Abstract
The effect of naloxone on fetal breathing and the respiratory sensitivity to CO 2 was tested on chronically prepared fetal lambs on days one and four post-surgery. After a control period the fetus was challenged with hypercapnia for 10 min and after another control period 9 mg naloxone was administered to the fetus followed by another CO 2 test 15 min later. An index of fetal breathing (Veq), tidal volume ( Vt) and frequency of breathing (f) was determined from tracheal pressure deflections and from the integrated diaphragmatic EMG, expressed as power of diaphragmatic activity per min. Naloxone consistently caused fetal arousal but the duration was variable. The respiratory response to naloxone was also variable and not statistically different from control. The respiratory sensitivity to CO 2(% Δ Veq/Torr ΔPa CO2 or % Δ Diaph. Power/min/Torr Δ Pa CO2) was not changed by naloxone on either day. We conclude that endorphins do not have a significant direct role in the fetal respiratory response to CO 2 but may be involved in the control of state.
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