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.