Fetal breathing movements and lung maturation in the congenitally abnormal human fetus

Fetal breathing movements have been studied in conjunction with features of anatomical and biochemical development of the lung at birth in fetuses with congenital abnormalities affecting the respiratory system. Total absence of fetal breathing movements or abnormal fetal breathing movements were associated with lung hypoplasia and failure of normal surfactant release into saline extracts of lung fluid. Surfactant synthesis was demonstrated regardless of the presence or absence of fetal breathing movements. The study supports the hypothesis that normal fetal breathing movements are important for fetal lung development and suggests that surfactant synthesis and its release are independent. The latter process may be dependent upon fetal breathing movements while the former is not.     

Denervation of peripheral chemoreceptors decreases breathing movements in fetal sheep

The role of the peripheral chemoreceptors in the control of fetal breathing movements has not been fully defined. To determine whether denervation of the peripheral chemoreceptors affects fetal breathing movements, we studied 14 chronically catheterized fetal sheep from 120 to 138 days of gestation. In seven fetuses the chemoreceptors were denervated by bilateral section of the vagus and carotid sinus nerves; in seven others, sham operations were performed. We compared several variables during two study periods: 0-5 and 6-13 days after operation. In the denervated fetuses there were significant decreases in the incidence and amplitude of fetal breathing movements during both study periods. There were no differences between the two groups in incidence of low-voltage electrocortical activity, arterial pH and blood gas tensions, fetal heart rate, mean arterial blood pressure, or duration of survival after operation or birth weight. We conclude that denervation of the peripheral chemoreceptors decreases fetal breathing movements. These results indicate that the peripheral chemoreceptors are active during fetal life and participate in the control of fetal breathing movements.     

Effect of hyperoxia (PaO2 50-90 mmHg) on fetal breathing movements in the unanaesthetized fetal sheep

Hypoxia inhibits fetal breathing movements but after birth it stimulates breathing. These differences have long been thought to involve central nervous inhibitory mechanisms. Such mechanisms might exert a tonic inhibition of fetal breathing movements at normal fetal PaO2 and the rise in PaO2 at birth might lift this inhibitory effect. To test this hypothesis 7 fetal sheep were chronically instrumented at 125-130 days for recording electrocortical activity (ECoG), and the electromyograph (EMG) activity of the diaphragm and neck muscles. Catheters were placed in a fetal carotid and a brachial artery and in the fetal trachea. For an extracorporeal membrane oxygenation system a 12 F gauge silastic catheter was placed in the right atrium for draining fetal blood and a 9.6 F gauge catheter was placed in a carotid artery to return oxygenated blood. Three days after operation the fetuses were connected to the extracorporeal membrane oxygenation system and fetal PaO2 was raised to 65.2 +/- 4.4 mmHg (SEM) for 6 to 19 h without changing pH or PaCO2. Neither the incidence of high voltage ECoG (48.5 +/- SEM 2.0% vs 52.8 +/- 3.3%) nor of fetal breathing movements (37.3 +/- 2.6% vs 23.8 +/- 5.9%) changed during the periods of hyperoxia. Since fetal breathing movements did not become continuous, we conclude that the lower PaO2 in the fetus compared to the neonate does not exert a tonic inhibitory influence on fetal breathing movements.     

Stimulation of fetal breathing activity by beta-adrenergic mechanisms

Experiments were done on chronically prepared fetal lambs, 125-135 days gestation, to test the effects of various catecholamines on fetal breathing (FB) as well as the influence of isoproterenol on the fetal respiratory response to hypoxemia. Bolus injections of epinephrine, norepinephrine, and isoproterenol (5-20 micrograms) were administered via the lingual artery or femoral or jugular vein during periods of FB activity or apnea. The effects of epinephrine and norepinephrine on FB were variable and not statistically significant. Isoproterenol produced a significant increase in FB, frequency of breathing, and mean inspiratory effort, when infused during rapid-eye-movement (REM) sleep but it failed to induce FB during non-rapid-eye-movement (NREM) sleep. The positive response during REM sleep was absent following pretreatment with 3-5 mg propranolol and after bilateral section of the sinus nerves. The effect of hypoxia on FB was tested before and during constant infusion of isoproterenol (1 microgram/min iv). A reduction of the fetal arterial PO2 by 3-10 Torr produced the characteristic depression of FB in either situation. These results indicate that the fetal carotid body chemoreceptors can reflexly stimulate FB under certain circumstances but that their effectiveness is limited by more powerful inhibitory mechanisms such as those operative during NREM sleep and hypoxemia.     

