The correlation between echocardiographic findings in mitral valve disease and in vivo anatomy

Echocardiography in mitral valve stenosis

The main aetiology of mitral stenosis (MS) in the adult population is rheumatic heart disease (RHD). Although the incidence of the latter has markedly dropped in the developed world, it is still a major concern in low and middle socioeconomic countries. Degenerative MS is less frequently encountered, observed in the elderly population, and associated with hypertension and atherosclerotic disease. Echocardiography is the main tool used for diagnosis, assessment of severity and planning of the appropriate intervention.

Echocardiographic assessment of mitral stenosis: how accurately does it reflect the in vivo anatomy?

Indices of mitral stenosis severity

Diastolic pressure gradient

Continuous wave Doppler (CWD) adjusted at the mitral inflow in the apical 4-chamber view is the recommended method for measuring transmitral pressure gradients. Peak and mean diastolic pressure gradients can then be estimated from the transmitral flow velocity curve using the simplified Bernoulli equation (ΔP = 4v2). Mean gradient is the relevant haemodynamic parameter rather than peak gradient which appears to be influenced by left atrial compliance and LV diastolic function. These measurements have shown a good correlation with invasive measurement using transseptal catheterisation [1]. Optimisation of beam orientation guided by colour Doppler and a good acoustic window are needed to obtain well-defined contours of the Doppler flow and to avoid underestimation of gradients, especially in case of eccentric diastolic mitral jets frequently encountered in cases of severe deformity of the valvular and subvalvular apparatus [2].

However, pressure gradient is not the best marker of MS severity as it is affected by heart rate, cardiac output and associated mitral incompetence. Nonetheless, it has its own prognostic value, particularly following balloon mitral valvuloplasty, and it may have a special benefit in case of poor quality of other variables (especially planimetry of valve area).

Mitral valve area (MVA) planimetry

Being a direct measurement of MVA, planimetry has been shown to have the best correlation with the anatomical valve area assessed on explanted valves [3]. Planimetry is obtained by direct tracing of the mitral orifice, including opened commissures, at a parasternal short-axis view, in mid-diastole. Gain setting should be adjusted to visualise the whole mitral orifice contour while avoiding excessive gain that may underestimate MVA in patients with dense or calcified mitral leaflets. Poor acoustic windows and markedly distorted valve anatomy may hinder accurate measurement of MVA in some patients. In addition, technical expertise is required to optimise the plane of tracing to be at the leaflet tips. Recently, real-time 3D echocardiography has been shown to be useful in adjusting the plane of measurement, hence improving reproducibility [4].

In clinical practice, some patients are seen with markedly elevated pressure gradients despite a mild or moderate narrowing of the mitral valve orifice (MVA >1.5 cm²). In these situations, subvalvular obstruction should be suspected, and other imaging modalities such as 3D transoesophageal echocardiography can be of added benefit to confirm the diagnosis.

Pressure half time (T_1/2).

This is defined as the time interval (in milliseconds) for the diastolic pressure gradient to drop to half of its initial maximal value. It is inversely proportional to valve area (in cm²), and MVA can be calculated using the equation: MVA= 220/ T_1/2.

MVA is automatically calculated by an integrated software following measurement of T_1/2 by tracing the deceleration slope of the E-wave on transmitral flow (obtained using CW Doppler). In some instances, the decline of transmitral flow velocity is bimodal, where it is more rapid in early diastole. In these cases, tracing should be obtained in mid-diastole rather than for the early rapid deceleration slope [5]. 

The use of T_1/2 for MVA estimation has been validated using catheterisation data initially and then Doppler data. However, the deceleration slope of the E-wave depends not only on MVA but also on left atrial compliance and left ventricle compliance and relaxation. Thus, in situations when there are abrupt changes in pressure gradients and compliance, such as immediately following balloon mitral valvuloplasty, T_1/2 may be unreliable [6]. Other conditions, e.g., severe aortic incompetence and diastolic dysfunction, might interfere with accurate estimation of MVA using T_1/2.

Continuity equation and proximal isovelocity surface area

These are other less frequently used methods of valve area estimation. They are technically demanding and require multiple measurements affecting the accuracy of the calculated MVA.

