A method of reconstruction of clinical gas-analyzer signals corrupted by positive-pressure ventilation

The use of sidestream infrared and paramagnetic clinical gas analyzers is w idespread in anesthesiology and respiratory medicine. For most clinical app lications, these instruments are entirely satisfactory. However, their abil ity to measure breath-by-breath volumetric gas fluxes, as required for meas urement of airway dead space, oxygen uptake, and so on, is usually inferior to that of the mass spectrometer, and this is thought to be due, in part, to their slower response times. We describe how volumetric gas analysis wit h the Datex Ultima analyzer, although reasonably accurate for spontaneous v entilation, gives very inaccurate results in conditions of positive-pressur e ventilation. We show that this problem is a property of the gas sampling system rather than the technique of gas analysis itself. We examine the sou rce of this error and describe how cyclic changes in airway pressure result in variations in the flow rate of the gas within the sampling catheter. Th is results in the phenomenon of "time distortion," and the resultant gas co ncentration signal becomes a nonlinear time series. This corrupted signal c annot be aligned or integrated with the measured flow signal. We describe a method to correct for this effect. With the use of this method, measuremen ts required for breath-by-breath gas-exchange models can be made easily and reliably in the clinical setting.