Correlation properties of tidal volume and end-tidal O2 and CO2 concentrations in healthy infants

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Abstract
We investigated whether breath-to-breath fluctuations in tidal volume (Vt) and end-tidal O2 and CO2exhibit long-range correlations and whether parameters describing the correlations can be used as noninvasive descriptors of control of breathing. We measured Vt and end-tidal O2 and CO2 over n = 352 ± 104 breaths in 26 term, healthy, unsedated infants (mean age ± SD: 36 ± 6 days) and calculated the detrended fluctuation function [F( n)]. The F( n) of the breath-to-breath time series of Vt, O2, and CO2 revealed a linear increase with a breath number on log-log plots with a slope that was significantly different from 0.5 (random) and thus consistent with scale-invariant behavior. Long-range correlations were stronger for O2 than for Vt and CO2. The F( n) of many individual signals exhibited a crossover behavior indicating that control mechanisms regulating fluctuations of Vt, O2, and CO2 may be different on different time scales. Thus breathing has a memory up to at least 400 breaths that can be characterized by the simple indicator α.