Measurement of Cardiac Output by Carbon Dioxide Rebreathing Methods
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Cardiac output may be measured noninvasively by applying the Fick principle to CO2; CO2 output is measured by expired gas analysis and the veno-arterial CO2 content difference is obtained from estimates of PVCO2 and PaCO2. PVCO2 is determined using the lung as a tonometer for the equilibration of CO2; two main methods are available. In the Defares or exponential method, a low concentration of CO2 is initially rebreathed. Complete equilibration is not reached between the lung and rebreathing bag and the PvCO2 is calculated as the asymptote of the exponential rise in PETCO2 during rebreathing and prior to recirculation. Even though several mathematical methods can be used to calculate PvCO2, the most precise is an iterative statistical analysis to obtain the best-fit curve for PETCO2 with time, from which PvCO2 is obtained from PETCO2 at 20 seconds after the start of rebreathing. In the Collier or equilibrium method, a bag having CO2 concentration higher than PvCO2 is rebreathed. If an appropriate initial bag volume CO2 has been selected, equilibration will occur in the lung-bag system, recognized as a plateau in the PCO2 rebreathing record. If a plateau is not obtained, PvCO2 can be estimated by extrapolating the line joining the points of expired PCO2 during the 8th and 12th seconds of rebreathing to that of the 20th second. With the equilibrium method, the plateau PCO2 is systematically higher than PvCO2 (downstream effect) and a correction is applied to obtain PvCO2. PaCO2 can be estimated from PETCO2 or from the mixed-expired PCO2 and an assumed physiologic dead space, except in patients with abnormal lung function, in whom PaCO2 must be measured directly. The content of CO2 in blood may be calculated from PCO2 by the equation: In(CCO2) = [0.396 X In(PCO2)] + 2.38 More complex algorithms are available to calculate CCO2 if the pH, hemoglobin, and arterial O2 saturation are widely divergent from resting values. The indirect Fick method applied to CO2 during exercise can be used to obtain a valid and reproducible measurement of Q comparable to that obtained by invasive methods.
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