Tidal breathing model describing end-tidal, alveolar, arterial and mixed venous CO₂ and O₂.
Thermodilution is the current standard for determination of cardiac output. The method is invasive and constitutes a risk for the patient. As an alternative CO₂ rebreathing allows non-invasive cardiac output estimation using Ficks principle. The method relies on estimation of arterial CO₂ partial pressure from end-tidal CO₂ pressure and estimation of mixed venous CO₂ partial pressure from end-tidal CO₂ during rebreathing. Presumably the oxygenation of blood in the lung capillaries increases lung capillary CO₂ pressure due to the Haldane effect, which during rebreathing may result in overestimation of the mixed venous CO₂ pressure. However, the Haldane effect is not discussed in the current literature describing cardiac output estimation using CO₂ 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 CO₂ rebreathing technique. The model simulations show agreement with previous studies describing end-tidal to arterial differences in CO₂ pressure and rebreathing with high and low O₂ fractions in the rebreathing bag. In conclusion the simulations show that caution has to be taken when using end-tidal measurements to estimate CO₂ pressures, especially during rebreathing where the Haldane effect causes mixed venous CO₂ partial pressure to be substantially overestimated.
Poulsen
Peter
P
Center for Model-Based Medical Decision Support, Aalborg University, Niels Jernes Vej 14, DK-9220 Aalborg East, Denmark.
Karbing
Dan S
DS
Rees
Stephen E
SE
Andreassen
Steen
S
D002245:Carbon Dioxide / Q000097:blood*; D006801:Humans; D008962:Models, Theoretical; D010100:Oxygen / Q000097:blood*; D012119:Respiration; D018570:Risk Assessment
D002245:Carbon Dioxide; D010100:Oxygen