Valvular Regurgitation

12


Valvular Regurgitation




BASIC PRINCIPLES



APPROACHES TO THE EVALUATION OF THE SEVERITY OF REGURGITATION



AORTIC REGURGITATION



MITRAL REGURGITATION



TRICUSPID REGURGITATION



PULMONIC REGURGITATION


SUGGESTED READING


Echocardiographic evaluation of the patient with valvular regurgitation includes an assessment of valve anatomy, the severity of regurgitation, chamber dilation due to the imposed volume overload, ventricular function, and the degree of pulmonary hypertension. In some cases, the clinical significance of valvular regurgitation is related to the presence of abnormal regurgitation, regardless of severity. For example, the detection of aortic regurgitation (AR) in a patient with chest pain and an enlarged aorta heightens the suspicion of aortic dissection. In other situations (e.g., mitral valve prolapse), the severity of regurgitation is an essential factor in clinical decision making regarding surgical intervention. In chronic regurgitation due to primary valve disease, regurgitant severity and the response of the left ventricle (LV) to chronic volume overload are the most important factors in deciding on the timing of valve surgery.



Basic Principles



Etiology of Valvular Regurgitation


Valvular regurgitation may be due to congenital or acquired abnormalities of the valve leaflets or to abnormalities of the associated supporting structures. For example, dilation of the ascending aorta or sinuses can result in AR even with anatomically normal valve leaflets. Similarly, LV dilation can result in mitral regurgitation (MR) even with normal valve leaflets and chordae. Echocardiographic examinations allow definition of the etiology of valvular regurgitation in most cases. Even when a single definite cause is not evident, the differential diagnosis of the etiology of regurgitation often can be narrowed to the few most likely possibilities. The examination also may provide clues as to whether regurgitation is acute or chronic in duration.


When transthoracic echocardiographic (TTE) images are not diagnostic for the evaluation of aortic or mitral valve anatomy and the etiology of regurgitation, transesophageal echocardiographic (TEE) imaging may be helpful. With diseases of the aorta, visualization of the ascending aorta often is suboptimal on TTE imaging, so TEE imaging typically is needed to fully define the extent and severity of disease.



Fluid Dynamics of Valvular Regurgitation


The fluid dynamics of a regurgitant valve (Fig. 12-1) are, in many ways, similar to the fluid dynamics of a stenotic valve and are characterized by:




Even though the anatomy of inadequate valve closure may be quite complex, the valve can be thought of as having a regurgitant orifice, which in simple physiologic terms is characterized by a high-velocity laminar jet (Table 12-1). The instantaneous velocity in this jet (v) is related to the instantaneous pressure difference (∆P) across the valve, as stated in the simplified Bernoulli equation: ∆P = 4v2. Recording this high-velocity jet with continuous-wave (CW) Doppler allows assessment of the time course of the difference in pressure between the two chambers on either side of the valve.



On the upstream side of the regurgitant valve, flow acceleration proximal to the regurgitant orifice is present, and a proximal isovelocity surface area (PISA) can be defined, similar to that seen on the left atrial (LA) side of the stenotic mitral valve. The proximal isovelocity surface area, multiplied by the aliasing velocity, provides a method for quantitative evaluation of regurgitant stroke volume. The narrowest segment of the regurgitant jet, the vena contracta, occurs just distal to the regurgitant orifice, with vena contracta diameter reflecting regurgitant orifice area.


As the high-velocity jet enters the chamber receiving the regurgitant flow, the flow pattern becomes disturbed with nonlaminar flow, multiple blood flow velocities, and multiple blood flow directions. The size of the downstream regurgitant flow disturbance is affected by both physiologic and technical factors and thus is less useful for the quantitation of regurgitant severity (Table 12-2). In addition, the shape and direction of the regurgitant jet are affected by the anatomy and orientation of the regurgitation orifice, the driving force across the valve, and the size and compliance of the receiving chamber. Jets are “pulled” toward adjacent walls (e.g., MR in the LA) if within a critical distance from the wall at the entry site and also are “pulled” toward other flowstreams (e.g., AR and mitral stenosis). Eccentric jets that adhere to the wall of the chamber will have a smaller color jet area on two-dimensional (2D) color flow imaging (and a smaller three-dimensional [3D] volume) because entrainment of additional fluid elements into the jet occurs on only one side, instead of on all sides, as with a central jet.