Fetal breathing and behavior measured through a double-wall Plexiglas window in sheep

The inability to see the fetus makes the assessment of fetal behavior difficult. To circumvent this problem we implanted a Plexiglas window in the left flank of the ewe. Fetuses were instrumented for measurements of sleep, breathing, and swallowing. Ten fetal sheep were studied on 32 occasions. Six fetuses were delivered through the window at term, and postnatal behavior was compared with intrauterine behavior. Fetuses observed during resting conditions alternated between periods of quiet sleep [high-voltage electrocortical activity (ECoG)] and active or rapid-eye-movement sleep (low-voltage ECoG). In quiet sleep, movements were absent except for periodic generalized electromyographic discharges. Eye and breathing movements were rare or absent. Swallowing was also absent. In active sleep, movements were increased with powerful breathing and swallowing activity. Fetal wakefulness defined by open eyes and purposeful movements of the head was never seen in utero but was clearly observed after delivery. We conclude that fetal wakefulness as defined postnatally was not able to be demonstrated in utero.     

Patterns of spontaneous activity in single rat olfactory receptor neurons are different in normally breathing and tracheotomized animals

Spontaneous firing of olfactory receptor neurons (ORNs) was recently shown to be required for the survival of ORNs and the maintenance of their appropriate synaptic connections with mitral cells in the olfactory bulb. ORN spontaneous activity has never been described or characterized quantitatively in mammals. To do so we have made extracellular single unit recordings from ORNs of freely breathing (FB) and tracheotomized (TT) rats. We show that the firing behavior of TT neurons was relatively simple: they tended to fire spikes at the same average frequency according to purely random (Poisson) or simple (Gamma or Weibull) statistical laws. A minority of them were bursting with relatively infrequent and short bursts. The activity of FB neurons was less simple: their firing rates were more diverse, some of them showed trends or were driven by breathing. Although more of them were regular, only a minority could be described by simple laws; the majority displayed random bursts with more spikes than the bursts of TT neurons. In both categories bursts and isolated spikes (outside bursts) occurred completely at random. The spontaneous activity of ORNs in rats resembles that of frogs, but is higher, which may be due to a difference in body temperature. These results suggest that, in addition to the intrinsic thermal noise, spontaneous activity is provoked in part by mechanical, thermal, or chemical (odorant molecules) effects of air movements due to respiration, this extrinsic part being naturally larger in FB neurons. It is suggested that spontaneous activity may be modulated by respiration. Because natural sampling of odors is synchronized with breathing, such modulation may prepare and keep olfactory bulb circuits tuned to process odor stimuli.     

Effects of hyperthermia on fetal breathing movements

High environmental temperature is known to impair fetal growth and development. We now report long lasting changes in fetal breathing activity following the exposure of pregnant ewes to an ambient temperature of 43 degrees C for 8 h. In 16 trials in 10 ewes (119-138 days gestation) heat exposure increased maternal and fetal core temperatures 1.5-2.0 degrees C, and the hyperventilation by the ewe produced a fall in fetal PaCO2 from 53.5 +/- 1.3 to 34.8 +/- 5.3 mmHg (P less than 0.05). Fetal breathing movements decreased in incidence during the hyperthermia but remained episodic (present during low-voltage electrocortical activity) with occasional brief episodes of breathing at high rates (greater than 4 breaths/s). However, 1-2 h after the end of heating, when maternal and fetal core temperature and PaCO2 had returned to normal, fetal breathing movements became continuous, and were augmented 30-100% in amplitude. Fetal breathing movements occurred during both low- and high-voltage electrocortical activity. The results show that a heat load similar to that experienced by sheep in sub-tropical regions in the summer months cause prolonged changes in the central regulation of fetal breathing.     

