Pressure Half-Time Method

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Pressure half time (PHT, T1/2) is defined as the time needed for the peak transvalvular pressure gradient to fall to its half value, in milliseconds (ms). According to the Bernoulli equation, when pressure is halved, velocity is equal to peak transvalvular velocity divided by the square root of 2. Thus, PHT is the time for the peak valocity to reach 0,707 of its initial value. The faster the gradient falls, the easier the passage of blood through a valve, and vice versa. Thus, high diastolic transmitral PHT indicates a narrowed valve area, while low diastolic transaortic PHT indicates a wide regurgitant valve area. Original studies of pressure half-time, using initially catheterization and then Doppler data, were performed in 1968 and showed a good correlation between atrioventricular PHT and effective mitral valve area, relatively insensitive to changes in heart rate or cardiac output [1]. PHT is always proportionally related to deceleration time (DT): PHT = 0,29 x DT.

PHT is usually obtained from transmitral and transaortic valve continuous wave Doppler signals to assess the severity of mitral valve stenosis and aortic valve regurgitation, respectively.


Contents

Mitral stenosis (MS) severity

The PHT measurement for assessing the severity of MS is an easy and widely acceptable method. The decline of the velocity of diastolic transmitral blood flow is inversely proportional to mitral valve area (MVA), and MVA is derived using the empirical formula [2]:


     MVA (cm²) = 220 / PHT


PHT is calcuated automatically by tracing the deceleration slope of the E-wave of transmitral flow, obtained with continuous wave Doppler echocardiography (CWD). In patients with atrial fibrillation, PHT is best measured from a recording with an adequate diastolic filling period or averaging different cardiac cycles (Fig.1).


Figure 1. Mitral stenosis Pressure half-time.
Figure 1. Continuous wave Doppler signal of transmitral flow, four chamber view. Pressure half-time is 188 msec and Mitral valve area (MVA) is automatically calculated as 220 / 188 = 1,17 cm2. Moderate mitral stenosis.










If the deceleration slope is bimodal (rapid initial decline in early diastole followed by slower fall in mid-diastole), the second (mid-diastolic) slope should be used (Fig.2)[3].


Figure 2. Mitral stenosis bimodal deceleration slope.
Figure 2. Bimodal deceleration slope of transmitral velocity, exhibiting a rapid initial fall followed by a slower rate of decrease in mid-diastole. The second slope is preferred for evaluation of mitral stenosis severity. Moderate mitral stenosis.









Limitations

1) LV diastolic dysfunction: Left ventricle (LV) diastolic filling rate (reflected by the deceleration slope of the E-wave) depends on MVA but also on transmitral pressure gradient in early diastole, left atrial compliance and LV diastolic function (relaxation and compliance). For example, short PHT can be observed despite severe MS in case of associated severe aortic regurgitation. Early diastolic deceleration time tends to be shortened in case of decreased LV compliance and prolonged when LV relaxation is impaired. LV diastolic dysfunction makes PHT method to assess MVA less reliable, especially in older patients with degenerative calcific MS associated with aortic valve stenosis and hypertension.

2) PHT cannot be measured in tachycardic rhythms or first degree atrioventricular block, when E and A velocities are merged or the diastolic filling period is short.

3) Immediately after balloon mitral commissurotomy: in this situation, gradient and compliance are subject to important and abrupt changes. There may be important discrepancies between the decrease in mitral gradient and the increase in net compliance, making PHT method unsuitable for evaluating MS severity.

4) Prosthetic valves: PHT method overestimates the area of normal prosthetic mitral valves. However, a large rise in PHT on serial studies or a markedly prolonged single measurement (>200ms) may be a clue to the presence of prosthetic valve obstruction, because the PHT seldom exceeds 130 ms across a normally functioning mitral valve prosthesis.[4]


Aortic regurgitation severity

The rate of deceleration of the diastolic regurgitant jet and the derived pressure half-time reflect both the degree of regurgitation and the ventricular end-diastolic pressures. As the degree of AR increases, the aortic diastolic pressure decreases and the LV end-diastolic pressure increases. The late diastolic jet velocity is thus reduced and the PHT shostened. A PHT 0f <200ms is consistent with severe AR, whereas a value of >500ms suggests mild AR.[5] These cut-off values can also be used in case of prosthetic aortic valve regurgitation. (Fig.3)