Volume Overload


In patients with a regurgitant valve, the term total stroke volume refers to the total volume of blood pumped by the ventricle on a single beat. Forward stroke volume is the amount of blood delivered to the peripheral circulation, and regurgitant volume is the amount of backflow across the abnormal valve (Fig. 12-2).



Chronic valvular regurgitation results in progressive volume overload of the ventricle. Volume overload of the LV results in chamber dilation with normal wall thickness so that total LV mass is increased. An important clinical feature of chronic LV volume overload is that an irreversible decrease in systolic function can occur in the absence of symptoms. In fact, an irreversible decrease in contractility can occur despite a normal ejection fraction because of the altered loading conditions of the ventricle when regurgitation is present.


Serial echocardiographic evaluation of LV size and systolic function is a standard method of clinical evaluation, but two factors potentially limit the reliability of this approach. First, suboptimal image quality or recording techniques may result in erroneous measurements. Care is needed to ensure that the dimensions are measured perpendicular to the long and short axes of the LV, and instrument settings must be adjusted for optimal endocardial definition. Accurate tracing of endocardial borders for the calculation of ventricular volumes depends on clear endocardial definition, standard image planes without foreshortening of the long axis of the ventricle, and a trained and experienced individual tracing the borders at end-diastole and end-systole. Three-dimensional echocardiography with semiautomated border detection (see Chapter 6) provides more accurate LV volumes and will become the standard as this instrumentation becomes more widely available.


Second, the reproducibility of LV measurements must be considered. Overall reproducibility includes variation in recording the data, variation in measuring the data, and physiologic variation (such as heart rate and loading conditions) that may affect the measurement. Reproducibility of 2D-guided M-mode measurements of the LV suggests that an interval change of greater than 8 mm in end-systolic or end-diastolic dimensions represents a definite clinical change. Using 2D echocardiography, a change in ventricular volume or a change in ejection fraction greater than 10% on serial studies performed in the same laboratory indicates a significant change. Three-dimensional volumes have lower variability in recording and measuring data but are still subject to physiologic variability.



Detection of Valvular Regurgitation


Valvular regurgitation can be detected with:



While anatomic imaging provides detailed information about the valve apparatus and ventricular function, it provides only indirect evidence for the presence or absence of valvular incompetence. The finding of an anatomically abnormal mitral valve in the presence of LA and LV dilation suggests that MR may be present, but Doppler examination is necessary for direct confirmation or exclusion of the diagnosis. Although a few M-mode findings have been shown to be specific for diagnosing valvular regurgitation (e.g., high-frequency fluttering of the anterior mitral leaflet in AR), these findings are not sensitive enough to reliably exclude regurgitation when suspected on clinical grounds.


With color flow imaging, detection of regurgitation is based on identification of the flow disturbance downstream from the regurgitant orifice. When instrument settings and examination technique are optimal, color flow imaging is extremely sensitive (>90%) and specific (nearly 100%) for the detection of valvular regurgitation as compared to angiography. In fact, color flow imaging is so sensitive that regurgitation often is detected that is not audible by auscultation. These cases most often are true positives, as evidenced by angiographic confirmation. False-positive results can occur with color flow imaging when the origin or timing of the flow signal is mistaken. For example, normal pulmonary venous inflow into the LA may be mistaken for MR. False-negative results occur when signal strength is low because of poor acoustic access or attenuation due to the depth of interrogation. False-negative results also occur if color flow processing parameters are set incorrectly or if the examiner fails to evaluate the valve in more than one tomographic plane. Additional parameters important in the detection of valvular regurgitation with color flow imaging include frame rate, Nyquist limit, color gain, and the color velocity-variance display.


CW Doppler detection of valvular regurgitation is based on the identification of the high-velocity jet through the regurgitant orifice. An advantage of CW Doppler is that beam width is broad at the level of the valves when studied from an apical approach. Identification of the regurgitant signal uses the velocity, shape, timing, and associated antegrade flow signal to correctly identify the origin of the signal (Fig. 12-3).




Valvular Regurgitation in Normal Individuals


A small degree of regurgitation, often termed physiologic, is present in a high percentage of otherwise normal individuals (Fig. 12-4). Typically, physiologic regurgitation is:




When meticulously searched for, MR can be detected in 70% to 80%, tricuspid regurgitation in 80% to 90%, and pulmonic regurgitation in 70% to 80% of normal individuals. This small degree of regurgitation is normal and has no adverse clinical implications. AR is found in only a small percentage (5%) of young individuals with an otherwise normal echocardiographic study, but the prevalence of detectable AR increases with age. The clinical significance of a small amount of AR is unknown.



