Cardiac valves

6 Cardiac valves




PITFALLS WHEN STUDYING THE CONDITION OF THE STENOTIC VALVE


These echocardiographic pitfalls comprise:






Incomplete visualization of the valves (Fig. 6.1)


In order to assess correctly the condition of the stenotic valve, it is necessary to visualize:









It may be necessary to resort to transoesophageal echocardiography (TEE), especially in the case of MS.





PITFALLS WHEN STUDYING THE DEGREE OF VALVULAR STENOSIS


The echocardiographic measurements that enable the quantification of valvular stenosis are:




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.


Table 6.1 Parameters that differ between the aortic surface area measured using echocardiography and that calculated using Doppler



























  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


Pitfalls when measuring the trans-stenotic pressure gradient (Box 6.1)








Pitfalls when maesuring the surface area of the stenotic orifice



Pitfalls when using planimetry (Box 6.2)




Planimetry of the stenotic mitral orifice


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:














It should be noted that mitral planimetry is valuable in cases of associated mitral regurgitation (MR) or aortic regurgitation (AR).



Pitfalls when using Hatle’s method


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:



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This method may be used with TTE or TEE, but the transthoracic approach is generally sufficient.


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.












Pitfalls when using the continuity equation (Box 6.4)


The continuity equation uses the principle of conservation of mass, with the formula:




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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:



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This examination requires a high degree of technical rigour and precision of measurement in order to avoid the pitfalls of quantifying the valvular stenosis.


The following measurements are involved (Fig. 6.14):





In practice, the maximum velocities and the VTI can be used interchangeably when calculating the aortic surface area.



Pitfalls when measuring the subaortic diameter





Imprecise measurement of the subaortic diameter

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):







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.


The following formula offers an additional possibility for calculating the diameter of the subaortic diameter (D):



image



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%.


Finally, in an extreme diagnostic situation, the operator may resort to TEE to measure the subaortic diameter with greater precision.



Pitfalls when recording subaortic flows


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).




Incorrect positioning of the Doppler sample volume in the left ventricular outflow chamber

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).






Pitfalls when recording the stenotic flow


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).









Jun 4, 2016 | Posted by in CARDIOLOGY | Comments Off on Cardiac valves
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