Journal of Applied Physiology, Vol 57, Issue 3 917-922, Copyright © 1984 by American Physiological Society
Measurement of fetal heat production using differential calorimetry
G. G. Power, H. Schroder and R. D. Gilbert
These experiments were undertaken to measure heat production of fetal lambs
in utero by using differential calorimetry. We used the principle that
fetal heat production, H(fetus), can be calculated from measurements of
base-line temperature difference between mother and fetus, delta T(fetus),
heat introduced from an external source, H(heater), and the increase in
body temperature, delta T(heater), that results, i.e., H(fetus) = H(heater)
X delta T(fetus)/delta T(heater). We placed microheaters (1.8 mm diam) in
the inferior vena cavae of eight near-term lambs and placed thermistors and
catheters into maternal and fetal vessels and amniotic fluid. Five days
later, fetal arterial temperature averaged 0.54 +/- 0.02 degrees C (SE)
higher than maternal arterial temperature. When the heater was turned on to
dissipate 29-103 cal/min, fetal temperature increased to approach 0.1-0.5
degrees C higher than control; the final temperature was estimated using
the rate of increase during the first 20 min. Fetal heat production
averaged 47.1 +/- 4.1 cal X min-1 X kg-1 during the warming phase in these
lambs, which weighed 3.26 +/- 0.36 kg. This value would be 3-4% less if
corrected for the increase in metabolic rate caused by heating, assuming a
Q10 of 2.5. Fetal heating did not alter fetal heart rate, blood pressure,
or blood gas values significantly, nor was hemolysis visible in plasma
samples. When heat production was calculated from the decrease in fetal
temperature after the heater was turned off, an average value of 41.2 +/-
2.5 cal X min-1 X kg-1 was found. Because this value is comparable to the
heating phase, fetal metabolic rate and the insulating properties of the
fetal shell are not likely to have been changed by the heating.
