What is the primary haemodynamic concern in Tetralogy of Fallot (TOF)?

Primary Haemodynamic Concern in Tetralogy of Fallot

The primary haemodynamic concern in Tetralogy of Fallot (TOF) is right-to-left shunting at the ventricular level due to right ventricular outflow tract (RVOT) obstruction, resulting in cyanosis. 1

Pathophysiology of Tetralogy of Fallot

Tetralogy of Fallot consists of four cardinal features that create its distinctive haemodynamic profile:

1. Ventricular Septal Defect (VSD) - A non-restrictive communication between ventricles
2. Overriding Aorta - The aortic root overrides the ventricular septum
3. Right Ventricular Outflow Tract Obstruction - Obstruction may be infundibular, valvular, or both
4. Right Ventricular Hypertrophy - Secondary to RVOT obstruction

Key Haemodynamic Consequences

The severity of RVOT obstruction determines the degree of right-to-left shunting across the VSD, which directly affects:

• Degree of cyanosis - More severe obstruction leads to more pronounced cyanosis 1
• Right ventricular pressure - RVOT obstruction causes increased RV pressure, often at systemic or suprasystemic levels
• Blood flow distribution - Decreased pulmonary blood flow with increased systemic flow of deoxygenated blood

Post-Repair Haemodynamic Concerns

After surgical repair, the haemodynamic concerns shift significantly:

1. Pulmonary Regurgitation - The most common long-term complication, occurring in 40-85% of patients within 5-10 years after repair, especially with transannular patch repair 2

2. Right Ventricular Dilation and Dysfunction - Secondary to chronic pulmonary regurgitation, leading to:

   • Progressive RV enlargement
   • Decreased RV function
   • Risk of right heart failure 2, 1

3. Residual RVOT Obstruction - May persist at the infundibular level, pulmonary valve, main pulmonary trunk, or branch pulmonary arteries 2

4. Arrhythmias - Particularly ventricular tachycardia, which can lead to sudden cardiac death (incidence approximately 2.5% per decade of follow-up) 2

Imaging Assessment of Haemodynamics

Comprehensive evaluation requires:

• Echocardiography - First-line assessment of RVOT obstruction, pulmonary regurgitation, RV function, and residual VSD 2

• Cardiac MRI - Reference standard for:

  • RV volume quantification
  • RV systolic function assessment
  • Pulmonary regurgitant fraction measurement
  • Branch pulmonary artery evaluation 2, 3

• Cardiac Catheterization - For direct pressure measurements and interventions:

  • RVOT gradient assessment
  • Dynamic evaluation of regurgitant flow
  • Hemodynamic assessment of diastolic dysfunction 2

Clinical Implications and Management

The haemodynamic status directly influences management decisions:

• Pulmonary Valve Replacement (PVR) - Indicated for severe pulmonary regurgitation with symptoms or RV dilation/dysfunction 2, 1

• Branch Pulmonary Artery Intervention - For stenosis causing unbalanced pulmonary blood flow (often with preferential flow to the right pulmonary artery) 4

• Arrhythmia Management - Risk factors include RV dilation, dysfunction, and QRS prolongation >180ms 2

Pitfalls in Assessment

Common pitfalls in haemodynamic assessment include:

• Underestimation of pulmonary regurgitation by echocardiography
• Failure to recognize branch pulmonary artery stenosis due to limited echocardiographic windows 2
• Overlooking coronary artery anomalies that may affect RVOT interventions 2
• Misinterpreting RV dilation as primarily volume overload when residual RVOT obstruction may be contributing 2

The long-term survival after TOF repair is excellent (approximately 85% at 35 years), but careful monitoring of these haemodynamic parameters is essential to determine the optimal timing for reintervention and to prevent complications such as irreversible RV dysfunction and sudden cardiac death 2, 1.