Cardiac Output During Exercise: Thermodilution Versus Direct Fick

Accurate cardiac output (CO) measurements during exercise are essential for the diagnosis of exercise pulmonary hypertension (PH), exercise precapillary PH, and exercise postcapillary PH. The purpose of this study is to compare performance of thermodilution CO to gold-standard direct Fick CO (dfCO) at rest and exercise.

A single-center prospective cohort study of patients undergoing invasive cardiopulmonary exercise test over a 3-year period. For the primary outcome, we predicted CO at each stage of exercise and recovery using generalized additive modeling. In secondary analysis, we assessed mean differences in CO across exercise stages and Bland-Altman analysis at rest and peak exercise. Finally, we assessed the impact in classification of exercise PH, exercise precapillary PH, and exercise postcapillary PH between the 2 CO methods.

A total of 302 patients were included. In the primary analysis, the generalized additive model smooth term was significantly different between thermodilution and dfCO (P<0.001), with thermodilution underestimating dfCO at rest and overestimating dfCO at max exercise. Wide limits of agreement were noted between thermodilution and dfCO, particularly at peak exercise (-0.08 (-4.5 to +4.35) L/min). The classification of exercise PH, exercise precapillary PH, and exercise postcapillary PH was not significantly different between CO methods (4, 3, and 1 patient misclassified by thermodilution, respectively). CO reserve using thermodilution was overestimated compared with dfCO (96.66% versus 88.45%, P<0.001).

There are differences and wide limits of agreement between thermodilution and dfCO both at rest and during exercise. Sequential measurement of thermodilution during exercise with computing of mPAP/CO and PAWP/CO slopes, reduced the clinical impact of the differences in the diagnosis of exercise PH or exercise postcapillary PH.