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Quantification of Mitral Stenosis
Echocardiography is the modality of choice for the diagnosis of mitral stenosis (MS). The joint American Society of Echocardiography and European Association of Echocardiography guidelines for native valvular stenosis feature an exhaustive review of echocardiographic methods for quantitative assessment of MS. Full echocardiographic evaluation of MS includes the following triad: (1) MVA; (2) mean diastolic transmitral pressure gradient; and (3) secondary changes, including measurements of relevant chamber sizes and estimation of right heart pressures. Most modern ultrasound systems contain built-in software packages for determining these parameters. Major methods for quantification of MS are presented in Figs. 91.1 and 91.2 . In most instances, evaluation of MS by invasive methods of cardiac catheterization is not necessary unless there is a discrepancy between clinical and echocardiographic findings.
Quantitative assessment of mitral stenosis (MS) by spectral Doppler. A, Assessment of the mean diastolic mitral gradient. This patient has severe MS (mean gradient, 11 mm Hg at a heart rate of 70 beats/min). B, Mitral valve area (MVA) by pressure halftime (PHT). This patient has severe MS (MVA, 0.94 cm 2 ). Note that a 50% drop in initial pressure (from 18 to 9 mm Hg) corresponds to a 70% drop in initial velocity (from 2.1 to 1.5 ms). (See
, which conceptually corresponds to this panel, albeit in a different patient.) C, MVA by deceleration time (DT). Because PHT is approximately 0.29 × DT, the formula MVA = 220/PHT may be expressed as MVA = 759/DT.
, which conceptually corresponds to this panel, albeit in a different patient.) C, MVA by deceleration time (DT). Because PHT is approximately 0.29 × DT, the formula MVA = 220/PHT may be expressed as MVA = 759/DT.
Quantitative assessment of mitral stenosis (MS) by mitral valve area (MVA) methods. A, MVA by two-dimensional planimetry at mitral leaflet tips in the transthoracic short axis. The patient has mild to moderate MS (MVA = 1.21 cm 2 ) (see
). B, MVA by three-dimensional (3D) planimetry in this patient with severe MS. The panel demonstrates the left ventricular aspect of the mitral valve (MV). On newer systems, MVA can be planimetered directly on a 3D image (0.6 cm 2 in this patient). On older systems, a grid is used to calculate MVA. Each rectangle on the grid corresponds to 0.25 cm 2 (5 × 5 mm) (see
). C, MVA by multiplane reconstruction (MPR) of a 3D transesophageal echocardiography clip. MPR allows for localizing true tips of mitral leaflets; thus, this method allows for most accurate assessment of anatomic MVA. D, MVA by proximal isovelocity surface area (PISA) method. Both color and continuous-wave spectral Doppler are required. Note that when the color Doppler baseline is shifted, the aliasing velocity used to calculate MVA is the one in the direction of blood flow (38 cm/s in this case). This patient has moderate MS (MVA = 1.12 cm 2 ). LA, Left atrium; LV, left ventricle; RA, right atrium.
). B, MVA by three-dimensional (3D) planimetry in this patient with severe MS. The panel demonstrates the left ventricular aspect of the mitral valve (MV). On newer systems, MVA can be planimetered directly on a 3D image (0.6 cm 2 in this patient). On older systems, a grid is used to calculate MVA. Each rectangle on the grid corresponds to 0.25 cm 2 (5 × 5 mm) (see ). C, MVA by multiplane reconstruction (MPR) of a 3D transesophageal echocardiography clip. MPR allows for localizing true tips of mitral leaflets; thus, this method allows for most accurate assessment of anatomic MVA. D, MVA by proximal isovelocity surface area (PISA) method. Both color and continuous-wave spectral Doppler are required. Note that when the color Doppler baseline is shifted, the aliasing velocity used to calculate MVA is the one in the direction of blood flow (38 cm/s in this case). This patient has moderate MS (MVA = 1.12 cm 2 ). LA, Left atrium; LV, left ventricle; RA, right atrium.
A major change in MS quantification occurred with the publication of the 2014 American Heart Association (AHA)/American College of Cardiology (ACC) valvular heart disease guidelines. The changes were threefold: (1) MS, like other valvular disorders, was given stages A through D; (2) MVA cutoff values for the severity of MS were changed; and (3) the role of transmitral pressure gradients is no longer considered the major determinant of MS severity ( Table 91.1 ).
TABLE 91.1
Staging of Mitral Stenosis by 2014 American Heart Association/American College of Cardiology Guidelines a
| Stage | Definition | Symptoms | MS Severity | MVA | PHT | LA Dilatation | PASP |
|---|---|---|---|---|---|---|---|
| A | At risk of MS | None | None or trivial | 4–6 cm 2 | None | Normal | |
| B | Progressive MS | Mild or moderate | >1.5 cm 2 | <150 ms | Mild to moderate | Normal at rest | |
| C | Asymptomatic severe MS | Severe | Severe MS: ≤1.5 cm 2 Very severe MS: ≤1.0 cm 2 |
Severe MS: ≥150 ms Very severe MS: ≤220 ms |
Severe | >30 mm Hg | |
| D | Symptomatic severe MS | Exertional dyspnea |
LA, Left atrium; MVA, mitral valve area; PASP, pulmonary artery systolic pressure; PHT, pressure halftime.
