Respiratory sinus arrhythmia in spontaneously breathing, unanesthetized newborn and adult Wistar rats.

We examined respiratory sinus arrhythmia (RSA) and possible interaction with respiratory frequency (f R) and heart rate (HR) in spontaneously breathing, unanesthetized newborn Wistar rats (2- to 5-day-old; n = 54) and the adult rats (8-week-old; n = 34). Instantaneous heart rate (inst -HR) was calculated as the reciprocal of the inter-beat-interval. For each breath, RSA was determined as the difference between the maximum and minimum inst -HR value. The absolute RSA or RSA% (RSA per HR) were calculated as the average RSA of 10 consecutive breaths. RSA (or RSA%) in the newborn rats was significantly lower than that in the adult rats. Correlation coefficient between RSA (or RSA%) and 1/ f R or HR/ f R , but not HR, was significant in newborn rats, whereas only that between RSA (or RSA%) and HR was significant in adult rats. The power spectrum density of heartbeat fluctuation was detectable in both age groups. The present findings suggest that RSA exists and could be influenced by f R , rather than HR, in newborn rats. ● Respiratory sinus arrhythmia (RSA) was examined in unanesthetized newborn rats. ● Heart rate (HR) and respiratory rate (f R) were averaged in ten consecutive breaths. ● RSA in the newborn rats was significantly lower than that in the adult rats. ● Correlations were significant between RSA and 1/ f R or HR/ f R , not HR, in newborns. ● RSA exists and could be influenced by f R , rather than HR, in newborn rats. [ABSTRACT FROM AUTHOR]