Increased peripheral chemoreceptor activity may be critical in destabilizing breathing in neonates

ATS core curriculum 2023. Pediatric pulmonary medicine: Respiratory disorders in infants

The American Thoracic Society Core Curriculum updates clinicians annually in pediatric pulmonary disease. This is a summary of the Pediatric Pulmonary Medicine Core Curriculum presented at the 2023 American Thoracic Society International Conference. The respiratory disorders of infancy discussed in this year's review include: the care of the patient with bronchopulmonary dysplasia in the neonatal intensive care unit, clinical phenotypes and comorbidities; diffuse lung disease; pulmonary hypertension; central and obstructive sleep apnea. The care of infants with respiratory disorders often poses significant challenges to the general pediatric pulmonologist, sleep clinician, and neonatologist. This review aims to highlight the most clinically relevant aspects of the evaluation, management, and outcomes of infants with these key respiratory disorders, while emphasizing the importance of multidisciplinary care. Furthermore, this document summarizes essential aspects of genetic testing, novel imaging and treatment modalities, and includes multiple resources for clinical practice.

Persistent and symptomatic periodic breathing beyond the neonatal period in full-term infants: A case series

INTRODUCTION

Periodic breathing (PB) is considered physiological in the neonatal period and usually disappears in the first months of life. There are few data available on persistent PB after the neonatal period. The objective of this study was to characterize infants born at term with persistent PB after the age of 1 month through polysomnography (PSG) performed during symptoms.

METHODS

This retrospective case series included infants born at term between 2012 and 2021, without an underlying disease, who presented with symptoms of persistent PB during a PSG. Persistent PB was defined as more than 1 % of total sleep time (TST) of PB after 1 month of life, and PB was defined as a succession of at least three episodes of central apnea lasting more than 3 s and separated by less than 20 s of normal breathing.

RESULTS

A total of 10 infants born at term were included. They underwent PSG for brief resolved unexplained events, desaturation, pauses in breathing, cyanosis, and/or signs of respiratory distress. The percentage of TST spent with PB was 18.1 % before 3 months of age (n = 7), and 4.7 % between 3 and 6 months of age (n = 10). During the first PSG, ≥3 % of desaturation events were observed in 77-100 % of the PB episodes. At the first PSG, nine of the 10 infants had an obstructive apnea-hypopnea index of >10/h and five of 10 infants had a central apnea index of >5/h. Gastroesophageal reflux (GER) was suspected in eight infants. All infants showed improvement in the initial symptoms during the first year of life.

CONCLUSION

This study presents cases of persistent and symptomatic PB after 1 month of life in infants born at term. The interesting finding was the presence of obstructive sleep apnea syndrome and/or central apnea syndrome in the majority of children, along with GER.

Maturational effect of leptin on CO2 chemosensitivity in newborn rats

BACKGROUND

Leptin augments central CO2 chemosensitivity and stabilizes breathing in adults. Premature infants have unstable breathing and low leptin levels. Leptin receptors are on CO2 sensitive neurons in the Nucleus Tractus Solitarius (NTS) and locus coeruleus (LC). We hypothesized that exogenous leptin improves hypercapnic respiratory response in newborn rats by improving central CO2 chemosensitivity.

METHODS

In rats at postnatal day (p)4 and p21, hyperoxic and hypercapnic ventilatory responses, and pSTAT and SOCS3 protein expression in the hypothalamus, NTS and LC were measured before and after treatment with exogenous leptin (6 µg/g).

RESULTS

Exogenous leptin increased the hypercapnic response in p21 but not in p4 rats (P ≤ 0.001). At p4, leptin increased pSTAT expression only in the LC, and SOCS3 expression in the NTS and LC; while at p21 pSTAT and SOCS3 levels were higher in the hypothalamus, NTS, and LC (P ≤ 0.05).

CONCLUSIONS

We describe the developmental profile of the effect of exogenous leptin on CO2 chemosensitivity. Exogenous leptin does not augment central CO2 sensitivity during the first week of life in newborn rats. The translational implication of these findings is that low plasma leptin levels in premature infants may not be contributing to respiratory instability.

