO segments had a mean relative reduction in T_Trans (-24.7±17.6%, P<0.001), elevated T_I (11.8±10.5%, P<0.001), and a small reduction in T_E (-3.9±8.0, P≤0.05). Compensatory increases in inspiratory period during PAO are primarily explained by reduced transition period and not by reduced expiratory period.

Pathogenesis of central and complex sleep apnoea

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

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.

Sudden unexpected death in infancy: aetiology, pathophysiology, epidemiology and prevention in 2015

Despite the fall in numbers of unexpected infant deaths that followed the 'Back to Sleep' campaigns in the early 1990 s in the UK and many other countries, such deaths remain one of the largest single groups of deaths in the postneonatal period in many Western countries. Changes in the ways in which unexpected infant deaths are categorised by pathologists and coroners, and increasing reluctance to use the term 'sudden infant death syndrome', make assessment of nationally and internationally collected data on incidence potentially inaccurate and confusing. In this paper, we review current understanding of the epidemiology and aetiology of unexpected deaths in infancy, and current hypotheses on the pathophysiology of the processes that may lead to death. We also review interventions that have been adopted, with variable degrees of effectiveness in efforts to reduce the numbers of deaths, and new approaches that offer the possibility of prevention in the future.

The integrative role of the sigh in psychology, physiology, pathology, and neurobiology

"Sighs, tears, grief, distress" expresses Johann Sebastian Bach in a musical example for the relationship between sighs and deep emotions. This review explores the neurobiological basis of the sigh and its relationship with psychology, physiology, and pathology. Sighs monitor changes in brain states, induce arousal, and reset breathing variability. These behavioral roles homeostatically regulate breathing stability under physiological and pathological conditions. Sighs evoked in hypoxia evoke arousal and thereby become critical for survival. Hypoarousal and failure to sigh have been associated with sudden infant death syndrome. Increased breathing irregularity may provoke excessive sighing and hyperarousal, a behavioral sequence that may play a role in panic disorders. Essential for generating sighs and breathing is the pre-Bötzinger complex. Modulatory and synaptic interactions within this local network and between networks located in the brainstem, cerebellum, cortex, hypothalamus, amygdala, and the periaqueductal gray may govern the relationships between physiology, psychology, and pathology. Unraveling these circuits will lead to a better understanding of how we balance emotions and how emotions become pathological.

The physiological determinants of sudden infant death syndrome

It is well-established that environmental and biological risk factors contribute to Sudden Infant Death Syndrome (SIDS). There is also growing consensus that SIDS requires the intersection of multiple risk factors that result in the failure of an infant to overcome cardio-respiratory challenges. Thus, the critical next steps in understanding SIDS are to unravel the physiological determinants that actually cause the sudden death, to synthesize how these determinants are affected by the known risk factors, and to develop novel ideas for SIDS prevention. In this review, we will examine current and emerging perspectives related to cardio-respiratory dysfunctions in SIDS. Specifically, we will review: (1) the role of the preBötzinger complex (preBötC) as a multi-functional network that is critically involved in the failure to adequately respond to hypoxic and hypercapnic challenges; (2) the potential involvement of the preBötC in the gender and age distributions that are characteristic for SIDS; (3) the link between SIDS and prematurity; and (4) the potential relationship between SIDS, auditory function, and central chemosensitivity. Each section underscores the importance of marrying the epidemiological and pathological data to experimental data in order to understand the physiological determinants of this syndrome. We hope that a better understanding will lead to novel ways to reduce the risk to succumb to SIDS.

The effect of sigh on cardiorespiratory synchronization in healthy sleeping infants

STUDY OBJECTIVES

Sighs are thought to have a role in regulating breathing control. They may preceed a central apnea (sigh-CA) or a pause (sigh-P), particularly in quiet sleep. Recent techniques characterizing cardiorespiratory synchronization (CRS) provide sensitive measures of cardiorespiratory coupling, which is an important factor in breathing control. We speculated that the strength of CRS and direction of cardiorespiratory coupling (DC), would differ between sigh-P and sigh-CA; before and after a sigh; and with maturation.

DESIGN

Prospective study. CRS and DC were calculated from the respiratory signal and heart rate before and after sighs recorded during overnight polysomnography.

SETTING

Sleep laboratory.

PARTICIPANTS

The data were selected from 15 subjects of a prospective cohort of 34 healthy infants at ages 2 weeks, 3 months and 6 months.

INTERVENTIONS

N/A.

MEASUREMENTS AND RESULTS

Both CRS and respiratory modulation on heart rate (RMH) (negative DC index) were decreased around sigh-CA compared with sigh-P at all ages. Short-term CRS decreased after both sigh-P and sigh-CA in infants aged 2 weeks and 3 months. Long term CRS did not change before and after sigh-P or sigh-CA. CRS and RMH were increased at 3 months and 6 months compared to 2 weeks.

CONCLUSIONS

A sigh was not found to be associated with apparent resetting of breathing control in healthy infants less than 6 months of age. Cardiorespiratory coupling appears to be a leading marker of changes in breathing control, preceding central apnea associated with a sigh.

