Fetal breathing movements in response to maternal inhalation of 5% carbon dioxide

Chronic hypoxia in pregnant mice impairs the placental and fetal vascular response to acute hypercapnia in BOLD-MRI hemodynamic response imaging

INTRODUCTION

Brief hypercapnic challenge causes acute placental hypoperfusion with fetal brain sparing on BOLD-MRI. We hypothesize that this non-invasive imaging strategy can distinguish between normal pregnancy and chronic placental hypoperfusion (using the maternal hypoxia model).

METHODS

Eighteen pregnant female ICR mice were randomized to three groups: normoxia, late-onset hypoxia (12%O₂;E13.5-17.5) and early-onset hypoxia (12%O₂;E10.5-17.5). On E17.5, animals were imaged in a 4.7-T Bruker-Biospec MRI scanner. Fast coronal True-FISP was performed to identify organs of interest (placenta and fetal heart, liver and brain). BOLD-MRI was performed at baseline and during a 4-min hypercapnic challenge (5%CO₂). %-change in placental and fetal signal was analyzed from T2*-weighted gradient echo MR images. Following MRI, fetuses and placentas were harvested, weighed and immuno-stained.

RESULTS

In normoxic mice, hypercapnia caused reduction in BOLD-MRI signal in placenta (-44% ± 7%; p < 0.0001), fetal liver (-32% ± 7%; p < 0.0001) and fetal heart (-54% ± 12%; p < 0.002), with relative fetal brain sparing (-12% ± 5%; p < 0.0001). These changes were markedly attenuated in both hypoxia groups. Baseline fetal brain/placenta SI ratio was highest in normoxic mice (1.14 ± 0.017) and reduced with increasing duration of hypoxia (late-onset hypoxia: 1.00 ± 0.026; early-onset hypoxia: 0.91 ± 0.016; p = 0.02). Both hypoxic groups exhibited fetal growth restriction with prominent placental glycogen-containing cells, particularly in early-onset hypoxia. There was increased fetal neuro- and intestinal-apoptosis in early-onset hypoxia only.

CONCLUSIONS

BOLD-MRI with brief hypercapnic challenge distinguished between normoxia and both hypoxia groups, while fetal neuroapoptosis was only observed after early-onset hypoxia. This suggests that BOLD-MRI with hypercapnic challenge can identify chronic fetal asphyxia before the onset of irreversible brain injury.

BOLD-MRI demonstrates acute placental and fetal organ hypoperfusion with fetal brain sparing during hypercapnia

INTRODUCTION

We evaluated changes in placental and fetal hemodynamics in rodents during acute hypercapnia using BOLD-MRI and Doppler ultrasound.

METHODS

Animals were anesthetized with pentobarbital and, in consecutive 4-min periods, breathed: air, 21%O2:5%CO2, and 95%O2:5%CO2.

BOLD-MRI

Pregnant ICR mice (n = 6; E17.5) were scanned in a 4.7-T Bruker Biospec spectrometer. Placenta and fetal liver, heart and brain were identified on True-FISP images. Percent change in signal intensity (SI) were analyzed every 30 s from T2*-weighted GE images (TR/TE = 147/10 ms). Doppler: Pregnant Wistar rats (n = 6; E18-20) were anesthetized with pentobarbital and received abdominal Doppler ultrasound. Umbilical artery pulsatility index (PI) and fetal heart rate were assessed at baseline and after two minutes of both hypercapnic challenges.

RESULTS

BOLD-MRI: Normoxic-hypercapnia caused immediate marked reduction in SI in placenta (-44% ± 5.5; p < 0.001), fetal liver (-32% ± 6.4; p < 0.001) and fetal heart (-53% ± 9.9; p < 0.001) but only minor changes in fetal brain (-13% ± 3.4; p < 0.01), suggesting fetal brain sparing. Doppler: Normoxic-hypercapnia caused a marked increase in umbilical artery PI (+27.4% ± 7.2; p < 0.001) and a reduction in fetal heart rate (-48 bpm; 95%CI -9.3 to -87.0; p = 0.02), suggesting acute fetal asphyxia.