a The transmitral mean pressure gradient should be obtained to further determine the hemodynamic effect of the mitral stenosis (MS) and is usually >5 to 10 mm Hg in severe MS; however, because of the variability of the mean pressure gradient with heart rate and forward flow, it has not been included in the criteria for severity
Mitral Valve Area Measurements
Normal MVA in an adult is approximately 4.0 to 6.0 cm 2 . MVA can be calculated using a variety of noninvasive and invasive methods, none of which is considered a true gold standard. Historically, severe MS was defined as MVA less than 1.0 cm 2 , moderate when MVA is 1.0 to 1.5 cm 2 , and mild when MVA is greater than 1.5 cm 2 . However, in the 2014 AHA/ACC valvular heart disease guideline, severe MS was defined as MVA 1.5 cm 2 or less and very severe MS as MVA 1.0 cm 2 or less (see Table 91.1 ). Indexing of MVA for body surface area has not been validated.
Invasive Method
The most common invasive method for estimating MVA is based on the Gorlin equation published in 1951:
in which Q is the diastolic transmitral flow rate (in mL/min), c is a constant (0.85 for MV), and ΔP is the mean diastolic transmitral gradient (in mm Hg).
This method requires invasive measurements of both the cardiac output and the diastolic transmitral pressure gradient. Ideally, the gradient should be measured directly as the difference between left ventricular (LV) and left atrial (LA) diastolic pressure, typically after a transseptal puncture. However, pulmonary artery wedge pressure is often used in lieu of LA pressure; this typically overestimates the transmitral gradient (and thus the severity of MS) compared with direct LA pressure measurements.
Echocardiography Methods
By echocardiography, MVA can either be measured directly (anatomic orifice area) or estimated from Doppler measurements (effective orifice area).
Pressure Halftime Method
Pressure halftime (PHT) is defined as the length of time required for the maximal early diastolic transmitral gradient to reach half its value (see Fig. 91.1B and ). PHT is inversely related to MVA. PHT is quite short in patients without significant MS because the transmitral (LA to LV) diastolic pressure gradient declines rapidly as the pressures in these two chambers quickly equalize. On the other hand, with severe MS, the pressure gradient declines very slowly, resulting in a long PHT.
A semiquantitative method for estimating MVA from PHT was originally a cardiac catheterization technique using direct pressure measurements (thus the PHT name). The technique was later adapted for quantitative MVA assessments from noninvasive Doppler measurements. Historically, pulsed-wave (PW) spectral Doppler was first used to measure PHT; continuous-wave (CW) Doppler is now preferred.
Video 91.1B. Quantitative assessment of mitral stenosis (MS) by spectral Doppler. Mitral valve area (MVA) by pressure half time (PHT). This patient has severe MS (MVA, 0.94 cm 2 ). Note that a 50% drop in initial pressure (from 18 to 9 mm Hg) corresponds to a 70% drop in initial velocity (from 2.1 to 1.5 m/s).
Using the simplified Bernoulli equation:
in which ΔP is the transmitral gradient (in mm Hg) and V is the velocity of blood (in m/s). One can demonstrate that PHT is reached when the initial maximal velocity of blood across the MV during diastole drops to 70% of its initial value. Hatle and colleagues developed an empirical equation for calculating MVA from PHT:
Thus, in a patient with PHT of 220 ms, the MVA is calculated to be 1.0 cm 2 . Because of its simplicity, PHT is the most used Doppler technique for estimating MVA.
It is important to emphasize that the 220/PHT formula was developed for rheumatic MS and should not be used for, for example, senile MS related to mitral annular calcification.
For patients in atrial fibrillation, an average value of PHT derived from (typically) five cardiac cycles should be used. Short cardiac cycles should be avoided because they may be too brief for the pressure to drop to half its value. In some instances, the spectral Doppler velocity decay has not one but two slopes; this may be seen in patients with both MS and mitral regurgitation (MR). In such instances, the initial slope (occurring typically within the first 300 ms of transmitral flow) can be ignored; subsequent (mid-diastolic) slope should be used to measure PHT.
Occasionally, the PHT method may not accurately calculate MVA (e.g., when the changes in LA and/or LV pressures are independent of MS, when the initial transmitral pressure gradient is very high, or after percutaneous mitral balloon valvuloplasty [PMBV]). The PHT method overestimates MVA in patients with large atrial septal defects, significant aortic regurgitation (AR), or LV diastolic dysfunction and when the initial transmitral pressure gradient is very high.
In patients with both MS and atrial septal defect (referred to as Lutembacher syndrome), a significant left-to-right shunt decompresses the leaf atrium, decreases the transmitral gradient, and shortens the PHT, leading to overestimation of MVA. Significant AR and/or LV diastolic dysfunction may lead to increased LV diastolic pressure; this in turn diminishes the transmitral gradient, shortens the PHT, and leads to overestimation of MVA.
In patients with LV diastolic dysfunction (who tend to be older adults), abnormal LV relaxation leads to either prolongation or shortening of PHT independent of MS. Abnormal LV relaxation prolongs PHT (leading to underestimation of MVA), whereas abnormal LV compliance shortens PHT (leading to overestimation of MVA). Thus, the PHT method should be used with caution in older adult patients with MS. One should not use PHT to estimate MVA after PMBV because LV diastolic pressure may rise significantly as the relatively noncompliant left ventricle experiences an abrupt increase in transmitral flow after balloon-mediated relief of MS. When PHT is unavailable, one can use mitral deceleration time (DT) instead.
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