CITATION

Nguyen CD; Dakin C; Yuill M; Crozier S; Wilson S. The effect of sigh on cardiorespiratory synchronization in healthy sleeping infants. SLEEP 2012;35(12):1643-1650.

Augmented breaths ('sighs') are suppressed by morphine in a dose-dependent fashion via naloxone-sensitive pathways in adult rats

Morphine treatment can eliminate augmented breaths (ABs; 'sighs') during spontaneous breathing. In the present study, unanesthetized rats were studied to: (1) determine the involvement of naloxone-sensitive receptor pathways, and (2) establish the dose-response relationship of this side effect. At a dosage of 5mg/kg (2-10mg/kg is recommended range for analgesia) morphine eliminated ABs from the breathing rhythm across nearly 100 min post-administration (vs. 6.2 ± 1.6 ABs in 15 min, control condition, p<0.001). This occurred despite no apparent effect on indices of ventilation. By contrast, when naloxone was co-administered with morphine, the occurrence of ABs was not different compared to control. The suppression of ABs by morphine followed a sigmoidal pattern across the low-mid dosage range (R(2)=0.83), whereas tidal volume and breathing frequency were unaffected. We conclude that the opioid-induced suppression of ABs is mediated by naloxone-sensitive opioid receptor pathways, and that this side effect is potent across the low-mid dosage range, and cannot be simply avoided by restricting dosage.

Acute exercise induce endothelial nitric oxide synthase phosphorylation via Akt and AMP-activated protein kinase in aorta of rats: Role of reactive oxygen species

BACKGROUND

Acute exercise increases reactive oxygen species (ROS) levels, including hydrogen peroxide (H2O2). H2O2 promotes endothelial nitric oxide synthase (eNOS) activation and phosphorylation in endothelial cells. With this in mind, the present study was designed to evaluate ex vivo eNOS phosphorylation in rat aortas incubated with H2O2 and to test this hypothesis in vivo in the aortas of rats submitted to acute exercise.

METHODS

For ex vivo studies, six groups of aortic tissue were formed: control, H2O2, N-acetylcysteine (NAC), LY294002, compound C, and LY294002 plus compound C. While incubation with H2O2 increased Akt, AMPK and eNOS phosphorylation, pre-incubation with NAC strongly reduced the phosphorylation of these enzymes. For in vivo studies, male Wistar rats were divided into four groups: control, cont+NAC, exercise, and exer+NAC. After a 3h swimming session, animals were decapitated and aortas were excised for biochemical and immunoblotting analysis.

RESULTS

Acute exercise increased superoxide levels and dichlorofluorescein (DCF) concentrations, and this increase was related to phosphorylation of Akt, AMPK and eNOS. On the other hand, use of NAC reduced superoxide levels and DCF concentration. Reduced superoxide levels and DCF in the exer+NAC group were associated with decreased Akt, AMPK and eNOS phosphorylation. These results appear to be connected with vascular function because VASP phosphorylation increased in acute exercise and decreased in exer+NAC.

CONCLUSION

Our results indicate that ROS induced by acute exercise play the important role of activating eNOS, a process apparently mediated by Akt and AMPK.

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.

Esophageal sensation in premature human neonates: temporal relationships and implications of aerodigestive reflexes and electrocortical arousals

Electrocortical arousal (ECA) as an effect of visceral provocation or of its temporal relationships with aerodigestive reflexes in premature neonates is not known. We tested the hypothesis that esophageal provocation results in both esophageal reflex responses and ECAs during sleep and that ECAs are dependent on the frequency characteristics of esophageal neuromotor responses. We defined the spatiotemporal relationship of ECAs in relation to 1) spontaneous pharyngoesophageal swallow sequences and gastroesophageal reflux (GER) events and 2) sensory-motor characteristics of esophageal reflexes. Sixteen healthy premature neonates born at 27.9 ± 3.4 wk were tested at 36.8 ± 1.9 wk postmenstrual age. Ninety-five midesophageal and 31 sham stimuli were given in sleep during concurrent manometry and videopolysomnography. With stimulus onset as reference point, we scored the response latency, frequency occurrence and duration of arousals, peristaltic reflex, and upper esophageal sphincter contractile reflex (UESCR). Changes in polysomnography-respiratory patterns and esophageal sensory-motor parameters were scored by blinded observers. Significantly (for each characteristic listed, P < 0.05), swallow sequences were associated with arousals and sleep state changes, and arousals were associated with incomplete peristalsis, response delays to lower esophageal sphincter relaxation, and prolonged esophageal clearance. GER events (73.5%) provoked arousals, and arousals were associated with response delays to peristaltic reflexes or clearance, sleep state modification, and prolonged respiratory arousal. Midesophageal stimuli (54%) provoked arousals and were associated with increased frequency, prolonged latency, prolonged response duration of peristaltic reflexes and UESCR, and increased frequency of sleep state changes and respiratory arousals. In human neonates, ECAs are provoked upon esophageal stimulation; the sensory-motor characteristics of esophageal reflexes are distinct when accompanied by arousals. Aerodigestive homeostasis is defended by multiple tiers of aerodigestive safety mechanisms, and when esophageal reflexes are delayed, cortical hypervigilance (ECAs) occurs.

