Anita Rigaux

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.

Role of vagal innervation on pulmonary surfactant system during fetal development

Vagally mediated afferent feedback and compliant lungs (surfactant system) play vital roles in the establishment of adequate alveolar ventilation and pulmonary gas exchange at birth. Although the significance of vagal innervation in the establishment of normal breathing patterns is well recognized, the precise role of lung innervation in the maturation of the surfactant system remains unclear. The specific aim of the present study was to investigate whether vagal denervation compromises the surfactant system during fetal development. Experiments were performed on 12 time-dated fetal sheep: 8 underwent cervical vagal denervation, and 4 were sham operated. Vagal denervation was performed at 110-113 days gestation. Fetal lambs were instrumented in utero to record arterial pH and blood-gas tensions. The animals were delivered by cesarean section under general anesthesia between 130 and 133 days gestation (term approximately 147 days). Lung samples were collected for wet-to-dry ratios, light and electron microscopy, and overall lung morphology. In addition, total proteins, total phospholipids, and surfactant proteins A and B were analyzed in both lung tissue and bronchoalveolar lavage fluid. Vagal denervation had no effect on alveolar architecture, including type II cells or the morphology of lamellar bodies within them. Furthermore, surfactant proteins A and B and total phospholipids were similar in lung tissue and bronchoalveolar lavage fluid between the two groups. A significant correlation was observed between circulating cortisol concentrations and surfactant proteins in the bronchoalveolar lavage fluid and lung tissue. We provide definitive evidence that vagal innervation at midgestation is not required for maturation of the pulmonary surfactant system during fetal development.

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.

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.

Effect of Vagal Denervation on Sleep States, Cardiorespiratory Responses and Surfactant System during Postnatal Period

Effect of Vagal Denervation on Sleep States, Cardiorespiratory Responses and Surfactant System during 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.