CONCLUSIONS

Brief maternal hypercapnic challenge caused BOLD-MRI changes consistent with acute placental and fetal hypoperfusion with fetal brain sparing. The same challenge caused increased umbilical artery PI and fetal bradycardia on Doppler ultrasound, suggestive for acute fetal asphyxia. BOLD-MRI may be a suitable noninvasive imaging strategy to assess placental and fetal organ hemodynamics.

Metabolism, temperature, and ventilation

In mammals and birds, all oxygen used (VO2) must pass through the lungs; hence, some degree of coupling between VO2 and pulmonary ventilation (VE) is highly predictable. Nevertheless, VE is also involved with CO2 elimination, a task that is often in conflict with the convection of O2. In hot or cold conditions, the relationship between VE and VO2 includes the participation of the respiratory apparatus to the control of body temperature and water balance. Some compromise among these tasks is achieved through changes in breathing pattern, uncoupling changes in alveolar ventilation from VE. This article examines primarily the relationship between VE and VO2 under thermal stimuli. In the process, it considers how the relationship is influenced by hypoxia, hypercapnia or changes in metabolic level. The shuffling of tasks in emergency situations illustrates that the constraints on VE-VO2 for the protection of blood gases have ample room for flexibility. However, when other priorities do not interfere with the primary goal of gas exchange, VE follows metabolic rate quite closely. The fact that arterial CO2 remains stable when metabolism is changed by the most diverse circumstances (moderate exercise, cold, cold and exercise combined, variations in body size, caloric intake, age, time of the day, hormones, drugs, etc.) makes it unlikely that VE and metabolism are controlled in parallel by the condition responsible for the metabolic change. Rather, some observations support the view that the gaseous component of metabolic rate, probably CO2, may provide the link between the metabolic level and VE.

Maternal carbon dioxide level during labor and its possible effect on fetal cerebral oxygenation: mini review

During pregnancy, and especially during labor, the maternal carbon dioxide level declines considerably. Maternal carbon dioxide levels show a close relation with fetal carbon dioxide levels. The latter affects fetal cerebral oxygenation by regulating cerebral blood flow and shifting the oxyhemoglobin dissociation curve. In addition, maternal hypocapnia appears to impair placental oxygen transfer. Thus, maternal hyperventilation may interfere with optimal fetal cerebral oxygenation. Here, we provide a brief overview of the literature relevant to this issue.

The management of the critically ill obstetric patient

Hypertensive disorders, postpartum hemorrhage, and sepsis are the most common indications for intensive care unit admission among obstetric patients. In general, ICU mortality is low, and better than would be predicted using available mortality prediction tools. Provision of care to this special population requires an intimate understanding of physiologic changes that occur during pregnancy. Clinicians must be aware of the way various diagnostic and treatment choices can affect the mother and fetus. Most clinically necessary radiographic tests can be safely performed and fall under the maternal radiation exposure limit of less than 0.05 Gray (Gy). Careful attention must be paid to acid-base status, oxygenation, and ventilation when faced with respiratory failure necessitating intubation. Cesarean delivery can be justified after 4 minutes of cardiac arrest and may improve fetal and maternal outcomes. The treatment of obstetric patients in the ICU introduces complexities and challenges that may be unfamiliar to many critical care physicians; teamwork and communication with obstetricians is crucial.

Fetal breathing is associated with increased umbilical blood flow

OBJECTIVES

In humans, fetal breathing movements affect blood velocities in the umbilical vein and artery, but it is not known whether fetal respiratory activity is associated with increased fetal blood flow through the placenta. We therefore tested this hypothesis in the present study.

METHODS

One-hundred and ten women with low-risk singleton pregnancies were each examined three times by ultrasound during the second half of pregnancy. Fetal heart rate, umbilical artery blood velocity, umbilical vein diameter and blood velocity, and umbilical blood flow at the placental end were determined during fetal rest and fetal respiratory movements.

