6 Cardiac valves
Figure 6.1 Normal valve appearance. Mitral valve viewed in (a) the longitudinal and transverse parasternal transthoracic echocardiography (TTE) cross-section and (c) in the multiplanar transoesophageal echocardiography (TEE) mode identifying the three mitral segments. Aortic valve viewed in (b) TTE (two-dimensional (2D)/M-mode) and (d) in the transverse TEE view showing the three open scallops and the commissures. AO, aorta; LA, left atrium; LV, left ventricle; RV, right ventricle.
Figure 6.2 Segmentation of the mitral valve (MV) explored in multiplanar TEE: three segments of the large mitral valve (A1, A2, A3) and three segments of the small mitral valve (P1, P2, P3). The two mitral valve leaflets are separated by two commissures: the anterior commissure (AC) between A1 and P1, and the posterior commissure (PC) between A3 and P3. The TEE planes that can be used to study the different mitral segments are: 0°, A1 and P1; 45°, AC and PC; 90°, A3 and P3; 140°, A2 and P2. AO, aorta; LAR, left auricle.
Figure 6.3 Two-dimensional cross-sections centred on the aortic orifice: (a) longitudinal and (b) transverse, parasternal TTE cross-sections; (c) major axis, multiplanar (110–130°) TEE cross-section; (d) minor axis multiplanar (60–80°) TEE cross-section. LA, left atrium; LCC, left coronary cusp; LV, left ventricle; NCC, non-coronary cusp; Pa, pulmonary artery; Pulm, pulmonary valve; RA, right atrium; RCC, right coronary cusp; RV, right ventricle; Tric, tricuspid valve.
Figure 6.4 Valvular stenoses in TTE. (a) Calcifications of the aortic cusps masking the systolic opening in M-mode TTE. (b) Calcified and stenotic aortic orifice, evaluated at 1.02 cm2 using planimetry. (c) Massive calcifications of the mitral annulus limiting valvular mobility. (d) Thickening and retraction of the mitral chordae in a case of mitral stenosis.
The differential diagnosis between calcifications and fibrous nodules is not always simple. Generally, calcifications are manifested as dense and bright echoes that persist after reducing the gain settings. They are definite in the presence of an adjacent shadow cone. However, they are nonetheless quite frequently overestimated in echocardiograms.
This difficulty is due to the fusion and/or retraction of the chordae as well as fibrosis of the papillary muscles responsible for a subvalvular MS. In fact, the study of the subvalvular apparatus is more difficult than that of the valves, probably because subvalvular lesions are more complex and are often not easily picked up by transthoracic echocardiography (TTE). Identification of such lesions is much more precise in TEE.
It is sometimes difficult to identify a bicuspid valve when using TTE, as the extent of calcification does not allow for good definition of the commissures or the number of semilunar cusps. TEE is useful in this diagnosis. A quadricuspid aortic valve is a rare cardiac anomaly, and is generally responsible for AS.
Each echo Doppler method has preferential indications and its own limitations. The elements that distinguish between the aortic surface area measured using echocardiography and that calculated using echo Doppler are summarized in Table 6.1.
|Echocardiographic surface area||Doppler surface area|
|Type of surface||Anatomical||Functional|
|Measurement mode||Planimetry||Continuity equation|
|Measurement site||Upstream of the vena contracta||At the level of the vena contracta|
|Relation to cardiac output||Independent of output||Dependent on output|
|Modification under Dobutamine||Fixed||Increased|
The mean pressure gradient better reflects the severity of the valvular stenosis than does the maximuml gradient. The mean value represents the integration of the instantaneous gradient over the entire duration of the diastole (MS) or systole (AS). To calculate the mean gradient requires the maximum velocities in the central and proximal parts of the stenotic jet must be recorded. These measurements can be done using TEE, but TTE is generally adequate.
