Effect of hyperoxic hypercapnia on variational activity of breathing

Dysrhythmias of breathing occur in several clinical disorders, but their mechanistic basis is obscure. To understand their pathophysiology, factors responsible for the variability of breathing need to be defined. We studied the effect of hyperoxic hypercapnia (CO₂) on the variational activity of breathing in 14 volunteers before and after delivering CO₂ nonobstrusively via a plastic hood. Compared with air, CO₂ increased the gross variability of minute ventilation (V̇I) and tidal volume (VT), and decreased that of inspiratory time (TI) and expiratory time (TE) (all p < 0.03). CO₂ increased the autocorrelation coefficient at a lag of one breath for V̇I (p < 0.05), the number of consecutive breath lags having significant autocorrelation coefficients for V̇I and VT (both p < 0.01), and the cycle time of oscillations in V̇I (p = 0.03) and VT (p = 0.04). Uncorrelated random behavior constituted ≥ 80% of the variance of each breath component, correlated behavior represented 9 to 20%, and oscillatory behavior represented < 1% during both air and CO₂. CO₂ increased the correlated behavior of volume components, which was accompanied by development of low-frequency oscillations with a cycle time consistent with central chemoreceptor activation.