Objective:Heart rate variability (HRV) reflects integrity of the autonomic nervous system. However, no study has investigated the impact of therapeutic hypothermia (TH) on HRV measures in infants with hypoxic-ischemic encephalopathy (HIE). In this study, we evaluate the influence of temperature on measures of HRV for a group of infants with favorable outcomes, as compared with a control group of infants with unfavorable outcomes.Study Design:Term-born infants with moderate-severe HIE underwent standard TH treatment and prospective electroencephalography (EEG) and electrocardiogram (ECG) recording. Infants with favorable outcome (no seizures, normal/mild EEG scores at 96 h, no magnetic resonance imaging brain injury and normal neurodevelopmental scores at 18 to 24 months) were matched on gestational age, sex and worst encephalopathy score to a group of infants with unfavorable outcomes. Time-and frequency-domain HRV measures were calculated from 60 min of ECG data obtained at three time points: 24 h (hypothermia), 48 h (hypothermia) and 96 h (normothermia). The effect of time and temperature were evaluated using repeated-measures analysis of variance.Results:Sixteen infants were included (8 favorable, 8 unfavorable). For both groups of infants, an increase in the HR, RR and HF power was associated with an increase in temperature, but was not associated with any other HRV measure. In contrast, measures of HRV increased over time, as encephalopathy decreased, for infants with favorable outcomes (reflecting increased cortical-autonomic integration), but not for those with unfavorable outcomes.Conclusions:In general, the effect of hypothermia on measures of HRV is limited to changes in heart rate (bradycardia) and respiratory rate, as opposed to changes in true variability. This supports the hypothesis that persistent changes in HRV are driven by the underlying brain injury and not by the process of TH.