RESULTS

Based on 330 observations obtained during fetal rest or breathing activity, no difference was found in the mean fetal heart rate (beats per minute (bpm)) during rest compared with breathing (142 bpm vs. 142 bpm, respectively). Although fetal breathing affected the umbilical artery waveform, there was no difference in the mean time-averaged maximum velocity between rest and breathing: 26.6 (95% CI, 25.1-28.3) cm/s vs. 28.9 (95% CI, 27.2-30.7) cm/s, respectively. The umbilical vein was 27% greater in cross-sectional area and the blood velocity 9% higher during breathing, resulting in a 42% increase in mean umbilical blood flow: 121.8 (95% CI, 109.5-135.0) mL/min at rest vs. 173.0 (95% CI, 158.0-188.6) mL/min during breathing. Venous velocity was calculated from recordings of mean duration 3.7 s at rest and 6.2 s of respiratory activity. Gestational age did not influence the relationship.

CONCLUSION

Fetal breathing is associated with increased umbilical blood flow during the second half of pregnancy. Umbilical vein distension during breathing suggests active endocrine regulation.

Intiation of lung ventilation at birth

The newborn baby draws its first postnatal breath either during or within seconds of delivery. Within minutes, a regular breathing rhythm is established and this remains virtually continuous for the remainder of postnatal life. The mechanisms responsible for these sudden, dramatic and vital changes in the respiratory system at birth are only partially understood. Since fetuses make intermittent breathing movements long before birth, understanding the control of the fetal respiratory system may be essential to understanding the rapid onset of respiratory efforts at delivery. In this review the stimuli present at birth will be considered and, based on our current understanding of the fetal and neonatal respiratory control systems, those factors which are likely to play an important role in the initiation of lung ventilation at this time will be examined. Normal respiratory events in the early postnatal period will be concentrated on, but it is important to recognize that in some cases problems occur: a neonate may fail to initiate breathing efforts rapidly, or an apparently healthy premature neonate may suddenly stop breathing a few days after birth. In both cases, clinical intervention may be required to maintain adequate gas exchange and to prevent brain damage or death. Clearly, greater knowledge of respiratory control during this critical time of life would assist in the development of more appropriate and successful treatments for these life-threatening disorders.

Fetal cerebral oxygenation: the role of maternal hyperoxia with supplemental CO2 in sheep

OBJECTIVE

We tested the hypothesis that supplemental CO2 can enhance the effect of maternal oxygen administration on fetal cerebral oxygenation.

STUDY DESIGN

In near-term fetal sheep (n = 6), we instrumented the cerebral cortex with tissue PO2-laser Doppler flow probes, and placed arterial catheters. Following a 30-minute control period, the ewe breathed 50% O2 for 15 minutes, followed by added 6% CO2 for 15 minutes. We examined fetal cortical tissue PO2, cerebral blood flow (CBF), and fetal and maternal blood gases and related variables.

RESULTS

In response to maternal O2 administration, fetal arterial PO2, O2 content, cerebral O2 delivery, and cortical tissue PO2 increased significantly. In response to supplemental CO2 inhalation, fetal cortical tissue PO2 increased further. Fetal CBF also increased in response to the elevated arterial CO2 level.

CONCLUSION

CO2 supplementation of maternal O2 administration enhanced fetal cerebral oxygenation. In contrast, it was considered that during labor maternal hyperventilation with hypocapnia may blunt the effect of maternal O2 inhalation.

Predictive value of pulse oximetry and fetal scalp blood pH in the case of meconium-stained amniotic fluid

OBJECTIVE

To compare the predictive value of intrapartum fetal pulse oximetry to that of fetal scalp blood pH for an abnormal neonatal outcome in the case of thick meconium.

STUDY DESIGN

A prospective multicenter observational study was performed from June 1994 to November 1995. Fetal oxygen saturation was monitored using a Nellcor N-400 fetal pulse oximeter in case of abnormal FHR. The last readings of fetal oxygen saturation and fetal scalp blood pH before birth were used to assess the ability of both techniques to predict an abnormal neonatal status.