Failure to interpret the stenotic gradient according to the blood output through the stenotic orifice and the heart rate
The severity of stenosis may be overestimated (increased gradient) due to a high cardiac output (anaemia, hyperthyroidism) or an associated valvular leak, or may be underestimated (reduced gradient) due to a low cardiac output linked to a systolic dysfunction of the left ventricle (LV). For example, a moderate mean gradient does not allow a tight valvular stenosis to be ruled out in a case of low cardiac output. Finally, for patients in atrial fibrillation, the averaging of several cycles is vital, taking into account the wide variability of the gradient according to the length of the cycles (Fig. 6.6).
Figure 6.6 Mitral condition. Recording in continuous Doppler coupled with 2D colour imaging. The stenotic gradient and pressure half-time (PHT) (and therefore the mitral surface area (MSA)) vary due to the continuous arrhythmia arising from atrial fibrillation. LA, left atrium; LV, left ventricle; MR, mitral regurgitation; MS, mitral stenosis.
The simplified Bernoulli equation (4V22) is only valid if the velocity upstream of the stenosis (V1) is insignificant compared with the velocity at the level of the stenosis (V2). Otherwise (e.g. when the subaortic velocity is above 1.5 m/s) the use of the simplified equation leads to an overestimation of the gradient. The ‘complete’ Bernoulli formula 4(V22 – V12) should be used in this case.
In general, this phenomenon is due to the difference in gradient between the vena contracta (Doppler), on the one hand, and the prestenotic zone and post-stenotic zone (catheterization) on the other. It equates to a retransformation, downstream of the vena contracta (zone of lowest pressure and highest velocity), of the kinetic energy into potential energy (remounting of pressure). This phenomenon leads to an overestimation of the gradients when using Doppler compared with those measured using catheterization. It can be observed in particular cases:
Planimetry of the aortic orifice using two-dimensional (2D) TTE and the fundamental imaging method is practically impossible (small, irregular, ill-defined orifice, hyperechoic valves). Harmonic imaging can be useful in better identifying the limits of the orifice (see Fig. 6.4(b)). Multiplanar TEE can be used to measure the aortic orifice by planimetry with good reliability. However, this method should be reserved for cases where the continuity equation cannot be used or where there is a discrepancy between the results obtained with different quantification methods.
Planimetry of the stenotic mitral orifice remains the most reliable method for determining what is known as the anatomical mitral surface area (MSA). It is carried out on the valve in the open position according to the transverse, parasternal, transthoracic cross-section, using the zoom and the cine loop function. This planimetric technique must be undertaken with particular care, as there are numerous possible pitfalls with this measurement, such as:
Figure 6.8 Planimetry of the stenotic mitral orifice according to the transverse, parasternal cross-section (zoomed 2D images). The mitral surface area (a) correctly measured using planimetry (1.06 cm2) and (b) overestimated (1.32 cm2) due to using the oblique 2D projection of the mitral orifice in the same patient.
Figure 6.9 Slack, soft-valve mitral stenosis. (a) Parasternal long axis view. (b) M-mode of mitral valve. (c) Correct assessment of the mitral surface area (MSA) using planimetry (1.54 cm2). (d) Incorrect assessment of the MSA using continuous Doppler (1.52 cm2). AO, aorta; LA, left atrium; LV, left ventricle; MO, mitral orifice; MS, mitral stenosis; RV, right ventricle.
Figure 6.10 Morphological forms of the mitral stenosis: (a) funnel shape; (b) membrane shape. There is a risk of a clear overestimation of the mitral surface area using planimetry passing through the body of the mitral valve in the form of a membrane. LA, left atrium; LV, left ventricle.
Figure 6.11 Tight mitral stenosis (MS) with clear remodelling of the subvalvular apparatus. The mitral surface area measured using planimetry (1.1 cm2) (a) is larger than that determined using continuous-wave Doppler (0.77 cm2) (b). LA, left atrium.
