High vagal tone is associated with more efficient regulation of homeostasis in low-risk human fetuses

Homeostasis is maintained primarily by the parasympathetic nervous system a nd is thought to provide a physiological substrate for the development of c omplex behaviors. This investigation was undertaken to test the hypothesis that infants with high parasympathetic tone are more efficient regulators o f homeostasis than infants with low parasympathetic tone. Respiratory sinus arrhythmia (RSA) was used as a measure of parasympathetic tone, and the ef ficiency of homeostatic control tvas quantified, for each infant, by the sl ope (S-RSA) and correlation coefficient (R-RSA) of the regression line rela ting fluctuations in heart period and fluctuations in RSA. To test our hypo thesis, we examined the relationship between RSA and both S-RSA and R-RSA i n 34 low-risk human fetuses between 36 and 40 weeks gestation. We found tha t fetuses who were parasympathetic-dominated had larger S-RSA and R-RSA val ues, and hence were more efficient regulators of homeostasis, than fetuses who were sympathetic-dominated. The results of our analyses are important b ecause they establish, very early in development, a physiological basis for the relationship between vagal tone and the development of complex behavio r's. (C) 1999 John Wiley & Sons, Inc.