The term “single ventricle” encompasses multiple anatomies in which two fully developed ventricles capable of sustaining the systemic and pulmonary circulations are absent. These cases represent 7.7% of congenital heart diseases. Currently, the standard surgical solution involves several interventions to separate both circulations, establishing what is known as Fontan circulation, where the systemic venous return from the caval veins is directly connected to the pulmonary arteries, creating a passive pulmonary circuit. However, many patients fail to complete Fontan circulation, remaining at intermediate stages of palliation or even retaining their native anatomy. The prognosis and natural history of these patients are not well described in the literature.

The study by Gordon et al. aims to analyze the survival and complications in these patients, categorized into three groups according to the palliation stage:

• G1: Patients with restrictive pulmonary flow (native pulmonary stenosis or banding).
• G2: Patients with Glenn (superior vena cava-to-pulmonary artery connection).
• G3: Patients with systemic-pulmonary shunts (with or without Glenn).

Patients with Eisenmenger syndrome or segmental pulmonary hypertension were excluded.

A total of 120 patients from seven reference centers in Spain were retrospectively included (mean age 32 years, mean follow-up 7.1 years). Mortality, heart transplant rates, hospitalizations for heart failure (HF), the development of atrial arrhythmias (atrial fibrillation and flutter), and reintervention rates were analyzed. Additionally, clinical, laboratory, and ventricular function parameters were assessed at baseline and follow-up, including their longitudinal changes.

The study population represented 22.6% of the total cohort of patients with single-ventricle physiology. The remaining patients were excluded for having completed Fontan circulation (72.5%) or for being in Eisenmenger physiology (4.9%).

Each group displayed a distinct prognostic profile:

• G3 (systemic-pulmonary shunts, n=35): This group exhibited the worst prognosis, with the highest mortality (34.3%), HF hospitalizations, atrial arrhythmias, and reintervention rates. Baseline ventricular and renal function parameters were worse in this group, and they experienced greater ventricular function deterioration during follow-up.
• G1 (restrictive pulmonary flow, n=55): This group had the lowest mortality (12.7%) and more favorable characteristics, including better ventricular function, functional class, and oxygen saturation.
• G2 (Glenn, n=30): This group presented an intermediate prognosis (mortality 16.7%), with a notably lower incidence of HF hospitalizations.

The authors concluded that the type of palliation in patients with single-ventricle physiology and restrictive pulmonary flow who do not undergo Fontan surgery defines distinct profiles. Patients palliated with systemic-pulmonary shunts have a worse prognosis, with increased morbidity and mortality.

COMMENTARY:

This is the largest study to date on single-ventricle patients with restrictive pulmonary flow who reach adulthood without completing Fontan circulation, establishing several phenotypes with distinct physiology. This classification allows for defining the characteristics and prognosis of each group based on the type of palliation.

One key finding is that this population represents nearly a quarter (22.6%) of single-ventricle patients. Reasons for not completing Fontan were varied and often predictable, such as the presence of pulmonary hypertension, hypoplastic pulmonary branches, or high surgical risk. However, the most frequent reason was patient or family refusal, particularly in patients with restrictive pulmonary flow, likely due to their well-balanced physiology and “stable” clinical status.

The better prognosis observed in G1 may stem from several factors:

1. 75% had a native anatomy with a well-balanced circulation requiring no intervention.
2. The dominant anatomy was double-inlet left ventricle, typically associated with a more favorable prognosis.
3. They were older at the first visit, suggesting a survival bias in this group.
4. Their oxygen saturation was better than patients with Glenn, where upper body circulation flows directly into the lungs without systemic mixing, reflecting good hemodynamic balance.

The poorer prognosis in G3 may be explained by greater baseline cyanosis and both systolic and diastolic volume overload due to systemic-pulmonary shunts (compared to systolic overload alone in G1). This likely leads to greater ventricular dilatation and dysfunction, increased arrhythmias, HF, and ultimately higher mortality.

Mortality was high across all groups (20% overall), highlighting the need for caution when managing these patients, regardless of palliation type. HF and atrial arrhythmias (31.7% and 17.5%, respectively) were also common. Despite the high mortality, the proportion of transplants (8.3%) seems low, particularly given that patients with native anatomy might be good candidates with potentially suitable anatomical and hemodynamic characteristics.

Sudden cardiac death (38%) and HF (33%) were the leading causes of mortality, with no differences between groups. Risk stratification and sudden death prevention in these patients remain complex, as causes may extend beyond ventricular arrhythmias. The inherent anatomical limitations of this population also challenge device implantation.

The primary limitation of the study lies in its retrospective design, with potential confounding and information biases, as well as the inclusion of a highly selected population of adult survivors, introducing survival bias.

REFERENCE:

Gordon B, Buendía-Fuentes F, Rueda-Soriano J, Merás Colunga P, Gallego P, González García AE, et al. Univentricular hearts not undergoing Fontan: the type of palliation matters. Rev Esp Cardiol (Engl Ed). 2023 Mar 9:S1885-5857(23)00066-X. English, Spanish. doi: 10.1016/j.rec.2023.03.001.