VAGAL TONE DURING QUIET SLEEP IN NORMAL HUMAN TERM FETUSES

The purpose of this paper was to calculate vagal tone (V) for 17 norma l human fetuses in quiet sleep (QS) between 36 and 40 weeks gestation. The fetal cardiac electrical signal was captured transabdominally in 3-min blocks at a rate of 833 times per second and fetal R-waves were extracted using adaptive signal processing techniques. Fetal R-wave in terbeat intervals were converted to equally spaced, time-based data, a nd the low-frequency component was removed using a 21-point third-orde r moving polynomial. The parameter V was calculated by taking the natu ral logarithm of the sum of the power densities between 0.3 Hz and 1.3 Hz. We found that fetal breathing was associated with an approximatel y 25% increase in V as compared to nonbreathing, 3.33 +/- 0.48 versus 2.57 +/- 0.47, p < 0.0001. Furthermore, there was a significant linear relationship between the mean single-fetus V during spontaneous respi ration and the mean single-fetus V during normally occurring apneic pe riods, r = 0.772, p < 0.002. We conclude that respiratory activity is associated with a significant increase in vagal tone for normal human fetuses in QS. (C) 1994 John Wiley and Sons, Inc.