Scalp heat flux in postmature and in growth-retarded fetuses.

Postterm and growth-retarded fetuses share a common problem which can be characterized by a discrepancy between the supply of oxygen and nutrients to, and the demand of the fetus. But, this "insufficient" placental exchange function may also extend to and affect its thermal homeostasis; e.g. when the capacity of convective (placenta) pathways is shifted towards conductive (surface) pathways for heat loss. Therefore, fetal scalp heat flux measurements, where heat serves as an intrinsic tracer for metabolic activity and placental exchange function, promised a new kind of information. In 81 pregnant women during labor we measured fetal scalp heat flux by means of an heat flux transducer attached to the fetal head and after the cervix had dilated to 3 cm. In the healthy fetuses we found a positive linear relationship between scalp heat flux and different anthropometric variables such as body length (r = 0.432, n = 65, P less than 0.01), head circumference and gestational age. In comparison, postmature and growth-retarded fetuses showed higher heat flux values than appropriately grown fetuses of the same length head size and gestational age. Moreover, in those fetuses scalp heat flux decreased by approximately 4 watt/m2 during the second stage and differed in this regard from the control group who showed stable values during labor and delivery. We conclude that scalp heat flux measurements may indicate disturbances of placental exchange before acute hypoxia occurs.