Direct accurate gas flow measurement in the patient: compensation for unavoidable error.

It is shown that the conditions for accurate flow measurement are not met if the resistant flow meter (e.g., Fleisch pneumotachograph or screen pneumotachograph) is attached directly at the mouth or endotracheal tube and the breath flows directly through it, firstly because its gas composition, temperature, and humidity change radically even within the course of one respiratory cycle, secondly because the expiratory peak flow of the patient being ventilated rapidly tends to become too high, and thirdly because the entire system is sensitive to turbulence. Methods are proposed to compensate continuously for the influence of the changing gas concentrations and to reduce expiratory peak flow without increasing resistance. With the resulting reduction in the error from 20% to about 2%, tidal volume can be more reliably determined, and the higher quality of primary data allows a more differentiated and more complex evaluation (N2-washout compartment analysis, VDS measurement, analysis of complicated patterns of spontaneous breathing or mechanical ventilation such as IMV, etc.).