Hypocapnia-dependent facilitation of augmented breaths: observations in awake vs. anesthetized rats

We investigated whether commonly used injectable laboratory anesthetics alter the regulation of augmented breaths (ABs) in different respiratory backgrounds. Male rats were studied on three separate experimental days, receiving one of three injections in randomized order: ethyl carbamate ('urethane'; 1.2mgkg(-1)), ketamine/xylazine (ket/xyl; 80/10mgkg(-1)), or normal saline. Following each of the three interventions, breathing was monitored during 15min exposures to normoxia (room air), hypoxia (10% O(2)) and hypoxia+CO(2) (10% O(2), 5% CO(2)). Urethane anesthesia completely eliminated ABs from the breathing rhythm in room air conditions (p<0.001), and decreased the hypocapnia-dependent component of this response (p<0.001). ket/xyl left the normal incidence of ABs in room air breathing intact but significantly suppressed the hypoxia-induced facilitation of ABs (p=0.0015). These results provide the first clear evidence that laboratory anesthesia can profoundly alter the regulation of ABs including the hypocapnia-dependent component of their facilitation.

The hypoxia-induced facilitation of augmented breaths is suppressed by the common effect of carbonic anhydrase inhibition

The typical respiratory response to hypoxia includes a dramatic facilitation of augmented breaths (ABs) or 'sighs' in the breathing rhythm. We recently found that when acetazolamide treatment is used to promote CO(2) retention and counteract alkalosis during exposure to hypoxia, then the hypoxia-induced facilitation of ABs is effectively prevented. These results indicate that hyperventilation-induced hypocapnia/alkalosis is an essential factor involved in the hypoxia-induced facilitation of augmented breaths. However, acetazolamide is also known to decrease the sensitivity of the arterial chemoreceptors. Therefore, the question remains as to whether acetazolamide prevents the facilitation of ABs during hypoxia by offsetting the effects of respiratory alkalosis, or alternatively by suppressing carotid body afferent activity. In the present study, we addressed this question by studying the effects of treatment with an alternative carbonic anhydrase inhibitor, methazolamide, which has been reported to leave carotid body responsiveness to hypoxia intact. Respiratory variables were monitored before, during and after 2 days of methazolamide treatment (10 mg kg(-1) IP, bid) in unsedated and unrestrained adult male rats. Pre-treatment, the number of ABs observed in a 5 min observation window was 1.2 + or - 0.8 and 17.4 + or - 3.8 in room air and hypoxia, respectively. During methazolamide treatment, the facilitation of ABs in hypoxia was rapidly and reversibly suppressed such that ABs we no longer significantly more frequent than they were in room air. The present results demonstrate that the hypoxia-induced facilitation of ABs can be suppressed via the general effects of carbonic anhydrase inhibition, which are common to both acetazolamide and methazolamide. We discuss these results as they pertain to the mechanisms regulating augmented breath production, and the possible association between hypocapnia/alkalosis and sleep disordered breathing.

Acetazolamide suppresses the prevalence of augmented breaths during exposure to hypoxia

Augmented breaths, or “sighs,” commonly destabilize respiratory rhythm, precipitating apneas and variability in the depth and rate of breathing, which may then exacerbate sleep-disordered breathing in vulnerable individuals. We previously demonstrated that hypocapnia is a unique condition associated with a high prevalence of augmented breaths during exposure to hypoxia; the prevalence of augmented breaths during hypoxia can be returned to normal simply by the addition of CO2to the inspired air. We hypothesized that counteracting the effect of respiratory alkalosis during hypocapnic hypoxia by blocking carbonic anhydrase would yield a similar effect. We, therefore, investigated the effect of acetazolamide on the prevalence of augmented breaths in the resting breathing cycle in five awake, adult male rats. We found a 475% increase in the prevalence of augmented breaths in animals exposed to hypocapnic hypoxia compared with room air. Acetazolamide treatment (100 mg/kg ip bid) for 3 days resulted in a rapid and potent suppression of the generation of augmented breaths during hypoxia. Within 90 min of the first dose of acetazolamide, the prevalence of augmented breaths in hypoxia fell to levels that were no greater than those observed in room air. On cessation of treatment, exposure to hypocapnic hypoxia once again caused a large increase in the prevalence of augmented breaths. These results reveal a novel means by which acetazolamide acts to stabilize breathing and may help explain the beneficial effects of the drug on breathing stability at altitude and in patients with central forms of sleep-disordered breathing.

Long-range correlations in rectal temperature fluctuations of healthy infants during maturation

Background

Control of breathing, heart rate, and body temperature are interdependent in infants, where instabilities in thermoregulation can contribute to apneas or even life-threatening events. Identifying abnormalities in thermoregulation is particularly important in the first 6 months of life, where autonomic regulation undergoes critical development. Fluctuations in body temperature have been shown to be sensitive to maturational stage as well as system failure in critically ill patients. We thus aimed to investigate the existence of fractal-like long-range correlations, indicative of temperature control, in night time rectal temperature (T_rec) patterns in maturing infants.