IMPACT

Exogenous leptin does not augment CO2 sensitivity during the first week of life in newborn rats, similar to the developmental period when feeding behavior is resistant to leptin. Exogenous leptin increases CO2 chemosensitivity in newborn rats after the 3rd week of life and upregulates the expression of pSTAT and SOC3 in the hypothalamus, NTS and LC. Low plasma leptin levels in premature infants are unlikely contributors to respiratory instability via decreased CO2 sensitivity in premature infants. Thus, it is highly unlikely that exogenous leptin would alter this response.

Quantitative and Qualitative Changes in Peripheral Chemoreceptor Activity in Preterm Infants

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

Immature control of breathing and apnea of prematurity: the known and unknown

This narrative review provides a broad perspective on immature control of breathing, which is universal in infants born premature. The degree of immaturity and severity of clinical symptoms are inversely correlated with gestational age. This immaturity presents as prolonged apneas with associated bradycardia or desaturation, or brief respiratory pauses, periodic breathing, and intermittent hypoxia. These manifestations are encompassed within the clinical diagnosis of apnea of prematurity, but there is no consensus on minimum criteria required for diagnosis. Common treatment strategies include caffeine and noninvasive respiratory support, but other therapies have also been advocated with varying effectiveness. There is considerable variability in when and how to initiate and discontinue treatment. There are significant knowledge gaps regarding effective strategies to quantify the severity of clinical manifestations of immature breathing, which prevent us from better understanding the long-term potential adverse outcomes, including neurodevelopment and sudden unexpected infant death.

Perinatal Hypoxemia and Oxygen Sensing

The development of the control of breathing begins in utero and continues postnatally. Fetal breathing movements are needed for establishing connectivity between the lungs and central mechanisms controlling breathing. Maturation of the control of breathing, including the increase of hypoxia chemosensitivity, continues postnatally. Insufficient oxygenation, or hypoxia, is a major stressor that can manifest for different reasons in the fetus and neonate. Though the fetus and neonate have different hypoxia sensing mechanisms and respond differently to acute hypoxia, both responses prevent deviations to respiratory and other developmental processes. Intermittent and chronic hypoxia pose much greater threats to the normal developmental respiratory processes. Gestational intermittent hypoxia, due to maternal sleep-disordered breathing and sleep apnea, increases eupneic breathing and decreases the hypoxic ventilatory response associated with impaired gasping and autoresuscitation postnatally. Chronic fetal hypoxia, due to biologic or environmental (i.e. high-altitude) factors, is implicated in fetal growth restriction and preterm birth causing a decrease in the postnatal hypoxic ventilatory responses with increases in irregular eupneic breathing. Mechanisms driving these changes include delayed chemoreceptor development, catecholaminergic activity, abnormal myelination, increased astrocyte proliferation in the dorsal respiratory group, among others. Long-term high-altitude residents demonstrate favorable adaptations to chronic hypoxia as do their offspring. Neonatal intermittent hypoxia is common among preterm infants due to immature respiratory systems and thus, display a reduced drive to breathe and apneas due to insufficient hypoxic sensitivity. However, ongoing intermittent hypoxia can enhance hypoxic sensitivity causing ventilatory overshoots followed by apnea; the number of apneas is positively correlated with degree of hypoxic sensitivity in preterm infants. Chronic neonatal hypoxia may arise from fetal complications like maternal smoking or from postnatal cardiovascular problems, causing blunting of the hypoxic ventilatory responses throughout at least adolescence due to attenuation of carotid body fibers responses to hypoxia with potential roles of brainstem serotonin, microglia, and inflammation, though these effects depend on the age in which chronic hypoxia initiates. Fetal and neonatal intermittent and chronic hypoxia are implicated in preterm birth and complicate the respiratory system through their direct effects on hypoxia sensing mechanisms and interruptions to the normal developmental processes. Thus, precise regulation of oxygen homeostasis is crucial for normal development of the respiratory control network. © 2021 American Physiological Society. Compr Physiol 11:1653-1677, 2021.

