Shabih U Hasan

The Effects of Lung Distension, Oxygenation, and Gestational Age on Fetal Behavior and Breathing Movements in Sheep

ABSTRACT: Lung distension with 100% O2 at a continuous positive airway pressure of 30 cm H2O may induce continuous fetal breathing movements (FBM) in sheep. The objectives of this study were 1) to investigate the relative roles of lung distension and oxygenation and 2) to test the hypothesis that FBM can be induced during labor, when normally they are greatly reduced or absent. We studied 13 chronically instrumented, unanesthetized fetal sheep between 128 and 144 d of gestation (term = 147 ± 2 d). Each fetus was instrumented to record sleep states, diaphragm electromyogram, blood pressure, arterial pH, and blood gas tensions. The fetal lungs were distended via an in situ endotracheal tube with four different concentrations of O2 (0, 21, 50 and 100%) at a continuous positive airway pressure of 10, 20 and 30 cm H2O in a randomized order. No change in any recorded physiologic variable was observed at 129 ± 1 or 132 ± 1 d of gestation. At 135 ± 1 and 138 ± 1 d, in response to a continuous positive airway pressure of 30 cm H2O and 100% O2, pH decreased (p = 0.0004 and 0.005, respectively) and arterial O2 tension increased (p = 0.004 and 0.02, respectively). However, increases in 1) breathing time, 2) breathing time/low-voltage electrocortical activity ratio, 3) duration of arousal, and 4) length of single breathing epochs were observed only at 138 ± 1 d. Lung distension with N2 resulted in a decrease in FBM. Six fetuses were studied during labor. Significant increase in total breathing time, stimulation of breathing during quiet sleep, and increased duration of arousal were observed with an increase in fetal arterial O2 tension. The onset of breathing always coincided with the onset of arousal. We conclude that fetal maturity, an increase in fetal arterial O2 tension, and a change in state from quiet sleep toward arousal are necessary conditions for the induction of FBM.

Arterial oxygen tension threshold range for the onset of arousal and breathing in fetal sheep.

ABSTRACT: Mechanisms for the control of episodic fetal breathing movements or the onset of continuous breathing at birth remain unknown. Lung distension with 100% O2 at a continuous positive airway pressure of 30 cm H2O may induce arousal and continuous breathing. To investigate 1) the threshold range of arterial oxygen tension (PaO2) for the onset of arousal and breathing and 2) the graded response of breathing to various levels of PaO2, we studied 10 fetal sheep between 135 and 142 d of gestation (term = 147 ± 2 d). Each fetus was instrumented to record sleep states, diaphragmatic electromyogram, arterial pH, and blood gas tensions. PaO2 threshold was determined through an indwelling O2 sensor catheter. Fetal lungs were distended at a continuous positive airway pressure of 40 cm H2O with 100%. N2 or with O2 ranging from 40 to 100% via an in situ endotracheal tube. At the onset of arousal (n = 10), PaO2, arterial carbon dioxide tension, and Hb O2 saturation increased from control values of 21.7 ± 0.75 torr (2.9 ± 0.09 kPa), 41.8 ± 1.1 torr (5.47 ± 0.15 kPa), and 52.9 ± 2.6% to 65.6 ± 9.6 torr (8.74 ± 1.28 kPa), 46.9 ±1.3 torr (6.25 ± 0.17 kPa), and 92.9 ± 2.06%, respectively, whereas the pH decreased from 7.31 ± 0.006 to 7.27 ± 0.009 (mean ± SEM; p = 0.001, 0.04, 0.002, and 0.001, respectively). Seven of 10 fetuses breathed continuously. In these fetuses, PaO2 and arterial carbon dioxide tension further increased and pH decreased; however, no further significant increase in Hb O2 saturation was observed. Breathing stopped at a PaO2 of 38.5 ± 9.5 torr (5.1 ±1.3 kPa) but could be restarted by increasing the PaO2. In response to an increase in fetal PaO2 (30–60 torr; 4–8 kPa), amplitude of breathing and total respiratory output significantly increased from the control values. Further increases in PaO2 or Hb O2 saturation did not significantly affect the respiratory output. Hb O2 saturation and arterial pH had the most confounding effects on frequency and amplitude of breathing, respectively. Among the various sleep states, arousal had the most profound effects on respiratory output. We conclude that 1) very high levels of PaO2 are not necessary to initiate arousal and continuous breathing and 2) further increments in Hb O2 saturation above the threshold level have no significant effect on breathing responses.