Approaches to the Evaluation of the Severity of Regurgitation


The severity of valvular regurgitation typically is described using semiquantitative measures as mild, moderate, or severe (Table 12-3). The size of the color Doppler jet is not accurate for the evaluation of regurgitant severity. Instead, semiquantitative measures include:




In addition, several quantitative measures (Table 12-4) of regurgitant severity have been well validated, including:



TABLE 12-4


Selected Studies Validating Quantitative Evaluation of Regurgitant Severity Using Doppler Echocardiography


image


image


Ao, aorta; AR, aortic regurgitation; CMR, cardiac magnetic resonance imaging; CO, cardiac output; EM-flow, volume flow rate measured by electromagnetic flowmeter; ID, indicator dilation; PISA, proximal isovelocity surface area method; RF, regurgitant fraction; RSV, regurgitant stroke volume; SV, stroke volume; TD, thermodilution.


Bland-Altman mean difference and limits of agreement.


Data sources: Spain et al: J Am Coll Cardiol 13:585-590, 1989; Tribouilloy et al: Circulation 85:1248-1253, 1992; Enriquez-Sarano et al: J Am Coll Cardiol 21:1211-1219, 1993; Tribouilloy et al: Circulation 102:558-564, 2000; Hall et al: Circulation 95: 636-642, 1997; Rescusani et al: Circulation 83:594-604, 1991; Utsunomiya et al: J Am Soc Echocardiol 4:338-348, 1991; Vandervoort et al: J Am Coll Cardiol 22:535-541, 1993; Giesler et al: AJC 71:217-224, 1993; Chen et al: J Am Coll Cardiol 21:374-383, 1993; Teague et al: J Am Coll Cardiol 8:592-599, 1986; Masuyama et al: Circulation 73:460-466, 1986; Ascah et al: Circulation 72:377-383, 1985; Kitabatake et al: Circulation 72:523-529, 1985; Rokey et al: J Am Coll Cardiol 7:1273-1278, 1986; Bougher et al: Circulation 52:874-879, 1975; Touche et al: Circulation 72:819-824, 1985; Marsan et al: JACC Cardiovasc Imaging 2(11):1245-1252, 2009; Zeng et al: Circ Cardiovasc Imaging 4(5):506-13, 2011; Perez de Isla et al: Int J Cardiol 2011 Dec 20. [Epub ahead of print].



Regurgitant volume (RV) is the retrograde volume flow rate across the valve, expressed either as an instantaneous flow rate in milliliters per second or (more correctly) averaged over the cardiac cycle in milliliters per beat. Regurgitant volume can be calculated by three different approaches:



Regurgitant fraction (RF) is:


image (12-1)


Regurgitant orifice area (ROA) is calculated, using the continuity equation, from regurgitant volume and the velocity-time integral of the regurgitant jet (VTIRJ). Because the regurgitant volume (RV) proximal to and in the regurgitant orifice are equal,


image (12-2)


so that, solving for regurgitant orifice area,


image (12-3)


with RV in cm3, VTIRJ in cm, and ROA in cm2.



Color Doppler Imaging



Jet Size and Shape


In the past, regurgitant severity often was graded based on the size of the flow disturbance in the chamber receiving the regurgitant jet on a 0 (mild) to 4+ (severe) scale. However, this grading system is most useful for the identification of patients with mild regurgitation; there is substantial overlap in jet areas between patients with moderate and severe regurgitation. Color flow “mapping” also is subject to marked variability due to gain and other instrument settings, as well as physiologic variability. Thus, the length that a regurgitant jet extends into the receiving chamber is an unreliable indicator of disease severity and should no longer be used in patient management.