P2X3 receptor antagonism reduces the occurrence of apnoeas in newborn rats

Hyperreflexia of the peripheral chemoreceptors is a potential contributor of apnoeas of prematurity (AoP). Recently, it was shown that elevated P2X3 receptor expression was associated with elevated carotid body afferent sensitivity. Therefore, we tested whether P2X3 receptor antagonism would reduce AoP known to occur in newborn rats. Unrestrained whole-body plethysmography was used to record breathing and from this the frequency of apnoeas at baseline and following administration of either a P2X3 receptor antagonist - AF-454 (5 mg/kg or 10 mg/kg s.c.) or vehicle was derived. In a separate group, we tested the effects of AF-454 (10 mg/kg) on the hypoxic ventilatory response (10 % FiO2). Ten but not 5 mg/kg AF-454 reduced the frequency of AoP and improved breathing regularity significantly compared to vehicle. Neither AF-454 (both 5 and 10 mg/kg) nor vehicle affected baseline respiration. However, P2X3 receptor antagonism (10 mg/kg) powerfully blunted hypoxic ventilatory response to 10 % FiO2. These data suggest that P2X3 receptors contribute to AoP and the hypoxic ventilatory response in newborn rats but play no role in the drive to breathe at rest.

Effects of Perinatal Hyperoxia on Breathing

Air-breathing animals do not experience hyperoxia (inspired O₂ > 21%) in nature, but preterm and full-term infants often experience hyperoxia/hyperoxemia in clinical settings. This article focuses on the effects of normobaric hyperoxia during the perinatal period on breathing in humans and other mammals, with an emphasis on the neural control of breathing during hyperoxia, after return to normoxia, and in response to subsequent hypoxic and hypercapnic challenges. Acute hyperoxia typically evokes an immediate ventilatory depression that is often, but not always, followed by hyperpnea. The hypoxic ventilatory response (HVR) is enhanced by brief periods of hyperoxia in adult mammals, but the limited data available suggest that this may not be the case for newborns. Chronic exposure to mild-to-moderate levels of hyperoxia (e.g., 30-60% O₂ for several days to a few weeks) elicits several changes in breathing in nonhuman animals, some of which are unique to perinatal exposures (i.e., developmental plasticity). Examples of this developmental plasticity include hypoventilation after return to normoxia and long-lasting attenuation of the HVR. Although both peripheral and CNS mechanisms are implicated in hyperoxia-induced plasticity, it is particularly clear that perinatal hyperoxia affects carotid body development. Some of these effects may be transient (e.g., decreased O₂ sensitivity of carotid body glomus cells) while others may be permanent (e.g., carotid body hypoplasia, loss of chemoafferent neurons). Whether the hyperoxic exposures routinely experienced by human infants in clinical settings are sufficient to alter respiratory control development remains an open question and requires further research. © 2020 American Physiological Society. Compr Physiol 10:597-636, 2020.

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.

Intermittent hypoxia and bronchial hyperreactivity

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

Premature birth, homeostatic plasticity and respiratory consequences of inflammation

Infants who are born premature can have persistent apnea beyond term gestation, reemergence of apnea associated with inflammation during infancy, increased risk of sudden unexplained death, and sleep disorder breathing during infancy and childhood. The autonomic nervous system, particularly the central neural networks that control breathing and peripheral and central chemoreceptors and mechanoreceptors that modulate the activity of the central respiratory network, are rapidly developing during the last trimester (22-37 weeks gestation) of fetal life. With advances in neonatology, in well-resourced, developed countries, infants born as young as 23 weeks gestation can survive. Thus, a substantial part of maturation of central and peripheral systems that control breathing occurs ex-utero in infants born at the limit of viability. The balance of excitatory and inhibitory influences dictates the ultimate output from the central respiratory network. We propose in this review that simply being born early in the last trimester can trigger homeostatic plasticity within the respiratory network tipping the balance toward inhibition that persists in infancy. We discuss the intersection of premature birth, homeostatic plasticity and biological mechanisms leading to respiratory depression during inflammation in former premature infants.

An age- and sex-dependent role of catecholaminergic neurons in the control of breathing and hypoxic chemoreflex during postnatal development

