How does the degree of pulmonary stenosis affect the haemodynamic waveform in Tetralogy of Fallot (TOF)?

Haemodynamic Waveform in Tetralogy of Fallot: Impact of Pulmonary Stenosis

The severity of pulmonary stenosis in Tetralogy of Fallot directly affects the haemodynamic waveform by creating a pressure gradient across the right ventricular outflow tract (RVOT), with more severe stenosis producing higher RV pressures, greater right-to-left shunting across the VSD, and characteristic pressure waveforms showing increased RV systolic pressure that may approach or exceed systemic levels. 1, 2

Pathophysiological Effects of Pulmonary Stenosis

Pulmonary stenosis in TOF can occur at multiple levels:

• Infundibular (subpulmonary) stenosis - the hallmark component of TOF
• Pulmonary valve stenosis
• Supravalvular stenosis
• Branch pulmonary artery stenosis

The degree of stenosis directly impacts:

1. Right Ventricular Pressure:

   • Moderate-to-severe PS: Elevated RV systolic pressure (may approach or exceed LV pressure)
   • Mild PS: Lower RV pressure gradients with less pronounced waveform changes

2. Right-to-Left Shunting:

   • More severe PS: Increased right-to-left shunting across the VSD
   • Less severe PS: Reduced shunting ("pink tetralogy") 1

3. Pulmonary Blood Flow:

   • Severe PS: Markedly reduced pulmonary blood flow with preferential flow often to the right pulmonary artery 2
   • Mild PS: Better preserved pulmonary blood flow with less cyanosis

Haemodynamic Waveform Characteristics

Right Ventricular Pressure Waveform

• Severe PS:

  • Elevated RV systolic pressure (may equal or exceed LV pressure)
  • Steep pressure rise in early systole
  • Prolonged ejection time
  • Right-to-left ventricular peak pressure ratio (PRV/LV) > 0.8 3

• Moderate PS:

  • Moderately elevated RV pressure
  • PRV/LV between 0.5-0.8 3

• Mild PS:

  • Mildly elevated RV pressure
  • PRV/LV < 0.5
  • Less pronounced waveform abnormalities

Pulmonary Artery Pressure Waveform

• Severe PS:

  • Low pulmonary artery pressure
  • Delayed upstroke (parvus et tardus)
  • Reduced pulse pressure
  • Post-stenotic dilation may be present 1

• Mild-to-Moderate PS:

  • Less pronounced waveform abnormalities
  • Better preserved pulse pressure

Post-Repair Haemodynamic Considerations

After surgical repair, the haemodynamic waveform is influenced by the balance between residual/recurrent PS and pulmonary regurgitation (PR):

1. Aggressive RVOT Reconstruction:

   • Minimal residual PS
   • Significant PR with characteristic diastolic flow reversal on Doppler
   • RV volume overload pattern 4

2. Conservative RVOT Reconstruction:

   • Some degree of residual PS
   • Less severe PR
   • Combined pressure and volume load on RV 4

3. Optimal Balance:

   • Studies suggest that mild residual PS (PRV/LV < 0.8) may be protective against severe PR and RV dilation 4, 3
   • This has led to more conservative RVOT reconstruction approaches

Clinical Implications of Haemodynamic Patterns

• Pre-repair: The severity of PS determines the degree of cyanosis and RV hypertrophy
• Post-repair: The balance between PS and PR affects long-term RV function

Common Pitfalls in Assessment

• Underestimating the protective effect of mild PS against severe PR 2, 4
• Focusing solely on relieving PS without considering the long-term effects of PR 5
• Overlooking branch pulmonary artery stenosis which can cause asymmetric pulmonary blood flow 6
• Misinterpreting RV dilation as purely volume overload when residual RVOT obstruction may be contributing 2

Monitoring Approaches

• Cardiac Catheterization: Provides direct pressure measurements of RVOT gradients and dynamic assessment of regurgitant flow 1, 2
• Cardiac MRI: Reference standard for RV volume quantification and pulmonary regurgitant fraction measurement 1, 2
• Echocardiography: First-line assessment for RVOT obstruction and PR 2

The optimal haemodynamic profile after TOF repair appears to be one with mild residual PS (PRV/LV < 0.8) that limits the degree of PR, as this balance may better preserve long-term RV function and reduce the risk of RV dilation and dysfunction 4, 3.