Contribution of chemical feedback loops to breath-to-breath variability of tidal volume.

We tested whether chemical feedback loops contribute to the breath-to-breath correlations seen in respiratory cycle parameters. We have analyzed tidal volume (VT) of seven anesthetized, vagotomized, spontaneously breathing rats and the peaks of the 'integrated' phrenic neurograms (P) of another twelve anesthetized, vagotomized, paralyzed and artificially ventilated rats. Animals were studied while breathing 100% O2, 4% CO2 in O2 and/or room air. Our analysis consisted of fitting a first-order autoregressive (AR1) model to each measured variable in steady-state conditions. We found that: (1) breath-to-breath fluctuations of VT could be described by a first-order autoregressive model in which the fluctuation of VT on each breath from its mean level is correlated with that of the immediately preceding breath; (2) breath-to-breath fluctuations of P were not correlated with those of previous breaths, because successive values of P were uncorrelated random variables. Hypercapnia enhanced correlations in VT (3 rats) but not in P (12 rats). We propose that breath-to-breath correlations of VT in anesthetized, vagotomized and spontaneously breathing rats may reflect the effects of noise in respiratory chemical feedback loops.