Cardiomegaly in Hypoplastic Left Heart Syndrome
Primary Mechanism
Cardiomegaly in HLHS results from compensatory right ventricular dilation and hypertrophy as the single functional right ventricle must pump to both the systemic and pulmonary circulations in parallel, creating a severe volume overload. 1
Pathophysiologic Mechanisms
Volume Overload
- The right ventricle serves as the sole functional pumping chamber, supplying both systemic and pulmonary blood flow simultaneously in the native state, imposing massive volume loads 1
- Tricuspid regurgitation, which is common in HLHS (present in approximately 6% with moderate-to-severe severity), further exacerbates volume loading and ventricular dilation 1
- The cardiac response to volume loading is ventricular dilation, which increases wall stress and predisposes to dysfunction 1
Compensatory Mechanisms
- Most CHD patients, including those with HLHS, maintain normal baseline cardiac output through compensatory mechanisms including ventricular dilation, hypertrophy, and increased filling pressures 1
- These compensatory mechanisms themselves contribute to symptoms and the enlarged cardiac silhouette seen on imaging 1
- The more negative intrathoracic pressure during inspiration in these patients increases left ventricular afterload and can contribute to hypertrophy (though the left ventricle is hypoplastic, this principle applies to the functional single ventricle) 1
Stage-Specific Considerations
Stage I (Norwood with Shunt):
- The single right ventricle pumps to both systemic and pulmonary circulations in parallel through the systemic-to-pulmonary or Sano shunt, creating maximal volume overload 1
- Systemic-to-pulmonary collaterals develop and can be quantified, adding additional volume load 1
Post-Stage II (Bidirectional Glenn):
- Although the ventricle no longer pumps directly to the pulmonary circulation, systemic-to-pulmonary collaterals persist and continue to impose volume loads 1
- Atrioventricular valve regurgitation remains a significant contributor to ventricular dilation 1
Additional Contributing Factors
- Chronic hypoxemia, hypercarbia, and acidosis increase cardiac output and stimulate the renin-angiotensin system, elevating afterload 1
- Metabolic effects of chronic cyanosis can produce myocardial changes 1
- Regional wall motion abnormalities and myocardial scarring (particularly after Sano shunt procedures) can alter ventricular geometry and contribute to dilation 1
Clinical Implications
- Ventricular dilation increases risk of arrhythmia, elevated wall stress, and predisposition to progressive dysfunction 1
- Serial assessment of ventricular volumes using cardiac MRI is the gold standard for monitoring progression, as it provides true 3D volumetric measurements without geometric assumptions 1
- Ventricular performance parameters correlate with exercise performance and transplant-free survival after Fontan completion 1