Comparison of Exercise Cardiac Output by the Fick Principle Using Oxygen and Carbon Dioxide

Theoretically, cardiacoutput (CO) calculated by the Fick principle should be the same using, O2 (CO[o2]) or CO2(CO[co2]) as the test gas. However, agreementdepends on the accuracy of gas exchange and blood gas measurements and the validity of the equations to convert measured variables into bloodgas contents. Considering the widespread use of indirect estimates ofpulmonary artery blood Pco2 and CO2content to measure Fick principle CO during exercise, we wished todetermine whether CO[o2] and , CO[co2] were equal during exercise and whether CO[co2] could be accurately and precisely determined using direct measures of pulmonary arteryblood. Five healthy youngnonsmoking volunteer men performed incremental exercise from rest topeak exercise on two separate occasions with intervening rest. Catheters were placed in brachial and pulmonary arteries to allowrepeated blood sampling every minute during concurrent breath-by-breathgas exchange measurements from rest to peak exercise. CO[o2] was compared with, CO[co2] at multiple levels of exercise. Usingstandard equations, arterial and mixed venous O2 contentswere calculated from hemoglobin concentration (Hb), oxyhemoglobinsaturation (So2), and , Po2, whereas CO2 contents werecalculated from Pco2, p, H, Hb, and , So2. Blood gas analyzers were used formeasurement of p, H, Pco2, and , Po2, and a co-oximeter was used for measurementof Hb and So2. Initial calculations suggestedthat exercise CO[co2] was 14% higher than CO[o2] and helped disclose smallsystematic measurement errors in Pco2 forvalues > 45 mm Hg detected by proficiency testing surveys and documented with blood tonometry in the blood gas analyzer. After correcting Pco2 forthe small systematic measurement error found, the measures and equations used to calculate arterial and mixed venous O2and CO2 contents were adequate to provide mean CO valuesthat are reasonably similar. However CO[co2]values were more than twice as variable as, CO[o2]. Theincreased variability of Fick principle, CO[co2] compared with, CO[o2] is attributable to the much lowerextraction ratio for CO2 and the greater complexity incalculation of blood CO2 than O2 contents. These results raise concerns about the accuracy and precision ofestimating CO and stroke volume using CO2 as a test gas, even with direct measurement of blood CO2 contents innormal subjects.