The respiratory system undergoes significant development during the postnatal phase. Maturation of brainstem catecholaminergic (CA) neurons is important for the control and modulation of respiratory rhythmogenesis, as well as for chemoreception in early life. We demonstrated an inhibitory role for CA neurons in CO₂ chemosensitivity in neonatal and juvenile male and female rats, but information regarding their role in the hypoxic ventilatory response (HVR) is lacking. We evaluated the contribution of brainstem CA neurons in the HVR during postnatal (P) development (P7-8, P14-15 and P20-21) in male and female rats through chemical injury with conjugated saporin anti-dopamine beta-hydroxylase (DβH-SAP, 420 ng·μL^-1) injected in the fourth ventricle. Ventilation (V̇_E) and oxygen consumption were recorded one week after the lesion in unanesthetized rats during exposure to normoxia and hypoxia. Hypoxia reduced breathing variability in P7-8 control rats of both sexes. At P7-8, the HVR for lesioned males and females increased 27% and 24%, respectively. Additionally, the lesion reduced the normoxic breathing variability in both sexes at P7-8, but hypoxia partially reverted this effect. For P14-15, the increase in V̇_E during hypoxia was 30% higher for male and 24% higher for female lesioned animals. A sex-specific difference was detected at P20-21, as lesioned males exhibited a 24% decrease in the HVR, while lesioned females experienced a 22% increase. Furthermore, the hypoxia-induced body temperature reduction was attenuated in P20-21 lesioned females. We conclude that brainstem CA neurons modulate the HRV during the postnatal phase, and possibly thermoregulation during hypoxia.

Contribution of relative leptin and adiponectin deficiencies in premature infants to chronic intermittent hypoxia: Exploring a new hypothesis

Chronic intermittent hypoxia (CIH) occurs frequently in premature infants who have apnea of prematurity. Immaturity of the respiratory network from low central respiratory drive and the greater contribution of the carotid body on baseline breathing leads to respiratory instability in premature infants presenting as apnea and periodic breathing. During the 2nd week after birth, the smallest and the youngest premature infants have increased frequency of apnea and periodic breathing and associated oxygen desaturations that can persist for weeks after birth. CIH increases the production of reactive oxygen species that causes tissue damage. Premature infants have decreased capacity to scavenge reactive oxygen species. Oxidative injury is the cause of many of the co-morbidities that are seen in premature infants. In this review we discuss who low fat mass and the resulting relative deficiencies in leptin and adiponectin could contribute to the increase frequency of oxygen desaturations that occurs days after birth in the smallest and youngest premature infants. Leptin is a central respiratory stimulant and adiponectin protects the lung from vascular leak, oxidative injury and vascular remodeling.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Cardiorespiratory events in preterm infants: etiology and monitoring technologies

Every year, an estimated 15 million infants are born prematurely (<37 weeks gestation) with premature birth rates ranging from 5 to 18% across 184 countries. Although there are a multitude of reasons for this high rate of preterm birth, once birth occurs, a major challenge of infant care includes the stabilization of respiration and oxygenation. Clinical care of this vulnerable infant population continues to improve, yet there are major areas that have yet to be resolved including the identification of optimal respiratory support modalities and oxygen saturation targets, and reduction of associated short- and long-term morbidities. As intermittent hypoxemia is a consequence of immature respiratory control and resultant apnea superimposed upon an immature lung, improvements in clinical care must include a thorough knowledge of premature lung development and pathophysiology that is unique to premature birth. In Part 1 of a two-part review, we summarize early lung development and diagnostic methods for cardiorespiratory monitoring.

Intermittent neonatal hypoxia elicits the upregulation of inflammatory-related genes in adult male rats through long-lasting programming effects

The long-term effects of neonatal intermittent hypoxia (IH), an accepted model of apnea-induced hypoxia, are unclear. We have previously shown lasting "programming" effects on the HPA axis in adult rats exposed to neonatal IH. We hypothesized that neonatal rat exposure to IH will subsequently result in a heightened inflammatory state in the adult. Rat pups were exposed to normoxia (control) or six cycles of 5% IH or 10% IH over one hour daily from postnatal day 2-6. Plasma samples from blood obtained at 114 days of age were analyzed by assessing the capacity to induce transcription in a healthy peripheral blood mononuclear cell (PBMC) population and read using a high-density microarray. The analysis of plasma from adult rats previously exposed to neonatal 5% IH versus 10% IH resulted in 2579 significantly regulated genes including increased expression of Cxcl1, Cxcl2, Ccl3, Il1a, and Il1b. We conclude that neonatal exposure to intermittent hypoxia elicits a long-lasting programming effect in the adult resulting in an upregulation of inflammatory-related genes.

