The autonomic condition of children with congenital hypothyroidism as indicated by the analysis of heart rate variability.

Early treatment by thyroid hormone in congenital hypothyroidism (CH) prevents for mental retardation, but the autonomic condition could well remain impaired. The aim of this study was to get insights about the cardiovascular regulation of CH children by analysing their heart rate variability (HRV) data. RR-interval data of 34 early treated CH children, 24 to 48 months of age, were collected at rest in a clinical environment. In all these cases, CH resulted from an abnormal development of thyroid gland. Data from 29 age matched-controls were also collected in a scholar environment. Short time series were then analysed in time and frequency domains. Detrended fluctuation analysis (DFA) was also applied to obtain the scaling exponent α(1). We found that the HRV obtained from children with CH is diminished as suggested by statistical differences (p<0.05) in the following parameters: RMSDD, pNN20, total power, LF and HF. However, no significant differences were found in the LF/HF ratio, the central frequencies of the LF and HF bands, as well as in α(1). Given that the significant differences were basically reflected in static parameters rather than in the frequency distribution or scale-invariant behaviour of the studied data, we consider that the analysis points toward the effect of treatment on rising heart rate by the thyroid hormone, and not necessarily toward any residual autonomic dysfunction, as the dominant underlying cause of the diminished HRV exhibited by CH children.

Repeatability of heart rate variability in congenital hypothyroidism as analysed by detrended fluctuation analysis.

The analysis of heart rate fluctuations, or heart rate variability (HRV), may be applied to explore children's neurodevelopment. However, previous studies have reported poor reliability (repeatability) of HRV measures in children at rest and during light exercise. Whether the reliability can be improved by controlling variables such as physical activity, breathing rate and tidal volume, or by selecting non-conventional techniques for analysing the data remains as an open question. We evaluated the short-term repeatability of RR-interval data from medicated children with congenital hypothyroidism (CH). The alpha(1) exponents, obtained by detrended fluctuation analysis (DFA), from the data of 21 children collected at two different sessions were compared. Elapsed days between sessions were 59 +/- 33, and data were obtained during 10 min, trying to restrict the children's activity while being seated. We found statistical agreement between the means of alpha(1) exponents for each session (p = 0.94) and no bias with a low-coefficient variation (9.1%); an intraclass correlation coefficient ri = 0.48 ([0.14 0.72], 95% confidence interval) was also estimated. These findings, which were compared with results obtained by conventional time and frequency techniques, indicate the existence of agreement between the alpha(1) exponents obtained at each session, thereby providing support concerning the repeatability of HRV data as analysed by DFA in children with congenital hypothyroidism. Of particular interest was also the agreement found by using the central frequency of the high-frequency band and the parameter pNN20, both showing better or similar ri than alpha(1) (0.77 [0.57 0.89] and 0.51 [0.17 0.74], respectively), yet considerably better repeatability than other conventional time and frequency parameters.