The purpose of this study was to longitudinally describe changes in heart rate variability (HRV) from 28 to 34 weeks postmenstrual age (PMA). A convenience sample of 31 low-risk preterm infants participated. HRV was quantified using a spectral analysis of heart periods and recorded during seven weekly test sessions from an electrocardiogram (ECG) signal. The total range of frequency components (0.04–2.0 Hz), high-frequency (HF) components (0.30–1.3 Hz), and ratio of low-to-high frequency (LF/HF) components (0.04–0.20/0.30–1.3 Hz) were measured. A mixed general linear model analysis revealed no significant change over weekly test sessions for the total, the high, and the ratio of LF/HF components. A significant interaction effect was, however, noted in the HF components for test session × gender (df = 1; F = 4.85; p = .030).With increasing age, the HF components for females increased or displayed a pattern of HRV indicative of a more mature autonomic nervous system (ANS). Study findings warrant further investigation of the impact of gender on normative descriptions of HRV.
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