Hypoxic Episodes in Bronchopulmonary Dysplasia

Hypoxic episodes are troublesome components of bronchopulmonary dysplasia (BPD) in preterm infants. Immature respiratory control seems to be the major contributor, superimposed on abnormal respiratory function. Relatively short respiratory pauses may precipitate desaturation and bradycardia. This population is predisposed to pulmonary hypertension; it is likely that pulmonary vasoconstriction also plays a role. The natural history has been well-characterized in the preterm population at risk for BPD; however, the consequences are less clear. Proposed associations of intermittent hypoxia include retinopathy of prematurity, sleep disordered breathing, and neurodevelopmental delay. Future study should address whether these associations are causal relationships.

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.

The effect of prenatal maternal infection on respiratory function in mouse offspring: evidence for enhanced chemosensitivity

Systemic maternal inflammation is implicated in preterm birth and bronchopulmonary dysplasia (BPD) and may induce morbidities including reduced pulmonary function, sleep-disordered breathing, and cardiovascular disorders. Here we test the hypothesis that antenatal maternal inflammation per se causes altered alveolar development and increased chemoreflex sensitivity that persists beyond infancy. Pregnant C57BL/6 mice were administered lipopolysaccharide (LPS) (150 μg/kg ip) to induce maternal inflammation or saline (SHAM) at embryonic day 16 (randomized). Pups were weighed daily. On days 7, 28, and 60 (D07, D28, and D60), unrestrained wholebody plethysmography quantified ventilation and chemoreflex responses to hypoxia (10%), hypercapnia (7%), and asphyxia (hypoxic hypercapnia). Lungs were harvested to quantify alveolar number, size, and septal thickness. LPS pups had reduced baseline ventilation per unit bodyweight (∼40%, P < 0.001) vs. SHAM. LPS increased ventilatory responses to hypoxia (D07: 66% vs. 28% increase in ventilation; P < 0.001) hypercapnia (170% vs. 88%; P < 0.001), and asphyxia (249% vs. 154%; P < 0.001); hypersensitive hypoxic responsiveness persisted until D60 (P < 0.001). LPS also increased apnea frequency (P < 0.01). LPS caused thicker alveolar septae (D07, P < 0.001), diminished alveolar number (D28, P < 0.001) vs. SHAM, but effects were minimal by D60. Pups delivered from mothers exposed to antenatal inflammation exhibit deficits in lung structure and hypersensitive responses to respiratory stimuli that persist beyond the newborn period. Antenatal inflammation may contribute to impaired gas exchange and unstable breathing in newborn infants and adversely affect long-term health.

Quantification of periodic breathing in premature infants

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

Ventilatory control and supplemental oxygen in premature infants with apparent chronic lung disease

OBJECTIVES

Our goal was to evaluate changes in respiratory pattern among premature infants born at <29 weeks gestation who underwent a physiological challenge at 36 weeks postmenstrual age with systematic reductions in supplemental oxygen and inspired airflow.

STUDY DESIGN

Subjects were all infants enrolled in the Prematurity and Respiratory Outcomes Project at St. Louis Children's Hospital and eligible for a physiological challenge protocol because they were receiving supplemental oxygen or augmented airflow alone as part of their routine care. Continuous recording of rib cage and abdominal excursion and haemoglobin oxygen saturation (SpO2%) were made in the newborn intensive care unit.

RESULTS

37 of 49 infants (75.5%) failed the challenge, with severe or sustained falls in SpO2%. Also, 16 of 37 infants (43.2%) who failed had marked increases in the amount of periodic breathing at the time of challenge failure.

CONCLUSIONS

An unstable respiratory pattern is unmasked with a decrease in inspired oxygen or airflow support in many premature infants. Although infants with significant chronic lung disease may also be predisposed to more periodic breathing, these data suggest that the classification of chronic lung disease of prematurity based solely on clinical requirements for supplemental oxygen or airflow do not account for multiple mechanisms that are likely contributing to the need for respiratory support.

Acute Neonatal Respiratory Failure

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

Central role of carotid body chemoreceptors in disordered breathing and cardiorenal dysfunction in chronic heart failure

Oscillatory breathing (OB) patterns are observed in pre-term infants, patients with cardio-renal impairment, and in otherwise healthy humans exposed to high altitude. Enhanced carotid body (CB) chemoreflex sensitivity is common to all of these populations and is thought to contribute to these abnormal patterns by destabilizing the respiratory control system. OB patterns in chronic heart failure (CHF) patients are associated with greater levels of tonic and chemoreflex-evoked sympathetic nerve activity (SNA), which is associated with greater morbidity and poor prognosis. Enhanced chemoreflex drive may contribute to tonic elevations in SNA by strengthening the relationship between respiratory and sympathetic neural outflow. Elimination of CB afferents in experimental models of CHF has been shown to reduce OB, respiratory-sympathetic coupling, and renal SNA, and to improve autonomic balance in the heart. The CB chemoreceptors may play an important role in progression of CHF by contributing to respiratory instability and OB, which in turn further exacerbates tonic and chemoreflex-evoked increases in SNA to the heart and kidney.

