Whose clock makes yours tick? Maternal-infant physiological adaptations during the first months of life

In this longitudinal study, we examined the existence of a direct maternal-infant physiological relationship in the synchrony of respiratory sinus arrhythmia (RSA) between mothers and infants. RSA has regularly been related with infant self-regulation and has become a well-used measure in infant research as it can be obtained in a non-invasive way by ECG-respiratory registration during experimental research (e.g., Feldman, 2006). RSA reflects a component of the natural heart rhythm variability (HRV), that is manifest during a respiration cycle as the result of the regulation of the autonomic nervous system, mediated by the parasympathetic branch, more specific the nervus vagus (Berntson et al., 1997). During inspiration, heart rate accelerates, whereas, during expiration, heart rate decelerates. The degree of RSA thus gives an indication of the vagal control of the heart (Berntson et al., 1997). RSA as a multiply determined physiological variable. Besides vagal outflow, RSA reflects, among others, respiratoryand somatomotor metabolic parameters, such as movement and talking speech (Berntson et al., 2007). Therefore, inclusion of respiratory and movement components in analyses are needed. In the present study, we conducted paced breathing experiments with 11 mother-infant dyads when the infant was 1, 2, 4, 8 and 12 weeks old. We instructed mothers to breathe at 5 different rates (6-12-15-20-6 cycles per minute) with their infant lying on their body. Maternal-infant ECG and respiration were registered with simultaneous video recording. RR-interval (RRI), respiratory sinus arrhythmia (RSA) and respiration frequency (Fresp) were calculated. We analyzed mother-infant RSA response profiles, using an ANOVA approach followed by post-hoc testing of interaction terms. Polynomial and Helmert contrasts were used to evaluate respectively maternal-infant paced-breathing response differences and developmental differences. Results showed that a maternal-infant physiological relationship is manifest during the first two months of life. During this period, an infant is more susceptible to fluctuations in the RSA curves of their mother than on a later age. At the age of 3 months, maternal-infant differences in RSA were too large to determine a relationship. These results are in line with the 2-months developmental shift of social orientation. Results are discussed with regard to infants' vagal maturation and the need to include respiratory components in cardiovascular infant research.