  The mammalian ventilatory behavior exhibits nonlinear dynamics as reflected
by certain nonlinearity or complexity indicators (e.g. correlation dimension,
approximate entropy, Lyapunov exponents...) but this is not sufficient to
determine its possible chaotic nature. To address this, we applied the noise
titration technique, previously shown to discern and quantify chaos in short
and noisy time series, to ventilatory flow recordings obtained in quietly
breathing normal humans. Nine subjects (8 men and 1 woman, 24-42 yrs) were
studied during 15-minute epochs of ventilatory steady-state (10.1 +/- 3.0
breaths/minute, tidal volume 0.63 +/- 0.2L). Noise titration applied to the
unfiltered signals subsampled at 5 Hz detected nonlinearity in all cases (noise
limit 20.2 +/- 12.5%). Noise limit values were weakly correlated to the
correlation dimension and the largest Lyapunov exponent of the signals. This
study shows that the noise titration approach evidences a chaotic dimension to
the behavior of ventilatory flow over time in normal humans during tidal
breathing.