Apnea of prematurity

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

Apnea of prematurity--perfect storm

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

Cardiorespiratory coupling in health and disease

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

Gestational stress promotes pathological apneas and sex-specific disruption of respiratory control development in newborn rat

Recurrent apneas are important causes of hospitalization and morbidity in newborns. Gestational stress (GS) compromises fetal brain development. Maternal stress and anxiety during gestation are linked to respiratory disorders in newborns; however, the mechanisms remain unknown. Here, we tested the hypothesis that repeated activation of the neuroendocrine response to stress during gestation is sufficient to disrupt the development of respiratory control and augment the occurrence of apneas in newborn rats. Pregnant dams were displaced and exposed to predator odor from days 9 to 19 of gestation. Control dams were undisturbed. Experiments were performed on male and female rats aged between 0 and 4 d old. Apnea frequency decreased with age but was consistently higher in stressed pups than controls. At day 4, GS augmented the proportion of apneas with O(2) desaturations by 12%. During acute hypoxia (12% O(2)), the reflexive increase in breathing augmented with age; however, this response was lower in stressed pups. Instability of respiratory rhythm recorded from medullary preparations decreased with age but was higher in stressed pups than controls. GS reduced medullary serotonin (5-HT) levels in newborn pups by 32%. Bath application of 5-HT and injection of 8-OH-DPAT [(±)-8-hydroxy-2-di-(n-propylamino) tetralin hydrobromide; 5-HT(1A) agonist; in vivo] reduced respiratory instability and apneas; these effects were greater in stressed pups than controls. Sex-specific effects were observed. We conclude that activation of the stress response during gestation is sufficient to disrupt respiratory control development and promote pathological apneas in newborn rats. A deficit in medullary 5-HT contributes to these effects.

Carotid chemoreceptor development and neonatal apnea

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

Sex, hormones, and stress: how they impact development and function of the carotid bodies and related reflexes

Progesterone and corticosterone are key modulators of the respiratory control system. While progesterone is widely recognized as an important respiratory stimulant in adult and newborn animals, much remains to be described regarding the underlying mechanisms. We review the potential implication of nuclear and membrane progesterone receptors in adults and in newborns. This raises intriguing questions regarding the contribution of progesterone as a protective factor against some respiratory control disorders during early life. We then discuss our current understanding of the central integration of stressful stimuli and the responses they elicit. The fact that this system interacts with the respiratory control system, either because both share some common neural pathways in the brainstem and hypothalamus, or because corticosterone directly modulates the function of the respiratory control network, is a fascinating field of research that has emerged over the past few years. Finally, we review the short- and long-term consequences of disruption of stress circuitry during postnatal development on these systems.

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.

Physiologic basis for intermittent hypoxic episodes in preterm infants

Intermittent hypoxic episodes are typically a consequence of immature respiratory control and remain a troublesome challenge for the neonatologist. Furthermore, their frequency and magnitude are commonly underestimated by clinically employed pulse oximeter settings. In extremely low birth weight infants the incidence of intermittent hypoxia [IH] progressively increases over the first 4 weeks of postnatal life, with a subsequent plateau followed by a slow decline beginning at weeks six to eight. Over this period of unstable respiratory control, increased oxygen-sensitive peripheral chemoreceptor activity has been associated with a higher incidence of apnea of prematurity. In contrast, infants with bronchopulmonary dysplasia [chronic neonatal lung disease] exhibit decreased peripheral chemosensitivity, although the effect on respiratory stability in this population is unclear. Such episodic hypoxia/reoxygenation in early life has the potential to sustain a proinflammatory cascade with resultant multisystem, including respiratory, morbidity. Therapeutic approaches for intermittent hypoxic episodes comprise careful titration of baseline or supplemental inspired oxygen as well as xanthine therapy to prevent apnea of prematurity. Characterization of the pathophysiologic basis for such intermittent hypoxic episodes and their consequences during early life is necessary to provide an evidence-based approach to their management.

