End-tidal estimates of arterial PCO₂ for cardiac output measurement by CO₂ rebreathing: A study in patients with cystic fibrosis and healthy controls

We set out to determine the effects of various estimates of arterial PCO₂ (PaCO₂) on calculation of cardiac output (Q) by the indirect Fick (CO₂) method in healthy children and children with cystic fibrosis (CF), and to develop a prediction equation for children for PaCO₂, based on end-tidal PCO₂ (PetCO₂). The study had 3 parts: 1) Twenty-three healthy children exercised lightly and moderately while arterialized capillary blood gases and PetCO₂ were measured simultaneously so that a prediction equation for PaCO₂ could be derived from PetCO₂. Cardiac output was measured by CO₂ rebreathing at each workload; different values for PaCO₂ (measured in arterialized capillary blood, end-tidal, and PaCO₂ derived from the Bohr equation assuming normal dead space) were used to calculate Q; 2) our equation PaCO₂ = 0.647 PetCO₂ + 12.4 was tested prospectively to measure Q in 9 healthy children; and 3) cardiac output based on arterialized capillary PaCO₂ was compared with that based on Jones-corrected PetCO₂ during light and moderate exercise in 16 CF patients whose forced expiratory volume in 1 second (FEV₁) range from normal to 37% predicted. Our results have shown that in healthy children end-tidal based-estimates of PaCO₂ tended to overestimate Q, whereas PaCO₂ values derived by the Bohr equation and assuming normal dead space tended to underestimate Q, compared with Q calculated from directly measured PaCO₂. Our prediction equation resulted in good agreement compared with directly measured PaCO₂ when used to calculate Q (mean difference, +1.3%; range, +9% to -13%). CF patients with little or no airway obstruction had results similar to healthy controls, but those with severe airway obstruction had lower values for Q when PetCO₂ was used instead of directly measured PaCO₂. We conclude that estimates of PaCO₂ from PetCO₂ are not reliable in patients with moderately severe pulmonary disease due to CF. In healthy children, the prediction equation for PaCO₂ from PetCO₂ derived in the present study gives results superior to other bloodless methods currently in use for computation of Q by the indirect Fick (CO₂) method.