Methodology/Principal Findings

We measured T_rec fluctuations in infants every 4 weeks from 4 to 20 weeks of age and before and after immunization. Long-range correlations in the temperature series were quantified by the correlation exponent, α using detrended fluctuation analysis. The effects of maturation, room temperature, and immunization on the strength of correlation were investigated. We found that T_rec fluctuations exhibit fractal long-range correlations with a mean (SD) α of 1.51 (0.11), indicating that T_rec is regulated in a highly correlated and hence deterministic manner. A significant increase in α with age from 1.42 (0.07) at 4 weeks to 1.58 (0.04) at 20 weeks reflects a change in long-range correlation behavior with maturation towards a smoother and more deterministic temperature regulation, potentially due to the decrease in surface area to body weight ratio in the maturing infant. α was not associated with mean room temperature or influenced by immunization

Conclusions

This study shows that the quantification of long-range correlations using α derived from detrended fluctuation analysis is an observer-independent tool which can distinguish developmental stages of night time T_rec pattern in young infants, reflective of maturation of the autonomic system. Detrended fluctuation analysis may prove useful for characterizing thermoregulation in premature and other infants at risk for life-threatening events.

Activation of opioid micro-receptors in medullary raphe depresses sighs

Sighs, a well-known phenomenon in mammals, are substantially augmented by hypoxia and hypercapnia. Because (d-Ala(2),N-Me-Phe(4),Gly-ol)-enkephalin (DAMGO), a mu-receptor agonist, injected intravenously and locally in the caudal medullary raphe region (cMRR) decreased the ventilatory response to hypoxia and hypercapnia, we hypothesized that these treatments could inhibit sigh responses to these chemical stimuli. The number and amplitude of sighs were recorded during three levels of isocapnic hypoxia (15%, 10%, and 5% O(2) for 1.5 min) or hypercapnia (3%, 7%, and 10% CO(2) for 4 min) to test the dependence of sigh responses on the intensity of chemical drive in anesthetized and spontaneously breathing rats. The role of mu-receptors in modulating sigh responses to 10% O(2) or 7% CO(2) was subsequently evaluated by comparing the sighs before and after 1) intravenous administration of DAMGO (100 microg/kg), 2) microinjection of DAMGO (35 ng/100 nl) into the cMRR, and 3) intravenous administration of DAMGO after microinjection of d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP, 100 ng/100 nl), a micro-receptor antagonist, into the cMRR. Hypoxia and hypercapnia increased the number, but not amplitude, of sighs in a concentration-dependent manner, and the responses to hypoxia were significantly greater than those to hypercapnia. Systemic and local injection of DAMGO into the cMRR predominantly decreased the number of sighs, while microinjection into the rostral and middle MRR had no or limited effects. Microinjecting CTAP into the cMRR significantly diminished the systemic DAMGO-induced reduction of the number of sighs in response to hypoxia, but not to hypercapnia. Thus we conclude that hypoxia and hypercapnia elevate the number of sighs in a concentration-dependent manner in anesthetized rats, and this response is significantly depressed by activating systemic mu-receptors, especially those within the cMRR.

Automated respiratory inductive plethysmography to evaluate breathing in infants at risk for postoperative apnea

PURPOSE

Although respiratory inductive plethysmography (RIP) is the method of choice for the assessment of sleep disordered breathing, it has not been applied to the study of infants at risk for postoperative apnea (POA). The purpose of this study was to apply RIP to evaluate breathing in these infants. An additional purpose was to implement, simultaneously, three novel algorithms to detect movement artifact, respiratory pauses, and thoracoabdominal asynchrony, since their combined output both detects respiratory pauses and classifies them as obstructive or central in origin.

METHODS

A prospective study design was employed to record the analogue output of RIP, saturation, and finger plethysmography in a convenience sample of infants. The data record underwent a dual analysis: 1) automated detection of respiratory events; and 2) visual coding of the cardiorespiratory data. A novel index, coined pause density, was calculated as the sum of all respiratory pauses.

RESULTS

Twenty infants, whose mean postconceptional ages and weights were 44.47 +/- 2.88 weeks and 4.21 +/- 0.99 kg, respectively, were recruited. Data recording ranged from four to 24 hr. Ten infants (term = 5) experienced POA: central apnea = 5, mixed obstructive apnea = 6, and two former premature infants experienced both. Twenty-five central apneic events were detected, and the majority followed a sigh. Infants who experienced apnea also had high values of pause density.

CONCLUSION

Respiratory inductive plethysmography may provide a useful method to evaluate breathing in infants at risk for POA. The study of short respiratory pauses may prove useful in predicting apnea risk.

