Effects of mechanical ventilation on the measurement of cardiac output by thermodilution

Phasic variation in cardiac output (CO) of 10-50% during mechanical ventilation was documented in dogs, and in 1 patient by injection of thermal indicator at 1-sec intervals throughout the ventilation cycle. Measured flow variation increased with inflation pressure when other variables were unchanged, and was not influenced to a significant degree by the injection of iced vs room temperature injectate. Such variation can occur in adult patients requiring only minimally increased airway pressure for ventilation. The technique of injecting indicator at a particular moment of the ventilation cycle is not appropriate to monitor absolute or even relative changes in CO because of occasional changes in the phase relation between the ventilation cycle and the flow variation. The mean CO by thermodilution technique is approximately best by the mean of values taken at regularly spaced intervals through the ventilation cycle.