RESULTS

At a 30% cutoff, the negative predictive value of fetal oxygen saturation was not altered in case of meconium when compared to clear amniotic fluid (79 and 83%, respectively). Fetal scalp blood pH at a 7.20 threshold had a poor negative predictive value in case of meconium when compared to clear amniotic fluid (56% versus 88%, respectively). The receiver operator characteristic curve showed similar performance of fetal scalp blood pH and pulse oximetry in cases with clear amniotic fluid. In cases with meconium, the performance of fetal scalp blood pH was poor, whereas that of pulse oximetry remained unchanged. In three cases with meconium below the vocal cords, a drop in fetal oxygen saturation was observed during labor whereas fetal scalp blood pH remained within normal values.

CONCLUSION

The predictive value of fetal scalp blood pH is poor in case of meconium, whereas the predictive value of pulse oximetry seems to be unchanged in this situation.

Managing patients with meconium-stained amniotic fluid

Meconium-stained amniotic fluid might signify underlying acute or chronic fetal hypoxia with adverse perinatal outcome, especially if associated with cardiotocographic abnormality. Management requires awareness of this potential risk, appropriate intrapartum care and a combined obstetricneonatal approach. Amnioinfusion can be an effective preventative measure.

Forty years of obstetric ultrasound 1957-1997: from A-scope to three dimensions

In this article, we record the history of obstetric ultrasound as it developed worldwide in the second half of the twentieth century. The technological advances during this period saw the evolution of equipment from the original adapted metal flaw detectors producing a simple A-scan to the modern, purpose built, real-time colour flow machines with three-dimensional capability (Fig. 1). Clinically, ultrasound began as a research tool, but the poor quality of the images led to the ridicule of many of the early investigators. However, because of their perseverance, ultrasound developed into an imaging modality providing immense diagnostic capabilities and facilitating with precision many invasive procedures, diagnostic and therapeutic, both of which have made significant contributions to patient care. In this history, we recall the people, the personalities, and the problems they encountered during the development of ultrasound and how these problems were resolved, so that ultrasound now is available for use in the care of pregnant women throughout the developed world.

The effect of hypercapnia and hypercapnia associated with central cooling on breathing in unanesthetized fetal lambs

In utero breathing activity is present periodically, but it must become continuous at birth. We investigated the effect of hypercapnia and of hypercapnia combined with central cooling on fetal breathing in seven chronically instrumented fetal lambs of 131-134-d gestation, using an extracorporeal membrane oxygenation (ECMO) system to control fetal blood gases and fetal temperature. During fetal hypercapnia (from a partial pressure of arterial CO2 (Paco2) 6.18 +/- 0.11 kPa to Paco2 7.39 +/- 0.15 kPa) frequency, amplitude, and incidence of fetal breathing movements during low voltage electrocortical activity (LV ECoG) increased significantly compared with isocapnic control on ECMO, but breathing remained absent during high voltage electrocortical activity (HV ECoG). During hypercapnia accompanied by central cooling (Paco2 7.90 +/- 0.13 kPa, temperature decreased by 2.1 degrees C) there were similar changes in fetal breathing movements during LV ECoG, but in four out of seven fetuses fetal breathing movements continued throughout HV ECoG. Hypercapnia accompanied by central cooling can thus override the inhibitory effects of HV ECoG on fetal breathing movements. This may be due to changes in sensitivity to CO2 produced by an increase in afferent input to the CNS.

Regulation of breathing and perception of dyspnea in healthy pregnant women

STUDY OBJECTIVE

To examine if the perception of dyspnea during normal pregnancy may be related to an inappropriate ventilatory response to the increased metabolic rate, due to a higher chemosensitivity.

PATIENTS AND INTERVENTIONS

At weeks 12, 24, and 36 of gestation and 4 months after delivery, 11 healthy pregnant women with dyspnea and 12 asymptomatic pregnant women were studied. Progesterone plasma levels, lung volumes, diffusion capacity, maximal respiratory pressures, rest oxygen uptake, breathing pattern, and mouth occlusion pressure (P0.1) were measured. Progressive isocapnic hypoxic stimulation and progressive hyperoxic hypercapnic stimulation were performed.

