How is the peak gradient measured using echo in aortic stenosis? What are the limitations of using the peak gradient to grade severity?

  • The peak gradient (maximum pressure difference across the valve) can be calculated from the maximum velocity (peak velocity of blood across the valve during systole) using the Bernoulli equation

 

ΔP= 4 × v2

  • In aortic stenosis:

 

Peak gradient = 4 × (peak velocity)2

 

  • Continuous-wave Doppler recordings allow peak velocity to be measure:
    • Obtained from blood flow in the left ventricular outflow tract
    • In practice, software built into the echo machine calculates the peak gradient from Doppler readings
  • The gradient is dependent on flow across the valve:
    • In high output states, such as coexisting aortic regurgitation, the severity will be overestimated
    • Once the ventricle starts to fail, the severity will be underestimated