Effect of Inhaled Nitric Oxide on Survival Without Bronchopulmonary Dysplasia in Preterm Infants: A Randomized Clinical Trial

Importance Bronchopulmonary dysplasia (BPD) occurs in approximately 40% of infants born at younger than 30 weeks’ gestation and is associated with adverse pulmonary and neurodevelopmental outcomes. Objective To test whether administration of inhaled nitric oxide to preterm infants requiring positive pressure respiratory support on postnatal days 5 to 14 improves the rate of survival without BPD. Design, Setting, and Participants This intent-to-treat study was a randomized clinical trial performed at 33 US and Canadian neonatal intensive care units. Participants included 451 neonates younger than 30 weeks’ gestation with birth weight less than 1250 g receiving mechanical ventilation or positive pressure respiratory support on postnatal days 5 to 14. Enrollment spanned from December 23, 2009, to April 23, 2012, and neurodevelopmental outcome studies were completed by April 4, 2014. Interventions Placebo (nitrogen) or inhaled nitric oxide initiated at 20 ppm was decreased to 10 ppm between 72 and 96 hours after starting treatment and then to 5 ppm on day 10 or 11. Infants remained on the 5-ppm dose until completion of therapy (24 days). Main Outcomes and Measures The primary outcome was the rate of survival without BPD at 36 weeks’ postmenstrual age (PMA). Secondary outcomes included BPD severity, postnatal corticosteroid use, respiratory support, survival, and neurodevelopmental outcomes at 18 to 24 months’ PMA. Results In total, 222 infants (52.3% male [n = 116]) received placebo, and 229 infants (50.2% male [n = 115]) received inhaled nitric oxide. Their mean (SD) gestation was 25.6 (1.5) vs 25.6 (1.4) weeks, and their mean (SD) birth weight was 750 (164) vs 724 (160) g. Survival without BPD at 36 weeks’ PMA was similar between the placebo and inhaled nitric oxide groups (31.5% [n = 70] vs 34.9% [n = 80]) (odds ratio, 1.17; 95% CI, 0.79-1.73). Rates for severe BPD (26.6% [55 of 207] vs 20.5% [43 of 210]) and postnatal corticosteroid use for BPD (41.0% [91 of 222] vs 41.5% [95 of 229]) and the mean (SD) days of positive pressure respiratory support (55 [40] vs 54 [42]), oxygen therapy (88 [41] vs 91 [59]), and hospitalization (105 [37] vs 108 [54]) were equivalent between the 2 groups. No differences in the incidence of common morbidities were observed. Respiratory outcomes on discharge to home, at 1 year, and at age 18 to 24 months’ PMA and neurodevelopmental assessments at 18 to 24 months’ PMA did not differ between groups. Conclusions and Relevance Inhaled nitric oxide, initiated at 20 ppm on postnatal days 5 to 14 to high-risk preterm infants and continued for 24 days, appears to be safe but did not improve survival without BPD at 36 weeks’ PMA or respiratory and neurodevelopmental outcomes at 18 to 24 months’ PMA. Trial Registration clinicaltrials.gov Identifier: NCT00931632

Cardiovascular Responses During Transition from Fetal to Neonatal Life in Intact and Vagotomized Lambs |[dagger]| 1664

Evidence suggests that pulmonary vagal innervation contributes more significantly to the alveolar ventilation in newborns than in older animals. To investigate whether vagal efferents play a role in the control of pulmonary blood flow during transition from fetal to neonatal life, we studied 12 unanesthetized, spontaneously breathing, chronically instrumented newborn lambs at 143-147 d gestation (Term=147±2d) after unassisted vaginal delivery. Surgery was performed between 130-134 d gestation to implant jugular venous and carotid arterial catheters, diaphragmatic electrode and left pulmonary artery blood flow probe (Transonic Inc). Six of 12 fetuses underwent intrathoracic vagotomy (VAG), whereas the remainder six were sham-operated(SHAM). Breathing frequency (breaths per minute) remained higher in the SHAM animals as compared with the VAG group, during the entire postnatal period(92-104 vrs 30-40, respectively; p=0.001). Inspiratory and expiratory times were significantly longer in the VAG group; p<0.01). Furthermore, similar to our previous studies, vagotomized animals were unable to establish adequate alveolar ventilation and remained hypothermic despite attempts to rewarm. Left pulmonary arterial (LPA) blood flow (ml/min/kg) increased from pre-birth values of 2.67±2.45 and 0.2±0.2 to 48±7 and 57±17 at five minutes after birth, in sham-operated and vagotomized animals, respectively (p<0.05). LPA continued to increase gradually reaching the maximum value of 101±10 and 126±19 ml/kg/min at 12 minutes of age in both SHAM and VAG groups, respectively. LPA blood flow pleatued at 85±8 and 84±6 ml/kg/min by 40 minute of age. Heart rate(beats/min) increased from 128±11(pre-birth) to 180±18 and 260±7(p<0.05),and from 134±15 (pre-birth), to 167±25 and 227±9 (p<0.05) at 10 and 30 minutes of age in SHAM and VAG groups, respectively. Heart rate was significantly higher in the SHAM group as compared with the VAG group at 30 minutes of age (p=0.02) whereas no significant changes were observed in systolic, diastolic or mean blood pressure between the two groups. We conclude that vagal innervation does not play significant role in cardiovascular adaptation at birth and inadequate pulmonary gas exchange in vagotomized animals is not due to lack of an increase in pulmonary blood flow. In addition, our study provides the first evidence of sequential changes in pulmonary blood flow during the first hour after birth in unanesthetized, spontaneously breathing newborn animals.

