Linda Day

Fetal Rehydration Via Intraamniotic Fluid: Contribution of Fetal Swallowing

ABSTRACT: Amniotic fluid volume is regulated by a balance of fetal fluid production and resorption. Although fetal swallowing is believed to be a major site of fluid resorption, additional routes of fluid exchange also may contribute. In our present study, five chronically prepared, water-restricted, pregnant ewes with singleton fetuses (128 ± 1 d) were rehydrated via an intraamniotic infusion (100 mL/h over 90 min) of 0.075 M saline. In response to the maternal water restriction, significant increases were noted in maternal and fetal plasma osmolalities (306.6 ± 1.2 to 315.4 ± 2.4; 300.5 ± 1.5 to 311.0 ± 1.6 mosmol/kg, respectively) and arginine vasopressin concentrations (1.9 ± 0.2 to 22.6 ± 5.0; 1.5 ± 0.1 to 8.5 + 2.2 pg/mL, respectively). After the intraamniotic infusion, fetal plasma osmolality (311.0 ± 1.6 to 303.0 ± 1.2 mosmol/kg) and hematocrit (36.7 ± 1.9 to 33.8 ± 1.4%) significantly decreased although there was no change in maternal arterial blood values. Fetal swallowing averaged 0.39 ± 0.10 mL/min during the basal period and 0.34 ± 0.17 mL/min at maximum dehydration, and decreased significantly to 0.19 ± 0.07 mL/min in response to the intraamniotic infusion. These results indicate the rapid absorption of intraamniotic fluid by the dehydrated ovine fetus, despite the suppression of fetal swallowing. The volume swallowed during and after the intraamniotic infusion was insufficient to account for the observed changes in fetal plasma osmolality and hematocrit. Thus, alternative routes of fluid absorption (i.e. intramembranous flow) likely predominate under conditions of increased fetal plasma to amniotic fluid osmotic gradients.

Assessments of Fetal Swallowred Volume: Tracer Disappearance Versus Esophageal Flow

OBJECTIVE: We sought to compare fetal swallowed volume determinations simultaneously by two techniques—amniotic fluid (AF) tracer disappearance and esophageal flow probe measure ments. METHODS: Six ovine fetuses (129 ± 1 days) were chronically prepared with a thoracic esophageal flow probe and vascular and two AF catheters. 125I-labeled albumin was injected into the AF cavity, and samples were withdrawn at timed intervals for 8 hours. The AF volume was calculated by time-0 extrapolation of the semilog of 125I disappearance. Tracer-determined swal lowed volume was calculated as the product of AF volume and the slope of isotope disappearance. Flow probe measurement of swallowed volume was determined by computer integration of cali brated flow probe velocity recordings. RESULTS: The AF volume averaged 805 ± 168 mL. The isotope disappearance rate from the AF was 2.8 ± 0.4%/hour. Average tracer-determined swallowed volume (547 ± 113 mL/day) was greater than flow probe volume (366 ± 81 mL/day), although these values were not significantly different. However, when corrected for estimated swallowed lung fluid, tracer- determined volume was significantly greater than flow probe volume (P < .05). CONCLUSION: Ovine fetal swallowed volume determinations by AF tracer techniques are greater than those determined by esophageal flow probe measurements. (J Soc Gynecol Invest 1994;1:37-44)

Ovine fetal breathing and swallowing activity in response to plasma glucose changes

Fetal swallowing activity generally occurs simultaneously with fetal breathing movements (FBM) in sheep. The present study investigated the FBM and swallowing responses to altered fetal plasma glucose. Fetal lambs were chronically prepared with laryngeal, esophageal and diaphragm electromyogram (EMG) wires, an esophageal flow probe and vascular catheters. Beginning at 138 +/- 1 day, FBM and swallowing were monitored during control periods and in response to intravenous glucose infusions (14 mg/kg/min for 120 min) to fetuses of fed and fasted ewes. Glucose infusions to fetuses of fed ewes resulted in significant increases in fetal plasma glucose (21.2 +/- 0.7 to 40.5 +/- 1.9 mg/dl) and time breathing (46.2 +/- 6.3 to 60.0 +/- 9.5 min/2 h). In response to maternal fasting, fetal glucose levels (13.4 +/- 1.0 mg/dl) and time breathing (23.0 +/- 7.2 min/2 h) decreased significantly. Glucose infusion to fetuses of fasted ewes resulted in significant increases in time breathing (50.3 +/- 13.4 min/2 h) and diaphragmatic EMG activity (1,295 +/- 654 to 3,012 +/- 1,182 spikes/2 h). There was no change from basal levels of fetal EMG swallows (83.2 +/- 4.3 swallows/2 h) or esophageal flow (40.8 +/- 7.9 ml/2 h) in response to maternal fasting or fetal glucose infusions.

Ovine Fetal Laryngeal Chemoreflex Thresholds and Respiratory Effects

In newborn infants, laryngeal contact with solutions of low chloride concentration or pH evokes swallowing, laryngeal adduction, and respiratory inhibition (laryngeal chemoreflex). To determine whether the laryngeal chemoreflex is present during fetal life and its effect on fetal respiratory activity, eight time-bred ewes (128 +/- 2 days) were prepared with fetal electrocortical diaphragm and esophageal electrodes and a nasopharyngeal catheter. After a 60-minute control period, increasing volumes (0.1 to 1.0 ml/kg) of 0.15 mol/L NaCl or distilled water (0.05 to 1.0 ml/kg) and decreasing concentrations of NaCl (0.15 to 0.02 mol/L) at a fixed volume (0.3 ml/kg) were sequentially administered through the nasopharyngeal catheter (38 degrees C). The minimum water volume that stimulated swallowing was significantly less than the minimum 0.15 mol/L NaCl volume (0.10 +/- 0.02 vs. 0.70 +/- 0.05 ml/kg). The maximum NaCl concentration that stimulated swallowing was 0.04 +/- 0.01 mol/L During the control period, respiratory activity averaged 14.6 +/- 0.7 breaths/minute and did not change during absent swallow responses or isotonic saline-induced swallows. However, respiratory activity significantly decreased during water (4.7 +/- 0.6 breaths/minute) and hypotonic saline-induced swallow responses (3.7 +/- 0.7 breaths/minute). Fetal electrocortical activity did not change during absent or stimulated swallows. We conclude that laryngeal water or hypotonic saline solution may stimulate fetal swallowing and suppress fetal respiratory activity, similar to the newborn laryngeal chemoreflex. We speculate that an exaggeration of the laryngeal chemoreflex apnea response in the newborn may predispose to sudden infant death syndrome.