Hatle’s method can be used to calculate the ‘functional MSA’ on the basis of the diastolic mitral flow recorded using continuous Doppler. This MSA value is based on the measurement of the pressure half-time (PHT), which varies in inverse proportion to the anatomical surface area of the mitral orifice. The MSA is derived from an empirical mathematical equation:
Hatle’s method is useful because it gives information about both valvular obstructions (commissural fusion) and subvalvular obstructions (lesions of the subvalvular apparatus), whereas planimetry gives information only about valvular obstructions. When the PHT is carefully measured, the reliability of Hatle’s method is excellent. Nevertheless, this method is not without several pitfalls (Box 6.3), which are described below.
This is a particular morphology of the Doppler curve: a biphasic slope with a steep, brief initial period, followed by a slower phase (Fig. 6.12). Given this ambiguity regarding which slope should be used, it is recommended that the PHT measurement is made using the second slope (Fig. 6.13). However, in the majority of patients, the deceleration slope is a straight line.
Figure 6.12 Mitral stenosis (MS) with a ‘biphasic slope’. The mitral surface area measured using the initial slope (2.53 cm2) and the second slope (1.8 cm2) of the spectrum, recorded using continuous Doppler. LA, left atrium; LV, left ventricle.
Immediately after percutaneous mitral dilatation, the PHT values are responsible for an overestimation of the MSA. This phenomenon is linked to the sharp modifications in atrioventricular compliance postdilatation. Hatle’s method must, therefore, not be used in the 48 hours following the dilatation procedure.
Planimetry is the preferred method in cases where there is satisfactory imaging, and Hatle’s method is preferred in cases of a severely calcified mitral orifice, subvalvular stenosis or poor quality imaging. When there is a discrepancy between the results obtained using mitral planimetry and Hatle’s method, it is necessary to resort to a third method (continuity equation). Finally, colour Doppler has a secondary role in assessing the tightness of the MS. It helps to position the Doppler beam in the central laminar part of the stenotic jet (the central core of the stenosis). Finally, Hatle’s method is no longer considered valid for native mitral valves.
where V1 and V2 are the subaortic and transvalvular stenotic velocities, and S1 and S2 are the aortic and subaortic (outflow) areas, respectively. The equation is based on the equality of the outputs:
Doppler TTE makes it possible to calculate the functional surface area of the stenotic orifice (mitral or aortic), which is equal to the output in the left ventricular outflow chamber divided by the velocity–time integral (VTI) of the trans-stenotic flow:
Figure 6.14 Three echo Doppler measurements enabling calculation of the functional surface area of the aortic orifice by means of the continuity equation. AO, aorta; D, diameter; LA, left atrium; LV, left ventricle; SAOS, subaortic surface; VTIAO, aortic velocity–time integral; VTISAO, subaortic velocity–time integral.
Imprecise measurement of the subaortic diameter using this approach is linked to the low lateral resolution of the ultrasonic waves in this view. The aortic orifice is approached in a tangential manner, and therefore there is a risk of underestimating the diameter of the left ventricular outflow chamber. For these reasons, the subaortic diameter should be measured in the longitudinal, parasternal cross-section, in systole, using the zoom and cine loop.
Normally, this measurement should be carried out between the two points of insertion of the aortic cusps and in parallel with the plane of the valve. Care must be taken to measure the subaortic diameter with the greatest possible precision, since, if a mistake is made, the squaring of the diameter will modify the calculated valve surface area by the same amount. The following situations may be responsible for errors in measuring the subaortic diameter (Figs 6.16 and 6.17):
Figure 6.16 Measurements of the subaortic diameter when using 2D echocardiography and the longitudinal, parasternal cross-section. (a) Correct angle; (b) oblique angle overestimating the diameter; (c) angle underestimating the diameter in the case of a subaortic septal rim included in the measurement. AO, aorta; LA, left atrium; LV, left ventricle.
