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

Abstract

ABSTRACT. Lung distension with 100% 0 2 at a continuous positive airway pressure of 30 cm H 2 0 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 f 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, , it has been known that fetal sheep make irregular, rapid, and episodic breathing movements (2). One of the many unique and vital changes occumng at birth is the establishment of continuous breathing. Mechanisms for the control of FBM or the initiation of continuous breathing at birth remain unknown. In sheep, during late gestation (1 15-120 d; term = 147 k 2 d), FBM occupy 40% of the total time and occur normally only during LV-ECoG and are absent during HV-ECoG. Although the precise control mechanism for FBM has yet to be explained, FBM are susceptible to modulation. Isocapnic hypoxemia suppresses and normoxic hypercapnia stimulates FBM (3, 4); morphine, meclofenamate, indomethacin, pilocarpine, doxapram, aminophylline, caffeine, and 5-hydroxytryptophan stimulate, whereas general anesthetics, barbiturates, diazepam, and prostaglandin E2 depress FBM (2, 5-8).We have recently shown that distending fetal lungs with 100% O2 at a CPAP of approximately 30 cm H 2 0 may induce continuous FBM (9). However, the relative roles of distending pressure and oxygenation remain unknown. Nor is it known if these FBM can be induced during spontaneous onset of labor when normally these are absent or greatly reduced. Therefore, we designed a study I ) to investigate the relative roles of oxygenation and distending pressure by using various concentrations of O2 and different levels of distending pressures and 2) to test the hypothesis that FBM can be induced during spontaneous onset of labor. MATERIALS AND METHODSAnimal preparation. We studied 13 fetal sheep between 128 and 144 d of gestation. Surgery was performed on time-dated, pregnant ewes of mixed breed between 124 and 132 (127 + 2) d of gestation using 4% halothane in O2 for induction of anesthesia and 1.5 to 2% for maintenance. After the skin was cleansed with iodine and 70% alcohol, the abdominal wall was opened by a midline incision using sterile technique. Avoiding major vessels, the uterine cavity was opened and the fetal head and neck were exteriorized. The uterine wall and fetal skin were brought together with several Babcock forceps to minimize loss of amniotic fluid. An incision was made at the upper dorsal part of the neck to tunnel four color-coded electrode wires to record ECoG, EOG, EMGNK, and diaphragm electromyogram. The elect...

Urorectal septum malformation sequence: Insights into pathogenesis

“…Twin lambs were delivered electively by Cesarean section at 126 days gestation (term ϭ 147 Ϯ 2 days) (Hasan and Rigaux, 1991). The ewe was mated on a specific day to allow precise knowledge of gestational age.…”

Effects of Lung Distension and Spontaneous Fetal Breathing on Hemodynamics in Sheep

“…We had ventilated fetuses with oscillating positive Recent studies have demonstrated that spontaneous negative pressures; the possibility that high intrathoracic pressures pressure pulmonary ventilation can be initiated and maintained adversely affected cardiac output could not be ruled out. in fetal sheep when the umbilical cord is occluded while oxygen To determine the effects of negative intrathoracic pressure is introduced into the fetal trachea (7)(8)(9)(10). We applied this ventilation, we occluded the umbilical cord of nine long-technique to determine the effects of spontaneous, rhythmic term instrumented fetal sheep at 135 f 1.2 d gestation pulmonary ventilation on blood flow patterns at birth.…”

Arterial Oxygen Tension Threshold Range for the Onset of Arousal and Breathing in Fetal Sheep

“…Breathing stopped at a Pao2 of 38.5 +: 9.5 torr (5.1 f 1.3 kPa) but could be restarted by increasing the Pao2. In response to an increase in fetal Pao, (4)(5)(6)(7)(8), 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.…”

“…In sheep, during late gestation, FBM occupy 40% of the total time and normally occur only during LV-ECoG but are absent during HV-ECoG (3). Recent studies have shown that an increase in fetal Pao2 achieved by distending the fetal lungs with 100% O2 at a CPAP of 30 cm H 2 0 may stimulate FBM during both LV-and HV-ECoG (4,5). In our previous study (5) initiation of breathing was always associated with the onset of fetal arousal and was critically dependent on fetal maturity.…”

“…However, in these studies, fetal Pao2 levels were as high as 250 ton (33.3 kPa). Furthermore, the range of Pa02 has also been very wide (4,5), and it is not known whether there is a graded response of FBM to various levels of Pao2. Therefore, we designed a study to investigate I ) the Pao2 threshold range for the onset of arousal and breathing and 2) the graded response of breathing to various levels of Pao2.…”