Transthoracic echocardiography (TTE) is an initial step in the cardiologist's diagnosis of the MV patient. Perioperatively TEE is most likely to be employed to assist the surgeon performing MV repair or replacement.6 Three-dimensional TEE is becoming more commonly available and is used increasingly to examine the mitral valve perioperatively (Video 7–7). The basic TEE views of the normal mitral valve were discussed in the introduction to echocardiography and should be briefly reviewed before proceeding with this section (Figure 7–4).
The anterior and posterior leaflets of the MV appear thickened and poorly mobile in patients with mitral stenosis. As the patient's valve becomes stenotic, the orifice narrows obstructing diastolic filling of the LV.
When performing a perioperative TEE examination in the MS patient it is also critical to look for additional echo findings associated with mitral stenosis such as a dilated left atrium with stagnant blood (Video 7–8). The smoky appearance or spontaneous echo contrast is indicative of decreased blood velocities in the LA. The two-chamber view demonstrates both the MV and the beak-like left atrial appendage (LAA). The LAA is often the site of clot formation; the clot can be removed at the time of surgery and the LAA subsequently ligated. Left and right ventricular function is often impaired in the MS patient secondary to LV underloading, pulmonary hypertension, and RV failure.
Cardiologists employ both TTE and TEE to examine and quantify the mitral valve.7,8 Perioperatively the valve area and pressure gradient across the MV will have already been determined before the patient is referred for surgery. Nonetheless, it is important to be able to assess the pressure gradient and estimate valve area in the operating room. Doppler measurements and the Bernoulli equation are used perioperatively to determine the transmitral pressure gradient between the LV and the LA.
where V is the velocity of flow between the LA and the LV across the stenotic MV. Recall, that in areas of stenosis, blood flows faster (the rapids!!) indicating a greater pressure gradient between the two heart chambers. Mean pressure gradients greater than 10 mm Hg are associated with severe MS and a valve area less than 1.0 cm2.
There are several techniques, which can estimate the valve area of the stenotic MV. Although the cardiologist will have performed these prior to surgery it is useful to understand the concepts especially when MS is detected unexpectedly during a perioperative TEE examination.
The pressure half-time (PHT) method can estimate the degree of mitral stenosis (Figure 7–5). Using continuous wave Doppler, as discussed in the introductory chapter, we can measure the blood velocity across the stenotic mitral valve, and by using the Bernoulli equation we can calculate the atrioventricular pressure gradient. PHT is the time required for the atrioventricular pressure gradient to decrease from the initial maximal value to one-half of that value. It is calculated by the software incorporated in the TEE machine as 0.7 × maximal velocity of the blood across the mitral valve. PHT is obtained from the spectral Doppler display of the blood flow across the mitral valve by tracing the slope from the maximal to the minimal velocity in the Doppler envelope.
Compared with normal diastolic inflow patterns, patients with mitral stenosis take longer to equilibrate pressures between the LA and the LV during diastole, as demonstrated by the relatively flat slope of line AB.
In patients with greater degree of stenosis, it takes longer for the pressures between the left atrium and left ventricle to equilibrate resulting in a longer pressure half-time. The stenotic valve area can be estimated from the PHT calculation as follows:
Hence, any PHT greater than 220 milliseconds is associated with a mitral valve area of less than 1 cm2 area and as such with severe mitral stenosis.
The proximal isovelocity surface area (PISA) method can likewise be used to estimate valve area in mitral stenosis (Figure 7–6).
The PISA method also is employed to estimate mitral valve area in mitral stenosis. As blood passes through the tight mitral valve orifice it accelerates generating a flow convergence hemisphere which can be visualized using color flow Doppler.
The blood flow across the mitral valve can be visualized by using color flow Doppler. As blood rushes toward the narrowed orifice of the stenotic mitral valve, it accelerates, with the formation of multiple shells of hemispheric shape. The area of a flow convergence hemisphere is known as proximal isovelocity surface area and the velocity on the surface of this hemisphere is the same. According to the conservation of mass principle, the flow rate at the surface of a hemispheric shell is the same with the flow rate through the stenotic mitral valve. Generally, the Nyquist limit determines the velocity on the color flow map where the blood flow appears to change direction known as "aliasing." By adjusting the Nyquist limit of the color flow Doppler map, the flow velocity at the hemispheric surface where the flow is "aliasing" can be known. The radius of the hemisphere where the flow is "aliasing" can be measured. The maximum velocity of the blood at the level of the stenotic valve can be also measured. Thus, it is possible to estimate valve area by knowing the velocity of blood flow at the point of the stenosis, the area of the PISA hemisphere, and the velocity of flow at the hemisphere.