Figure 6.17 Aortic stenosis. Pitfalls in measuring the subaortic diameter using the longitudinal parasternal cross-section (zoomed images). (a) Oblique angle overestimating (2.0 cm) the actual diameter (1.8 cm). (b) Underestimation of the aortic diameter (1.5 cm) measured at the level of the subaortic septal rim. Correct value obtained downstream of the rim: 1.8 cm. AO, aorta; LA, left atrium; LV, left ventricle.
In practice, the measurement of the left ventricular outflow chamber diameter should be repeated at least three times and the mean value calculated; any extreme, non-reproducible values should be eliminated.
Values of D calculated using this formula are relatively reliable. However, use of the fixed value of 2 cm for the subaortic diameter should be avoided, as this is a major source of errors. An incorrectly enlarged subaortic diameter will lead to an overestimation of the valve surface area (mitral or aortic) calculated using the continuity equation (Box 6.5). Conversely, a value for the subaortic diameter that is falsely too low will lead to an underestimation of the valve surface area.
Finally, in cases of AS where the measurement of the subaortic diameter is not possible via the transthoracic route, it is possible to quantify the stenosis using the permeability index. This may be done using the VTI ratio: subaortic VTI/transaortic VTI. This easily calculated parameter is independent of the cardiac output and its sensitivity is satisfactory, but its specificity remains poor. In fact, a ratio of £ 0.25 identifies an aortic surface area of £ 0.75 cm2 with a sensitivity of 92% and a specificity of 68%.
Normally, these flows should be recorded using pulsed Doppler across the apical cross-section passing through the aortic root. It is important to be aware of the potential pitfalls of this technique (see Box 6.4).
In order to minimize the angle between the Doppler beam and the subaortic flow, it is often necessary to move the echocardiographic probe towards the armpit of the patient being examined. This manipulation makes it possible to verticalize the outflow chamber and to obtain a better alignment of the Doppler beam with the aortic ejection flow.
In practice, the small Doppler sample volume (4–6 mm) should be positioned in the middle of the left ventricular outflow chamber, approximately 5 mm upstream of the aortic cusps, a position that corresponds fairly well with the level used for measuring the subaortic diameter in the parasternal view. Colour Doppler may make it easier to locate the site of collection of the subaortic flow. This site usually corresponds to the small first aliasing zone (passage from blue to red) in the absence of low output. By modifying slightly the position of the Doppler sample volume in the outflow chamber, it is possible to optimize the collection of subaortic velocities. This process enables the operator to record subaortic flows integrally within the exclusively laminar zone, at the level of the vena contracta.
An underestimation of the subaortic velocities is due to the Doppler sample volume being too distant from the aortic valve. This leads to an underestimation of the valve surface area calculated using the continuity equation. Bringing the Doppler sample volume too close to the aortic orifice leads to a sharp enlargement of the spectrum linked to the entry into the acceleration zone of the ejection flow. This in turn leads to an overestimation of the subaortic velocities, and therefore to an erroneous increase in the valve surface area calculated using the continuity equation (Fig. 6.18).
Figure 6.18 Aortic stenosis. Pitfalls due to recording subaortic velocities using pulsed Doppler; all views are in the same patient. (a) Correct recording site (velocity–time integral (VTI): 28 cm). (b) Incorrect site, too far from the aortic orifice (VTI underestimated: 23 cm). (c) Incorrect site, too close to the aortic orifice (VTI overestimated: 36 cm). AO, aorta; LA, left atrium; LV, left ventricle.
The poor quality of the subaortic spectrum recorded using pulsed Doppler renders the subaortic VTI measurement using planimetry imprecise. It is important to obtain a clear spectral envelope of the subaortic flows, with well-defined and homogenous contours, and without echoes within the spectrum. The presence of the aortic closing click is not a required condition for recording; this click is often muffled in the case of a tight AS. Finally, the measurement of the subaortic VTI should be done over at least three cycles and the mean value calculated.
Figure 6.19 Aortic stenosis. Variability of the subaortic velocity–time integral (VTI) measured using pulsed Doppler (a) and of the transaortic VTI measured using continuous Doppler (b) in the case of continuous arrhythmia due to atrial fibrillation.
