FOURIER-ANALYSIS VERSUS MULTIPLE LINEAR-REGRESSION TO ANALYZE PRESSURE-FLOW DATA DURING ARTIFICIAL-VENTILATION

Respiratory resistance (Rrs) and elastance (Ers) are commonly measured in artificially-ventilated patients or animals by multiple linear reg ression of airway opening pressure (Pao) versus flow (V') and volume ( V), according to the first order model: Pao=P-0 + Ers.V + Rrs.V', wher e P-0 is the static recoil pressure at end-expiration. An alternative way to obtain Rrs and Ers is to derive them from tbe Fourier coefficie nts of Pao and V' at the breathing frequency. A potential advantage of the second approach over the first is that it should be insensitive t o a zero offset on V' and to the corresponding volume drift. The two m ethods were assessed comparatively in six tracheotomized, paralysed an d artificially ventilated rabbits with and without adding to V' an off set equal to 5% of the mean unsigned flow. The 5% flow offset did not modify the results of Fourier analysis, but increased Rrs and Ers from linear regression by 15.8+/-14.6% and 4,55+/-0.64%, respectively. Wit hout additional offset, differences between the two methods averaged 3 0.2+/- 14.0% for Rrs and 9.3+/-6.2% for Ers. The differences almost co mpletely disappeared (2.47 and 0.61%, respectively) when the flow sign al was zero-corrected using the assumption that inspired and expired v olumes were the same. After induced bronchoconstriction, however, Ers was still slightly larger by linear regression than by Fourier analysi s, which may result from nonlinearities and/or frequency dependence of the parameters. We conclude that the regression method requires zero flow correction and that Fourier analysis is an attractive alternative .