The effect of fetal lung inflation on fetal heart rate

The effect on the fetal heart by inflating the fetal lungs with liquid or air while the animal was being maintained in utero by its normal placental circulation was investigated in 10 healthy, chronically catheterized fetal sheep of gestational age 126-137 days. It was found that initial attempts to inflate the lungs with volumes of air as small as 10 ml (i.e., with less than a predicted normal tidal volume) caused abrupt, powerful slowing of the fetal heart with, usually, an associated hypotension. Inflations with similarly small volumes of saline were ineffective. Atropine pretreatment abolished the cardiac slowing caused by the air inflations, indicating the operation of a neural reflex. An analysis of the pressure changes induced by the air and liquid inflations in airway, intrathoracic and intra-amniotic pressures showed that the cardiac slowing was primarily related to the level of mechanical stress applied across the fetal airway.