Correlation Between Fontan Pathway Diameter and Inferior-Superior Vena Cava Gradients in Adults Undergoing Exercise Catheterization

The ideal Fontan pathway size remains unknown, with some advocating optimal conduit sizes of 18 to 20 mm during implantation and 16 to 18 mm late post-palliation.¹ A well-recognized long-term complication, diagnosing significant Fontan obstruction remains challenging given the lack of established criteria and common discrepancies between angiographic and hemodynamic data. Potential explanations for these discrepancies include the inherently underwhelming gradients seen in venous stenoses, nonpulsatile low-output state post-Fontan, and decompressing venovenous collaterals. We reported using exercise catheterization, an established tool for diagnosing occult heart failure and pulmonary vascular disease, to unmask the hemodynamic burden of Fontan conduit obstruction in Fontan-associated liver disease.² Following these preliminary observations, we report herein the correlation between Fontan pathway size and invasive exercise transconduit/pathway gradients.

All supporting data are available within the article. Eighteen consecutive adults (age ≥18 years) post-Fontan undergoing exercise venous catheterization using a supine bicycle protocol³ between July 2021 and June 2022 with inferior vena cava (IVC) pressure measured at peak exercise were analyzed. To allow simultaneous measurements, superior vena cava (SVC) pressures were measured as surrogates for pulmonary artery pressures using the sidearm of an 8F internal jugular sheath, whereas resting and exercise IVC pressures were measured via 7F balloon-tipped catheters or a pressure-wire. Pressure measurements represent averages of ≥5 consecutive cardiac cycles obtained under spontaneous breathing. Minimal Fontan pathway/conduit diameters on anteroposterior or lateral views using biplane angiography⁴ were abstracted from catheterization reports or measured offline when unavailable (n=3). If invasive angiography was not performed (n=1), computerized tomography was used instead. Comparisons between variables were performed by simple linear regression or paired analyses. To account for multiple linear regression analyses, P<0.01 were considered significant (Bonferroni method). The institutional review board approved the study; only patients providing research authorization were included.

Age at catheterization was 28 (interquartile range, 23–35) years and at Fontan palliation 3 (interquartile range, 2–5) years; 10 (56%) were female. Body mass index was 23.9±5.4 kg/m². Double inlet left ventricle was present in 6 patients (33%), double outlet right ventricle in 4 (22%), tricuspid atresia and pulmonary atresia/intact ventricular septum in 3 each (17%), and hypoplastic left heart syndrome in 2 (11%). Fontan connections were extracardiac conduit in 12 (67%), lateral tunnel in 5 (28%), and intra-atrial in 1 (6%). Two (11%) patients underwent Fontan stenting during a prior procedure and 2 (11%) had patent fenestrations. Prior atrial arrhythmias were present in 7 patients (39%) and pacemaker implantation in 8 (44%); 9 (50%) were on loop diuretics. Ventricular ejection by echocardiography was 55% (interquartile range, 41–62).

Resting pulmonary artery pressure was 11.8±2.6 mm Hg, and pulmonary artery wedge pressure was 7.6±2.7 mm Hg. Cardiac index was 2.8±0.9 L/(min·m²). Simultaneous resting IVC and SVC pressures were 13.1±2.9 and 12.0±3.0 mm Hg, respectively, and the IVC-SVC gradient was 1.1±1.5 mm Hg. At peak exercise (62.2±27.3 W), the IVC-SVC gradient increased to 4.6±5.3 mm Hg (P<0.0001; bottom panel) and IVC pressure to 26.7±7.8 mm Hg. Cardiac output increased by 3.6±3.0 L/min, being related to ΔIVC-SVC gradients (r=0.74; P=0.005).

Minimal Fontan conduit/pathway diameter was 15.9±5.8 mm. The original conduit size was known in 10 patients, being larger than at the time of catheterization (19.1±1.9 versus 12.6±3.0 mm; P=0.002). Minimal Fontan diameter was significantly related to exercise (r=-0.61, P=0.007; upper right) but not resting (r=−0.54, P=0.02; upper left) IVC-SVC gradients. Minimal Fontan diameter indexed for body surface area was 9.1±3.4 mm/m², being inversely related to exercise (r=-0.66; P=0.003) but not resting (r=-0.43; P=0.07) IVC-SVC gradients (Figure).

We acknowledge the heterogeneous sample and the complexities of Fontan hemodynamics going beyond pathway/conduit size. However, to the best of our knowledge, this is the first attempt to correlate Fontan pathway size and invasive exercise IVC hemodynamics in adults. Several observations deserve highlighting: (1) exercise IVC-SVC gradients were inversely related to Fontan pathway size and appeared to better correlate with minimal Fontan diameter than resting values; (2) exercise provided incremental hemodynamic information in those with low (1–2 mm Hg) resting gradients, whereas no significant gradient was provoked in those with no resting gradient; (3) most patients with minimal Fontan diameter <16 mm at catheterization had exercise IVC-SVC gradients ≥5 mm Hg, with several demonstrating exercise gradients >10 mm Hg. Conversely, none with a minimal diameter ≥16 mm experienced resting or exercise IVC-SVC gradient >3 and 5 mm Hg, respectively; (4) similar to the concerns regarding indexing cardiac chamber size by body surface area in current practice,⁵ indexed Fontan diameter did not appear to outperform nonindexed values.

Our initial experience agrees with prior reports of Fontan diameters <16 mm being associated with unfavorable hemodynamics late post-palliation.¹ Accordingly, it suggests that hemodynamically significant obstruction should be suspected in this subset of adults and that exercise provides incremental information regarding transconduit/pathway hemodynamics in those with minimal resting gradients. We hope our observations stimulate subsequent studies on the correlation between Fontan conduit size and exercise IVC hemodynamics as well as their impact on functional capacity and liver disease.