Fetal breathing is not initiated after cord occlusion in the unanaesthetized fetal lamb in utero.

We investigated the role of cord occlusion in the initiation of breathing at birth using an extracorporeal membrane oxygenator system to control fetal blood gases independently of the placenta in 12 chronically instrumented fetal lambs. In group IA (n = 9; exp = 12) PaCO2 was kept constant (5.62 +/- 0.21 to 5.70 +/- 0.23 kPa) during cord occlusion. Group IB (n = 7; exp = 8) were cord occlusion experiments from group IA in which no fetal breathing movements had occurred; CO2 flow to the membrane was increased and fetal PaCO2 rose significantly (5.45 +/- 0.24 to 8.27 +/- 0.56 kPa). In group II (n = 7; exp = 12) PaCO2 was allowed to increase from 5.98 +/- 0.24 kPa to 8.09 +/- 0.48 kPa after cord occlusion. Within 5 min of cord occlusion, FBM did not occur in 11 out of 12 experiments in group IA or in 11 out of 12 experiments in group II. In contrast in group IB breathing did occur in 5 out of 8 experiments. When they occurred, fetal breathing movements were always associated with low voltage electrocortical activity. Our results do not support the hypothesis that the initiation of breathing within 5 minutes of birth is dependent on an inhibitory factor of placental origin. Furthermore these data suggest an association between the presence of breathing and a substantial rise in PaCO2.     

Drug-induced changes in fetal breathing activity and sleep state

Drugs reported to stimulate fetal breathing (FB) were injected into a femoral vein of near-term fetal lambs during rapid eye movement (REM) and non-REM (NREM) sleep. The primary response to NaCN, 0.25-0.5 mg, a dose which did not flatten the electrocorticogram, was a brief burst of gasping in any sleep state. When injected during REM sleep, NaCN caused the cessation of spontaneous FB and the onset of gasping. Stimulation of FB was observed infrequently. Caffeine (10 mg) and doxapram (3 mg) frequently caused an immediate change in sleep state or arousal. The incidence of FB increased concomitantly with a change to REM sleep or wakefulness (W), but FB still ceased with the onset of NREM sleep. When administered during an episode of spontaneous FB during REM sleep, both caffeine and doxapram caused stimulation of the frequency and depth of breathing. Pilocarpine (4 mg) caused arousal and gasping followed by prolonged vigorous breathing that was dependent on intact carotid sinus nerves. Indomethacin (120 mg over several hours) did not affect sleep states but induced FB in both NREM and REM sleep. In summary, in the fetus the primary effect of NaCN is to suppress spontaneous FB and induce gasping and the effects of pilocarpine, caffeine, and doxapram are intimately related to sleep states or arousal. Indomethacin causes the conversion from episodic fetal to continuous postnatal-type breathing. These data indicate the importance of assessing fetal state of consciousness in interpreting the respiratory response to drugs.     

Increased oxygen consumption associated with breathing activity in fetal lambs

To examine the relationship between fetal O2 consumption and fetal breathing movements, we measured O2 consumption, umbilical blood flow, and cardiovascular and blood gas data before, during, and after fetal breathing movements in conscious chronically catheterized fetal lambs. During fetal breathing movements, O2 consumption increased by 30% from a control value of 7.7 +/- 0.7 (SE) ml X min-1 X kg-1. Umbilical blood flow was 210 +/- 21 ml X min-1 X kg-1 before fetal breathing movements; in 9 of 16 samples it increased by 52 +/- 12 ml X min-1 X kg-1, while in the other 7 it decreased by 23 +/- 9 ml X min-1 X kg-1. Umbilical arterial and venous O2 partial pressures and pH fell during fetal breathing movements, and the fall was greater when umbilical blood flow was decreased. Partial CO2 pressure rose in both vessels, and again the increase was greatest when umbilical blood flow fell during fetal breathing movements. Also associated with a fall in umbilical blood flow was the transition from low-amplitude irregular to large-amplitude regular fetal breathing movements. It is concluded that fetal breathing movements increase fetal O2 demands and are associated with a transient deterioration in fetal blood gas status, which is most severe during large-amplitude breathing movements.     

