State-related changes in lung liquid secretion and tracheal flow rate in fetal lambs

The volume of liquid in the fetal lungs depends on the rate of liquid secretion (Vs) across the pulmonary epithelium and the rate of flow out of the trachea (Vtr). We measured Vs, by an isotope-dilution technique, and Vtr, with a bubble flowmeter, during low-voltage (LV) and high-voltage (HV) electrocortical activity. In nine chronically instrumented fetal lambs, Vtr was greater during the transition to and at LV (16.98 +/- 1.98 ml/h, mean +/- SE, n = 23) than values during the transition to and at HV (8.69 +/- 0.8 ml/h). A pronounced peak in Vtr of 22.3 +/- 1.8 ml/h (n = 197) occurred at the transition to LV and early in the LV state. Ten minutes or more into LV, Vtr had declined to 10.3 +/- 1.8 ml/h (n = 235). Vtr remained low throughout the HV state. Vs values were not significantly different throughout the LV (11.83 +/- 1.34 ml/h, n = 216) and the HV (13.61 +/- 2.34 ml/h, n = 174) states. Diaphragmatic burst rate during LV (146.9 +/- 6.7 bursts/5 min, n = 432) was greater than during HV (26.5 +/- 4.6 bursts/5 min, n = 348), but burst rate was not correlated with Vtr. In summary, Vtr reaches a peak during the early part of LV when breathing commences and Vs remains constant throughout the behavioral cycle. As a result, lung liquid volume increases slightly during HV and decreases by a similar amount in the early part of LV.