Pulmonary artery pressure (PAP)

Estimation of PAP, using CW Doppler estimation of pressure gradient between the right ventricle and right atrium, may reflect the effect of mitral stenosis rather than its severity. Although a wide range of PAP could be present for a given mitral valve area, PAP should always be estimated and documented in all cases of MS as it might be essential for management decisions in some patients [7]. 

Assessment of mitral valve anatomy

This represents a major component of echocardiographic assessment of the mitral valve as it guides the choice of the appropriate intervention strategy.

Commissural fusion

This is assessed in the short-axis parasternal view used for planimetry. Complete fusion of both commissures usually suggests severe MS.

Commissural fusion may be difficult to assess, particularly in patients with severe valve deformity. Moreover, lack of commissural fusion does not exclude significant MS, especially in degenerative aetiologies or in restenosis following balloon valvuloplasty where stenosis may be related to leaflet rigidity [3]. Real-time 3D echocardiography can visualise the commissures better and detect any commissural calcification [4].

Leaflet mobility, thickness, calcification and subvalvular apparatus affection

Echocardiographic assessment should also include comment on leaflet mobility and thickness (usually assessed in the parasternal long-axis view). The extent of shortening and affection of subvalvular apparatus can be evaluated in parasternal long-axis and apical 4- and 3-chamber views. Areas of increased echo brightness suggest calcification. Echocardiography can locate the precise site of calcification, which can have a direct impact on the management, as commissural calcifications have been associated with poor results in balloon valvuloplasty [8]. In this context, real-time 3D echocardiography can be of added value in determining whether calcifications are extending to and obscuring the commissures or just confined to the leaflet tips.

Grading of mitral stenosis

Grading of MS severity depends on the combined measurements of planimetry, T_1/2 and pressure gradients. The planimetry result should be considered the reference value in case of discrepancy. Various scoring systems have been developed to assess suitability for balloon valvuloplasty. In general, according to recent ESC 2017 guidelines, patients with clinically significant MS (MVA <1.5 cm²) should be considered for intervention. Balloon mitral valvuloplasty would be the treatment of choice when the valve morphology is favourable. Less commonly, symptomatic patients with valve area more than 1.5 cm² may be considered for percutaneous intervention if symptoms cannot be explained by other causes and the valve morphology is favourable [7].

Echocardiography in mitral valve regurgitation

Mitral regurgitation (MR) remains an important cause of morbidity and mortality. MR can result from various aetiologies that lead to failure of proper and complete apposition of the valve leaflets. In general, the aetiologies are divided into organic valve disease (primary regurgitation) which is related to structural abnormalities of the valvular apparatus itself, and functional (secondary) regurgitation where left ventricular remodelling affects a structurally normal mitral valve leading to failure of leaflet coaptation. Recognising the mechanism of MR is obviously significant as both the management and outcome would vary according to whether it is a primary or secondary regurgitation.

Echocardiography is the cornerstone of MR evaluation as it can provide sufficiently accurate data about the degree of severity, the aetiology of MR and the consequences on cardiac performance. In addition, advances in 3D echocardiography have further enhanced the role of echocardiography in MR assessment; hence, the in vivo anatomy and pathophysiology of valve insufficiency can be more accurately described.

Grading severity of mitral regurgitation: how far do echocardiographic parameters correlate with in vivo anatomy?

There are several methods for assessing MR severity using echocardiography; each one of them has its own advantages and limitations.

Colour flow Doppler

Regurgitant jet area

This method is excellent for excluding MR; however, it is not reliable for grading its severity. In patients with eccentric MR jets impinging the wall of the left atrium, jet area correlated poorly with regurgitant volume and fraction compared to those with centrally directed regurgitant jets [9]. Moreover, hypertensive patients with mild MR may have a large jet area, while patients with acute severe MR may have a small colour jet area.

