Mitral Stenosis

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5 Mitral Stenosis

Valid Methods for Mitral Valve Area Determination (Need ≥2 Concordant)

Continuity (if < mild AI)

Pressure half time (PHT), if no factors are present that alter left ventricle (LV) compliance, such as left ventricular hypertrophy, left ventricular wall motion abnormalities, LV dysfunction, recent CBV, or more than mild AI

Proximal isovelocity surface area (PISA)

Real-time three-dimensional (3D) echocardiography

Scanning Notes

Planimetry

Optimize the gain settings for planimetry—over-gain results in a falsely low estimation of mitral valve area (MVA) due to “blooming” of the margins.

Measure the area at the narrowest part of the funnel-like orifice of MS—this may be at the level of the leaflet tips or at the subvalvar level.

Be cautious if there was a prior commissurotomy—planimetry underrepresents area if the orifice has deep “splits” into the commissures (or leaflets), which are difficult so see by ultrasound, especially if they extend off the plane of imaging.

Pressure Half Time

Do not accept PHT estimates of MVA if any of the following are present:

• Left ventricular hypertrophy

• Left ventricular wall motion abnormalities

• LV dysfunction

• Recent commissurotomy

• More than mild AI

All are factors that reduce the accuracy of the PHT relation to MVA.

Proximal Isovelocity Surface Area

where r = PISA radius.

The PISA method is based on the assumption that there is a hemisphere of flow acceleration such as would occur with a planar orifice. This often is not the case in mitral stenosis, however, particularly when the greatest stenosis is subvalvar. To adjust for the effect of the orifice on the volume of the PISA, use of an angle correction factor has been suggested (Fig. 5-1), multiplying the preceding calculation by α/180°. Not all echo systems carry angle measurement software.

where α = the measured angle of the “plane” of the orifice and r = PISA radius.

Transesophageal echocardiography (TEE) generally affords excellent depiction of PISA in mitral stenosis.

Reporting Issues

The mean gradient is the gradient seen, by the left atrium, on average, through diastole and is, as with AS, the only suitable expression of the physiologic pressure burden on the downstream chamber. Many MS profiles have an initial, brief, higher velocity/gradient that is not borne out through the rest of diastole. Therefore, emphasize the mean gradient, and avoid mentioning the peak gradient. It is clinically useful to offer the heart rate and rhythm, both of which are apparent on echocardiography, when stating the mean gradient: “The mean gradient was 13 mm Hg at an average heart rate of 75 bpm (atrial fibrillation).” When reporting, compare gradients, areas and RVSP, and rhythm to any previously recorded measurements.

Area Issues

Place less emphasis on the valve area, which is invariably calculated (other than by the planimetry method) and, therefore, is subject to larger error.

Ensure MVA is concordant by at least two suitable methods.

Pressure Half Time Method

The PHT relation (220) was validated in a relatively small series and is best applied for MVA between 1 and 1.5 cm²; r = 0.75.¹ The actual precision of the correlation is less than has been thought.

The 220 constant is actually different for both MVA <1 cm² and MVA >1.5 cm²; therefore, use of the relationship at higher or lower areas is essentially an extrapolation of the known relationship, and assumes that a known variable (220) is a constant, which it is not.

In the presence of AI, there are two ventricular inflows determining the spectral inflow:diastolic relationship. The PHT relation, therefore, is less representative of MS alone, in the presence of more than moderate AI. Correlation and SEE for MVA by PHT (vs. Gorlin) for patients both without and with AI are as follows:

• r = 0.69, SEE = 0.44 cm² for patients with AI

• r = 0.91, SEE = 0.24 cm² for patients without AI.²

PHT is less accurate in patients older than 65 years of age, because of effects of hypertension on the ventricular diastolic properties, leading to overestimation of MVA.³

PHT is less accurate for 3 months after CBV,⁴ because passive ventricular diastolic properties and atrial diastolic properties are adapting to the changes in load (i.e., less atrial/more ventricular).

Planimetry Method

When images are amenable to planimetry, it is an accurate technique. It requires attention to scan extensively along the short axis of the orifice, to ensure sampling at the narrowest part. This may be at the leaflet tips, or in the subvalvar zone, which is important not to miss.

Correlation for MVA by planimetry (vs. cardiac catheterization): r = 0.92 – 0.95.^5,⁶

Planimetry of MVA is an AI-independent technique.

Planimetry is less accurate with prior commissurotomy,^3,^7,⁸ because the split often is eccentric: it may lead out onto another plane of imaging and be missed, or may go into a bright commissure that is difficult to visualize.

Continuity Method

MVA by the continuity method is less accurate when the ideal reference site (the LVOT) has more than mild AI. The tricuspid reference site is harder to use, and is at least as likely to have TR. For all-comers, the correlation of continuity is very good: r = 0.91, SEE = 0.24 cm²,² especially in the absence of AI: r = 0.93.² However, in the presence of AI it is less: r = 0.84.²

Real-Time Three-Dimensional Echocardiography

The real-time 3D technique allows depiction of the mitral orifice on any plane, and thus should assist with the task of planimetry. Although studies are limited to date, real-time 3D echocardiography shows very good correlation with Gorlin (average difference of 0.08 cm²), and low inter- and intraobserver variability (κ of 0.84 and 0.96, respectively), which in comparative studies was better than with other methods.¹⁰

Descriptors of Mitral Stenosis Severity

After it has been established that MS is present, the next important task is to determine its severity. This involves a composite assessment of symptoms, physical findings, and hemodynamics. It must be recalled that MVA < 1 cm² always indicates severe MS, but larger patients or those with higher cardiac output (CO) needs are commonly “severe” above an area of 1 cm² and receive intervention for average MVAs of 1.2 or 1.3 cm².