Hypocapnia increases the prevalence of hypoxia-induced augmented breaths

Augmented breaths promote respiratory instability and have been implicated in triggering periods of sleep-disordered breathing. Since respiratory instability is well known to be exacerbated by hypocapnia, we asked whether one of the destabilizing effects of hypocapnia might be related to an increased prevalence of augmented breaths. With this question in mind, we first sought to determine whether hypoxia-induced augmented breaths are more prevalent when hypocapnia is also present. To do this, we studied the breath-by-breath ventilatory responses of a group of freely behaving adult rats in a variety of different respiratory background conditions. We found that the prevalence of augmented breaths was dramatically increased during hypocapnic-hypoxia compared with room air conditions. When hypocapnia was prevented during exposure to hypoxia by adding 5% CO2 to the inspired air, the rate of occurrence of augmented breaths was no greater than that observed in room air. The addition of CO2 alone to room air had no effect on the prevalence of augmented breaths. We conclude that in spontaneously breathing rats, hypoxia promotes the generation of augmented breaths, but only in poikilocapnic conditions, where hypocapnia develops. Our results, therefore, reveal a means by which CO2 exerts a stabilizing influence on breathing, which may be of particular relevance during sleep in conditions commonly associated with respiratory instability.

Postnatal development of periodic breathing cycle duration in term and preterm infants

Previous studies of the maturation of periodic breathing cycle duration (PCD) with postnatal age in infants have yielded conflicting results. PCD is reported to fall in term infants over the first 6 mo postnatally, whereas in preterm infants PCD is reported either not to change or to fall. Contrary to measured values, use of a theoretical respiratory control model predicts PCD should increase with postnatal age. We re-examined this issue in a longitudinal study of 17 term and 22 preterm infants. PCD decreased exponentially from birth in both groups, reaching a plateau between 4 and 6 mo of age. In preterm infants, PCD fell from a mean of 18.3 s to 9.8 s [95% confidence interval (CI) is +/- 3.2 s]. In term infants, PCD fell from 15.4 s to 10.1 s (95% CI is +/- 3.1 s). The higher PCD at birth in preterm infants, and the similar PCD value at 6 mo in the two groups, suggest a more rapid maturation of PCD in preterm infants. This study confirms that PCD declines after birth. The disagreement between our data and theoretical predictions of PCD may point to important differences between the respiratory controller of the infant and adult.

Altered respiratory pattern and hypoxic response in transgenic newborn mice lacking the tachykinin-1 gene

Substance P is known to be involved in respiratory rhythm and central pattern-generating mechanisms, especially during early development. We therefore studied respiratory responses in transgenic newborn mice (Tac1(-/-)) lacking substance P and neurokinin A (NKA). In vivo, the effects of intermittent isocapnic hypoxia (IH) and hypercapnia were studied using whole body flow plethysmography at P2-3 and P8-10. In vitro, anoxic responses and the effects of hypocapnic and hypercapnic conditions were studied in brain stem-spinal cord preparations (C4 activity) at P2. Hypoxic challenge considerably modified the respiratory activity in transgenic mice displayed in vivo as an attenuated increase in tidal volume during IH. Transgenic mice also showed a more prominent posthypoxic frequency decline in vivo, and posthypoxic neuronal arrests appeared more often in vitro. We recognized two types of sigh activity: with or without a following pause. During IH, the amount of sighs with a pause decreased and those without increased, a redistribution that became stronger with age only in controls. Intermittent anoxia induced long-term facilitation effects in controls, but not in Tac1(-/-) animals, manifested as an increase in burst frequency in vitro and by an augmentation of ventilation during posthypoxic periods in vivo. Thus our data demonstrate that a functional substance P/NKA system is of great importance for the generation of an adequate respiratory response to hypoxic provocation in newborn mice and during early maturation. It also indicates that substance P (and/or NKA) is involved in the development of the plasticity of the respiratory system.

Early postnatal changes in respiratory activity in ratin vitroand modulatory effects of substance P

Developmental changes in the respiratory activity and its modulation by substance P (SP) were studied in the neonatal rat brainstem-spinal cord preparation from the day of birth to day 3 (P0-P3). The respiratory network activity in the ventrolateral medulla was represented by two types of bursts: basic regular bursts with typical decrementing shape and biphasic bursts appearing after augmented biphasic discharges in inspiratory neurons. With advancing postnatal age the respiratory output was considerably modified; the basic rhythm became faster by 20%, whereas the biphasic burst rate, which was originally 15 times slower, declined further by 180% and the C4 burst duration significantly decreased by 20% due to reduced decay time without preceding changes in the central inspiratory drive. SP had an age-dependent excitatory effect on respiratory activity. In the basic rhythm, SP could induce transient rhythm cessations on P0-P2 but not on P3. For the biphasic burst frequency, the sensitivity to SP significantly decreased from P0 to P3, whereas the range of SP-induced changes increased. In both types of bursts, SP prolonged C4 burst duration due to increasing decay time. This effect was three times greater on P3 and did not depend on the central inspiratory drive. Our results suggest that the potency of SP to regulate the respiratory activity elevates during the early postnatal period. The developmental changes in the respiratory activity appear to represent the transient stage in the maturation of rhythm and pattern generation mechanisms facilitating adaptive behavior of a quickly growing organism.