RESULTS

Oxygen ventilation equivalent during pregnancy was significantly higher for the dyspneic group than for nondyspneic pregnant women. Dyspneic patients exhibited greater minute ventilation, tidal volume, and P0.1 than the nondyspneic group. The mean values of ventilatory and P0.1 slopes to hypoxia and CO2 during pregnancy were significantly greater in the patients with dyspnea than in asymptomatic subjects. These changes were not due to differences in progesterone plasma levels. A significant relation among the Borg score, inspiratory drive, and chemosensitivity was found.

CONCLUSIONS

In some pregnant women, a higher sensitivity to CO2 and hypoxia may induce excessive ventilation to metabolic demand, which would contribute to dyspnea.

Patterns of development in fetal breathing activity in the latter third of gestation of the baboon

Patterns of fetal breathing activity were examined in a longitudinal study of the fetal baboon over the latter third of gestation. More than 1400 h of recorded tracheal fluid pressure in 16 or 24 h records from seven fetuses over a range in gestation from 121 to 172 days (term, 175-180 days) were analyzed. In these 81 records, there was a high degree of variability in the percent of time spent breathing by the fetuses (range, 14-83%) with no apparent influence of gestational age (mean +/- S.D., 45.6 +/- 17.6%). Nonetheless, the mean amplitude of fetal breaths increased with gestation from absolute values of about 5-10 mmHg (r = 0.73, P < 0.001) and the mean inspiratory time interval increased from about 0.45-0.55 s (r = 0.40, P < 0.001). During epochs of breathing, the mean rate decreased from about 42-36 breaths per min (r = -0.54, P < 0.001) and the indices of both short term (r = -0.54, P < 0.001) and long term (r = -0.73, P < 0.001) variability in rate decreased. These results demonstrate a clearly defined pattern of development in the breathing activity of the fetal baboon which is comparable to the pattern described for the human fetus in the third trimester of gestation. These similarities suggest that the progressive functional maturation of the mechanisms generating respiratory patterns are comparable among primate species.

Effects of maternal administration of 3% carbon dioxide on umbilical artery and fetal renal and middle cerebral artery Doppler waveforms

OBJECTIVE

The null hypothesis is that umbilical, middle cerebral, and renal artery pulsed Doppler velocity waveforms in the normal term fetus may be affected during short-term maternal inhalation of 3% carbon dioxide gas mixture.

STUDY DESIGN

Seventy-two observations were made on 14 term fetuses before and during maternal 3% carbon dioxide gas mixture inhalation. The umbilical, middle cerebral, and renal arteries of these fetuses were sampled with pulsed Doppler velocity waveforms and recorded on a strip chart at a preset speed of 50 mm/sec. Doppler waveforms were analyzed for differences in the systolic peak to end-diastolic velocity ratio for these three vascular beds. Peak flow velocity and time velocity integral were also analyzed for the cerebral and renal vascular beds. The data were analyzed with the paired t test.

RESULTS

A significant decrease in the systolic-to-diastolic-velocity ratio of the middle cerebral artery occurred with 3% carbon dioxide inhalation (p < 0.02). The other vascular beds had no demonstrable change.

CONCLUSION

Transient maternal breathing of 3% carbon dioxide gas mixture selectively causes a decrease in resistance in the fetal cerebral circulation.

Meconium aspiration syndrome: reflections on a murky subject

Meconium-stained amniotic fluid occurs in approximately 12% of live births. In approximately one third of these infants meconium is present below the vocal cords. However, meconium aspiration syndrome develops in only 2 of every 1000 live-born infants. Ninety-five percent of infants with inhaled meconium clear the lungs spontaneously. Recent investigations have suggested that a reexamination of our assumptions about the etiology of meconium aspiration syndrome is in order. Several authors have provided evidence that support the hypothesis that it is not the inhaled meconium which produces the primary pathologic condition of meconium aspiration syndrome but rather it is fetal asphyxia that is the etiologic agent. Asphyxia in utero produces pulmonary vasospasm and hyperreactivity of the pulmonary vessels. With severe asphyxia the fetal lungs undergo pulmonary vascular damage with pulmonary hypertension. The damaged lungs are then unable to clear the meconium. In the most severe cases there is right-to-left shunting and persistent fetal circulation with subsequent fetal death. The incidence of meconium aspiration may thus be essentially unaffected by current obstetric and pediatric interventions at birth. For the asphyxiated or distressed infant we recommend suctioning at birth and tracheal intubation. In the healthy fetus observation may be sufficient.

