Repeated acute hypoxia temporarily attenuates the ventilatory respiratory response to hypoxia in conscious newborn rats.

We asked whether repeated hypoxic exposures during the early neonatal periods could affect the ventilatory control, such as the lung volume-dependent ventilatory inhibition (HBR), pulmonary ventilation (VE), and CO2 production (VCO2). Within each litter of rats, one group of pups (experimental group H) was exposed to 6% O2 (30-min duration twice a day from postnatal d 1 to 4). The other group (control group C) was exposed to air. At 5 d after birth, the HBR was triggered by lung inflation via negative body surface pressure (10 cm H2O). Measurements of VE and VCO2 were done by plethysmography and the inflow-outflow CO2 difference, respectively. At 2 wk of age, VE and VCO2 measurements were repeated by the barometric technique and the inflow-outflow CO2 difference, respectively. Each conscious pup was breathing normoxia (21% O2) and then hypoxia (10% O2). Results were as follows: 1) during normoxia, HBR was stronger and both VE and VCO2 were higher in H pups than in C pups; 2) during hypoxia, the HBR of C was as in normoxia, whereas that of H was increased above the normoxic value; 3) during hypoxia, C maintained VE, whereas H decreased it; 4) in hypoxia, VCO2 was reduced significantly in both groups; 5) at 2 wk of age, VE and VCO2 did not differ between H and C during normoxia or in response to 10% hypoxia. We conclude that in rat pups, repeated hypoxic episodes can modify the HBR and, at least temporarily, reduce the VE response to hypoxia with a decrease in VCO2. The findings are in agreement with the view that repeated hypoxic exposures in the neonatal period could interfere with the development of respiratory control and could possibly be involved in the mechanisms of neonatal apnea or sudden infant death syndrome.     

Maturation of human fetal responses to airborne sound in low- and high-risk fetuses

The purpose of the study was to characterize the onset and maturation of airborne sound-elicited responses in low- and high-risk preterm fetuses. In Study 1, a total of 91 low-risk fetuses at 27, 30, 33, and 36 weeks GA received three sound trials at 90, 100, 105 and 110 dB and three no-stimulus control trials. The onset of cardiac acceleration and body movement responses occurred at 30 weeks GA. Maturation of the cardiac response was observed with a decrease in threshold from 105-110 dB at 33 weeks GA to 100-105 dB at 36 weeks GA. In Study 2, the procedure was similar except that the 43 high-risk fetuses at 27, 30 and 33 weeks GA did not receive sounds at 90 dB. For the high-risk fetuses, the onset of cardiac and motor responses also occurred at 30 weeks GA. At 33 weeks GA, those high-risk fetuses subsequently born at term showed an increased magnitude of the cardiac acceleration response compared to low-risk fetuses. The results indicate that both low- and high-risk fetuses begin responding to sounds at the same gestational age. Differential responses observed over gestation in the high-risk group most likely indicate differential functional development of the auditory-response system.     

Respiratory and arousal responses to hypoxia in apnoeic infants reinvestigated

Respiratory and arousal responses to mild hypoxia (15% oxygen in nitrogen) were recorded in 18 healthy infants and 33 infants who had sustained severe sleep related apnoeic events (ALTE). Respiratory movements and transcutaneous gas pressures (tcPO₂ and tcPCO₂) were continuously monitored during the 10 min test. The changes in tcPCO₂ in relation to the decrease in tcPO₂ were used as an index of the ventilatory and metabolic responses to hypoxia. We found that the response of apnoeic infants was within the range of the controls although the distribution of the individual response slopes was shifted towards the lower end of the range. Arousal was observed in 33% of apnoeic infants and 32% of the controls. Regular periodic breathing occurred in 42% of apnoeic infants compared to 28% of controls. In contrast to the controls, periodic breathing in apnoeic infants was not associated with a drop in tcPCO₂ to below baseline levels. Apnoeic infants also alternated between regular and periodic breathing during the test. These findings are suggestive of a weak feed back control of breathing but do not support former views of a deficient hypoxic response in infants with ALTE.Abbreviations ALTE apparently life-threatening event - SID sudden infant death     

