Reduction of correlation dimension in human respiration by inhaling a mixture gas of 5% carbon dioxide and 95% oxygen

In order to investigate the effect of the respiratory control system on deterministic behavior in respiration, we used nonlinear analysis in subjects breathing a mixture gas of 5% carbon dioxide (CO 2) and 95% oxygen (O 2) (CO 2–mixed gas). The respiratory movements during breathing air or CO 2–mixed gas in eight healthy volunteers were measured. We estimated the values of the correlation dimension (D2) in respiratory movement using Grassberger–Procaccia algorithm. The respiratory movements during inhaling either air or CO 2–mixed gas showed a nonlinear behavior using surrogate data method. The values of D2 in respiratory movement during inhaling CO 2–mixed gas (1.77 ± 0.17) were significantly smaller than those during inhaling air (2.52 ± 0.60) ( P