Perinatal exposure to nicotine causes deficits associated with a loss of nicotinic receptor function

We investigated the role played by beta2-containing neuronal nicotinic receptors [nicotinic acetylcholine receptors (nAChRs)] in mediating nicotine's side effects in the fetus and newborn. Pregnant WT and mutant mice lacking the beta2 nAChR subunit were implanted with osmotic minipumps that delivered either water or a controlled dose of nicotine. Subsequently, we compared the development of the sympathoadrenal system and breathing and arousal reflexes of offspring shortly after birth, a period of increased vulnerability to nicotine exposure. Newborn WT pups exposed to nicotine exhibited all of the deficits associated with maternal tobacco and nicotine use, and linked to poor neonatal outcome: growth restriction, unstable breathing, and impaired arousal and catecholamine biosynthesis. Remarkably similar deficits were detected in pups lacking beta2-containing nAChRs. Loss-of-function of these nAChRs consequently reproduces with astonishing fidelity many of the abnormalities caused by perinatal nicotine exposure. We propose that the underlying mechanisms of nicotine's detrimental side effects on a range of crucial defensive reflexes involve loss of function of nAChR subtypes, possibly via activity-dependent desensitization.

Effect of sighs on breathing memory and dynamics in healthy infants

Deep inspirations (sighs) play a significant role in altering lung mechanical and airway wall function; however, their role in respiratory control remains unclear. We examined whether sighs act via a resetting mechanism to improve control of the respiratory regulatory system. Effects of sighs on system variability, short- and long-range memory, and stability were assessed in 25 healthy full-term infants at 1 mo of age [mean 36 (range 28–57) days] during quiet sleep. Variability was examined using moving-window coefficient of variation, short-range memory using autocorrelation function, and long-range memory using detrended fluctuation analysis. Stability was examined by studying the behavior of the attractor with use of phase-space plots. Variability of tidal volume (Vt) and minute ventilation (V̇e) increased during the initial 15 breaths after a sigh. Short-range memory of Vt decreased during the 50 breaths preceding a sigh, becoming uncorrelated (random) during the 10-breath presigh window. Short-range memory increased after a sigh for the entire 50 breaths compared with the randomized data set and for 20 breaths compared with the presigh window. Similar, but shorter duration, changes were noted in V̇e. No change in long-range memory was seen after a sigh. Coefficient of variation and range of points located within a defined attractor segment increased after a sigh. Thus control of breathing in healthy infants shows long-range stability and improvement in short-range memory and variability after a sigh. These results add new evidence that the role of sighs is not purely mechanical.

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

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

Autonomic responses to sighs in healthy infants and in victims of sudden infant death

OBJECTIVE

Sigh, defined as an isolated breath with an increased tidal volume, can be associated with abrupt changes in heart rate (HR) or blood oxygenation. Sigh may be followed by a central apnea. As impairment of autonomic control was postulated in future SIDS victims, we hypothesized that their autonomic responses to sighs were different from those of healthy control infants.

METHODS

Sighs followed by central apnea were studied in the sleep recordings of 18 infants who eventually died of SIDS and of 18 control infants. The infants of the two groups were matched for sex, gestational age, postnatal age, weight at birth and sleep position during sleep recording. HR autoregressive power spectral analysis was performed on RR intervals preceding and following sighs.

RESULTS

In all infants, most sighs followed by an apnea were found in NREM sleep. Compared to the control infants, the future SIDS victims were characterized by a greater sympathovagal balance and a lower parasympathetic tonus before the sighs. Following the sighs, no more differences were found in NREM sleep.

CONCLUSION

Based on the present findings, it can be postulated that sighs contribute to reset autonomic tonus during NREM sleep.

Correlation properties of tidal volume and end-tidal O2 and CO2 concentrations in healthy infants

We investigated whether breath-to-breath fluctuations in tidal volume (VT) and end-tidal O2 and CO2 exhibit long-range correlations and whether parameters describing the correlations can be used as noninvasive descriptors of control of breathing. We measured VT and end-tidal O2 and CO2 over n = 352 +/- 104 breaths in 26 term, healthy, unsedated infants (mean age +/- SD: 36 +/- 6 days) and calculated the detrended fluctuation function [F(n)]. The F(n) of the breath-to-breath time series of VT, O2, and CO2 revealed a linear increase with a breath number on log-log plots with a slope that was significantly different from 0.5 (random) and thus consistent with scale-invariant behavior. Long-range correlations were stronger for O2 than for VT and CO2. The F(n) of many individual signals exhibited a crossover behavior indicating that control mechanisms regulating fluctuations of VT, O2, and CO2 may be different on different time scales. Thus breathing has a memory up to at least 400 breaths that can be characterized by the simple indicator alpha.

