Analysis of end-tidal and arterial PCO2 gradients using a breathing model

The aim of this paper was to analyse the difference between end-tidal carbon dioxide tension (PETCO2) and arterial carbon dioxide tension (PaCO2) at rest and during exercise using a homogeneous lung model that simulates the cyclic feature of breathing. The model was a catenary two-compartment model that generated five non-linear first-order differential equations and two equations for gas exchange. The implemented mathematical modelling described variations in CO2 and O2 compartmental fractions and alveolar volume. The model also included pulmonary capillary gas exchange. Ventilatory experimental data were obtained from measurements performed on a subject at rest and during four 5-min bouts of exercise on a cycle ergometer at 50, 100, 150 and 200 W, respectively. Analysis of the PETCO2-PaCO2 difference between experimental and sinusoidally adjusted ventilatory flow profiles at rest and during exercise showed that the model produced similar values in PETCO2-PaCO2 for different respiratory flow dynamics (P approximately equal to 0.75). The model simulations allowed us to study the effects of metabolic, circulatory and respiratory parameters on PETCO2-PaCO2 at rest and during exercise. During exercise, metabolic CO2 production, O2 uptake and cardiac output affected significantly the PETCO2-PaCO2 difference from the 150-W workload (P < 0.001). The pattern of breathing had a significant effect on the PETCO2-PaCO2 difference. The mean (SD) PETCO2-PaCO2 differences simulated using experimental profiles were 0.80 (0.95), 1.65 (0.40), 2.40 (0.20), 3.30 (0.30) and 4.90 (0.20) mmHg, at rest and during exercise at 50, 100, 150 and 200 W, respectively. The relationship between PETCO2-PaCO2 and tidal volume was similar to data published by Jones et al. (J Appl Physiol 47: 954-960, 1979).