Umbilical Arterial Blood Flow and Plasma Prostaglandin E2 Concentrations during Arousal and Breathing Movements in Fetal Sheep

To investigate the effects of lung distension and oxygenation on umbilical blood flow (UBF) and plasma prostaglandin E2 (PGE2) in relation to arousal and stimulation of breathing movements, we studied eight chronically instrumented, unanesthetized fetal sheep between 137 and 143 d of gestation. Electrocorticogram, electro-oculogram, nuchal and diaphragmatic electromyograms, arterial pH and blood gas tensions, Hb oxygen saturation, body temperature, and UBF were recorded in each fetus. Electrocorticogram, electro-oculogram, and nuchal electromyograms were used to define sleep states. No sooner than 4 d after surgery, fetal lungs were distended with 100% O2 or N2 in a randomized order via an in situ Y-endotracheal tube. PGE2 concentrations were analyzed by RIA. A significant increase in fetal arousal and stimulation of breathing during nonrapid eye movement sleep was observed during lung distension with O2 as compared with control periods and lung distension with nitrogen. In all sleep states, UBF significantly decreased during oxygenation as compared with the control values. However, no significant correlation was observed between the time of the onset of arousal and the decrease in UBF. Lung distension with N2 resulted in increased plasma PGE2 concentrations, whereas, no change was observed during oxygenation. Our data suggest that an increase in fetal partial pressure of arterial O2 leads to a decrease in UBF. However, the onset of arousal and stimulation of breathing during lung distension and oxygenation are not dependent on a decrease in plasma PGE2 concentrations.

Methemoglobin levels in umbilical cord blood of women with intrauterine growth restriction and preeclampsia.

Abstract Objective: To measure nitric oxide (NO) levels, as assessed by methemoglobin (MetHb), in the umbilical cord arterial blood of women with preeclampsia (PE), normotensive pregnancies with intrauterine growth restriction (IUGR, <10th percentile for birth weight) and normotensive pregnancies with appropriate for gestational age (AGA) infants. Design/Methods: Prospective analysis of MetHb levels in umbilical arterial blood of pregnancies with PE, normotensive IUGR and normotensive AGA infants using spectrophotometry. Exclusion criteria included cigarette smoke exposure, hemolytic disorders, chronic hypertension, any significant medical illness and medications affecting NO or MetHb levels. Results: There were 42 women in the PE, 42 in the AGA and 37 in the IUGR group. Maternal age, mode of delivery, gravidity, parity and infant gender were similar in the three groups. MetHb levels were significantly lower in the IUGR group compared to the AGA group (p < 0.01). MetHb levels, while higher in the PE group compared to the AGA group, did not reach statistical significance. Conclusion: Our data suggests that NO may play a key role in the pathogenesis of IUGR. We also, for the first time, provide information on umbilical arterial MetHb levels in PE, normotensive IUGR and normotensive AGA pregnancies.

Prenatal cigarette smoke exposure and postnatal respiratory responses to hypoxia and hypercapnia

Prenatal cigarette smoke (CS) exposure, in combination with hypoxia and/or hyperthermia can lead to gasping and attenuated recovery from hypoxia in 7 days old rat pups. We studied 95 unanesthetized spontaneously breathing 14 days old rat pups to investigate if the destabilizing effects of increased ambient temperature and prenatal CS exposure on respiratory control observed in 7 days old rats were still evident at day 14. This postnatal age was selected as it is beyond the analogous risk period for SIDS in human. Furthermore, we investigated if the breathing responses to hypercapnia are affected by prenatal CS exposure. Since high ambient (HA) temperature can lead to gasping and aberrant respiratory control, we recorded respiratory patterns at low (24–25°C) and high (29–30°C) ambient temperatures, and under hypoxic or hypercapnic states. No gasping was observed in 14 days old rat pups. During hypoxia, breathing frequency increased in the CS‐exposed group under low and HA temperatures. Rectal temperature decreased only in the sham group in response to low ambient temperature hypoxia. At HA temperature, breathing frequency increased in both sham and CS‐exposed groups during hypercapnia, however, it remained elevated during washout period only in the sham group. We demonstrate that prenatal CS exposure continues to have profound effects on respiratory and thermoregulatory responses to hypoxia and hypercapnia at day 14. The attenuated respiratory and thermoregulatory responses to acute hypoxia and hypercapnia on day 14 demonstrate a strong interaction between CS exposure, respiratory control, and thermoregulation during postnatal maturation. Pediatr Pulmonol. 2012; 47:487–497.

