Tracheal fluid in fetal lambs: spontaneous decrease prior to birth.

We studied tracheal fluid (TF) production in 14 fetal lambs: 6 controls, 6 receiving atropine on 1 or more of the last 7 days before birth, and 2 with bilateral section of the cervical vagosympathetic trunk. A cannula diverted all TF into an intrauterine bag; we collected TF intermittently and measured its volume. All ewes delivered spontaneously at 128-150 days' gestation. TF production decreased before birth in all fetuses except one control. TF production decreased before birth in all fetuses except one control. TF production did not correlate with fetal arterial blood gas tensions, hematocrit, or plasma proteins. In controls only, TF production correlated with fetal arterial pH (P less than 0.02); however, the pH range was small and the correlation has questionable physiological significance. For all fetuses, TF production during the 7 days before birth correlated inversely with the plasma cortisol concentration of 48 h previously (n = 36; r = -0.603; P less than 0.001). We conclude a) TF production in fetal lambs decreases before spontaneous term or preterm labor; b) this decrease is not affected by atropine or by section of the cervical vagosympathetic trunk; and c) the decrease in TF production may be related to increased secretion of cortisol.