[Measuring intratracheal pressure in sheep fetuses in utero]

In pregnant sheep between the 100th and 138th gestational day the fetal tracheal pressure was measured in utero. For this purpose the uterus was opened and catheters inserted into the trachea and also into the amniotic sac. The average values of the intratracheal pressure during resting periods of the fetus were 9.8 +/- 1.2 mm Hg (1.3 +/- 0.135 kPa). During long periods only maximal pressure changes of 2 mmHg (0.26 kPa) without any greater alterations in pressure were seen, displaying a pressure gradient of 5.3 mmHg (0.7 kPa) between trachea and amniotic sac. Beside this also phases of stronger breathing excursions were observed, the intratracheal pressures ranging between + 25 mmHg and - 10 mmHg (+ 3.3 kPa, - 1.33 kPa) ("physiological gasps") usually starting with positive pressures. After ligature of the umbilical cord similar breathing excursions could be seen, displaying values as cited above, but mostly starting with negative pressures("pathological gasps"). By a short unique electrical stimulation of the isolated fetal ischiadic nerve, only an unique intratracheal pressure change was seen, ranging between + 25 mmHg and - 8 mmHg.