Color flow imaging of the regurgitant jet remains clinically useful for the detection of valve regurgitation, evaluation of the timing of flow, and for insights about the cause of regurgitation (Figs. 12-5 and 12-6). Because color flow imaging basically is pulsed Doppler ultrasound with somewhat different signal processing and display formats, it is important to remember that signal aliasing still occurs. However, the usefulness of flow imaging depends on the timing and spatial location of the Doppler signals and not absolute blood flow velocity. Thus, signal aliasing does not limit the utility of flow imaging and, in fact, may enhance the appreciation of abnormal flow patterns. In addition, flow imaging can be performed from windows where the intercept angle between the ultrasound beam and the direction of regurgitant flow is nonparallel. These windows often allow a shorter distance from the transducer to the flow region of interest, resulting in a better signal-to-noise ratio. For example, AR is best evaluated from the parasternal approach (Fig. 12-7). Although the direction of an AR jet in the parasternal long-axis view is nearly perpendicular to the ultrasound beam, multiple flow directions within the jet allow detection of the diastolic flow disturbance. Of course, an accurate blood velocity determination cannot be made both because of the nonparallel intercept angle and because the velocity exceeds the Nyquist limit of the pulsed Doppler mode.





The appearance of a regurgitant jet with color flow imaging will vary depending on the ultrasound system, transducer frequency, and specific instrument settings. Correct visual interpretation depends on experience with a particular instrument and knowledge of the influence of instrument settings on the visual display. On most systems, a “variance” color scale results in a green regurgitant signal superimposed on the normal red-blue flow patterns. A “velocity” scale results in a mosaic of red, blue, and white pixels in the regurgitant jet. Because the goal of this application is to identify the location and timing of abnormal flow signals in a tomographic format, the exact color scale used is not particularly important as long as it displays the boundaries of the flow disturbance accurately.


Given the physics of pulsed Doppler color flow imaging, it is self-evident that an abnormal color pattern is not synonymous with abnormal flow. An abnormal color pattern can be seen even with normal intracardiac flow patterns; for example, the normal antegrade flow velocity of laminar flow across the aortic valve exceeds the Nyquist limit, resulting in aliasing and an “abnormal” color pattern. Conversely, abnormal flow signals may not demonstrate variance or a mosaic pattern if the flow velocities are within the Nyquist limit for that interrogation depth. For example, the low velocities seen in pulmonic regurgitation result in a uniform color display even though the flow pattern is abnormal. Interpretation of the color images will be most consistent from study to study if instrument settings and flow maps are standardized for each laboratory.


Recommended instrument settings for color flow imaging are:



Evaluation of the exact timing of a flow signal in relation both to valve closing and to the QRS complex can be helpful in correct identification of the signal. With color flow imaging, temporal resolution is sacrificed for spatial resolution because frame rates are far lower than the sampling rate of pulsed or CW Doppler. Simultaneous recording of an electrocardiographic lead is essential for frame-by-frame analysis of the color flow images to verify the timing of the disturbance.



Vena Contracta


The vena contracta, the narrowest diameter of the flowstream, reflects the diameter of the regurgitant orifice with the advantages that it is independent of volume flow rate and driving pressure and that it is relatively unaffected by instrument settings. However, because vena contracta diameters have a narrow range of values, care is needed to obtain adequate images for measurement. In order to optimize both temporal and spatial resolution, the recommended approach to measurement of vena contracta is to use a view:



Angulation out of the standard image planes may be needed to depict both the proximal acceleration region and downstream flow expansion for accurate identification of the vena contracta (Fig. 12-8).



Vena contracta diameter may vary with dynamic changes in regurgitant orifice area, for example, with late-systolic MR due to mitral valve prolapse. However, vena contracta width remains accurate in the setting of acute regurgitation, when jet area may be misleading. Three-dimensional visualization of vena contracta area shows promise, particularly for nonsymmetrical regurgitant orifices, but this technique currently is challenging and limited by low frame rates (Fig. 12-9).




Proximal Flow Convergence


Color flow imaging allows for the calculation of the retrograde volume flow rate based on measurement of the flow convergence region proximal to the regurgitant orifice. Acceleration of flow occurs proximal to the valve plane with, in concept, a series of isovelocity “surfaces” leading to the high-velocity jet in the regurgitant orifice. Immediately adjacent to the orifice, these surfaces are small with higher flow velocities; at increasing distances from the orifice, areas are larger and velocities are lower. Based on the principle of volume flow calculation by Doppler techniques, the volume flow rate (in this case, regurgitant flow) for a proximal isovelocity surface area (PISA), when averaged over the temporal flow period, is (Fig. 12-10):


image (12-4)


The velocity of the PISA can be determined from the color flow image as the aliasing velocity where a distinct red-blue interface is seen (Fig. 12-11). At this interface, the velocity is known, being equivalent to the Nyquist limit on the velocity color scale. The size of the PISA can be maximized to allow more accurate regurgitant flow rate calculations by decreasing the velocity range, shifting the velocity baseline, or both.




