The regulation of flow of pulmonary fluid in fetal sheep

The flow of fluid within the trachea of fetal sheep (121-135 days) has been measured using a newly developed flowmeter. The flowmeter, which is basically a miniature servo-controlled peristaltic pump, is connected in series with an extra-corporeal tracheal loop. Integrated tracheal flow was measured for periods of at least 8 h in 5 control fetuses during which we determined its relationship to fetal breathing movements detected by EMG's of inspiratory muscles or tracheal pressure fluctuations. The overall flow of fluid away from the lungs was 14.4 ml/h; on average outward flow was 5.3 times greater during episodes of breathing movements than during apnea. Interruption of the motor innervation of the larynx in 5 fetuses led to a 25% reduction in mean overall flow and, compared with control fetuses, there was a reduction in net flow associated with breathing movements and an increased flow during apnea. These findings suggest that tracheal flow is normally retarded by a laryngeal mechanism during apnea, giving rise to an elevated pressure within the trachea and probably resulting in increased pulmonary distension. Paralysis of the fetus with gallamine triethiodide reduced the flow of liquid from the lungs and abolished variations in flow rate related to fetal electrocortical states.