Changes in thoracic dimensions induced by breathing movements in fetal sheep

The dimensions of the three major axes of the thorax (longitudinal, transverse and antero-posterior) were monitored in utero from the ultrasonic transit-time between pairs of piezo-electric transducers chronically implanted at opposite sides of the thorax in five fetal sheep at 119-122 days of gestation; tracheal and vascular catheters and diaphragmatic EMG electrodes were also implanted. To measure thoracic length, ultrasound transducers were implanted on the diaphragm and over the upper sternum and manubrium sterni. A pair was implanted on opposite sides of the chest to measure thoracic width, and another pair was implanted over the lower thoracic spine and lower sternum (antero-posterior dimension). The width of the thorax either decreased (mean 0.6 +/- 0.2 mm) or increased (mean 0.7 +/- 0.1 mm) during fetal breathing movements (FBM). The depth of the thorax (antero-posterior dimension) usually decreased (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 between the upper thorax and a fixed point on the lower thoracic spine decreased by a similar amount. Overall, fetal thoracic dimensions changed by 1-3%. The largest changes measured, and the most surprising, were reductions, rather than increases, in the separation between the dome of the diaphragm and the upper thorax; this suggests that, during inspiratory efforts, the upper thorax moves caudally by a greater distance than the diaphragm. FBM induce complex and variable changes in thoracic dimensions; these are likely to induce small alterations in the shape of the lungs that may act as a stimulus to lung growth.