Figure 3. Aortic regurgitation Pressure half-time.
Figure 3. CW Doppler of the AR jet obtained from the apical five-chamber view. CW density does not provide useful information about the severity of AR. Pressure half-time is 213 msec, consistent with moderate aortic regurgitation









Limitations

1) The PHT is influenced by chamber compliance in addition to chamber pressures. For a given severity of AR, pressure half-time is shortened with increasing left ventricular diastolic pressure, vasodilator therapy, and in patients with a dilated compliant aorta, or lengthened in chronic aortic regurgitation. Thus, this parameter serves only as a complementary finding for the assessment of AR severity.[6]

2) The assessment of PHT requires good Doppler beam alignment and an adequately dense spectral envelope of the regurgitant jet.


Tricuspid stenosis severity

Although there are similarities between mitral and tricuspid stenosis, the PHT method has not been as extensively validated for the calculation of tricuspid valve area. However, as a general rule, a PHT>190ms is frequently associated with significant stenosis. The PHT is not used in pulmonary regurgitation severity assessment.


Conclusion

Pressure half-time is a simple and widely used method. In spite of the aforementioned limitations, it remains an excellent tool in the assessment of mitral valve stenosis severity (Level 1 Recommendation), and a significant complementary parameter for the assessment of aortic regurgitation severity. Its use is recommended for native, as well as prosthetic valve disease.


References

  1. Circulation. 1968; 38: 144-150 Atrioventricular Pressure Half-Time Measure of Mitral Valve Orifice Area ALBERT J. LIBANOFF, M.D.; SIMON RODBARD, M.D., PH.D.
  2. J Am Coll Cardiol. 1987 Oct;10(4):923-9. Doppler mitral pressure half-time: a clinical tool in search of theoretical justification. Thomas JD, Weyman AE.
  3. Eur J Echocardiogr. 2009 Jan;10(1):1-25. Epub 2008 Dec 8. Echocardiographic assessment of valve stenosis: EAE/ASE recommendations for clinical practice. Baumgartner H, Hung J, Bermejo J, Chambers JB, Evangelista A, Griffin BP, Iung B, Otto CM, Pellikka PA, Quiñones M; EAE/ASE.
  4. J Am Soc Echocardiogr. 2009 Sep;22(9):975-1014; quiz 1082-4. Recommendations for evaluation of prosthetic valves with echocardiography and doppler ultrasound: a report From the American Society of Echocardiography's Guidelines and Standards Committee and the Task Force on Prosthetic Valves, developed in conjunction with the American College of Cardiology Cardiovascular Imaging Committee, Cardiac Imaging Committee of the American Heart Association, the European Association of Echocardiography, a registered branch of the European Society of Cardiology, the Japanese Society of Echocardiography and the Canadian Society of Echocardiography, endorsed by the American College of Cardiology Foundation, American Heart Association, European Association of Echocardiography, a registered branch of the European Society of Cardiology, the Japanese Society of Echocardiography, and Canadian Society of Echocardiography. Zoghbi WA, Chambers JB, Dumesnil JG, Foster E, Gottdiener JS, Grayburn PA, Khandheria BK, Levine RA, Marx GR, Miller FA Jr, Nakatani S, Quiñones MA, Rakowski H, Rodriguez LL, Swaminathan M, Waggoner AD, Weissman NJ, Zabalgoitia M;
  5. Eur J Echocardiogr. 2010 Apr;11(3):223-44. European Association of Echocardiography recommendations for the assessment of valvular regurgitation. Part 1: aortic and pulmonary regurgitation (native valve disease). Lancellotti P, Tribouilloy C, Hagendorff A, Moura L, Popescu BA, Agricola E, Monin JL, Pierard LA, Badano L, Zamorano JL; European Association of Echocardiography.
  6. Am Heart J. 1991 Oct;122(4 Pt 1):1049-56. The effects of regurgitant orifice size, chamber compliance, and systemic vascular resistance on aortic regurgitant velocity slope and pressure half-time. Griffin BP, Flachskampf FA, Siu S, Weyman AE, Thomas JD.
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