CHANGES IN THORACIC DIMENSIONS INDUCED BY BREATHING MOVEMENTS IN FETAL SHEEP

The dimensions of the three major axes of the thorax (longitudinal, tr ansverse and antero-posterior) were monitored in utero from the ultras onic transit-time between pairs of piezo-electric transducers chronica lly implanted at opposite sides of the thorax in five fetal sheep at 1 19-122 days of gestation; tracheal and vascular catheters and diaphrag matic EMG electrodes were also implanted. To measure thoracic length, ultrasound transducers were implanted on the diaphragm and over the up per sternum and manubrium sterni. A pair was implanted on opposite sid es of the chest to measure thoracic width, and another pair was implan ted over the lower thoracic spine and lower sternum (antero-posterior dimension). The width of the thorax either decreased (mean 0.6+/-0.2 m m) or increased (mean 0.7+/-0.1 mm) during fetal breathing movements ( FBM). The depth of the thorax (antero-posterior dimension) usually dec reased (mean 0.9+/-0.1 mm) during FBM. The distance between the upper sternum and the diaphragm decreased by 1.0+/-0.1 mm (left side) and 1. 6+/-0.3 mm (right side) during inspiratory efforts. The distance betwe en the upper thorax and a fixed point on the lower thoracic spine decr eased by a similar amount. Overall, fetal thoracic dimensions changed by 1-3%. The largest changes measured, and the most surprising, were r eductions, rather than increases, in the separation between the dome o f the diaphragm and the upper thorax; this suggests that, during inspi ratory efforts, the upper thorax moves caudally by a greater distance than the diaphragm. FBM induce complex and variable changes in thoraci c dimensions; these are likely to induce small alterations in the shap e of the lungs that may act as a stimulus to lung growth.