EFFECT OF ELASTIC LOADING ON VARIATIONAL ACTIVITY OF BREATHING

To examine the effect of elastic loading on variational activity of br eathing, we studied 11 healthy subjects breathing at rest and with ins piratory elastic loads of 9 and 18 cm H2O/L, applied randomly for 1 h each. Compared with rest, a load of 18 cm H2O/L decreased gross variab ility, quantitated as standard deviation, of tidal volume (VT) and exp iratory time (TE) (p < 0.01 in both instances) but increased that of i nspiratory time (TI) (p < 0.03). The autocorrelation coefficients at a lag of 1 breath for each breath component were not altered by elastic loading, although the number of breath lags with significant serial c orrelations for TE tended to increase with a load of 18 cm H2O/L (p = 0.08). A load of 18 cm H2O/L decreased only the fraction of variationa l activity of VT and TE due to uncorrelated, random behavior (white no ise), while it increased that fraction for TI (p < 0.05 in each instan ce); the correlated and oscillatory fractions did not change. Uncorrel ated random behavior constituted > 87% of the variance of each breath component, correlated behavior represented 3 to 11%, and oscillatory b ehavior represented < 1.5% during both rest and loaded breathing. Elas tic loading changed the gross variability of each primary breath compo nent by altering the random fraction of variational activity; it had n o significant effect on the structured, correlated fraction. We specul ate that the observed changes in variational activity may reflect an a ttempt by the controller to compensate for the increased load while si multaneously minimizing load-induced dyspnea.