Growth and motor development in fetuses of women with type-1 diabetes. III. First trimester quantity of fetal movement patterns

Delayed emergence of movement patterns and disturbances in the development of behavioural states are often observed in the fetuses of diabetic women. The developmental course of specific fetal movement patterns was studied in 20 women with type-1 diabetes during the first trimester of pregnancy. The aim was to investigate whether maternal diabetes also has an effect on the rate of occurrence and temporal patterning of fetal movements. Real-time ultrasound recordings with a duration of 1 h were made once a week between the 7th and 17th week postmenstrual age in order to quantify fetal movements. The data were compared to those obtained in a group of uncomplicated pregnancies. The developmental trends in the occurrence of movement patterns were similar to those in the control fetuses, with the exception of startles. Before the 9th week of gestation, fetal movements occurred less frequently; this was related to the quality of maternal glucose control. After 12 weeks, the overall incidence was higher than in the control group, due to an increase in the incidence of breathing movements. The rate of breathing movements was generally slower than in the control group.

The role of carbon dioxide in the generation of human fetal breathing movements

To determine the role of carbon dioxide in the generation of fetal respiratory movements, the effect of induced maternal hypocapnia and hypercapnia on fetal breathing movements, gross body movements, and fetal heart rate was studied in 12 healthy pregnant women near term. Patients were studied for a 1-hour control period breathing room air followed by four randomized 15-minute study periods with patients breathing either room air, a prepared gas mixture with 2% or 4% carbon dioxide, or undergoing controlled hyperventilation as determined by monitoring end-tidal PCO2. The percentage of time fetal breathing movements correlated significantly with maternal end-tidal PCO2 (r = 0.62, p less than 0.01), increasing with maternal breathing of 2% and 4% carbon dioxide and decreasing with maternal hyperventilation. Fetal gross body movements, fetal heart rate, and fetal heart rate variability showed no significant changes. It is concluded that as in adults, the carbon dioxide level in fetuses is an important stimulus for the generation of respiratory movements, acting independent of a change in behavioral state. It is hypothesized that tonic carbon dioxide level input is an important determinant of fetal respiratory center drive, but little or no phasic carbon dioxide input exists because of continuous placental excretion, thus resulting in the episodic occurrence of breathing movements with changes in the fetal behavioral state.

The presence or absence of fetal breathing movements as a predictor of outcome in preterm labor

The presence or absence of fetal breathing movements may be helpful in differentiating between true and false preterm labor. We attempted to demonstrate the clinical utility of this simple ultrasonic observation in predicting short-term delivery outcome during suspected preterm labor. A total of 50 pregnancies between 26 and 34 weeks' gestation with presumed preterm labor were observed in a prospective manner. During 20 minutes of observation with real-time ultrasound at the time of admission, fetal breathing movements were observed in 33 patients and considered absent in the remaining 17 patients. In those pregnancies with absent fetal breathing movements, true labor with subsequent delivery occurred in 16 patients. Of the 33 pregnancies with fetal breathing movements present, 29 continued for greater than 48 hours. It appears that the absence of fetal breathing movement is a reliable indicator of imminent preterm delivery, irrespective of fetal membrane status (p less than 0.0001). The observed mean sensitivity and specificity of this phenomenon in predicting short-term delivery outcome are 96.6% +/- 3.3% (mean +/- SD) and 80.0% +/- 8.9%, respectively. A multivariant statistical model based on the frequency of contractions, white blood cell counts, initial cervical examination results, and premature rupture of membranes could successfully predict delivery outcome in 40% to 75% of cases. The addition of fetal breathing movement analysis to the model allowed for the correct prediction of outcome in 90% of the cases.

