Tidal breathing model describing end-tidal, alveolar, arterial and mixed venous CO 2 and O 2

Abstract

Thermodilution is the current standard for determination of cardiac output. The method is invasive and constitutes a risk for the patient. As an alternative CO 2 rebreathing allows non-invasive cardiac output estimation using Ficks principle. The method relies on estimation of arterial CO 2 partial pressure from end-tidal CO 2 pressure and estimation of mixed venous CO 2 partial pressure from end-tidal CO 2 during rebreathing. Presumably the oxygenation of blood in the lung capillaries increases lung capillary CO 2 pressure due to the Haldane effect, which during rebreathing may result in overestimation of the mixed venous CO 2 pressure. However, the Haldane effect is not discussed in the current literature describing cardiac output estimation using CO 2 rebreathing. The purpose of this study is to construct and verify a compartmental tidal breathing lung model to investigate the physiological mechanisms that influence the CO 2 rebreathing technique. The model simulations show agreement with previous studies describing end-tidal to arterial differences in CO 2 pressure and rebreathing with high and low O 2 fractions in the rebreathing bag. In conclusion the simulations show that caution has to be taken when using end-tidal measurements to estimate CO 2 pressures, especially during rebreathing where the Haldane effect causes mixed venous CO 2 partial pressure to be substantially overestimated.