Comparison of cardiac output measured by two automated methods of CO2 rebreathing

Purpose: The aim of the present study was to investigate the reproducibilit y of the exponential method of CO, rebreathing with the use of automated cu rve fitting and to determine whether this method is superior to the equilib rium method in terms of reproducibility and clinical practicability. Method s: Repeated measurements of cardiac output were performed using the automat ed equilibrium and exponential methods. These measurements were compared in 12 healthy male subjects at rest and during incremental exercise tests. Re sults: Estimated cardiac output was not significantly different between dup licate measurements at rest nor at any level of exercise with either method . At rest the exponential method showed a tendency reward larger variabilit y than the equilibrium method. The exponential method produced significantl y higher (P less than or equal to 0.001) estimates at rest (averaging up to 9.8 L.min(-1)) compared with the equilibrium method (averaging up to 6.5 L .min(-1)). Reproducibility improved for both methods with increasing worklo ads, and a second measurement at rest also seemed more reproducible and val id than the first. During exercise, both methods produced comparable values for cardiac output, and highly significant relations between cardiac outpu t and oxygen uptake were observed for both methods (ranging from r(2) = 0.7 9 to r(2) = 0.88, P less than or equal to 0.001). The equilibrium method pr oduced unpleasant side effects more frequently (75% vs 21%, P less than or equal to 0.001) compared with the exponential method and lead more subjects to premature interruption of the exercise test because of the rebreathing maneuver (42% vs 17%, P = 0.058). Conclusions: Automated curve fitting for the exponential method gave reproducible and valid results during submaxima l and maximal exercise but not at rest. The equilibrium method on the other hand interfered with exercise. Therefore, the equilibrium method is recomm ended at rest and at lower levels of exercise and the exponential method at higher intensities.