Impaired ventilatory responses to hypoxia in mice deficient in endothelin-converting-enzyme-1

Endothelin-converting-enzyme (ECE-1) catalyzes the proteolytic activation of big endothelin-1 to mature endothelin-1. Most homozygous ECE-1-/- embryos die in utero and show severe craniofacial, enteric, and cardiac malformations precluding ventilatory function assessment. In contrast, heterozygous ECE-1+/- embryos develop normally. Their respiratory function at birth has not been studied. Taking into account previous respiratory investigations in mice with endothelin-1 gene disruption, we hypothesized that ECE-1-deficient mice may have impaired ventilatory control. We analyzed ventilatory responses to hypercapnia (8% CO(2)) and hypoxia (10% O(2)) in newborn and adult mice heterozygous for ECE-1 deficiency (ECE-1+/-) and in their wild-type littermates (ECE-1+/+). Ventilation, breath duration, and tidal volume were measured using whole-body plethysmography. Ventilatory responses to hypoxia were significantly weaker in ECE-1+/- than in ECE-1+/+ newborn mice (percentage ventilation increase: 1 +/- 25% versus 33 +/- 29%, p = 0.010). Baseline breathing variables and ventilatory responses to hypercapnia were normal in the ECE-1+/- newborn mice. No differences were observed between adult ECE-1+/- and ECE-1+/+ mice. We conclude that ECE-1 is required for normal ventilatory response to hypoxia at birth.     

Immunological responses to respiratory syncytial virus infection in infancy

OBJECTIVES: To determine whether there is evidence of immunological responses in infants with respiratory syncytial virus (RSV) bronchiolitis by measuring inflammatory mediators in peripheral blood and, if found, whether these related to the severity of illness. PATIENTS AND METHODS: Blood was taken from 94 children with RSV infection during the acute episode and 10 or more days later when the child was well. Control serum samples were obtained from well children of similar ages. Serum samples were assayed for mediators of lymphocyte activity (interleukin-4 (IL-4), soluble interleukin-2 receptor (sCD25), soluble intercellular adhesion molecule-1 (sICAM-1), eosinophil activity (eosinophil cationic protein) and neutrophil activity (myeloperoxidase). Symptoms were assessed as very mild (coryza only), mild (symptoms of lower respiratory tract infection), moderate (requiring nasogastric or intravenous fluids), and severe (requiring oxygen or ventilation). RESULTS: IL-4 concentrations were at the lower limits of detection of the assay. The concentrations of sCD-25 were greater in samples from patients with acute illness than from convalescent patients and both were greater than in control samples. sICAM-1 concentrations were similar in samples from patients with acute illness and convalescent patients, but both were greater than in samples from controls. Eosinophil cationic protein concentrations were lower in samples from patients with acute illness than in those from convalescent patients; there was no difference between samples from convalescent and control patients. Myeloperoxidase concentrations were similar in all samples. There was no correlation between the severity of infection and the concentrations of any inflammatory mediators. CONCLUSIONS: There is evidence of an inflammatory response in the peripheral blood of infants with acute bronchiolitis which may affect lymphocytes and eosinophils, but an association between this response and the severity of illness was not shown here.     

Respiratory and metabolic responses to early postnatal chronic intermittent hypoxia and sustained hypoxia in the developing rat

Exposure to sustained hypoxia (SH) differentially modifies the hypoxic ventilatory response (HVR) in adults and developing rats. We examined the possibility that postnatal intermittent hypoxia (IH), a more prevalent clinical condition than SH, may lead to significant modifications of ventilatory patterning during development. Sprague-Dawley rat pups were exposed as of the d 1 of life to either SH (10% O2) or IH [alternating room air (RA) and 10% O2 every 90 s] for up to 30 d; controls were exposed to normoxia. HVR (10% O2 for 20 min) was assessed in unrestrained pups at 5, 10, 15, and 30 d of age using whole-body plethysmography. IH pups displayed higher normoxic ventilation (VE) at all ages (p < 0.001 versus control; n = 12 per group), which was not observed in SH animals until 10 d of exposure (p < 0.001 versus control; n = 12 per group). Furthermore, both SH and IH modified properties of peak HVR (pHVR), as well as those of the ensuing hypoxic ventilatory decline (HVD); however, the ventilatory strategies adopted after SH and IH greatly differed. We conclude that both postnatal IH and SH modify normal ventilatory patterning and induce altered HVR, but differ in the ventilatory strategies adopted to mount HVR responses.     

