Ovine fetal swallowing and behavioral state responses to sublingual water

Ovine fetal swallowing primarily occurs in bouts of activity associated with low voltage electrocorticogram activity and breathing movements. Despite similar rates of electromyographic swallowing activity, there are significant differences in the net esophageal fluid flow among fetuses. To determine if variations in the volume of fluids accessible to the fetus (amniotic fluid, lung fluid, salivary secretions) affect fetal swallowing, we studied the effects of sublingual distilled water infusions on ovine fetal swallowing rates and esophageal flow. Seven pregnant ewes at 127 +/- 2 days gestation were chronically instrumented with maternal and fetal vascular catheters, fetal electrocortical, electro-ocular and electromyograph electrodes, an esophageal flow probe and a sublingual infusion catheter. Following a 2-hour control period, consecutive sublingual infusions of room temperature distilled water (10 and 20 ml/kg/h) were administered for 2 h each. In response to sublingual infusions, fetal plasma sodium (142.5 +/- 0.5 to 140.8 +/- 0.8 mEq/l), chloride concentrations (109.2 +/- 0.4 to 107.7 +/- 0.7 mEq/l) and osmolality (302 +/- 2 to 298 +/- 2 mOsm/kg) decreased significantly. Swallowing activity and esophageal flow did not change significantly from basal values (47 +/- 9 swallows/h, 19.2 +/- 4.2 ml/h) during the sublingual infusions. However, fetal low-voltage electrocortical activity increased significantly during both infusion periods (42 +/- 3 to 56 +/- 6%). Sublingual distilled water infusion decreases fetal plasma osmolality and alters fetal behavioral state without affecting swallowing frequency or esophageal flow.