DISCUSSION Healthy controls: Time d

DISCUSSION
Healthy controls: Time domain measures of HRV increase with age throughout the pediatric age range, achieving adult values by aolescence.¹³ Our findings were similar to those of Massin and von Bernuth. As mean RR interval varies significantly with age, use of a method of normalization for RR interval would be useful to compare time domain values across different ages. We found that the quotient of SDNN and mean RR intervals tended to remain stable across most of the pediatric age range.
In the frequency domain our findings were similar to those of Finley and Nugent where the LF/HF ratio decreased in early childhood, followed by a later increase. However, unlike their report, absolute values of LF and HF power tended to change in opposite directions in our subjects, with HF power increasing in early childhood and showing a later decline into adolescence. Again, these differences are most likely related to the duration of electrocardiographic recordings. Furlan at al derived frequency domain values for GRV using an averaging technique spanning 6-hour segments of data, which could obscure detailed information about autonomic modulation of HRV. This observation and concern about the lack of ‘’stationarity’’ of the signal partly underlie the recommendations of the task Force of the European Society of Cardiology and the North American Society of pacing and Electrophysiology to preferably use of short-term recordings for these derivations. We selected stable segments of sinus rhythm at the lowest mean heart rate in an attempt to standardize condition. The continuous supine position or controlled ventilation, as is used in adults , was not possible in our younger population.
Preoperative congenital heart disease: HF power has been shown to correlate with parasympathetic modulation. The finding of decreased HF power in children with congenital heart disease before surgery suggests relative parasympathetic withdrawal. The observed decrease in total power was only party contributed to by a decrease in HF power. This suggests that there could be alterations in other factors thought to contribute to HRV including sympathetic tone and the renin-angiotensin and thermoregulatory systems. These findings are in agreement with the hypothesis that decreased HRV is a nonspecific marker of cardiovascular stress, reflecting an alteration in autonomic nervous system input to the heart
Postoperative congenital heart disease: The significant decrease in HRV and HF power in the immediate postoperative period suggests that further parasympathetic withdrawal is an important component of these changes. This could partly be the result of decreased sensitivity of the sinus node to autonomic nervous in put immediately after surgery for congenital heart disease. None of these patients was taking β blockers or any specific antiarrhymic medications, which are known to alter HRV.
Reduced HRV, independent of heart rate, in subjects who had a prolonged postoperative course suggests that HRV provides additional information on the well being of such patients. Whereas all measures of HRV reflected this, only long-term differences in time domain measures reached statistical significance. It has been observed that the values of these measures of HRV increase with increasing duration of recording. These differences in time domain measures could reflect differences in the renin-angiotensin system. Importantly, pleural drainage was longer in subject who had longer lengths of hospital stay. Higher renin and angiotensin levels have been found in postoperative pediatric cardiac patients who had longer duration of pleural drainage. The difference in timing of the postoperative analyses between the 2 groups was to ensure that all subjects were in a similar stable hemodynamic state and off all vasoactive medications. Because the recordings were done later in group II, it would be expected that the differences in HRV would be reduced. Hence, the observed differences in HRV between these 2 patient groups assumes more significance. In this study it was not possible to control for differences in diagnoses and type of surgery because of small number of subject in each diagnostic category; however, these groups were apparently homogenous from a heart rate control or autonomic standpoint , because there were no preoperative differences in heart rate or HRV. In the only other report on HRV in perioperative critically ill pediatric cardiac patients, Gordon et al¹¹ showed that children who died were more likely to have a “lower respiratory quotient” i.e., a higher respiratory-to-lower frequency power ratio. This was interpreted as being due to a relative increase in parasympathetic tone and possibly tone and possibly sympathetic exhaustion.¹¹ Data were gathered in 2 centers using different analytic methods, standard measures of HRV were not used, and the duration of recording varied from 1 to 27 house. Significant(38% to 62 %) and possibly important segments were excluded from analysis .most their patients were likely to have been receiving vasopressors and artificial ventilation, both of which affect HRV. Our data represent a more complete evaluation of longer term stable recordings thus cannot be compared with data from that report
Acknowledgment: We are grateful to Cathy Tipton, RCDS, Mary Drury, RN, RNP, Susan Stollzman, RN, RNP, Claudio Ramaciotti, MD, and Lynn Mahony, MD for their assistance