Spontaneous arousal activity in infants during NREM and REM sleep

The infant arousal response involves subcortical and cortical responses occurring as a sequence of stereotyped behaviour regardless of the eliciting stimulus. The spontaneous activity of these responses during sleep, however, is uncertain. We examined the spontaneous arousal pattern in normal infants to determine the sequence of responses, and to examine their periodicity and the effects of sleep state. We performed a nap polysomnographic study on 10 normal infants between 2 and 10 weeks of age. Electroencephalographic and electro-oculographic activity, and respiratory airflow and movements were measured, and video recordings were made throughout each study. Different levels of arousal behaviour were examined. We found that spontaneous arousal activity occurred frequently and the majority of responses occurred as a sequence involving an augmented breath followed by a startle and then cortical arousal. Subcortical arousals as reflected by augmented breaths and startles were more common than cortical arousals. Additionally, augmented breaths followed by apnoea were recorded and were not usually associated with other arousal responses. All of the responses occurred periodically either as bursts of activity or as isolated responses. Each of the responses occurred more frequently during rapid eye movement (REM) sleep than during non-rapid eye movement (NREM) sleep. We conclude that there is an endogenous rhythm of spontaneous activity in infants involving excitatory processes from the brainstem, which may or may not be closely followed by cortical excitation. The spontaneous arousal responses occur periodically but with a high level of irregularity and the level of activity is affected by sleep state.

The morphology of periodic breathing in infants and adults

To test the hypothesis that the crescendo-decrescendo type of pattern of periodic breathing is more common in infants than in adulthood, we examined the morphologies of periodic breathing in four groups of subjects: group 1 (n=10, gestational age 30+/-1 week), group 2 (n=10, GA 31+/-1 week), group 3 (n=10, GA 38+/-1 week), and group 4 (n=10, age 50+/-4 years). Respiratory pattern and ventilation were measured using a flow-through system. The breathing morphologies were defined according to the respiratory flow. We found (1) a predominant crescendo-decrescendo pattern in preterm infants (groups 1 and 2, >50%) and this changed to a predominant decrescendo breathing in adults (group 4, 50%); (2) total breathing cycle and its phases did not change significantly among the neonatal groups, but they almost doubled in adult subjects; however, the number of breaths per breathing interval remained the same (crescendo-decrescendo) or less (flat and decrescendo) in adults as compared to preterm infants; (3) the duty cycle (breathing interval/cycle duration) remained consistent with age; and (4) at the beginning of each breathing interval, alveolar P(CO2) was highest and alveolar P(O2) and O2 saturation lowest. The findings suggest a change in the strategy of the respiratory control system during periodic breathing between the infant and the adult, perhaps dictated by mechanical and chemoreceptor limitations early in age, with a switch from a crescendo-decrescendo to a predominantly decrescendo pattern.

Reflexes from airway rapidly adapting receptors

Rapidly adapting receptors (RARs) occur throughout the respiratory tract from the nose to the bronchi. They have thin myelinated nerve fibres, an irregular discharge and adapt rapidly to a maintained volume stimulus, but often slowly to a chemical stimulus. They are polymodal, responding to mechanical and chemical irritant stimuli, and to many inflammatory and immunological mediators. RARs show very varied sensitivities to different stimuli, and diverse reflex responses. Those in the larynx are usually called 'irritant' receptors. They probably cause cough, the expiration reflex and other laryngeal reflexes: cardiovascular, mucus secretion, bronchoconstrictor and laryngoconstrictor. Those in the trachea and larger bronchi are very mechanosensitive; they cause cough, bronchoconstriction and airway mucus secretion. Those in the larger bronchi are more chemosensitive; they may cause cough, but also stimulate hyperventilation, augmented breaths, mucus secretion, bronchoconstriction and laryngeal closure. Most of the stimuli to RARs also affect other airway receptors, especially those with C-fibre afferents, and the total reflex response will be the additive affect of all these reflexes.

Depression of ventilatory responses after daily, cyclic hypercapnic hypoxia in piglets

Ventilatory responses (VRs) were measured via a sealed face mask and pneumotachograph in 30 unsedated, mixed-breed miniature piglets at 12.6 +/- 2.3 days of age (day 1) and then repeated after seven daily 24-min exposures to 10% O(2)-6% CO(2) [hypercapnic hypoxia (HH)]. Arterial blood was sampled at baseline, after 10 min of exposure, and after 10 min of recovery. VRs included hypoxia (10% O(2) in N(2)), hypercapnia (6% CO(2) in air), and HH (10% O(2)-6% CO(2)-balance N(2)). Treatment groups (n = 10 each) were exposed to 24 min of HH from day 2 to 8 as sustained HH (24 min of HH and then 24 min of air) or cyclic HH (4 min of HH alternating with 4 min of air). Day 1 and 9 data were compared in treatment and control groups. After cyclic HH, respiratory responses to CO(2) were reduced during hypercapnia and during HH (P < 0.001 vs. control for minute ventilation in both). In both treatment groups, time to peak minute ventilation was delayed in hypoxia (P = 0.02, ANOVA), and response amplitude was increased (P < 0.001 and P = 0.003, sustained and cyclic HH, respectively, vs. control). Respiratory pattern was also altered during the VRs and among treatment groups. Stimulus presentation characteristics exert effects on VRs that are independent of those elicited by daily HH.

