The developmental character of cardiac autonomic responses to an acute noxious event in 4- and 8-month-old healthy infants

Heart rate (HR) has been widely studied as a measure of an individual's res ponse to painful stimuli. It remains unclear whether changes in mean HR or the variability of HR are specifically related to the noxious stimulus (i.e , pain). Neither is it well understood how such changes reflect underlying neurologic control mechanisms that produce these responses, or how these me chanisms change during the first year of life. To study the changes in card iac autonomic modulation that occur with acute pain and with age during ear ly infancy, the relationship between respiratory activity and short-term va riations of HR (i.e. respiratory sinus arrhythmia) was quantified in a long itudinal study of term born healthy infants who underwent a finger lance bl ood collection at 4 months of age (n = 24) and again at 8 months of age (n = 20). Quantitative respiratory activity and HR were obtained during baseli ne, lance, and recovery periods. Time and frequency domain analyses from 2. 2-min epochs of data yielded mean values, spectral measures of low (0.04-0. 15 Hz) and high (0.15-0.80 Hz) frequency power (LF and HF), and the LF/HF r atio. To determine sympathetic and parasympathetic cardiac activity, the tr ansfer relation between respiration and HR was used. At both 4 and 8 months, mean HR increased significantly with the noxious ev ent (p > 0.01). There were age-related differences in the pattern of LF, HF , and LF/HF ratio changes Although these parameters all decreased (p > 0.01 ) at 4 months, LF and LF/HF increased at 8 months and at 8 months HF remain ed stable in response to the noxious stimulus, Transfer gain changes with t he lance demonstrated a change from predominant vagal baseline to a sympath etic condition at both ages. The primary finding of this study is that a re sponse to an acute noxious stimulus appears to produce an increase in respi ratory-related sympathetic HR control and a significant decrease in respira tory-related parasympathetic control at both 4 and 8 months. Furthermore; w ith increasing age, the sympathetic and parasympathetic changes appear to b e less intense, but more sustained.