Respiratory inductive plethysmography to assess respiratory variability and complexity in humans.

Human ventilation is aperiodic, exhibiting a breath-by-breath variability and a complexity of which the characteristics may be interesting physiologically and clinically. In the present study, we tested the ability of respiratory inductive plethysmography (RIP) to describe these properties. Indeed, RIP does not have the effects on ventilation described with mouthpiece measurements. We compared the ventilatory flow recorded with a pneumotachograph (V'PNT) and the ventilatory flow derived from the mathematical treatment of the thoracoabdominal motion signals obtained from a particular type of RIP (V'RIP, Visuresp, Meylan, France) in 8 freely breathing normal subjects. Using the Z correlation coefficient, Passing-Bablock regressions and Bland and Altman graphical analyses, we compared the coefficients of variation of the main discrete respiratory variables determined with V'PNT and V'RIP and a set of nonlinear descriptors including the noise limit (chaotic nature of the signal), largest Lyapunov exponent (sensitivity to initial conditions), the Kolmogorov-Sinai entropy (unpredictability) and the correlation dimension (irregularity). When the recordings were obtained with the two techniques simultaneously, all the measurements were correlated and interchangeable. RIP can be safely used to quantify the breath-by-breath variability of ventilation and to study the complexity and the chaotic behavior of the ventilatory flow.