PISA area = 2πR2 where R is the radius of the PISA (cm) The PISA velocity = the Nyquist limit (cm/s)
The velocity of blood flow through the stenotic valve is measured with continuous wave Doppler at the level of the mitral valve in diastole.
MV area = (PISA area × Nyquist limit/maximal MV flow velocity) × α/180
The angle correction α/180 is added to the calculation to adjust for the reality that the PISA is not a complete hemisphere. The alpha (α) angle reflects the reality that the mitral valve leaflets do not sit as a straight line but are angled from the baseline.
Both acute and chronic MR can be diagnosed with the aid of either TTE or TEE. Knowledge of MV anatomy is necessary in determining the mechanism and significance of MR in the perioperative period.
The anterior and posterior mitral leaflets are joined at both the anterolateral and posteromedial commissures. The posterior leaflet has been described as consisting of three scallops: anterolateral (P1), medial (P2), and posteromedial (P3). These three posterior leaflet scallops coapt with their respective areas of the anterior leaflet of the MV (A1, A2, and A3). The various TEE views of the MV allow the visualization of these scallops. As such, it is possible to identify areas of valvular prolapse or restriction and use the TEE to identify the mechanism of regurgitation. The mitral valve fails to coapt during systole secondary to one of the following possible mechanisms:
The mitral annulus is dilated such that the leaflets do not come together sufficiently to fully close. In this instance, the color flow Doppler jet of MR is usually centrally directed.
The leaflets of the mitral valve move too much and prolapse or flail above the point of coaptation. This results in an eccentric color flow Doppler jet directed away from the prolapsing/flail valve leaflet.
The mitral valve leaflets do not move enough. Restriction of movement of one or more of the leaflets prevents the valve from coapting resulting similarly in an eccentric regurgitant jet.
Often disease processes of the papillary muscles and chordae can lead to the development of acute MR secondary to ruptured chordae and/or ischemic papillary muscles resulting in a "flail" mitral leaflet (Video 7–9).
TEE allows the determination of both the location and cause of MR as well as an estimation of its severity. Using the Carpentier nomenclature of the mitral valve leaflets as outlined above, it is possible to identify the source of leaflet pathology. Mastery of this takes some time and only those with the appropriate training and certification in perioperative TEE should undertake to guide surgery by the identification of MV pathology. Still, it is quite possible to readily discern if leaflets are restricted, flail, prolapsed, intact, or unable to coapt secondary to annular dilatation.
There are a number of TEE techniques, which can be employed to assess the severity of MR including9:
- Jet Area: The area of the regurgitant color flow jet is compared with that of the area of the left atrium. If the area is greater than 40% the MR is thought severe. However, if the jet hugs the wall of the LA this method will underestimate the severity of MR (Video 7–10).
- Vena Contracta: The vena contracta is the narrowest part of the regurgitant jet as it passes through the valve. A vena contracta greater than 6 mm in diameter is thought to be associated with significant MR (Video 7–11).
- PISA: The concept of PISA has been described in the earlier section. As the LV contracts during systole, blood accelerates toward the incompetent mitral valve forming multiple shells of hemispheric shape (Figure 7–7). Using the same formula as above, the area of the regurgitant orifice can be calculated by knowing the Nyquist limit or velocity where the regurgitant flow is aliasing, the radius of the hemisphere where the flow is aliasing, and the maximal velocity of the regurgitant flow at the level of the regurgitant orifice. In a simpler manner, it can be estimated that if the Nyquist limit is set at 40 cm/s a PISA hemisphere radius of >1cm is associated with severe MR.
- Pulmonary Vein Doppler: Blood flows from the pulmonary veins into the LA both in systole and in diastole. The systolic flow in the pulmonary vein is normally greater than that of diastolic flow (Figure 7–8A). When severe mitral regurgitation is present, because of greatly increased LA pressure the systolic pulmonary vein flow is often blunted or reversed (Figure 7–8B).
A PISA is seen in this patient with mitral regurgitation. In patients with mitral stenosis, the PISA sphere is seen in the LA during diastole as flow accelerated to pass through the narrowed mitral orifice into the LV. However, during mitral regurgitation the 0.951-cm PISA radius is seen on the ventricular side of the mitral valve as flow accelerates to pass through the leaky mitral valve during systole.
The normal pulmonary vein pattern is seen above. Pulse wave Doppler (PWD) is used in the left upper pulmonary vein. Flow is normally toward the left atrium throughout systole (S) and diastole (D). There is reversal of flow during atrial contraction at the end of diastole (AR).
In patients with severe mitral regurgitation the S wave is often blunted or reversed in comparison to the D wave during pulse wave Doppler interrogation of the pulmonary vein.