Predata processing involved NIRS data filtering to reduce undesired parts of measured data, such as noise or trends. A moving Gaussian filter (average) was applied, which is a generic smoothing filter that reduces high frequency noise and has the advantage of corresponding to the weighted mean rather than the unweighted mean, giving a smoother result. The continuously measured regional cerebral oxygenation parameters were then truncated to 30 seconds before the end of B1 period and 300 seconds past the end of the procedure (last touch by a laboratory technician). Some epochs were of constant time length for each infant (eg, recovery phase—300 seconds), the heel lance/squeezing phase varied between infants (eg, 70 seconds ID-17 vs 160 seconds ID-30). To remove remaining artifactual spikes, traces were passed through a Hampel filter with a moving window of ±3 seconds and a threshold of K = 2. This replaces any outliers that are more than 2 median absolute deviation units away from the median of the points in the 3 seconds preceding and 3 seconds postwindow, and replaces these outliers with the median value in the window.32 Traces were then band pass filtered using a third degree Savitzky–Golay filter by removing low-frequency noise with a window of 80 seconds and higher frequency noise with a window of 10 seconds.34 These windows were chosen to remove very short artifactual spikes across the entire TSI collection period. Different filtering parameters did not change which infants were considered to have had significant dips in the TSI during either the heel lance/squeeze procedure or recovery phases. Data processing and statistical analyses were conducted blindly (A.A.).

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