Comparison of cardiac output determined by different rebreathing methods at rest and at peak exercise
Several rebreathing methods are available for cardiac output (QT) measurement. The aims of this study were threefold: first, to compare values for resting QT produced by the equilibrium-CO2, exponential-CO2 and inert gas-N2O rebreathing methods and, second, to evaluate the reproducibility of these three methods at rest. The third aim was to assess the agreement between estimates of peak exercise QT derived from the exponential and inert gas rebreathing methods. A total of 18 healthy subjects visited the exercise laboratory on different days. Repeated measures of QT, measured in a seated position, were separated by a 5 min rest period. Twelve participants performed an incremental exercise test to determine peak oxygen consumption. Two more exercise tests were used to measure QT at peak exercise using the exponential and inert gas rebreathing methods. The exponential method produced significantly higher estimates at rest (averaging 10.9 l min-1) compared with the equilibrium method (averaging 6.6 l min-1) and the inert gas rebreathing method (averaging 5.1 l min-1; P < 0.01). All methods were highly reproducible with the exponential method having the largest coefficient of variation (5.3%). At peak exercise, there were non-significant differences between the exponential and inert gas rebreathing methods (P = 0.14). The limits of agreement were -0.49 to 0.79 l min-1. Due to the ability to evaluate the degree of gas mixing and to estimate intra-pulmonary shunt, we believe that the inert gas rebreathing method has the potential to measure QT more precisely than either of the CO2 rebreathing methods used in this study. At peak exercise, the exponential and inert gas rebreathing methods both showed acceptable limits of agreement. © Springer-Verlag 2007.