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.

A neonatal mouse model of intermittent hypoxia associated with features of apnea in premature infants

A neonatal mouse model of intermittent hypoxia (IH) simulating the recurring hypoxia/reoxygenation episodes of apnea of prematurity (AOP) was developed. C57BL/6 P2 pups were culled for exposure to either intermittent hypoxia or intermittent air as control. The IH paradigms consisted of alternation cycles of 20.9% O2 and either 8.0% or 5.7% O2 every 120 or 140s for 6h a day during daylight hours from day 2 to day 10 postnatally, i.e., roughly equivalent to human brain development in the perinatal period. IH exposures elicited modest to severe decrease in oxygen saturation along with bradycardia in neonatal mice, which were severity-dependent. Hypomyelination in both central and peripheral nervous systems was observed despite the absence of visible growth retardation. The neonatal mouse model of IH in this study partially fulfills the current diagnostic criteria with features of AOP, and provides opportunities to reproduce in rodents some of the pathophysiological changes associated with this disorder, such as alterations in myelination.

Chronic Lung Disease of Childhood: Control of Breathing During Wake and Sleep

Control of breathing in infants during wake and sleep is immature at birth and undergoes rapid maturation over the first year of life. Infants with chronic lung disease (CLD) have multiple control of breathing impairments leaving them particularly vulnerable to hypoxic and asphyxic events. These impairments in the control of breathing are thought to contribute significantly to increased morbidity and the increased incidence of sudden infant death in infants with CLD. This review provides an overview of factors integral to the control of breathing during wake and sleep and factors that influence the development of control of breathing with a focus on the impact of CLD.

Development of ventilatory control in infants

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

Caffeine reduces apnea frequency and enhances ventilatory long-term facilitation in rat pups raised in chronic intermittent hypoxia

The mechanisms underlying the therapeutic function of caffeine on apneas in preterm neonates are not well determined. To better understand these effects, we exposed rat pups from postnatal d 3-12 to chronic intermittent hypoxia (5% O2/100 s every 10 min; 6 cycles/h followed by 1 h at 21% O2, 24 h/d), a model mimicking hypoxemic exposure in apneic neonates. Then, using whole-body plethysmography, we evaluated minute ventilation, apnea frequency, and duration after i.p injection of caffeine citrate (20 mg/kg) or saline under normoxia and in response to either sustained (FiO2 12%, 20 min) or brief (FiO2 5%, 60 s, total 10 episodes of 8 min each) hypoxia. These tests were used to assess peripheral and central components of hypoxic response. The latter also assessed the ventilatory long-term facilitation during recovery (2 h). Caffeine injection increased minute ventilation under baseline and during recovery. This effect was correlated with a decrease in apnea frequency (not duration). On the contrary, caffeine did not change the ventilatory response to sustained or brief hypoxic exposure. These results suggest that the effects of caffeine on apnea depend on increased central normoxic respiratory drive and enhancement of ventilatory long-term facilitation rather than on higher hypoxic ventilatory response.

A model investigation of the impact of ventilation-perfusion mismatch on oxygenation during apnea in preterm infants

Ventilation-perfusion (V/Q) mismatch is a prominent feature of preterm infants and adults with lung disease. V/Q mismatch is known to cause arterial hypoxemia under steady-state conditions, and has been proposed as the cause of rapid arterial oxygen desaturation during apnea. However, there is little evidence to support a role for V/Q mismatch in the dynamic changes in arterial oxygenation that occur during apnea. Using a mathematical model, we quantified the effect of V/Q mismatch on the rate of desaturation during apnea to ascertain whether it could lead to rates of up to 10%s(-1) as observed in preterm infants. We used a lung-body model for the preterm infant that incorporated 50 parallel alveolar-capillary units that were ventilated and perfused with the severity of V/Q mismatch (sigma) defined conventionally according to sigma=S.D. of the distribution of V/Q ratios. Average desaturation rate 10s from apnea onset was strongly elevated with worsening V/Q mismatch as a result of an earlier desaturation of low V/Q units compared with high V/Q units. However, V/Q mismatch had little impact after apnea onset, with peak desaturation rate only substantially increased if mismatching caused a lowered resting arterial O(2) saturation. In conclusion, V/Q mismatch causes a more immediate onset of desaturation during apnea, and therefore places preterm infants and adults with lung disease at risk of hypoxemic dips. However, V/Q mismatch does not accelerate desaturation rate beyond apnea onset and cannot, therefore, explain the rapid desaturation observed during recurrent apnea in preterm infants.