The effects of long- and short-term maternal caffeine ingestion on human fetal breathing and body movements in term gestations

The possible effects of long- and short-term maternal ingestion of caffeine during normal human pregnancy on the breathing and body movements of third-trimester fetuses were studied in 14 patients, selected by a dietary questionnaire, and divided into two equal groups: high consumers (greater than 500 mg/day) (group 1) and low consumers (less than 250 mg/day) (group 2). All mothers followed a standard study protocol and underwent overnight fasting; studies began with a 30-minute control period, followed by oral administration of 200 mg caffeine, and a 180-minute subsequent observation period with continued maternal fasting. Blood samples for glucose and caffeine were obtained every 30 minutes and continuous recording of fetal breathing and body movements were entered on a microcomputer for off-line analysis. The two groups were similar for all obstetric outcome features. Plasma glucose levels were similar and constant in both groups whereas caffeine levels increased significantly at 60 minutes after administration; mean plasma caffeine levels were significantly higher in group 1 than in group 2 at all intervals. Fetal breathing rates and body movement incidences were similar in both groups before and after caffeine administration. Fetal breathing movement incidence decreased significantly in group 2 but was sustained at baseline levels in group 1 throughout the study. High long-term ingestion of caffeine during pregnancy is associated with higher maternal plasma caffeine levels and fetal breathing activity when compared with low caffeine ingestion. Short-term administration of 200 mg caffeine does not appear to have a significant physiologic impact on these activities.

Fetal breathing movements and the response to carbon dioxide in patients on methadone maintenance

Fetal breathing movements were monitored on six methadone maintenance patients and ten healthy control patients, studied while breathing room air and while breathing a prepared gas mixture with 5% carbon dioxide, both before and 2 hours after either the usual daily dose of methadone or a diet drink. There was no difference before and after the diet drink in control patients, who showed a significant increase in the incidence of fetal breathing movements from 37.1% +/- 5.9% (SEM) on room air to 69.4% +/- 2.8% while breathing 5% carbon dioxide (p less than 0.01). In the patients studied on room air before receiving methadone, fetal breathing movements were significantly decreased from those of the control group, 4.7% +/- 1.2% (p less than 0.01), with a further decrease when studied after receiving methadone, 1.3% +/- 0.7%. Fetal breathing movements did increase significantly in response to 5% carbon dioxide both before and after receiving methadone; however, in both instances the incidence was significantly less that that of the control group. The findings of the present study on methadone subjects demonstrate that abnormal function of the respiratory control network is evident in utero, which may be predictive of subsequent neonatal respiratory development.

The rate and regularity of breathing movements in the normal and growth-retarded fetus

The rate and regularity of fetal breathing movements (FBM) were determined in 14 women with uncomplicated singleton pregnancies, eight of whom were between 30 and 33 weeks gestation and six between 37 and 40 weeks gestation. Similar observations were made in 19 women with pregnancies complicated by severe intrauterine growth retardation, 11 of whom were between 30 and 33 weeks and eight between 37 and 40 weeks. In normal pregnancy recordings of breath-to-breath intervals showed that FBM became more regular with advancing gestational age, and the rate [breaths/min, mean (SEM)] slowed from 57.2 (1.3) at 30-33 weeks to 47.9 (0.8) at 37-40 weeks. FBM in the growth-retarded group were regular at each gestation studied and the rate was even slower than in the normal group at term, being 41.9 (1.2) at 30-33 weeks and 41.1 (1.0) at 37-40 weeks. Hyperoxia and hypercapnia appeared to have no consistent effect on fetal breathing rate. Fasting for greater than 12 h considerably reduced the rate of FBM in the normal fetus but only marginally so in those with growth retardation. It is concluded that the pattern of FBM provides more information about the fetus than the amount of time spent breathing, particularly when growth is retarded.

The effect of induced maternal hypercapnia on gross fetal body movements

Gross fetal body movements and fetal heart rate were studied serially during the last trimester of pregnancy by real-time ultrasound in 11 healthy women during the inhalation of 4% carbon dioxide in air. We observed that induction of maternal hypercapnia during the last 10 weeks of pregnancy had no effect on the percentage of time of gross fetal body movements or on fetal heart rate accelerations.

