Hrv analysis software version 3

ECG data was sampled at 256Hz and 1h recordings at 12h, 18h, 24h, 30h, 36h, 42h and 48h postnatal age were extracted. Our cohort included only outborn infants, thus recordings at 6h after birth were rarely available. Time epochs with electrographic seizures in corresponding EEG recording were excluded from the analysis. Signal processing was performed using Kubios HRV analysis software, version 3.3 (Kubios Oy, Kuopio, Finland).¹⁹ A built-in QRS detection algorithm based on the Pan–Tompkins algorithm was applied to detect R-peaks and compute RR interval (Figure 1). Cubic spline interpolation at a rate of 4Hz generated an equidistantly sampled RR time series. An automatic artefact correction algorithm was used to detect technical and physiological artefacts and remove ectopic/misplaced beats from the normal sinus rhythm.¹⁹ The detrending method based on smoothness prior to regularization with a cutoff frequency of < 0.04Hz was used to remove non-stationarities. Frequency domain measures were estimated by Welch’s periodogram method using window width of 300 seconds with 50% overlap and smoothing window of 0.02Hz.