Induction of fetal breathing by metabolic acidemia and its effect on blood flow to the respiratory muscles

Physiological control of fetal heart rate variability during labour: Implications and controversies

The interpretation of fetal heart rate (FHR) patterns is the only available method to continuously monitor fetal wellbeing during labour. One of the most important yet contentious aspects of the FHR pattern is changes in FHR variability (FHRV). Some clinical studies suggest that loss of FHRV during labour is a sign of fetal compromise so this is reflected in practice guidelines. Surprisingly, there is little systematic evidence to support this observation. In this review we methodically dissect the potential pathways controlling FHRV during labour-like hypoxaemia. Before labour, FHRV is controlled by the combined activity of the parasympathetic and sympathetic nervous systems, in part regulated by a complex interplay between fetal sleep state and behaviour. By contrast, preclinical studies using multiple autonomic blockades have now shown that sympathetic neural control of FHRV was potently suppressed between periods of labour-like hypoxaemia, and thus, that the parasympathetic system is the sole neural regulator of FHRV once FHR decelerations are present during labour. We further discuss the pattern of changes in FHRV during progressive fetal compromise and highlight potential biochemical, behavioural and clinical factors that may regulate parasympathetic-mediated FHRV during labour. Further studies are needed to investigate the regulators of parasympathetic activity to better understand the dynamic changes in FHRV and their true utility during labour. This article is protected by copyright. All rights reserved.

The role of CO(2) and central chemoreception in the control of breathing in the fetus and the neonate

Central chemoreception is active early in development and likely drives fetal breathing movements, which are influenced by a combination of behavioral state and powerful inhibition. In the premature human infant and newborn rat ventilation increases in response to CO(2); in the rat the sensitivity of the response increases steadily after ∼P12. The premature human infant is more vulnerable to instability than the newborn rat and exhibits periodic breathing that is augmented by hypoxia and eliminated by breathing oxygen or CO(2) or the administration of respiratory stimulants. The sites of central chemoreception active in the fetus are not known, but may involve the parafacial respiratory group which may be a precursor to the adult RTN. The fetal and neonatal rat brainstem-spinal-cord preparations promise to provide important information about central chemoreception in the developing rodent and will increase our understanding of important clinical problems, including The Sudden Infant Death Syndrome, Congenital Central Hypoventilation Syndrome, and periodic breathing and apnea of prematurity.

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.

Influence of fetal breathing movements on pulmonary hemodynamics in fetal sheep

During fetal development, pulmonary vascular resistance (PVR) is high, and, as a result, blood flow through the fetal lungs is low. Although PVR markedly decreases at the time of birth, the factors that regulate pulmonary blood flow (PBF) and PVR before and immediately after birth are not clear. Our aim was to examine the relationship between episodes of fetal breathing movements (FBM) and pulmonary hemodynamics during late gestation to further understand the relationship among lung luminal volume, phasic changes in intrapulmonary pressure, and PVR before birth. In chronically catheterized fetal sheep (120-128 d gestation; n = 5; term approximately 147 d), PBF and PVR were measured during periods of FBM and apnea. Episodes of FBM were divided into periods of accentuated (amplitude of >3.5 mm Hg change in tracheal pressure) and nonaccentuated periods of FBM. During accentuated episodes of FBM, mean PBF was increased to 159.5 +/- 23.4% (p < 0.0025) of the preceding apneic period and was associated with a 19.1 +/- 5.2% reduction in PVR. In addition, during accentuated episodes of FBM, the retrograde flow of blood through the left pulmonary artery was reduced to 90.1 +/- 1.0% of the preceding apneic period, which most likely contributed to the increase in mean PBF at this time. Although a change in PBF and PVR could not be detected during nonaccentuated FBM, compared with the preceding apneic period, PBF was linearly and positively correlated with the amplitude (change in pressure) of FBM. We conclude that PVR is decreased and PBF is increased during accentuated episodes of FBM, possibly as a result of phasic reductions in intrapulmonary pressures.

Respiratory muscle blood flow in the fetal lamb during apnoea and breathing

We measured blood flow to the respiratory muscles of the fetal lamb using the radioactively-labelled microsphere technique in order to assess whether fetal breathing is an energetically costly activity as has been reported. Diaphragm flow ranged from 6.4-35.2 ml.min-1.100 g-1 during fetal apnoea and rose to 21.1-615 ml.min-1.100 g-1 during fetal breathing (P < 0.02; n = 7). Parasternal muscle flow also increased significantly (P < 0.02) between fetal apnoea and breathing while external and internal intercostal flows did not change. Expressed as a percentage of cardiac output the diaphragm received 0.08-0.28% during apnoea and 0.22-2.2% during fetal breathing. Neither placental blood flow nor fetal O2 consumption increased significantly between fetal apnoea and breathing. We conclude that the levels of perfusion required by the respiratory muscles for breathing in the fetus are inconsistent with fetal breathing costing a large proportion of the fetal O2 budget.

The effects of acute alcohol intoxication on biophysical activities: a case report

A computer-assisted biophysical assessment was performed in a woman with chronic alcoholism at 37 weeks' gestation. She was first seen in a state of acute alcohol intoxication (322 mg/dl). Although fetal breathing movement incidence was normal (30%), fetal tachypnea (67 breaths per minute) and decreased fetal body movements (0.11%) were seen. Because these findings differ from those previously reported with regard to normal pregnancies exposed to lower levels of alcohol, their implications are discussed.

