Development of patterns of respiratory activity in unanesthetized fetal sheep in utero

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Abstract
Respiratory activity (diaphragmatic electromyogram) was recorded in 6 unanesthetized, in utero, fetal lambs between 0.7 of gestation and term. Respiratory patterns generated by the fetus showed developmental changes that included the following: an emergence of a periodic modulation of respiratory rate producing alternating active and quiet phases (mean cycle length of 37 min between 130 and 140 days'' gestation); an increase in percentage apnea (expiratory time > 10 s) from 20% at 110 days to 60% at 140 days; a linear decrease in the 2 h average respiratory rate, while mean rate during active phases showed no consistent gestational decline. Electrocortical and electroocular activity was monitored in 3 of 6 fetuses; discrete sleep state patterns were not consistently identified. Results demonstrate a gestational change in the respiratory patterns of the developing fetus and suggest an orderly maturation of the mechanisms controlling respiratory neuronal output.

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