Effect of graded hypoxia on cerebral cortical genomic DNA fragmentation in newborn piglets

Previous studies have shown that hypoxia is associated with modification of the cerebral cortical nuclear membrane, leading to increased intranuclear calcium. The increased intranuclear calcium activates calcium-dependent endonucleases, resulting in DNA fragmentation. The present study tests the hypothesis that the fragmentation of neuronal genomic DNA increases with an increase in the degree of cerebral tissue hypoxia. Sixteen newborn piglets were anesthetized, ventilated and divided into normoxic and hypoxic groups with varying degrees of hypoxia. Cerebral hypoxia was documented biochemically by measuring tissue levels of ATP and phosphocreatine. Isolation of cerebral cortical neuronal nuclei and DNA and their purity was confirmed by standard techniques. DNA samples were separated by electrophoresis on 1% agarose gel and stained with ethidium bromide. In the hypoxic samples, multiple low-molecular-weight DNA fragments were present as a smear pattern from 200 to 2,000 base pairs. Levels of high-energy phosphates were compared to the area of each smear for each animal to correlate the degree of hypoxia with the degree of DNA fragmentation. DNA fragmentation increased when high-energy phosphate levels decreased. We conclude that there is a critical threshold value of oxidative metabolism beyond which there are progressive changes in the cortical neuronal cells, leading to DNA fragmentation.     

Effect of graded hypoxia on brain cell membrane injury in newborn piglets.

Alterations in brain cell membrane structure and function during cerebral hypoxia were investigated by measuring Na+,K(+)-ATPase activity and levels of lipid peroxidation products in brain cell membranes obtained from newborn piglets following exposure to 60 min of hypoxic hypoxia in vivo. Cerebral hypoxia was documented as a decrease in the ratio of phosphocreatine to inorganic phosphate (PCr/Pi) using 31P-NMR spectroscopy. During hypoxia (FiO2 0.07-0.11), PCr/Pi decreased 28-47% compared to the corresponding baseline value without a decrease in cerebral ATP levels. No change in brain cell membrane Na+,K(+)-ATPase activity was observed for changes in PCr/Pi of less than 30%. When PCr/Pi was at least 30% lower than baseline, Na+,K(+)-ATPase activity decreased linearly as a function of the decrease in PCr/Pi (r = 0.95). Levels of lipid peroxidation products (conjugated dienes and fluorescent compounds) increased significantly as PCr/Pi decreased. These data suggest that below a critical threshold value of oxidative metabolism there are progressive changes in brain cell membrane structure and function during cerebral hypoxia, and demonstrate that membrane alterations occur prior to changes in cellular ATP levels.     

NMDA receptor modification during graded hypoxia in the cerebral cortex of newborn piglets

Previous studies have shown that the cerebral N-methyl-D-aspartate (NMDA) receptor is altered during hypoxia in newborn piglets. The present study tests whether modification of the glutamate and ion channel sites of the NMDA receptor correlates with the progressive decrease in cerebral energy metabolism induced by hypoxia. Degrees of cerebral hypoxia were attained by exposure of ventilated piglets to decreased oxygen at different concentrations and confirmed by tissue phosphocreatine levels. During graded hypoxia, the number of glutamate sites decreased, the affinity of the ion channel site increased, the inhibition by Zn(2+) increased, the activation by glutamate increased, and the activation by glycine decreased. Therefore, modification of the NMDA receptor correlates with the energy state of the tissue. Alterations in receptor phosphorylation may gradually modify the NMDA receptor and may be initiated by subtle decreases in tissue oxygenation in the newborn brain.     

