Cardiac Index Measured Using the Fick Principle Versus Thermodilution in Children Following Orthotopic Heart...

Cardiac index (CI) is used to assess graft function and calculate pulmonary vascular resistance (PVR) following orthotopic heart transplantation (OHT). The gold standard is thermodilution (TD), but in practice, the Fick method using the LaFarge VO₂ estimation is commonly used. We hypothesize that the difference between cardiac index and PVR using the estimated Fick principle versus thermodilution decreases over time following OHT in pediatric patients. This is a single center prospective observational cohort study of patients who received an orthotopic heart transplant between 5 and 18 years of age. We performed serial pair-wise comparisons of cardiac index measured using the Fick method (LaFarge VO₂ estimation) and thermodilution. PVR was calculated using both the Fick CI and the thermodilution CI. Descriptive statistics, multivariable model, linear mixed-effects model, intra-class correlation, absolute difference, and unpaired t-test statistical methods were utilized to analyze the data. In 48% of cases, cardiac index was normal by Fick but abnormal using thermodilution. In 27% of cases, PVR was normal by Fick but abnormal using thermodilution. The ICC between Fick CI and TDCI was 0.13, indicating low agreement between the two methods. The absolute difference between Fick and TD did not vary significantly by pre-transplant use of VAD, milrinone, inpatient hospitalization, or indication for transplant. There was not a significant relationship between days since transplantation and absolute difference between Fick and TDCI. Upon controlling for patient level differences, TDCI did not significantly change over time since OHT. There is poor agreement between the estimated Fick method, using the LaFarge equation, and thermodilution in determining cardiac index after pediatric heart transplantation. When providers only use the Fick method, they are under-estimating cardiac dysfunction and pulmonary hypertension, two very important measures of graft function and post-transplant health. These differences between Fick and thermodilution persist for at least the first year after heart transplant.