Central chemical regulation of breathing movements in fetal lambs

In 12 chronically prepared fetal lambs between 126 and 136 days of gestation, 17 ventriculocisternal perfusions (123 microliter/min) were performed. The concentration of bicarbonate ([HCO3-]) in the mock cerebrospinal fluid (CSF) perfusate was altered to manipulate the [HCO3-] in the cerebral ventricles. These perfusions did not systematically alter fetal arterial PCO2, PO2, pH, heart rate or mean blood pressure. Fetal breathing movements (FBM) were noted as rhythmic negative intrathoracic pressures with reference to amniotic fluid pressure. The incidence of FBM was determined and expressed as the percent of the 3 h observation period during which breathing movements were present. Perfusions with mock CSF having an approximately normal [HCO3-], resulted in cisternal [HCO3-] of 23.9 +/- 0.8 (SEM) meq/L and an incidence of fetal breathing movements (FBM) of 22.1 +/- 10.0%. Perfusions with an elevated [HCO3-] in the mock CSF increased the cisternal [HCO3-] to 30.7 +/- 0.4 meq/L and lowered the incidence of FBM to 0.6 +/- 0.2%. Perfusions with a lowered [HCO3-] mock CSF decreased the cisternal [HCO3-] to 17.3 +/- 0.8 meq/L and increased the incidence of FBM to 45.7 +/- 6.8%. These perfusions did not alter the relative incidence of low voltage (LV) versus high voltage (HV) electrocortical (ECOG) state. In 4 animals, low [HCO3-] perfusions induced FBM in the normally apneic HV ECOG state. We conclude that in the near-term fetal lamb, central chemoreceptors can modulate the incidence of rhythmic respiratory efforts, that tonic afferent stimuli arising from these receptors are critical for the generation of spontaneous FBMs and that central acidosis is capable of evoking FBMs in the normally apneic HV ECOG state.

Fetal breathing movements and maternal hyperoxia in the growth retarded fetus

The effect of fetal breathing movements (FBM) of maternal breathing of 50% oxygen was examined. The procedure caused a two-fold rise in maternal arterial Po2 and did not alter FBM in normal pregnancy. In pregnancy complicated by intrauterine growth retardation (IUGR) a substantial increase in the incidence of FBM was recorded during hyperoxia (control 29.8 +/- 3.9%, hyperoxia 68.2 +/- 3.6). No such change was observed in pregnancies where IUGR was suspected but not proven at delivery.

Braxton-Hicks contractions and fetal breathing movements

Fetal breathing movements were observed for periods of approximately 100 minutes in 10 women at 36-42 weeks' gestation. Simultaneous recordings of uterine activity were made using an external tocograph. Fetal breathing movements were present at the onset of 82 Braxton-Hicks' contractions. There was a significant change in the rate of fetal breathing with a trough before the acme of the contraction, followed by a peak as uterine activity decreased. It is suggested that these changes are related to pressure on the fetus during the contraction and probably are unrelated to changes in fetal oxygenation.

Effects of glucose concentration on fetal breathing movements and electrocortical activity in fetal lambs

Tracheal pressure and electrocortical activity were recorded in 10 unanesthetized fetal lambs in utero to determine the effect on fetal breathing movements (FBMs) of fasting and a subsequent infusion of glucose. The incidence of FBMs decreased significantly from 37.9% +/- 4.3% during fed control studies to 23.6% +/- 3.6% (P less than 0.05) after 24 to 36 hours of maternal fasting, with a related fall in fetal blood glucose. A 2-hour infusion of glucose to the fetuses of fasted animals resulted in a significant increase in FBMs to 39.5% +/- 4.9%, but only to a level similar to that in the fed control studies despite a twofold increase in the fetal concentration of glucose above fed control values. Changes in the incidence of FBMs paralleled the changes in fetal low-voltage electrocortical activity with alterations in glucose. In conclusion, FBM, although correlated with the concentration of glucose, may be influenced more by fetal hypoglycemia than hyperglycemia. Glucose appears to affect the incidence of FBM by altering the incidence of low-voltage electrocortical activity.