Effects of naloxone on breathing movements during hypercapnia in the fetal lamb

To determine whether endogenous opioids influence the fetal breathing response to CO2 we have investigated the effect of the opiate antagonist, naloxone on the incidence, rate, and amplitude of breathing movements during hypercapnia in fetal lambs in utero. In 20 experiments in six pregnant sheep (130-145 days gestation) hypercapnia was induced by giving the ewe 4-6% CO2-18% O2 in N2 to breathe for 60 min. After 30 min of hypercapnia either naloxone (13 experiments) or saline (7 experiments) was infused intravenously for the remaining 30 min. During hypercapnia breath amplitude increased from 5.8 +/- 0.5 to 9.1 +/- 1.2 mmHg (P less than 0.001), and infusion of naloxone was associated with a further significant increase to 15.7 +/- 1.2 mmHg (P less than 0.001). Naloxone had no effect on the incidence or rate of breathing movements during hypercapnia. After hypercapnia there was a significant decrease in the incidence of fetal breathing movements in the naloxone group (14.7 +/- 3.2%). Infusion of saline during hypercapnia had no effect on incidence, rate, or amplitude of fetal breathing movements. These results suggest that endogenous opioids act to suppress or limit breath amplitude during hypercapnia but do not affect rate or incidence of breathing movements.     

Fetal breathing movements and other tests of fetal wellbeing: a comparative evaluation.

Sixty pregnant women whose fetuses were considered to be at high risk were intensively studied with fetal and placental function tests. Fetal breathing movements were studied with real-time ultrasound and the amount of time spent breathing and the variability of the breath-to-breath interval were measured. A reduction in the amount of time the fetus spent making breathing movements and decreased variability were indicative of fetal compromise. When these results were compared with those of other tests of fetal wellbeing measurement of fetal breathing movements and ultrasound assessment of growth were more sensitive tests of fetal wellbeing than the biochemical measures (urinary oestrogen, human placental lactogen, pregnancy-specific beta-1-glycoprotein, and unconjugated oestriol concentrations) or fetal heart rate. The predictive value was highest with serum unconjugated oestriol but the results of other tests were similar. Study of fetal breathing movements or an ultrasonic assessment of growth may provide a better screening test for fetal compromise than biochemical estimations.     

Stimulation of breathing movements in fetal sheep by inhibitors of prostaglandin synthesis

We studied the effects of inhibitors of prostaglandin synthesis on fetal breathing movements on 17 occasions in 11 lambs (gestational age 125-141 days). We gave 12 h infusions of sodium mechlofenamate (8.6-22.2 mg.kg-1) in 13 studies and indomethacin (21.8-38.8 mg.kg-1) in four studies. Results were similar with both agents and did not correlate with drug dosage. There were no changes in fetal arterial blood pressure, pH or blood gas tensions. We assessed fetal breathing movements by measurements of tracheal pressure for a control period of 224 h prior to and 208 h during the infusion of inhibitors of prostaglandin synthesis; their administration caused a marked stimulation of fetal breathing movements judged from the following four variables: (1) incidence of fetal breathing movements increased from 38.4 to 69.2% of the time (P < 0.001); (2) average amplitude of change in tracheal pressure during fetal breathing movements increased from 4.1 to 6.0 torr (P < 0.01); (3) maximal amplitude of change in tracheal pressure during fetal breathing movements increased from 8.8 to 13.4 torr (P < 0.01); and (4) the duration of the longest continuous episode of fetal breathing movements increased from 37 to 229 min (P < 0.05). Two fetuses had electrocorticogram (ECoG) recordings. In control periods, fetal breathing movements occurred only during low voltage, high frequency ECoG activity; however, during infusions of inhibitors of prostaglandin synthesis, fetal breathing movements occurred also during high voltage, low frequency ECoG activity. We conclude that inhibitors of prostaglandin synthesis stimulate fetal breathing movement in fetal sheep. These results suggest that a component of the prostaglandin system is a factor which inhibits breathing movements during fetal life.     

