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.