The effects of doxapram on breathing, heart rate and blood pressure in fetal lambs

In experiments to assess its value in the study of fetal arterial chemoreceptor reflexes, doxapram (0.5 – 2mg i.a.) given to unanaesthetized fetal lambs in utero in low voltage electrocortical activity stimulated fetal breathing in 73% of trials. The response began within a few seconds and lasted up to 5 min. This response to doxapram was present in intact or mid-collicular brainstem transected fetuses and after bilateral section of the carotid sinus nerves and cervical vagosympathetic trunks (denervation). In intact and brainstem transected fetuses, doxapram injections were followed by a modest rise in arterial pressure and a small fall in heart rate. After denervation the rise in arterial pressure was fourfold and the heart rate increased. Electrocortical activity and limb movements were not affected in any group of lambs, and fetal breathing was not induced during isocapnic hypoxia. It is concluded that doxapram stimulates fetal breathing by a central action below the pons, and that it can therefore not be used to study peripheral arterial chemoreceptor function in utero.