AJP - Regulatory, Integrative and Comparative Physiology, Vol 249, Issue 1 133-R138, Copyright © 1985 by American Physiological Society
Amniotic fluid volume and fetal swallowing rate in sheep
S. Tomoda, R. A. Brace and L. D. Longo
To investigate amniotic fluid (AF) dynamics and volume regulatory
mechanisms, we measured the concentration of radioiodinated (125I) serum
albumin (RISA), 51Cr-labeled red blood cells (Cr-RBC), and 103Ru-labeled
microspheres after injection into the amniotic cavity and determined AF
volume and fetal swallowing rate in 22 singleton pregnant sheep. Under
normal conditions 2-3 h were required for complete mixing of RISA and
Cr-RBC within AF; however, when the fetus was dead only 3-5 h were
required. AF volume of 17 sheep on the 5th postoperative day averaged 975
+/- 128 ml by RISA and 986 +/- 130 ml by Cr-RBC. AF volume determined with
RISA and Cr-RBC correlated well. In contrast, AF volume measurement with
microspheres produced erratic results. The disappearance rate of the labels
in 17 ewes on the 5th postoperative day averaged 4.9 +/- 0.7%/h for RISA
and 5.5 +/- 0.7 for Cr-RBC, and the calculated rates of fetal swallowing
were 935 +/- 78 ml/day by RISA and 1,085 +/- 102 by Cr-RBC. In dead fetuses
the disappearance rates were almost zero, suggesting that the labels
disappear mainly by swallowing. Absolute volume swallowed and swallowed
volume per fetal weight correlated with gestational age. AF volume
correlated with fetal weight. Radiolabeled albumin or red blood cells may
be used to simultaneously measure amniotic fluid volume and the rate of
fetal swallowing. Furthermore it appears that fetal swallowing increases
with gestational age.
