CHAPTER 12: Mitral Stenosis

PRESSURE HALF TIME

• The pressure half time principle was adopted by echo from the 1968 cath paper by Libanoff and Rodbard, which provides elegant pressure figures.

Source

• Libanoff AJ, Rodbard S. Atrioventricular pressure half-time. Measure of mitral valve orifice area. Circulation. 1968;38:144–150. Full PDF: www.ahajournals.org/doi/pdf/10.1161/01.CIR.38.1.144

• Remember: Cath is good in measuring pressures but cannot measure blood flow velocities. Echo measures velocities and pressure gradients, but not actual pressures.

• Pressure half time principle: The time that it takes for the diastolic pressure across the mitral valve to drop by half is proportional to the mitral stenosis valve area—irrespective of heart rate. The worse the stenosis, the longer the half time.

• Translation: Pressure half time formula works in mitral stenosis patients even after they develop atrial fibrillation. It is fairly consistent from diastole to diastole, irrespective of cardiac cycle length. You can choose to use the longer cycles for technically easier measurements.

• Sample calculation of pressure half time in a mitral stenosis patient with a measured E velocity of 2 m/s:

  1. Go from velocities to pressures.

     4V² “takes you from velocities to pressures”.

     “Pressure question” in this case would be:

     • - How long does it take for the pressure gradient to drop from 16 mm Hg (4 × 2 × 2) to 8 mm Hg?

  2. Now that you calculated the target half time pressure, go back from pressures to velocities.

     To go back to velocities:

     • - “UnBernoulli” the calculated 8 mm Hg pressure gradient in this patient back to a velocity:

       1. Divide 8 by 4.

       2. Calculate the square root:

          8 ÷ 4 = 2; square root of 2 = 1.4 m/s.

  3. Same question (now a “velocity question”):

     • - How long in milliseconds does it take for the original 2 m/s E velocity in this patient to drop to 1.4 m/s?

  4. This pressure half time in milliseconds is easily measured on the Doppler mitral inflow tracing in mitral stenosis. Do it yourself! Don’t let the computer do it for you, or you will never learn to “cook the numbers on the fly” during a conversation about the severity of the stenosis (see examples below).

     • 3. Calculate mitral stenosis area by dividing the accepted constant of 220 ms by the calculated pressure half time in this patient.

       Formula: 220 ÷ calculated pressure half time = mitral stenosis area.

  5. Examples:

     • - Severe mitral stenosis: A patient with a long calculated pressure half time of 440 ms has a valve area of 0.5 cm².

     • - Mild mitral stenosis: A patient with a short calculated pressure half time of 110 ms has a valve area of 2.0 cm².

  6. Pressure half time pitfalls:

  7. In ...