The borderline left ventricle (LV) encompasses a broad spectrum of congenital heart conditions. Controversy exists over what constitutes a borderline ventricle, raising uncertainties about whether to attempt a biventricular repair or commit to a univentricular pathway. When referring to small left ventricles, hypoplastic left heart syndrome (HLHS) often comes to mind, with its most severe forms involving atresia of the aortic and/or mitral valves. Cases with valve atresia are not candidates for biventricular repair; therefore, a borderline LV must have patent valves and some degree of left ventricular development. Other conditions involving small LVs include critical aortic stenosis, congenital mitral stenosis, unbalanced atrioventricular canals (frequently associated with coarctation of the aorta), and hypoplasia of the aortic arch. Additionally, variations such as double-inlet ventricles, double-outlet right ventricles with mitral valve hypoplasia, or overriding atrioventricular valves often lead to a univentricular approach.
The decision between pathways is often challenging, with dire consequences if a biventricular strategy is chosen for an LV unable to support it. Conversely, the univentricular pathway poses long-term complications that are difficult to manage.
A universal guideline for dichotomizing cases into biventricular or univentricular pathways cannot be established. This review aims to synthesize current evidence on algorithms and scoring systems to guide management strategies tailored to specific pathologies.
In cases of critical aortic stenosis, the Rhodes score, developed by the Boston Children’s group, considers the heart’s long axis, indexed diameter of the aortic root, mitral valve area, and indexed LV mass to determine the most appropriate pathway. Application of this score concluded that neonates with critical aortic stenosis, a mitral valve area above Z < -2, and biventricular physiology had a survival rate exceeding 90%. Following the Rhodes score, the American Congenital Heart Surgeons Society introduced the CHSS-1 score, identifying several risk factors for hospital mortality. This was later refined into the CHSS-2 score, emphasizing the minimum left ventricular outflow tract diameter for biventricular circulation suitability, identifying the risks of pursuing a circulation pathway not aligned with the patient’s anatomy.
Unbalanced atrioventricular canals with right ventricular dominance constitute a challenging cohort. Here, the indexed left-to-right atrioventricular valve area ratio determines the feasibility of a biventricular repair. Ratios above 0.67 make biventricular repair viable, while those below 0.5 preclude survival with such an approach.
In cases of HLHS, the 2V score incorporates echocardiographic parameters—mitral and aortic annulus dimensions, LV and right ventricular lengths, pulmonary artery diameter, and body surface area—to guide clinical decisions. Studies based on this score concluded that the mitral and aortic annuli are critical determinants in choosing a circulation pathway. Building on this research, Tchervenkov’s group identified a subset favorable for biventricular repair in 1998: patients with hypoplastic LVs and outflow tracts, small but non-stenotic aortic and mitral valves, and no evidence of endocardial fibroelastosis. Select HLHS patients demonstrated growth of cardiac structures, particularly the aortic valve, during the first two years post-surgery, supporting individualized approaches to enhance LV development for biventricular circulation.
Based on this premise of growth potential, various strategies have emerged to precondition borderline LVs for biventricular circulation. The Boston strategy begins with conventional neonatal Norwood palliation. At 4–6 months, during the bidirectional Glenn procedure, valve repairs and fibroelastosis resection are performed. In select cases, an obstructive membrane is placed in the pulmonary artery to prevent high Glenn pressures, a technique termed “super-Glenn.” A restrictive atrial septal defect (ASD) of 4–5 mm is left in all patients. Follow-up every 2–4 months monitors left heart cavity growth. Candidates with an LV end-diastolic volume >40 mL/m² and an end-diastolic pressure <12 mmHg are considered for biventricular conversion.
The Giessen strategy involves a hybrid approach with bilateral pulmonary artery banding and ductal stent implantation during the neonatal period. A restrictive ASD ensures left atrial pressure remains below 15 mmHg, with a 5–10 mmHg gradient across the ASD. This approach buys time to decide on the definitive circulation pathway at 4–6 months.
Birmingham’s reverse double switch strategy for patients with Shone’s syndrome and elevated LV end-diastolic pressures involves leaving the LV as the subpulmonary ventricle, combined with a bidirectional Glenn. While innovative, its mid-term outcomes are pending.
Determining whether a borderline LV can support biventricular circulation is complex and pathology-specific. Decisions must be informed by cardiac structure measurements and growth potential.
COMMENTARY:
Our left ventricle (LV) traces its origins to an ancient ancestor—a fish from the Chordata phylum (animals whose embryos exhibit a dorsal cord, precursor to a backbone) appearing 500 million years ago during the Devonian period. These fish had a univentricular heart pumping blood to the systemic circulation and respiratory organs, the gills. Mammals, emerging 180 million years ago in the Jurassic period, developed a right ventricle, which pumps blood to the lungs, alongside the ancestral LV, now responsible for systemic circulation. The LV’s evolutionary refinement through natural selection has resulted in a lower frequency of left-sided congenital defects compared to right-sided abnormalities or septal defects.
The univentricular pathway remains a double-edged solution. Despite improved surgical outcomes across stages—10–20% mortality for Norwood, 95% survival for Glenn, and 90% survival for Fontan—long-term results are less encouraging: 50–70% mortality at 10 years, with 5% of patients requiring transplantation. Complications such as protein-losing enteropathy, plastic bronchitis (occasionally unresolved even post-transplant), arrhythmias, valvular insufficiencies, thrombosis, cirrhosis, and ventricular dysfunction often emerge, with limited therapeutic options. Consequently, striving for biventricular circulation, whenever feasible, is imperative.
Ultimately, while the decision is binary, it need not be made during the neonatal period, when the patient’s evolution remains uncertain. The mentioned strategies allow deferring the decision after assessing the growth potential of cardiac structures.
REFERENCE:
Albrahimi E, Korun O. Contemporary management of borderline left ventricle. Eur J Cardiothorac Surg. 2024 Jul 1;66(1):ezae247. doi: 10.1093/ejcts/ezae247.