Maturation alters cyclic nucleotide and relaxation responses to nitric oxide donors in ovine cerebral arteries

To examine the hypothesis that maturation modulates nitric oxide (NO)-induced relaxation in cerebral arteries, we quantified concentration-relaxation relations and the corresponding dynamic responses of guanosine 3':5'-cyclic monophosphate (cGMP) and adenosine 3':5'-cyclic monophosphate (cAMP) levels following administration of nitroglycerin and S-nitroso-N-acetyl-penicilamine (SNAP), an NO donor, in posterior communicating and middle cerebral arteries from newborn (3-7 days) and adult sheep. The results offer 5 main observations: (1) the efficacy and potency of NO donors were generally greater in newborn than in adult cerebral arteries; (2) rates of relaxation, and presumably rates of NO release, were faster for equimolar concentrations of SNAP than for nitroglycerin in both newborn and adult arteries; (3) basal concentrations were greater for cAMP than for cGMP, and both were greater in newborn than adult cerebral arteries; (4) in adult cerebral arteries, NO-induced increases in cGMP occurred faster but relaxation developed more slowly than in newborn cerebral arteries, and (5) responses to NO donors involved significant cross-reactivity between cGMP and cAMP, the characteristics of which were age, artery, and agent specific. From these results, we conclude that postnatal changes in reactivity to NO reflect corresponding changes in soluble guanylate cyclase activity and possible decreases in NO half-life. We also conclude that maturation slows the mechanisms mediating NO-induced relaxation, and that this effect is more pronounced in distal than in proximal cerebral arteries. The data also suggest that the rate-limiting step governing rates of response to NO is probably downstream from cGMP synthesis. From the basal cyclic nucleotide levels, we conclude that basal ratios of synthesis to hydrolysis were greater in fetal than adult arteries. Because NO increased both cGMP and cAMP, we speculate that Type III phosphodiesterase has a possible influence upon cerebrovascular responses to NO, and that this influence varies with postnatal age and artery type. Together, these findings emphasize that the cerebrovascular effects of NO are highly age dependent and artery specific, and should be carefully considered when administering NO therapeutically in the neonate.     

IgG and IgM antibodies to viral glycoproteins in respiratory syncytial virus infections of graded severity.

Serum antibodies to the fusion (F) and large glycoprotein (G) of respiratory syncytial virus in the serum of 57 infected infants were measured by enzyme linked immunosorbent assay (ELISA). Most serum samples taken at the time of admission to hospital contained antibodies to both glycoproteins, and overall there was no significant evidence of a selective deficiency of antibody to either viral antigen. Less than a quarter of the infants showed rising IgG antibody titres to either glycoprotein after infection, whereas over threequarters produced an IgM response. There was a significant correlation between IgG response to viral glycoproteins and the age of the infant. The correlation of age with the IgM response was less pronounced, and there was no correlation between serum IgG antibody derived transplancentally in the acute phase of infection and IgM response to either glycoprotein. Neither IgG or IgM responses correlated with a clinical assessment of the severity of infection in the infants. IgM responses, however, were weakly correlated with reduced secretion of infectious virus in the upper respiratory tract.     

