What are the dangers of Hypoplastic Left Heart (HLH) and Fontan circulation?

Dangers of Hypoplastic Left Heart Syndrome and Fontan Circulation

Hypoplastic left heart syndrome (HLHS) with Fontan circulation is inherently dangerous due to its unique physiological demands, resulting in nearly one-third of patients either dying or requiring heart transplantation within 35 years post-Fontan procedure. 1

Fundamental Physiological Challenges

Abnormal Circulatory Dynamics

• The Fontan circulation relies on elevated, nonpulsatile venous pressure to drive passive blood flow through the pulmonary vascular bed 1
• This creates chronically elevated central venous pressure
• Decreased preload to the systemic ventricle occurs
• Impaired cardiac output augmentation during exercise or stress 1

Single Ventricle Burden

• In HLHS, the right ventricle must support the systemic circulation, a role it's not anatomically designed for 1
• The absence of normal interaction between two ventricles results in:
  • Marked differences in myocardial twist and strain
  • Reduced systolic ventricular function despite normal mass and volume 1

Four Major Categories of Complications

1. Cardiac/Systemic Ventricular Dysfunction

• Systemic right ventricle is ill-designed for supporting systemic circulation 1
• Ventricular dysfunction develops over time
• Diastolic filling abnormalities are common regardless of ventricular morphology 1
• Atrioventricular valve regurgitation can develop and worsen outcomes 1

2. Fontan Pathway Dysfunction

• Pathway obstruction can develop
• Elevated pulmonary vascular resistance impairs passive flow
• Thromboembolic complications are common 1
  • Inconsistent anticoagulation increases risk 1
  • Rates of thrombosis are higher in patients who don't achieve target INR levels 1

3. Lymphatic Dysfunction

• Chronically elevated venous pressure disrupts lymphatic drainage
• Can lead to:
  • Protein-losing enteropathy (PLE)
  • Plastic bronchitis
  • Persistent pleural effusions 1

4. Extracardiac Dysfunction

• Progressive multi-organ dysfunction affects:
  • Liver: Fontan-associated liver disease (FALD) 1
  • Kidneys: Renal dysfunction
  • Lungs: Abnormal pulmonary function 1, 2

HLHS-Specific Concerns

Beyond standard Fontan complications, HLHS patients face additional risks:

• Aortic obstruction related to the anastomosis of pulmonary artery and aorta 1
• Neoaortic dilation 1
• Coronary ischemia risk due to coronary arteries arising from a small aortic root 1

Exercise and Quality of Life Impact

• Cardiorespiratory responses to exercise are subnormal 1
• Lower-than-expected VO2max, cardiac output, and heart rate responses
• Abnormal reduction of arterial O2 saturation during exercise 1
• Significant impact on daily activities and quality of life 1

Long-term Survival Concerns

• Nearly one-third of patients will either die or require transplant within 35 years post-Fontan 1
• Major adverse events are common by 20 years following Fontan operation 1
• Dysrhythmias both early and late after Fontan procedure contribute to morbidity 1
• Multiorgan dysfunction leads to early mortality and need for heart transplantation 2

The combination of these physiological challenges creates a precarious circulatory system that requires vigilant monitoring and specialized care throughout the patient's lifetime. Early detection of complications and standardized management algorithms are essential for improving outcomes in this high-risk population.