Distinguishing moderate from mild is really of little consequence, as both are “nonsevere” and would not prompt intervention.

“Severe” = mean gradient >12 mm Hg at rest, and an area of <1.3 cm².

• A subset of clinically severe cases have MVA > 1 cm² (1.2–1.3 cm²).

“Critical” = mean gradient > 12 mm Hg at rest, and an area of <1.0 cm²

Moderate = mean gradient < 12 mm Hg at rest, and an area of ≥1.3 cm²

Surgical Issues

Establish the degree of submitral (annular) calcification. There is a greater risk of periprosthesis (paravalvar) MR when submitral (annular) calcification is severe.

Determine the degree of AI: in severe MR, IABP should not be used if there is concurrent AI ≥2+.

Describe the RV in detail (RVH/systolic dysfunction/size), as well as the RVSP.

RA size is a reasonable descriptor of RV diastolic function, if neither TR nor atrial fibrillation is present.

Provocative Testing to Enhance Transmitral Gradients and Right Ventricular Systolic Pressure

Simple bedside maneuvers (e.g., a few sit-ups) or more sophisticated ones (e.g., supine bicycle, upright treadmill testing, or dobutamine) may be used to increase the heart rate and cardiac output, and thereby enhance the mitral gradient and RVSP. The role of such testing is unclear: in mitral stenosis the gradient would be expected to increase, and generally it does, about two-fold, when patients are subjected to a Bruce or modified Bruce protocol: from 9 ± 7 mm Hg at rest to 17 ± 8 mm Hg, as does the RVSP, from 41 ±1 9 mm Hg to 70 ± 32 mm Hg.¹¹ The optimal interpretation of the provoked hemodynamics is unclear. Advocates suggest that it better describes the basis of exertional symptoms and may be contributory when the resting hemodynamics are less striking than the (exertional) symptoms and findings.

Dobutamine stress echo (a gradient cut-off of 18 mm Hg) has been shown to predict clinical events (90% sensitivity, 87% specificity, and 90% accuracy) over a 5-year period, and increases the detection of medium- and higher-risk cases.¹² Dobutamine stress echocardiography may assist with the evaluation of patients whose symptoms are out of proportion to the calculated valve area, and who may benefit from “medical or invasive intervention.”¹³

Notes on Mitral Stenosis

Pathophysiology

The normal mitral valve area is 4.5 to 6 cm². A diastolic transmitral valve gradient occurs with loss of about half of the original area. There is increased left atrial pressure at rest with MVA of 1.5 cm².

Two thirds (at least) of MS cases occur in women. Of all cases of rheumatic heart disease, 25% are pure MS and 40% are mixed MS/mitral insufficiency.

The course of disease proceeds as follows:

• There is a 5- to 15-year latent period after rheumatic fever before early findings (auscultatory abnormalities) become apparent.

• Usually, a decade of asymptomaticity (with auscultatory abnormalities) passes before symptoms develop.

• Over 5 years, most patients with NYHA Class 2 symptoms progress to Class 3 to 4.

• Ten-year survival of patients with NYHA Class 2 symptoms is about 80%; for patients with NYHA Class 3 symptoms, it is about 38%. A more rapid progression has been noted among certain groups, such as in the subtropical areas of India and the Philippines, and among the Inuit.

Role of Transthoracic and Transesophageal Electrocardiography in Mitral Stenosis

Transthoracic Echocardiography

Nearly all cases of MS can be hemodynamically assessed using TTE alone. Both apical and lower esophagus sampling sites are, however, ideally aligned for Doppler interrogation, and generally generate accurate estimates of gradient. However, if the plane of the mitral orifice is oblique (because of LA dilation), TEE may struggle to achieve optimal alignment for sampling.

Mitral valve scoring is better performed by TTE than TEE, because posterior long-axis and apical views show subvalvar (chordal) involvement better than TEE does. TEE images from the esophagus, acquired through thickened (and potentially calcified) leaflets, fail to clearly depict subvalvar disease. Transgastric long-axis views may afford adequate views of the important subvalvar apparatus.

TEE is better than TTE for the following:

Detection of LAA and LA thrombus, which is necessary information when contemplating CBV

Estimation of MR severity (TTE may underestimate MR severity)

The rare truly technically difficult study case

TEE is not superior for the evaluation of subvalvar disease. The thickened and often calcified leaflets impair subvalvar imaging from the esophagus. Transgastric views are helpful to circumvent this imaging problem.

Cardiac Catheterization for Assessment of Mitral Stenosis

Usual Technique

Pulmonary artery (PA) catheter(s) for right-sided pressures and thermodilution-estimated CO

Femoral arterial access for retrograde cannulation into the LV via the aorta

• LV to aortic (femoral) pressure gradients

• Diastolic filling period (DFP): the average width (time) of the gradients

Verification of pressure waveforms (see later discussion)

Gorlin Equation

The Gorlin equation is employed, using the following variables: CO, DFP, HR, gradient. These variables are readily obtained at cardiac catheterization.

Historically, the Gorlin equation constant was developed for MS, because there is less flow dependence with MS than for AS.¹⁴

In a small initial validation (11 cases), variation of serial measurements averaged ±0.0 cm², but ranged from +0.2 to –0.4 cm².¹⁵

In the initial series, autopsy standard was ±0.5 to ±0.1 cm² and the variation against autopsy was ≤0.2 cm².¹⁴

The Gorlin equation purportedly estimates anatomic, not effective, orifice, because it was validated against surgery and autopsy, but there is evidence that it does not truly reflect anatomic area.

The Gorlin equation is less accurate in the presence of the following:

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