The effect of relative hypoxemia on the pattern of breathing movements in fetal lambs

Mechanisms regulating hypoxic respiratory depression during fetal and postnatal life

Selected topics in the respiratory response to acute hypoxia in the fetus and newborn are reviewed. Peripheral chemoreceptors acting through ionotrophic glutamate receptors play an important role in affecting the initial augmentation phase. Whether fall off in peripheral chemoreceptor activity contributes to the secondary depressive phase remains controversial. A number of approaches including permanent electrolytic and reversible cooling lesions, Fos protein activation, and double-labeling immunohistochemistry has converged to show that an area in and around the locus ceruleus in the rostral pons affects the central depression. There is evidence that this is mediated by catecholamines acting at alpha(2)-adrenergic receptors. Tonic activity in early expiratory (postinspiratory) neurons may contribute to hypoxia-induced apneic episodes in the fetus and newborn. Desensitization of alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid receptors has been demonstrated in respiratory-related neurons both in vivo and in vitro. The role that this process might play in the depressive phase of the hypoxic ventilatory response has not been established. In vitro experiments with isolated brain stem-spinal cord preparations or transverse brain stem slices usually involve anoxia, whereas whole animal experiments use 8-15% O(2). Therefore, caution must be exercised in attempting to construct a unifying framework from these two approaches.

The pattern of breathing of fetal sheep in graded hypoxia

We examined the effects on fetal breathing movements (FBM) and electrocortical activity (ECoG) of maintaining fetal arterial PO2 (PaO2) at 16.9 +/- 1.9 mmHg for 20 min in unanaesthetized, chronically instrumented fetal sheep in utero at 122-132 days gestation. In 82% of trials, a pattern of regular FBM occurred characterised by its large (35 +/- 7 mmHg) excursions in tracheal pressure and its low frequency (ca. 3 per min) compared to the irregular FBM occurring during normoxia (ca. 5 mmHg and ca. 90 per min, respectively) (PaO2 ca. 23 mmHg). The occurrence of FBM in mild hypoxia was independent of the ECoG state of the fetus and it also occurred in fetuses in which the carotid sinus nerves and cervical vagosympathetic trunks had been sectioned bilaterally. Reducing PaO2 to between 16 and 11 mmHg produced a cessation of FBM, as reported by other workers. The FBM in mild hypoxia were distinct from 'gasping', which occurs only when PaO2 was reduced to much lower levels. The FBM in mild hypoxia are discussed in terms of central respiratory mechanisms excited by hypoxia, as opposed to the well-documented mechanisms which inhibit FBM in more intense hypoxia.