Effect of intermittent hypercapnia on respiratory control in rat pups

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

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

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

Breath-to-breath hypercapnic response in neonatal rats: temperature dependency of the chemoreflexes and potential implications for breathing stability

The breathing of newborns is destabilized by warm temperatures. We hypothesized that in unanesthetized, intact newborn rats, body temperature (T(B)) influences the peripheral chemoreflex response (PCR response) to hypercapnia. To test this, we delivered square-wave challenges of 8% CO(2) in air to postnatal day 4-5 (P4-P5) rats held at a T(B) of 30 degrees C (Cold group, n = 11), 33 degrees C (Cool group, n = 10), and 35 degrees C thermoneutral zone group [thermoneutral zone (TNZ) group, n = 11], while measuring ventilation (Ve) directly with a pneumotach and mask. Cool animals were challenged with 8% CO(2) balanced in either air or hyperoxia (n = 10) to identify the PCR response. Breath-to-breath analysis was performed on 30 room air breaths and every breath of the 1-min CO(2) challenge. As expected, warmer T(B) was associated with an unstable breathing pattern in room air: TNZ animals had a coefficient of variation in Ve (Ve CV%) that was double that of animals held at cooler T(B) (P < 0.001). Hyperoxia markedly suppressed the hypercapnic ventilatory response over the first 10 breaths (or approximately 4 s), suggesting that this domain is dominated by the PCR response. The PCR response (P = 0.03) and total response (P = 0.04) were significantly greater in TNZ animals compared with hypothermic animals. The total response had a significant, negative relationship with Vco(2) (R(2) = 0.53; P < 0.001). Breathing stability was positively related to the total response (R(2) = 0.36; P < 0.001) and to a lesser extent, the PCR response (R(2) = 0.19; P = 0.01) and was negatively related to Vco(2) (R(2) = 0.34; P < 0.001). ANCOVA confirmed a significant effect of T(B) alone on breathing stability (P < 0.01), with no independent effects of Vco(2) (P = 0.41), the PCR response (P = 0.82), or the total Ve response (P = 0.08). Our data suggest that in early postnatal life, the chemoreflex responses to CO(2) are highly influenced by T(B), and while related to breathing stability, are not predictors of stability after accounting for the independent effect of T(B).

Reactive oxygen species-dependent endothelin signaling is required for augmented hypoxic sensory response of the neonatal carotid body by intermittent hypoxia

We previously reported that intermittent hypoxia (IH) augments hypoxic sensory response (HSR) and increases the number of glomus cells in neonatal carotid bodies. In the present study, we tested the hypothesis that recruitment of endothelin-1 (ET-1) signaling by reactive oxygen species (ROS) plays a critical role in IH-evoked changes in neonatal carotid bodies. Experiments were performed on neonatal rats exposed either to 10 days of IH (P0-P10; 8 h/day) or to normoxia. IH augmented HSR of the carotid bodies ex vivo and resulted in hyperplasia of glomus cells. The effects of IH were associated with enhanced basal release of ET-1 under normoxia, sensitization of carotid body response to exogenous ET-1, and upregulation of ET(A) but not an ET(B) receptor mRNA without altering the ET-1 content. An ET(A) but not ET(B) receptor antagonist prevented augmented HSR by IH. ROS levels were elevated in carotid bodies from IH-treated rat pups as evidenced by increased levels of malondialdehyde. Systemic administration of manganese (III) tetrakis(1-methyl-4-pyridyl)porphyrin pentachloride (MnTMPyP; 5 mg/kg ip), a scavenger of O(2)(*-), prevented IH-induced elevation of ROS, basal release of ET-1, upregulation of ET(A) mRNA, and augmented HSR. In striking contrast, MnTMPyP treatment had no significant effect on IH-induced hyperplasia of glomus cells. These results demonstrate that IH-evoked increase in HSR involve a ROS-mediated increase in basal ET-1 release and upregulation of ET(A) receptor mRNA.