Immunological responses to respiratory syncytial virus infection in infancy

Accepted 4 November 1996
OBJECTIVES—To determine whether there is evidence of immunological responses in infants with respiratory syncytial virus (RSV) bronchiolitis by measuring inflammatory mediators in peripheral blood and, if found, whether these related to the severity of illness.
PATIENTS AND METHODS—Blood was taken from 94 children with RSV infection during the acute episode and 10 or more days later when the child was well. Control serum samples were obtained from well children of similar ages. Serum samples were assayed for mediators of lymphocyte activity (interleukin-4 (IL-4), soluble interleukin-2 receptor (sCD25), soluble intercellular adhesion molecule-1 (sICAM-1), eosinophil activity (eosinophil cationic protein) and neutrophil activity (myeloperoxidase). Symptoms were assessed as very mild (coryza only), mild (symptoms of lower respiratory tract infection), moderate (requiring nasogastric or intravenous fluids), and severe (requiring oxygen or ventilation).
RESULTS—IL-4 concentrations were at the lower limits of detection of the assay. The concentrations of sCD-25 were greater in samples from patients with acute illness than from convalescent patients and both were greater than in control samples. sICAM-1 concentrations were similar in samples from patients with acute illness and convalescent patients, but both were greater than in samples from controls. Eosinophil cationic protein concentrations were lower in samples from patients with acute illness than in those from convalescent patients; there was no difference between samples from convalescent and control patients. Myeloperoxidase concentrations were similar in all samples. There was no correlation between the severity of infection and the concentrations of any inflammatory mediators.
CONCLUSIONS—There is evidence of an inflammatory response in the peripheral blood of infants with acute bronchiolitis which may affect lymphocytes and eosinophils, but an association between this response and the severity of illness was not shown here.

     

Fetal hind limb oxygen consumption and blood flow during acute graded hypoxia

Hind limb blood flow and O2 uptake, mean blood pressure, and heart rate were measured in six fetal sheep at 127-141 d gestation in the control state and at different levels of hypoxia that were induced by partial occlusion of the maternal terminal aorta. Blood flow was measured by an ultrasonic flow transducer. Control fetal arterial O2 content ([O2]a) in the descending aorta was 3.25 + 0.17 mM. In response to graded acute hypoxia, blood flow first increased (22.2 versus 19.9 mL.min-1.100 g-1, p = 0.003) and then decreased abruptly at approximately 1.5 mM [O2]a.O2 uptake decreased about 12% (14.74 versus 16.71, p = 0.03) as [O2]a decreased to 1.5 mM, and then fell markedly, tending toward zero for [O2]a = 0.9 mM. In the 2.7- to 1.5-mM [O2]a range, heart rate increased above control (194 versus 169 min-1, p = 0.0024), whereas mean blood pressure did not change significantly. For [O2]a less than 1.5 mM, heart rate decreased to 148 min-1 (p = 0.0005) and mean blood pressure increased above baseline (55 versus 47 torr, p = 0.0001). In conjunction with previous evidence, these results define a state of acute moderate hypoxia in which the whole fetus can sustain a relatively high rate of oxidative metabolism, and a state of acute severe hypoxia ([O2]a between 1.5 and 1.0 mM) in which O2 uptake by some fetal organs is selectively and markedly decreased.     

The effect of theophylline on regional cerebral blood flow responses to hypoxia in newborn piglets

Theophylline attenuates cerebral hypoxic hyperemia in several adult models and this is thought to be due to receptor-mediated antagonism of adenosine, a proposed mediator of hypoxic hyperemia. This attenuation of hypoxic hyperemia reduces cerebral oxygen delivery and may thus jeopardize cerebral oxidative metabolism. With these considerations in mind, and because theophylline is widely used in neonatal medicine, the present study was designed to investigate the effect of theophylline on regional cerebral blood flow, cerebral oxygen delivery, and cerebral metabolic rate for oxygen during normoxia and hypoxia in the newborn piglet model. In 16 newborn piglets, regional cerebral blood flow (microspheres) increased 250-350% during hypoxia (PaO2 20-30 torr), while cerebral oxygen delivery and cerebral metabolic rate for oxygen were maintained at normoxic levels. Eight of these piglets were then given 10 mg/kg theophylline ethylenediamine intravenously and studies during normoxia and hypoxia were repeated; the remaining eight piglets served as time controls. Regional cerebral blood flow, cerebral oxygen delivery, and cerebral metabolic rate for oxygen during normoxia and hypoxia were not influenced by theophylline, despite plasma theophylline levels of 55-65 mumol/liter, and cerebrospinal fluid theophylline levels of 30-40 mumol/liter. These negative results are reassuring with respect to hypoxic cerebral blood flow control in theophylline-medicated infants. However, they do not support a role for adenosine as a mediator of cerebral hypoxic hyperemia in this model.     

