Figure 2. Infant heart period delay (HPD) values measured from cross-spectral analysis.

Values of HPD calculated from the gain of the transfer function between spontaneous fluctuations in blood pressure and heart period in a low frequency band of 0.04–0.15 Hz. Blood pressure and heart period were recorded continuously in 1–2 min epochs (n= 180) in 30 infants aged 2–4 weeks, 2–3 months and 5–6 postnatal age. Numbers on the x-axis represent the individual infants studied numbered 1 to 30. Note the variability both within and between infants.