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 p lacental circulation was investigated in 10 healthy, chronically cathe terized 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 mi (i.e., with less than a predicted normal tidal volume) caused a brupt, powerful slowing of the fetal heart with, usually, an associate d hypotension. Inflations with similarly small volumes of saline were ineffective. Atropine pretreatment abolished the cardiac slowing cause d by the air inflations, indicating the operation of a neural reflex. An analysis of the pressure changes induced by the air and liquid infl ations in airway, intrathoracic and intra-amniotic pressures showed th at the cardiac slowing was primarily related to the level of mechanica l stress applied across the fetal airway.