Researchers at Ann & Robert H. Lurie Children’s Hospital of Chicago found poor agreement between two common methods of measuring cardiac index in children who received heart transplants. When clinicians relied on the Fick method with the LaFarge formula, they missed signs of reduced heart function in 48% of cases and missed elevated pulmonary vascular resistance in 27% of cases that thermodilution, the gold-standard method, correctly identified as abnormal. The two methods showed only weak statistical agreement overall, and this gap did not narrow during the year following transplant.
Cardiac Index Guides Critical Decisions After Pediatric Heart Transplant
Cardiac index measures how much blood the heart pumps relative to a patient’s body size. Physicians use it to judge how well a transplanted heart is functioning and to calculate pulmonary vascular resistance, a measure of pressure in the lung’s blood vessels. Both values shape decisions about medications, including drugs used to treat pulmonary hypertension in transplant recipients.
Thermodilution, which involves injecting a small amount of cold saline into the bloodstream and tracking how it disperses, is considered the reference standard for measuring cardiac index during cardiac catheterization. In routine practice, however, clinicians often use the Fick method instead, because it works faster and costs less. The Fick method calculates cardiac index using an estimate of oxygen consumption rather than a direct measurement, and the LaFarge formula is the most common way to generate that estimate. The formula relies only on a patient’s age, sex, and heart rate.
Adult Studies Linked Heart Failure to Inaccurate Oxygen Estimates
Prior research in adults established that resting oxygen consumption changes with congestive heart failure, pulmonary hypertension, and low cardiac output. Other adult studies found that resting oxygen consumption drops as heart failure severity increases. No comparable studies had examined this relationship in children. Because pediatric heart transplant recipients are frequently in heart failure both before and immediately after surgery, the study authors set out to measure how well the estimated Fick method matched thermodilution in this specific population. They hypothesized that any gap between the two methods would shrink over time as patients recovered from transplant.
Researchers Compared Two Measurement Methods Across 174 Catheterizations
The study team conducted a single-center, prospective observational cohort study of patients who received heart transplants between the ages of 5 and 18. They excluded patients with significant shunting between the heart’s chambers, since that condition can distort the measurements. The team analyzed catheterization data collected from November 2021 through February 2024.
During each patient’s post-transplant catheterization, the researchers measured cardiac index using both methods side by side. Clinicians typically performed these procedures at 2 weeks, 4 weeks, 3 months, 6 months, 9 months, and 12 months after transplant as part of standard post-transplant monitoring, and the study made no changes to that schedule. The team recorded thermodilution results as the average of three separate measurements to reduce variability, then used the same catheterization data to calculate pulmonary vascular resistance under both methods. Statistical analysis included a linear mixed-effects model to assess agreement between the two methods and a multivariable model to track changes over time.
Fick Method Overestimated Cardiac Index by Nearly 1 Liter
The final sample included 26 patients and 174 catheterizations, with a median of six procedures per patient. The median cardiac index measured by thermodilution was 2.7 L/min/m², compared with a median of 3.7 L/min/m² by the Fick method — a median difference of just under 1 L/min/m². Statistical agreement between the two methods was low, with an intra-class correlation coefficient of 0.13.
In 84 of 174 cases, the Fick method classified cardiac index as normal when thermodilution classified it as abnormally low. In 47 cases, the Fick method classified pulmonary vascular resistance as normal when thermodilution indicated pulmonary hypertension. This pattern held regardless of whether patients had used a ventricular assist device, taken milrinone, been hospitalized before transplant, or received their transplant for congenital heart disease versus cardiomyopathy; none of these factors significantly affected the size of the gap between methods. The gap also did not shrink over the roughly one-year follow-up period, and thermodilution-measured cardiac index itself did not change significantly over time once the researchers controlled for individual patient differences.
Study Authors Recommend Thermodilution Over Fick Estimation for Transplant Monitoring
The researchers concluded that the LaFarge formula’s assumptions about oxygen consumption do not hold up well in pediatric heart transplant recipients. They proposed several possible explanations, none of which the study directly tested. The LaFarge formula was originally derived from patients who were awake and breathing on their own, while post-transplant catheterizations in this cohort typically involved general anesthesia and mechanical ventilation — conditions known to lower oxygen consumption. Transplanted hearts also lack normal nerve connections, which raises resting heart rate and, in turn, inflates the LaFarge oxygen estimate, since heart rate is one of the formula’s inputs. The authors further noted that heart failure with preserved ejection fraction, common after transplant due to graft ischemia or rejection, has been linked to reduced oxygen consumption capacity in other patient groups.
Because no direct quotes from the authors were available in the published article, this summary reflects the conclusions and reasoning the researchers reported in the text. Based on their findings, the authors recommended that clinicians use thermodilution rather than the Fick method when measuring cardiac index and pulmonary vascular resistance to guide treatment decisions in pediatric heart transplant patients, particularly when adjusting pulmonary hypertension medications.
Single-Center Design and Indirect Oxygen Measurement Limit the Findings
The authors identified several limitations of their work. The study drew on a single center, which restricted the sample size to 26 patients. The team never measured oxygen consumption directly with a metabolic cart, so the results speak only to the LaFarge estimation method and do not evaluate the direct Fick method. Thermodilution itself carries some risk of variability between operators, though the researchers reduced this by averaging three measurements per catheterization. Finally, the team did not test an alternative estimation approach, the Seckeler formula, because it is not standard practice in pediatric cardiology and has not been validated in heart transplant patients. The authors called for further research into what drives the oxygen consumption differences seen in this population.
Citation
Cohen, E., O’Halloran, C., Pilli, N., & Tannous, P. (2026). Cardiac Index Measured Using the Fick Principle Versus Thermodilution in Children Following Orthotopic Heart Transplantation. Pediatric Cardiology. https://doi.org/10.1007/s00246-026-04302-3
This is a sensitive medical topic. Readers with questions about their own or a family member’s post-transplant care should consult their treating cardiologist rather than adjust monitoring or treatment based on this study alone.