Age-related difference in immune responses to respiratory syncytial virus infection in young children

There have been longitudinal studies of the developmental change of the immune system during the first year of life. The aim of this study was to investigate if there is any age-related difference in cytokine responses to respiratory syncytial virus (RSV) infection between the patients under 6 months of age and the patients over 12 months of age compared with age-matched controls. Forty-five children < or =24 months of age who were admitted with acute RSV bronchiolitis were enrolled. The patients were divided into two groups: the infants < or =6 months old and the young children > or =12 months old. Immune response to RSV infection was determined by measuring the serum concentrations of cytokines and compared with age-matched controls. Serum samples were obtained on admission and analyzed for interferon (IFN)-gamma, interleukins (IL)-10, -13, and -4 using ELISA. Comparing the cytokine levels of two control groups, both IFN-gamma and IL-13 were lower in the children > or =12 months of age than in the infants < or =6 months of age. IL-10 and IL-4 showed no significant changes with age. Comparing with age-matched controls, IFN-gamma levels were significantly higher in RSV group > or =12 months of age, but showed a tendency toward lower levels in RSV group < or =6 months of age. Both IL-10 and IL-13 levels were significantly higher in RSV group > or =12 months of age, but showed no significant difference in RSV group < or =6 months of age. Our study demonstrated a significant age-related difference in immune response to RSV infection during early life. It suggests that the developmental changes in cytokine responses to RSV infection may be considered in the control of RSV bronchiolitis in young children.     

Ventilatory responses to hypercapnia and hypoxia in conscious cystic fibrosis knockout mice Cftr-/-

This study was designed to examine the ventilatory performance and the lung histopathology of cystic fibrosis knockout mice (Cftr-/-) compared with heterozygous (Cftr+/-) or wild-type (Cftr+/+) littermates. Ventilation was recorded in conscious animals using whole-body plethysmography. Tidal volume (VT), respiratory frequency (f), and minute ventilation (VE) were measured during air breathing and in response to various levels of hypercapnia (2, 4, 6, or 8% CO2) or hypoxia (14, 12, 10, or 8% O2). The results for Cftr+/- and Cftr+/+ were pooled into one control group because they did not differ. In air and in response to hypercapnia, VE, VT, and f were similar in Cftr-/- mice and in controls. During graded hypoxia, VE was decreased in Cftr-/- mice at 10 and 8% O2 because of a lower f. Histology showed neither inflammation nor obstruction of airways in Cftr-/- mice. Morphometric analysis showed alveolar dilation as a result of either distension or impaired development. In conclusion, cystic fibrosis knockout mice have normal baseline breathing and ventilatory response to hypercapnia but a decreased ventilatory response to severe hypoxia. This latter result associated with the morphometric analysis suggests that Cftr-/- mice may exhibit immaturity of the respiratory system.     

Ventilatory responses to hypercapnia and hypoxia in Mash-1 heterozygous newborn and adult mice.

Normal control of breathing is characterized by maintenance of CO2 and O2 arterial pressures at constant levels by appropriate ventilatory responses to changes in CO2 production and O2 consumption. Abnormal development of this regulatory system during embryogenesis may produce early impairments in chemosensitivity, as in congenital central hypoventilation syndrome. The present study addresses the role of the mammalian achaetescute homologous gene (Mash-1) in the development of respiratory control. We analyzed ventilatory responses to hypercapnia (8% CO2, 21% O2, 71% N2) and hypoxia (10% O2, 3% CO2, 87% N2) in newborn and adult Mash-1 heterozygous mice (Mash-1+/-) and their wild-type littermates (Mash-1+/+). Ventilation, breath duration, and tidal volume were measured using whole-body plethysmography. Ventilatory responses to hypercapnia were significantly weaker in newborn male Mash-1+/- compared with Mash-1+/+ mice as a result of a weaker breath-duration response. No differences were observed between adult Mash-1+/- and Mash-1+/+ mice. Our data suggest that Mash-1 may be involved in respiratory control development via mechanisms linked to the X chromosome.