The relationship between fetal breathing movements and prostaglandin E2 during ACTH-induced labour in sheep

We measured fetal breathing movements and fetal carotid arterial prostaglandin E concentrations during adrenocorticotrophin-induced labour in 6 pregnant sheep and in 6 control animals starting at day 127. The 6 ACTH-treated animals went into labour on average 97 h after the onset of infusion and the incidence of fetal breathing movements diminished during the last 12h before the onset of labour. There was a significant negative relationship between the incidence of fetal breathing movements and fetal carotid arterial prostaglandin E concentrations (r = -0.88; P less than 0.001) in ACTH treated animals. These data suggest a role for prostaglandin E in the diminution of fetal breathing movements prior to the onset of labour.     

Effects of prostaglandins on fetal breathing do not involve peripheral chemoreceptors

In sheep, prostaglandin (PG) E2 inhibits fetal breathing movements and meclofenamate, a PG synthetase inhibitor, causes a marked stimulation of fetal breathing movements; the site of action of these agents is not known. To determine whether these effects are mediated through the peripheral chemoreceptors, we studied 13 fetal sheep at gestational ages of 127 to 138 days. Seven fetuses had bilateral section of the carotid sinus and vagus nerves (denervated); six had sham operations. Beginning at least 6 days after the operation, we infused PGE2 (0.6 microgram X kg-1 X min-1) into five denervated and five sham-operated fetuses and meclofenamate (0.4 mg X kg-1 X h-1) into six denervated and four sham-operated fetuses. Infusions averaged 20 h in duration. During preinfusion control periods, the incidence of fetal breathing movements (% of time) was lower in denervated than in sham-operated fetuses (18.9% vs. 31.5%; P less than 0.005). In both groups, the incidence of fetal breathing movements was decreased by PGE2 and was increased by meclofenamate; when expressed as absolute values, the magnitude of the changes with both agents was greater in sham-operated fetuses than denervated fetuses. However, the effects were similar in both groups when the changes were expressed as a percent of the respective control values. The incidence of fetal breathing movements (% of control) was decreased by PGE2 to 25.4% in denervated and to 28.2% in sham-operated fetuses and was increased by meclofenamate to 297.3% in denervated and to 304.0% in sham-operated fetuses.(ABSTRACT TRUNCATED AT 250 WORDS)     

Decrease in plasma prostaglandin E2 is not essential for the establishment of continuous breathing at birth in sheep

Depression of prostanoid concentrations by indomethacin induces continuous breathing in fetal sheep, but it is not known whether this is associated with changes in fetal behaviour. Furthermore, the relationship between changes in prostaglandin E2 (PGE2) concentration after delivery and the appearance of continuous breathing has not been examined. We hypothesized that the decrease in fetal PGE2 by infusion of indomethacin would induce continuous breathing and a change in behaviour such that the fetus should come to resemble a newborn lamb; and coinciding with the establishment of continuous breathing at birth, PGE2 concentrations would decrease to a critical level below that present in the fetus. We found that continuous breathing in fetal sheep induced by infusion of indomethacin was related to a decrease in PGE2 from 436 +/- 114 to 189 +/- 73 pg/ml (P less than 0.005) but that this was not associated with fetal wakefulness. In addition, measurements of carotid arterial PGE2 concentrations showed that the beginning of continuous breathing after birth occurred at a plasma concentration of PGE2 of 1245 +/- 260 pg/ml, a value about three times higher than the 422 +/- 53 pg/ml measured in the fetus during breathing activity. Together these findings suggest that PGE2 is not primarily involved in the establishment of continuous breathing at birth.     

Fetal and neonatal breathing movements in man after betamethasone.