Ionized calcium levels in umbilical cord blood of women with preeclampsia and normotensive pregnancies

Calcium is essential for normal fetal growth and development. During intrauterine life, the fetus is entirely dependent on the mother and a normally functioning placenta for calcium accretion. Preeclampsia is associated with abnormal calcium metabolism and placental dysfunction. The objective of our study was to investigate ionized calcium levels in the umbilical cord arterial blood of women with preeclampsia and normotensive pregnancies. There were 24 women in the preeclampsia group and 25 in the normotensive group. There was no difference in the cord pH and fetal growth restriction between the two groups. Ionized calcium levels were significantly lower in the preeclampsia group (p < 0.001). Our results emphasize the need for further studies on the calcium status of infants born to mothers with preeclampsia.

Afferent neural feedback overrides the modulating effects of arousal, hypercapnia and hypoxaemia on neonatal cardiorespiratory control

Evidence obtained at whole animal, organ‐system, and cellular and molecular levels suggests that afferent volume feedback is critical for the establishment of adequate ventilation at birth. As a result of the irreversible nature of the vagal ablation studies performed to date, it was difficult to quantify the roles of afferent volume input, arousal and changes in blood gas tensions on neonatal respiratory control. During reversible perineural vagal block, profound apnoeas and hypoxaemia and hypercarbia were observed, necessitating the termination of perineural blockade. Respiratory depression and apnoeas were independent of sleep state. We demonstrate that profound apnoeas and life‐threatening respiratory failure in vagally denervated animals do not result from a lack of arousal or hypoxaemia. A change in sleep state and concomitant respiratory depression result from a lack of afferent volume feedback, which appears to be critical for the maintenance of normal breathing patterns and adequate gas exchange during the early postnatal period.

Vagal Denervation Decreases the Large Surfactant Aggregates and mRNAs for SP-A and SP-B. 1951

We have previously shown that fetal lambs which have undergone intrathoracic vagal denervation have increased surface tension and develop respiratory failure at birth. However, the mechanism of surfactant dysfunction remains unknown. To quantify the relative roles of surfactant synthesis, release and activity, we studied 15 chronically instrumented unanesthetized newborn lambs; eight of 15 fetuses underwent intrathoracic vagal denervation whereas seven were sham-operated. Each fetus was instrumented to record heart rate, blood pressure, diaphragmatic electromyogram, arterial pH, and blood gas tensions between 131-133 d gestation (term=147±2 d) and studied at 142-143 d after spontaneous vaginal delivery. Various cardiorespiratory variables were recorded immediately after birth and the animals were sacrificed at 1 hr of age. Surfactant associated proteins (SP-A, -B and -C) mRNAs were analyzed in the right middle lobe whereas bronchoalveolar lavage(BAL) was performed on the remaining lungs to determine total and various phospholipid (PL) species, large aggregates (LA), small aggregates (SA) and plasma proteins. Recovery of BAL was similar in both groups (91±1 vrs 90±2%). Total phospholipids were also similar in the sham-operated and vagotomized animals. However, LA (mg/kg) in sham group (23±4.24) were significantly higher than in the vagotomized group (10±1.9; p=0.01). Also, the LA as a percentage of total PL was significantly higher in the sham operated animals (52±6 vrs. 25±8, respectively, p=0.009). The percentage of phosphatidylcholine (n=9) was similar in both groups, however, phosphatidylinositol (PI) was higher (p=0.02) and phosphatidylglycerol (PG) was lower (p=0.05) in the vagotomized group. Northern blot analysis showed decreased mRNA levels of SP-A and SP-B, whereas no significant differences were observed for SP-C. We conclude that vagally denervated animals have an increased proportion of SA, an inactive form of surfactant, decreased LA, and delayed surfactant maturation as indicated by the PI and PG ratios, and decreased expression of surfactant apoproteins mRNAs. These changes in surfactant system may play a role in respiratory failure in vagally denervated newborn lambs.