A Simple Method for Monitoring the End-organ Function in Patients With Fontan Circulation.

This is a multicenter, observational, cross-sectional study designed to investigate whether a parsimonious set of low-cost, accessible functional, anthropometric, and physiological measurements can help identify multisystem clinical-physiological impairment in patients with Fontan circulation.

Fontan circulation is associated with long-term multisystem complications affecting cardiac, cardiopulmonary, hepatic, renal, nutritional-lymphatic, hematological, and metabolic function. Current surveillance may require multiple specialized, costly, and time-consuming investigations. This study will explore whether simple and easily repeatable measurements can provide an accessible complementary approach for the initial functional and clinical assessment of these patients.

Approximately 150 participants aged 14 to 60 years with Fontan circulation, recruited from 18 Spanish healthcare centers, will be included. Participants must have undergone Fontan surgery at least 2 years before inclusion and be clinically stable in outpatient follow-up.

Each participant will attend one study-specific visit lasting approximately 45-60 minutes. During this visit, functional, anthropometric, and physiological variables will be assessed. Functional measurements will include squat jump performance using the Samozino method and the five-times sit-to-stand test (5xSTS) using the Alcázar method. Anthropometric measurements will include body mass index, age- and sex-adjusted anthropometric indicators of muscularity, and calf and thigh circumferences. Physiological measurements will include resting heart rate and oxygen saturation, heart rate recovery after the 1-minute sit-to-stand test, and post-exercise oxygen saturation.

Clinical, laboratory, echocardiographic, and functional data obtained as part of routine clinical follow-up will also be collected. Whenever possible, routine clinical measurements will correspond to assessments performed on the same day as the functional evaluation; measurements obtained within a maximum window of ±15 days will also be considered.

The primary analysis will evaluate the association between the candidate low-cost measurements and the presence of clinical-physiological impairment affecting two or more organ or functional domains. Additional analyses will investigate their ability to identify patients with impairment in at least one domain and to discriminate increasing levels of multisystem clinical-physiological burden.

The study will not involve any therapeutic intervention or prospective study-specific follow-up. The results will be interpreted as cross-sectional associations and will not establish causal or longitudinal prognostic relationships.