Stimulation of breathing movements by L-5-hydroxytryptophan in fetal sheep during normoxia and hypoxia

1. In fetal lambs in late gestation, systemic infusion of L-5-hydroxytryptophan (L-5-HTP) during normoxia greatly increases the incidence of fetal breathing movements (FBM) and high-voltage electrocortical activity (HV ECoG). It also induces FBM during HV ECoG and increases blood pressure. To investigate its mechanism of action, L-5-HTP was administered in conjunction with the 5-hydroxy-tryptamine (5-HT) antagonists ketanserin or cyproheptadine. L-5-HTP was also infused with or without the antagonists during hypoxia, to test whether it would overcome the inhibition of FBM by hypoxia. 2. When L-5-HTP was given in normoxia, cyproheptadine blocked and ketanserin reduced the increase in blood pressure, both drugs blocked the stimulation of FBM, but neither drug prevented the induction of prolonged episodes of HV ECoG. 3. In hypoxia, L-5-HTP similarly stimulated FBM. This effect was also blocked by cyproheptadine and was delayed by ketanserin. 4. The antagonism of the effects of L-5-HTP on blood pressure and the incidence of FBM in normoxia and hypoxia is consistent with the action of L-5-HTP via 5-HT receptors. At present there is no clear explanation of the mechanism by which L-5-HTP induces HV ECoG.

The effect of carbonic anhydrase inhibition on breathing movements and electrocortical activity in fetal sheep

Fetal breathing movements (FBM) indicated by repetitive negative intrathoracic pressures and biparietal electrocorticograms (ECoG) were recorded from 8 fetal sheep for 3 h before (control) and 3 h after the administration of a carbonic anhydrase inhibitor, acetazolamide. FBM and the low voltage (LV) ECoG state occurred 36 +/- 5% (SEM) and 60 +/- 3% of the control period, respectively. Virtually no FBM occurred during high voltage (HV) ECoG while in 57 +/- 6% of the LV state the fetuses were making FBM. The peak magnitude of the negative intrathoracic (tracheal) pressure deflections was 4 +/- 1 Torr. Following acetazolamide the incidence of FBM rose to 53 +/- 4% (P less than 0.01) but there was no significant change in the incidence of the LV state (58 +/- 3%). Most of the increase in the incidence of FBM remained confined to periods of LV ECoG activity so that an increased proportion of this state (88 +/- 2%, P less than 0.001) was occupied with respiratory efforts. The amplitude of the FBM also increased to 8 +/- 1 Torr (P less than 0.05). The increased incidence and depth of FBM is most likely due to an elevated hydrogen ion concentration and differs from a fetal respiratory acidosis induced by increasing the inspired CO2 fraction to the ewe in that the respiratory stimulation induced by acetazolamide is not associated with an increased incidence of the permissive LV ECoG state.

Central stimulation of breathing movements in fetal lambs by prostaglandin synthetase inhibitors

In unanaesthetized fetal lambs at 125-135 days gestation in utero central acidosis caused by perfusion of the cerebral ventricular system with a solution containing less than 1 mM-HCO3- (cerebrospinal fluid (c.s.f.) pH 6.98) or intravenous infusion of ammonium chloride (c.s.f. pH 7.1) produced an increase in the depth and frequency of episodic breathing but no change in electrocortical activity, heart rate or arterial pressure. Administration of prostaglandin synthetase inhibitors, sodium meclofenamate (0.8-10 mg/kg I.V. or 0.6-2.6 mg/kg intracerebrally) or acetylsalicylic acid (6.7 mg/kg I.V.) caused prolonged episodes of fetal breathing during low and high voltage electrocortical activity, with a large increase in breath amplitude. Blood gas values, heart rate, blood pressure, electrocortical activity and eye movements were not altered. In fetuses whose brain stems had been sectioned in the upper pons or the inferior colliculus, sodium meclofenamate induced prolonged deep breathing. Intravenous prostaglandin E2 abolished the continuous breathing induced by meclofenamate, but not breathing movements enhanced by hypercapnia or hypoxia. It is concluded that the central chemoreceptors respond to acidosis in near-term lamb fetuses qualitatively as in adult animals. Secondly, the results suggest that prostaglandin E2 and the inhibitors of prostaglandin synthesis also act centrally in the lower pons or medulla to modulate fetal breathing.

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.

Cerebral blood flow and metabolism during morphine-induced stimulation of breathing movements in fetal lambs

Brain blood flow increased in the fetal lamb during morphine-induced stimulation of breathing. The increase in flow was 60% in the cerebral hemispheres and the cerebellum, and 100% in the midbrain plus rhinencephalon, pons, medulla and cervical spinal cord. Oxygen content of arterial blood decreased in all experiments and the arterial carbon dioxide tension increased in all but one of the experiments. The increase in cerebral blood flow observed is predicted by the changes in arterial oxygen content and carbon dioxide tension. Cerebral oxygen consumption and glucose utilization were not changed by morphine treatment. These results suggest that there is no direct effect of morphine upon cerebral blood flow and metabolism in the fetal lamb during morphine-stimulated breathing.

Hemodynamic responses to asphyxia in spontaneously breathing newborn term and premature lamb

The comparative effects of asphyxia were studied in spontaneously breathing premature and full term newborn lambs. The premature lambs exhibited significantly higher baseline blood flow to several organs. Asphyxia induced similar changes in regional organ blood flow in preterm and term newborn lambs. Regional differences were found in baseline brain blood flow with an increase from cephalad to caudad. Asphyxia resulted in preferential increases in blood flow to the lower brain structures (cerebellum, medulla, midbrain, and spinal cord). Blood flow to most organs returned to baseline 140 min after recovery from asphyxia.