Low-frequency respiratory rhythms in infants during the first six months of life

The aim of the study was to investigate characteristics of low-frequency components in respiration. Sixteen healthy term infants were examined from the first day up to the 6th month of life. The respirogram, instantaneous respiratory frequency and respiratory amplitude of undisturbed segments of quiet sleep phases and periodic breathing (PB) were analysed via fast Fourier transformation. The peak frequency (PF) in the low-frequency range (0.04-0.2 Hz) was determined. PF for PB ranged from 0.056 to 0.1 Hz. Further, low-frequency rhythms (LFR) of the respirogram, which were stable during the recordings as well as during development, were found ranging from 0.045 to 0.067 Hz. The LFR of the respirogram is correlated with rhythmic changes in the relationship between inspiratory and expiratory amplitudes. The frequency of the LFR was significantly lower than that of the PB. The data indicate that LFR and PB are low-frequency respiratory rhythms which are separately controlled and perform independently.

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

AIMS

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

METHODS

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

RESULTS

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

Dynamic ventilatory responses in rats: normal development and effects of prenatal nicotine exposure

Infants of smoking mothers are at increased risk of SIDS, one cause of which is thought to be due to impaired ventilatory responses. We tested the hypotheses that prenatal nicotine exposure impairs the development of dynamic carotid chemoreceptor-driven ventilatory responses, and reduces the ability to lower metabolic rate in hypoxia. Osmotic minipumps were implanted into 20 pregnant rats at day 3 of gestation to deliver nicotine (6 mg/kg per day free base) or saline for 4 weeks. Minute ventilation was recorded breath by breath in rat pups at 3, 8 and 18 days (n = 6, 8 and 6) postnatal in response to 5-sec challenges of 100% O2 (Dejours test) and 5% O2 + 5% CO2. Carotid sinus nerve (CSN) responses to hypoxia and CO2 were recorded from 22 control and 17 nicotine-exposed preparations at ages between 3-20 days. Oxygen consumption (V(O)2) was measured in groups of pups at 3 days (n = 7 each for nicotine and control) and 8 days (n = 5 each for nicotine and control) in room air and 10% O2. There was no detectable effect of nicotine exposure on the development of CSN responses. Ventilatory responses to 5% O2-5% CO2 increased with age but did not differ between nicotine and control groups. Ventilatory responses to 100% O2 were unaffected by nicotine exposure at 8 and 18 days. However, the 3-day nicotine group showed no significant response to 100% O2 whereas V(E) was significantly reduced in the control group by 100% O2. There was no significant effect of nicotine exposure on the ability to reduce oxygen consumption in hypoxia at 3 or 8 days, but at 3 days, baseline (room air) variability in oxygen consumption was greater in the nicotine group. We conclude that nicotine exposure appears to result in abnormal ventilatory responses to withdrawal of baseline peripheral chemoreceptor drive during a period of early postnatal life. We speculate that a transient abnormality could contribute to a period of instability and increased vulnerability to challenges.

Ventilatory response of the newborn infant to mild hypoxia

The transition from an immature (biphasic) to a mature (sustained hyperpneic) response to a brief period of sustained hypoxia is believed to be well advanced by postnatal day 10 for newborn infants. However, a review of the supporting evidence convinced us that this issue warranted further, more systematic investigation. Seven healthy term infants aged 2 days to 8 weeks were studied. The ventilatory response (VR) elicited by 5 min breathing of 15% O2 was measured during quiet sleep. Arterial SaO2 (pulse oximeter) and minute ventilation (expressed as a change from control, delta V'i) were measured continuously. Infants were wrapped in their usual bedding and slept in open cots at room temperature (23 degrees-25 degrees). Infants aged 2-3 days exhibited predominantely a sustained hypopnea during the period of hypoxia (delta V'i = -2% at 1 min, -13% at 5 min). At 8 weeks of age, the mean response was typically biphasic (delta V'i = +9% at 1 min, -4% at 5 min). This age-related difference between responses was statistically significant (two-way ANOVA by time and age-group; interaction P < 0.05). These data reveal that term infants studied under ambient conditions during defined quiet sleep may exhibit an immature VR to mild, sustained hypoxia for at least 2 months after birth. This suggests that postnatal development of the O2 chemoreflex is slower than previously thought.

Periodic breathing induced on demand in awake newborn lamb

Spontaneous periodic breathing, although a common feature in fullterm and preterm human infants, is scarce in other newborn mammals. The aim of this study was to induce periodic breathing in lambs. Four 10-day-old and two <48-h-old awake lambs were instrumented with jugular catheters connected to an extracorporeal membrane lung aimed at controlling arterial PCO2 (PaCO2). Arterial PO2 (PaO2) was set and maintained at the desired level by changing inspired O2 fraction and providing O2 through a small catheter into the "apneic" lung. At a critical PaO2/PaCO2 combination, the four 10-day-old lambs exhibited periodic breathing that could be initiated, terminated, and reinitiated on demand. In the 2-day-old lambs with low chemoreceptor gain, periodic breathing was hardly seen, regardless of the trials done to find the critical PO2/PCO2 combination. We conclude that periodic breathing can be induced in lambs and depends on critical PaO2/PaCO2 combinations and maturity of the chemoreceptors.