Gaining insight into the factors that influence the variability of breathing

Remarkably little is known about the mechanisms that regulate the pattern of breathing, and most research on the control of breathing has been confined to measuring mean responses of the respiratory controller to physiological perturbations. We analyzed variability of breathing in healthy subjects under different conditions by partitioning variational activity in a random and a non-random (correlated and oscillatory) fraction. For volume components, hyperoxic hypercapnia decreased the relative contribution of random variability and increased the non-random behavior; it also increased the number of consecutive breath lags displaying significant autocorrelation coefficients ("short-term memory"). The increases in correlated behavior and "short-term memory" during hyperoxic hypercapnia may have been the consequence of carbon dioxide-mediated increases in afterdischarge of the respiratory controller. Elastic loading increased the random fraction of variational activity in inspiratory time whereas it decreased the random variability of tidal volume. As a means of minimizing dyspnea when compensating for an inspiratory load, the advantage of prolonging inspiratory time as opposed to increasing the magnitude of inspiratory muscle effort may explain the opposing changes in the random fractions of tidal volume and inspiratory time. A resistive load of 3 cm. H2O/L/sec decreased the random variability of breath components, whereas a load of 6 cm. H2O/L/sec increased random variability compared to both rest and the smaller load. The proximity of the smaller resistive load to the perception threshold can explain this apparent paradox. In conclusion, we have shown that hyperoxic hypercapnia, elastic loading, and resistive loading cause specific and unique changes in different fractions of variational activity; the insight gained from analysis of respiratory variability advances our understanding of the control of breathing.