Fetal distress does not affect in utero defecation but does impair the clearance of amniotic fluid

PURPOSE: An experimental study was performed to evaluate the effect of fetal distress on in utero defecation and clearance of amniotic fluid (AF). METHODS: Sixteen pregnant New Zealand white rabbits underwent laparotomy at 25 days' gestation (full term, 31 to 32 days) as group A (n = 8) and B (n = 8). Uteroplacental ischemia was achieved by constriction of the aorta below the renal arteries to cause fetal distress in group B, whereas sham operation was done in group A. In both groups, 0.1 mL of technetium-99m (99mTc)-HIDA containing 1 mCi of radioactivity was injected into the gluteus muscle of each fetus, which had been exposed through the uterus. Beginning 2 hours after injection, a live fetus was killed every 2 hours for 48 hours in both groups. Tissue samples from the reference organs (lung, heart, stomach, kidney, bladder) and liver, meconium in proximal, mid and distal bowels, AF, and maternal blood were taken. The radioactivity of each sample was determined by a gamma counter and the percentage injected dose (uptake) per gram of tissue (%ID/g) was calculated. The total uptake and mean transit time (MTT) showing intestinal transport were calculated using the linear trapezoidal approximation and extrapolation. The peak concentration (Cmax, %ID/g) and time corresponding to the peak (tmax, h) were obtained. RESULTS: (1) Significant difference was noted between the groups with regard to uteroplacental perfusion pressure and blood pH (51.0+/-2.6 mm Hg; pH, 6.9+/-0.1 in group B; 80.1+/-2.7 mm Hg, pH, 7.3+/-0.1 in group A; P < .05). (2) 99mTc-HIDA was predominantly trapped by the liver and excreted into the gastrointestinal tract and AF in both groups. (3) In liver and bowel, shape of the profile was bimodal because of fetal swallowing and similar in both groups, tmax was the same in both groups, Cmax was lower in group B than in group A, the total uptake was smaller in group B than in group A, and intestinal transport time was similar (44.2 hours in group A and 43.0 hours in group B). In amniotic fluid, shape of the profile was sigmoidal in group B and reached a Cmax value of 11.6% ID/g, whereas unimodal profiles were observed in group A with a Cmax value of 12.6% ID/g; radioactivity was eliminated from the AF with a rate constant of 0.48% ID/g h in group A (AUC, 273% ID/g h); whereas accumulation of radioactivity was noted in group B (AUC, 308% ID/g h). (5) In maternal blood, shape of the profile was sigmoidal in group A with a Cmax value of 2.9% ID/g and unimodal in group B (Cmax, 1.6% ID/g), accumulation of radioactivity was noted in group A (AUC, 93% ID/g h), whereas a rapid decline of radioactivity (k, 0.06% ID/g h) was noted in group B (AUC, 47% ID/g h). CONCLUSIONS: Fetal distress did not affect the intestinal transport dynamics and in utero defecation but impaired the clearance of AF and the passage into the maternal circulation, which was shown by the accumulation of radioactivity in AF only in group B and in maternal blood only in group A without any elimination rate. This finding suggests that meconium-stained AF is not related to meconium passage after fetal distress; rather, it reflects impaired clearance of AF, which already has containing meconium caused by physiological in utero defecation.