Vena contracta

The vena contracta (VC) is the narrowest portion of the regurgitant jet that occurs at or immediately downstream from the regurgitant orifice. Both single-plane VC width from the parasternal long-axis view and biplane VC width from apical views correlated well with regurgitant volume and regurgitant orifice area [10]. Though in vitro validation studies showed that colour Doppler estimates of the VC significantly overestimate regurgitant orifice and true VC, measurement of the VC remains a helpful semi-quantitative measure of valve regurgitation severity [11]. A VC width <0.3 cm usually denotes mild MR; ≥0.7 cm is specific for severe MR.

Currently, 3D echocardiography can be used to measure VC area (VCA) directly, which is much more representative of regurgitant orifice area. 3D echocardiography has shown that the regurgitant orifice is usually crescent-shaped in secondary MR, which may lead to underestimation of MR severity using VC width (2D echocardiography) that assumes a circular orifice geometry. In a recent study, 3D VCA >0.4 cm2 denoted severe MR; however, as yet there are no studies correlating 3D VCA to clinical or surgical outcomes [12].

Flow convergence (proximal isovelocity surface area [PISA]) method

Over recent years, the flow convergence method has gained increasing interest and has become one of the most recommended methods for quantitative assessment of MR severity. The PISA method can be used for calculation of effective regurgitant orifice area (EROA) and regurgitant volume (R Vol). In vitro studies initially validated the use of the flow convergence method in determining regurgitant flow rate and regurgitant orifice area [13]. Using a continuity equation, the PISA method for orifice estimation was then validated in mitral stenosis patients, where mitral valve area calculated using the flow convergence method correlated well with area measured via planimetry, pressure half time and Gorlin’s formula [14]. Furthermore, several studies showed that EROA measured using the flow convergence method in MR correlated very well with angiographic grade of MR and with EROA calculated by the Doppler echocardiographic method [15,16].

In general, MR is considered severe when EROA is ≥0.4 cm² and R Vol is ≥60 ml. PISA is more accurate for central regurgitant jets than for eccentric jets and for circular orifices more than for non-circular orifices. In secondary MR, as mentioned earlier, the regurgitant orifice is crescent in shape leading to underestimation of EROA calculated by the flow convergence method. This may be one of the reasons why lower values of EROA by 2D PISA are associated with worse prognosis in secondary MR [17]. Another limitation of the PISA method is that, being calculated from a single frame image, it will overestimate MR severity when MR is not holosystolic.

Continuous wave (CW) Doppler of MR jet

CW Doppler of the MR jet is a qualitative method for grading MR severity. In most patients, maximal MR velocity is 4-6 m/sec due to the high pressure gradient between the left ventricle and left atrium. Although the velocity itself does not correlate with severity of MR, it may point towards haemodynamic consequences of MR. A low MR peak velocity might suggest haemodynamic compromise (high left atrial pressure and low systolic pressure). Moreover, a dense CW Doppler signal with full envelope suggests significant MR, whereas a faint signal is likely to indicate mild MR.

Pulsed Doppler

Pulsed Doppler can be used to determine R Vol and regurgitant fraction (RF) by one of two methods. The first one is by determining stroke volume (SV) at both the left ventricular outflow tract (LVOT) and at the mitral valve level, and calculating R Vol as the difference between the two values. The other method is by comparing Doppler LVOT SV to total LV SV obtained from LV volumes.

Pulsed Doppler was validated as an accurate tool for determination of stroke volume at both mitral and LVOT levels with good correlation to thermodilution measurements [18]. In addition, a highly significant correlation was shown between R Vol and RF estimated using pulsed Doppler echocardiography on the one hand and measurements determined by combined left ventricular angiography and thermodilution on the other [19]. Pulsed Doppler echocardiographic evaluation of MR severity was also validated against magnetic resonance imaging (MRI) and was proved to be a reliable method of quantitating MR [20].

Conclusions

Echocardiography remains the cornerstone of mitral valve disease evaluation. Several echocardiographic parameters have been well validated and correlated with in vivo anatomy. Moreover, advances in 3D echocardiography have provided more insight into the geometry of the valve in mitral valve disease. However, further studies are still needed to elucidate the correlation between 2D and 3D echocardiography parameters and in vivo anatomy objectively, and to delineate the link between these measures and clinical and surgical outcomes.