The shape of the isovelocity surface proximal to a regurgitant valve typically is hemispherical with a tendency toward a hemielliptical shape closer to the orifice. Assuming a hemispherical shape, the PISA is calculated from measurements of the distance from the aliasing velocity to the regurgitant orifice as the surface area of a hemisphere:


image (12-5)


Note that the PISA method for calculating regurgitant volume is analogous to calculation of stroke volume proximal to a stenotic valve. The differences between these approaches are (1) the differing shapes of the proximal velocity stream lines; (2) the use of color flow, rather than pulsed Doppler, to measure velocity at a given location; and (3) the need for temporal averaging when color data from single images are used.


The PISA method can be combined with the velocity-time integral of CW Doppler flow through the regurgitant orifice to calculate regurgitant orifice area using Equation 12-3. Instead of averaging PISA over the duration of flow, most clinicians calculate the maximum instantaneous regurgitant orifice area (ROAmax in cm2) based on the maximum regurgitant flow rate (RFR in milliliters per second) combined with maximum MR jet velocity (VMR in centimeters per second):


image (12-6)


This approach assumes that RFR and VMR occur at the same time point in the cardiac cycle. The PISA should be recorded in a view parallel to the flowstream, typically an apical four-chamber view for MR, using a narrow sector and zoom mode, with the aliasing velocity adjusted to optimize visualization of a hemispherical aliasing boundary. If the PISA is hemielliptical or if the valve is nonplanar, an alternate approach should be used or appropriate corrections made in the calculations.



CW Doppler Approach


Several types of information regarding the severity of valvular regurgitation can be derived from the spectral display of the CW Doppler signal:



First, signal intensity is proportional to the number of blood cells contributing to the regurgitant signal. Because the ultrasound beam is relatively broad and signals from the entire length of the beam are recorded, much of the regurgitant jet can be encompassed in the beam with appropriate adjustment of beam direction. It is particularly helpful to compare the intensity of the regurgitant signal to antegrade flow across the same valve as a qualitative estimate of regurgitant severity (see Fig. 12-11). A weak signal reflects mild regurgitation, whereas a signal nearly equal in intensity to antegrade flow reflects severe regurgitation. Moderate regurgitation has intermediate signal strength relative to antegrade flow.


Second, the associated antegrade velocity across the regurgitant valve provides useful information. Regurgitation results in an increase in the antegrade volume flow rate across the valve, which is reflected in an increase in the antegrade velocity across the valve. The greater the severity of regurgitation, the higher is the antegrade velocity. Of course, the possibility of coexisting valvular stenosis also must be considered.


Third, the shape of the velocity curve depends on the time-varying pressure gradient across the regurgitant valve. Each instantaneous velocity is related to the instantaneous pressure gradient across the valve, as stated in the Bernoulli equation. Normal LV systolic pressure is 100 to 140 mm Hg, and normal LA pressure is 5 to 15 mm Hg, so the LV to LA pressure difference in systole is 85 to 135 mm Hg. Thus, the MR velocity curve typically shows a maximum velocity of 5 to 6 m/s. When ventricular function is normal, there is rapid acceleration to peak velocity, with a maintained high velocity in systole and with rapid deceleration before diastolic opening of the mitral valve. An increase in end-systolic LA pressure (v-wave) results in a late-systolic decline in the instantaneous pressure gradient and in the instantaneous velocity (Fig. 12-12).



Similarly, the shape of the AR velocity curve depends on the time course of the diastolic pressure difference across the aortic valve. When LV end-diastolic pressure is low and aortic end-diastolic pressure is normal or mildly reduced, a large pressure difference (and high velocity) across the valve is present throughout diastole with a slow rate of pressure decline (Fig. 12-13). Acute or severe regurgitation results in more rapid equalization of LV and aortic pressures with a more rapid velocity decline in diastole.



The utility of the CW Doppler curve depends, in large part, on technical factors in data recording in addition to correct data interpretation. The high-velocity regurgitant signal is optimized by:


Jul 10, 2018 | Posted by in CARDIOLOGY | Comments Off on Valvular Regurgitation
Premium Wordpress Themes by UFO Themes