Nineteen women in the last trimester of pregnancy were treated with betamethasone 12 mg daily for three consecutive days. The fetal breathing movements were monitored by an ultrasonic method before and after the treatment and the patterns of movements were compared to those in controls. In seven treated cases the recording of breathing was performed also in the neonatal period. Betamethasone in the given dose, which causes pronounced alterations in the fetal corticosteroid balance, does not influence the pattern of breathing movements in the fetus and newborn. Although corticosteroids are known to affect several functions of CNS, the maturation of mechanisms regulating breathing movements in the perinatal period is apparently not accelerated by betamethasone.     

Indomethacin reversal of ethanol-induced suppression of ovine fetal breathing movements and relationship to prostaglandin E2

The effects of indomethacin on the ethanol-induced suppression of fetal breathing movements and fetal arterial plasma and cerebrospinal fluid (CSF) PGE2 concentrations and maternal arterial plasma PGE2 concentration were determined in the near-term fetal lamb. Eight conscious instrumented pregnant ewes (between 130 and 133 days of gestation; term, 147 days) received 1-h maternal intravenous infusion of 1 g ethanol/kg total body weight, and the fetus received 6-h intravenous infusion of indomethacin (1 mg/h per kg fetal body weight) commencing 30 min later. Serial fetal and maternal arterial blood samples (n = 8) and fetal CSF samples (n = 5) were collected at selected times throughout the 12-h study for the determination of PGE2 concentration. Fetal breathing movements were monitored continuously throughout the experimental period. Maternal ethanol infusion resulted in initial suppression (P less than 0.05) of fetal breathing movements for 2 h below pretreatment value, followed by a rapid increase in the incidence of fetal breathing movements after the onset of fetal indomethacin treatment. Fetal and maternal plasma PGE2 concentrations and fetal CSF PGE2 concentration were increased (P less than 0.05) above the pre-infusion value during the administration of ethanol and 1 h thereafter. Fetal indomethacin treatment suppressed (P less than 0.05) to undetectable levels fetal plasma and CSF PGE2 concentrations, which then became similar (P greater than 0.05) to pretreatment by 12 h. There was a positive correlation between fetal plasma and CSF PGE2 concentrations. There was an inverse correlation between the incidence of fetal breathing movements and fetal CSF PGE2 concentration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regulation of breathing at birth

The factors which regulate the transition to lung gas exchange in the newborn are not well understood. The transition begins within seconds of birth with the newborn's first breath and is largely complete by 30 min of age at which time breathing is continuous, and arterial blood gas tensions and pH approach stable newborn values. Experiments indicate that sensory stimulation caused by cutaneous cooling or sciatic nerve stimulation can result in the initiation of breathing within seconds. Thus, massive sensory stimulation of the newborn caused by labour and delivery probably plays an important role in promoting the rapid onset of lung ventilation. Any delay in the onset of lung gas exchange causes a rise in arterial PCO2 and fall in pH which would stimulate breathing probably via stimulation of the central chemoreceptors. Since an impairment of CO2 elimination is usually observed after birth, a rise in arterial PCO2 likely stimulates breathing in the newborn. However, this impairment is transient and is usually corrected within 30 min to 2 h of age. Recent experiments suggest that placental perfusion inhibits the fetal central respiratory system and that this effect may be mediated by a placentally-produced respiratory inhibitor. Thus, withdrawal of a respiratory inhibitor from the circulation may play an important role in maintaining breathing in the newborn after sensory stimulation wanes and arterial PCO2 returns to normal fetal levels.     

Almitrine inhibits breathing movements in fetal sheep in utero

Whilst hypoxia stimulates fetal peripheral chemoreceptors, fetal breathing movements do not increase as hypoxia also has central effects. We wondered whether specific stimulation of the arterial chemoreceptors by almitrine would produce a stimulation of fetal breathing movements. When almitrine was given to 5 intact and 3 peripherally-chemodenervated fetal sheep in utero, fetal breathing movements rapidly ceased for 1-12 h. There was also a decrease in the amount of time spent in low voltage electrocortical activity. The effects of almitrine are therefore similar to those of hypoxia, and are independent of the peripheral chemoreceptors. Thus it may be a valuable tool in the study of the control of fetal breathing.