The transvalvular stenotic flow (mitral or aortic) is recorded using continuous Doppler. It is important to understand the pitfalls of this type of recording in order to avoid incorrect results (see Box 6.4).
Poor alignment of the Doppler beam with the stenotic flow leads to an incomplete recording of the stenotic jet and a ‘hoarse’ and vibrant Doppler sound. It is therefore necessary to increase the number of echocardiographic views, especially when exploring AS. When done with care, it is possible to achieve correct alignment and to capture the maximum velocity of the stenosis. The pure and sharp acoustic signal indicates proper alignment. Prior localization of the stenotic flow using colour Doppler, wherein it is seen in the form of a mosaic, enables precise adjustment of the Doppler beam angle (Fig. 6.20).
Failure to use the 2 MHz, Pedoff-type, continuous, stand-alone Doppler probe (pen-shaped probe) without 2D imaging (‘blind’)
The great ease of manipulating of this probe allows for optimal alignment of the beam with the central jet of the stenosis. Poor alignment (above 20°) leads to a clear underestimation of the transvalvular VTI (Fig. 6.21), which leads to an overestimation of the valve surface area as calculated using the continuity equation (see Box 6.5).
Figure 6.21 Aortic stenosis. (a) Underestimation of the transvalvular gradient (maximum gradient: 37 mmHg) calculated using the flow recorded with continuous Doppler coupled with 2D imaging (incorrect alignment). (b) Correct evaluation of the gradient (63 mmHg) obtained using the Pedoff probe, which enables good alignment with the stenotic jet (recorded in the same patient as in (a)). AO, aorta; LA, left atrium; LV, left ventricle.
As with subaortic flows, it is recommended that at least three cardiac cycles in sinus rhythm are recorded, avoiding post-extrasystolic complexes, and the mean VTI value calculated. In the case of atrial fibrillation, VTI values from a minimum of five cycles should be averaged, due to the variability in the VTI of the stenotic flow during continuous arrhythmia (see Fig. 6.19).
Confusion between the flow in aortic stenosis and the flow in mitral regurgitation when using the Pedoff probe (Fig. 6.22)
Figure 6.22 Differential diagnosis between aortic stenosis (AS) and mitral regurgitation (MR), and between tricuspid regurgitation (TR) and left intraventricular obstruction (hypertrophic obstructive cardiomyopathy (HOCM)), using flows recorded with continuous Doppler. Note the short ejection times (AS and HOCM) in relation to the length of MR or TR, and the isovolumetric contraction time (the period between the QRS wave and the start of the flow) in the case of AS or HOCM. AOC, aortic closing; AOO, aortic opening; MC, mitral closing; MO, mitral opening; TC, tricuspid closing; TO, tricuspid opening.
Confusion between the flow in aortic stenosis and the flow in tricuspid regurgitation when using the Pedoff probe (see Fig. 6.22)
The differentiation between these flows is much easier than that between the AS and MR flows. The maximum flow velocity in tricuspid regurgitation (TR) is generally lower than that in AS, except in cases of severe pulmonary hypertension, or in cases of non-tight AS. The flow time in TR is longer than that of the aortic flow.
In order to differentiate between these flows, the operator must make use of echocardiographic imaging and continuous Doppler. The obstructive subaortic flow shows an acceleration crescendo during the systole, which has a characteristic sabre-shaped appearance (see Fig. 6.22). However, in cases of subvalvular obstruction associated with AS, the continuity equation is invalid, as the upstream velocity is no longer insignificant. The only usable method in this situation is planimetry of the aortic orifice.
Finally, in the presence of a subvalvular AS (e.g. in the membrane), the flow is difficult to differentiate from AS flow in continuous Doppler. In this case, pulsed or colour Doppler can be used to locate the site of the flow acceleration downstream of the aortic orifice.