Determinants of distortions in CO2 catheter sampling systems: a mathematical model.

A mathematical model was developed to predict the distortions in the measured expiratory CO2 concentration time profile caused by catheter sampling systems. Parameters considered included the expiratory flow and concentration profiles, the sample flow rate, the sample tube dimensions and the sample cell volume. Unless care is taken in choosing the parameters of a catheter sampling system, particularly in the presence of low expiratory flow rates and short expiratory times, the CO2 concentration profile may be severely distorted. In worst case situations end tidal CO2 plateaux may not be recognized or alternately artifactual plateaux may be produced. Sample flow rate and sample cell volume are the most important parameters while distortion within the sample tube itself is relatively less important.