Effect of medullary lesions, vagotomy and carotid sinus denervation on fetal breathing

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.

Placental transfusion insult in the predisposition for SIDS: a mathematical study

A difference has been observed between the newborn hearing screening tests of thirty-one SIDS cases versus control infants that survived the first year of life [Rubens DD, Vohr BV, Tucker R, O'Neil CA, Chung W. Newborn oto-acoustic emission hearing screening tests. Preliminary evidence for a marker of susceptibility to SIDS. Early Hum Dev 2008;84(4);225-9]. This study is motivated by the hypothesis that the predisposition for SIDS may be caused by inner ear and brainstem damage from a high venous pressure insult at birth that disrupts an infant's ability to detect rising CO(2) levels following the first month of life. The injury is not immediately lethal due to the persistence of fetal physiological responses during the early postnatal period [Guntheroth WG. Crib death, the Sudden Infant Death Syndrome. Armonk NY: Futura Publishing Co.; 1995. p. 291]. Elastic vessels are assumed in the umbilical vein and newborn venous circulation at the time of a potential high pressure placental transfusion insult and pulse wave propagation is simulated using the nonlinear one-dimensional equations of blood flow in elastic vessels. Peak pressures in the auricular veins increase with the amplitude and length of the umbilical surge, reaching over 60 mm Hg when two consecutive surges separated by 100 ms, of a peak pressure of 100 mm Hg, and a pulse interval of 200 ms are propagated in a network with low peripheral reflections. Our findings support the proposed mechanism for inner ear damage in SIDS and the potential benefit of a newborn hearing screening test in identifying susceptibility and early preventative measures following birth.

Metabolic and cardiorespiratory responses to hypoxia in fetal sheep: adenosine receptor blockade

8-Phenyltheophylline (PT), a potent and specific inhibitor of adenosine receptors, was infused intra-arterially into unanesthetized fetal sheep to determine the role of adenosine in hypoxic inhibition of fetal breathing. PT in normoxic fetuses increased heart rate and the incidence of low-voltage electrocortical activity, rapid eye movements (REM), and breathing. Mean breath amplitude increased by 44%. Hypoxia (preductal arterial PO2 = 14 Torr) induced a metabolic acidemia, a transient bradycardia, and hypertension while virtually eliminating REM and breathing. PT administration during hypoxia enhanced the metabolic acidemia, blocked the bradycardia and hypertension, increased the incidence of REM and breathing, and elevated mean breath amplitude. The results indicate that 1) adenosine is involved in fetal glycolytic and cardiovascular responses to hypoxia, 2) activation of central adenosine receptors mediates about one-half the inhibitory effects of hypoxia on REM and breathing, and 3) the depression of breathing may critically depend on a hypoxia-induced reduction in phasic REM sleep.

Thalamic locus mediates hypoxic inhibition of breathing in fetal sheep

The effects of lesions rostral to the brain stem on breathing responses to hypoxia were determined in chronically catheterized fetal sheep (>0.8 term). These studies were designed to test the hypothesis that the diencephalon is involved in hypoxic inhibition of fetal breathing. As in normal fetuses, hypoxia inhibited breathing with transection rostral to the thalamus or transection resulting in virtual destruction of the thalamus but sparing most of the parafascicular nuclear complex. Neuronal lesions were produced in the fetal diencephalon by injecting ibotenic acid through cannulas implanted in the brain. Hypoxic inhibition of breathing was abolished when the lesions encompassed the parafascicular nuclear complex but was retained when the lesions spared the parafascicular nuclear region or when the vehicle alone was injected. A new locus has been identified immediately rostral to the midbrain, which is crucial to hypoxic inhibition of fetal breathing. This thalamic sector involves the parafascicular nuclear complex and may link central O2-sensing cells to motoneurons that inhibit breathing.

Hypoxia sensitive neurons in the caudal hypothalamus project to the periaqueductal gray

Previous studies have demonstrated that the caudal hypothalamus modulates the respiratory responses to hypoxia and hypercapnia. In addition, many of the neurons in this area have a basal discharge related to the cardiac and/or respiratory cycles and are stimulated by hypoxia or hypercapnia. The purpose of the present study was to determine if these hypothalamic neurons project to a known cardiorespiratory area, the periaqueductal gray in the rat. In a first set of experiments, rhodamine-tagged microspheres were injected into the periaqueductal gray (PAG) to determine the areas of the caudal hypothalamus that project to the PAG. These studies revealed that the caudal hypothalamus sends strong ipsilateral and weak contralateral projections to the PAG. In a second set of experiments, single unit recordings were made from neurons in the caudal hypothalamus; the basal discharge of these neurons were examined with signal averaging techniques. Each neuron (n = 79) was tested for a response to inhalation of a hypoxic (10% O2) and a hypercapnic (5% CO2) gas. Antidromic activation techniques were then used to determine if neurons in the caudal hypothalamus send projections to or through the PAG. Nineteen percent (n = 15) of the hypothalamic neurons studied could be activated from the PAG; approximately 53% (n = 8) of these were excited by hypoxia and 27% (n = 4) by hypercapnia. Most of these neurons tested (42 of 64 neurons) had a basal discharge related temporally to the cardiac and/or respiratory cycles. These findings suggest that a caudal hypothalamic to periaqueductal gray projection is involved in the integrated response to hypoxia.