|Year : 2019 | Volume
| Issue : 3 | Page : 156-162
Raised Prosthetic Valve Gradients: What Should be the Approach?
Department of Non-invasive Cardiology, NH Rabindranath Tagore International Institute of Cardiac Sciences, Kolkata, West Bengal, India
|Date of Submission||04-Mar-2019|
|Date of Decision||14-Mar-2019|
|Date of Acceptance||02-May-2019|
|Date of Web Publication||18-Dec-2019|
Department of Non-invasive Cardiology, NH Rabindranath Tagore International Institute of Cardiac Sciences, Kolkata, West Bengal
Source of Support: None, Conflict of Interest: None
The introduction of valve replacement surgery has dramatically improved the outcome of patients with valvular heart disease. Echocardiography plays an important role to determine the outcome of the surgery and follow up of these patients with prosthetic heart valve. Flow across the prosthesis is determined by using Doppler echocardiography. Whenever there is a high gradient across a prosthetic valve, echocardiography becomes challenging, as there are many causes, which may give rise to high prosthetic valve gradient. Some of these causes are prosthesis-related which need urgent intervention and some are non-prosthesis-related. A careful systematic echocardiographic approach, using 2D, 3D, Doppler and transesophageal echocardiography is crucial to find out the exact cause of high gradient.
Keywords: Doppler echocardiography, prosthetic valve, raised gradients
|How to cite this article:|
Chatterjee D. Raised Prosthetic Valve Gradients: What Should be the Approach?. J Indian Acad Echocardiogr Cardiovasc Imaging 2019;3:156-62
|How to cite this URL:|
Chatterjee D. Raised Prosthetic Valve Gradients: What Should be the Approach?. J Indian Acad Echocardiogr Cardiovasc Imaging [serial online] 2019 [cited 2020 May 29];3:156-62. Available from: http://www.jiaecho.org/text.asp?2019/3/3/156/273293
| Introduction|| |
Doppler echocardiography is the standard noninvasive tool for the assessment of hemodynamics across the prosthetic heart valve (PHV) after valve replacement. The most basic Doppler assessment of PHV is with transvalvular gradients. An unusually raised prosthetic valve gradient must raise suspicion of prosthetic valve malfunction.
Flow across prosthetic valve is governed by delicate hemodynamic rules. High prosthetic valve gradient does not always mean prosthetic dysfunction. There are many causes other than the obstruction of the prosthesis by thrombus/pannus/vegetation, which can elevate the gradient. It is also to be remembered that all PHVs are inherently stenotic and cause higher gradient compared to native valves, the degree of which depends on the model and size of the PHV as well as on the patient's body size. All these factors make the echocardiographic evaluation of high prosthetic valve gradient complex and challenging. A careful, systematic approach and interpretation of echocardiographic findings is necessary for such patients with a special knowledge regarding the normal functioning of different types of PHV and their dysfunction and hemodynamic situations, which may contribute to high prosthetic valve gradient.
When echocardiography reveals a high prosthetic valve gradient, the first question which should come in mind is, what is the cause of raised gradient?
The possible causes of raised gradient across the prosthetic valves are:
- Intrinsic valve dysfunction resulting in true stenosis by a thrombus, pannus, vegetation, and calcific degeneration in bioprosthetic valve
- Patient–prosthesis (PPM) mismatch
- High cardiac output state
- Pressure recovery phenomenon
- Central jet artifact in bileaflet PHV.
Essential clinical data to know before starting echocardiographic examination of any PHV are:
- Symptoms of heart failure, neurologic events, and febrile illness which can give clues to valve dysfunction
- Type and size of PHV
- Date of surgery
- Height, weight, and body surface area (BSA) of the patient to assess the possibility of PPM
- BP and heart rate of the patient
- Coagulation status of the patient including the latest international normalized ratio (INR).
Keeping in mind the above-mentioned possible causes of high prosthetic valve gradient and the essential clinical data, echocardiographic approach to a patient with high gradient to find out its cause can be divided into the following steps.
| Step 1: Evaluation of Leaflet Morphology and Mobility|| |
Evaluation of morphology and mobility of the PHV is the cornerstone of identification of cause of raised gradient. Two-dimensional (2D) transthoracic echocardiography (TTE) is the first line of imaging using multiple conventional as well as unconventional views. However, a TTE has many limitations in assessing PHV due to reverberation artifact and acoustic shadowing by the prosthetic material. Hence, transesophageal echocardiography (TEE) is mandatory whenever there is a high transvalvular gradient as TEE allows a more detailed assessment of valve morphology and mobility., It is essential to obtain images in multiple views and multiple planes to ensure complete visualization of the valvular and paravalvular regions.
3D echocardiography, particularly during TEE, is useful and provides incremental advantage over 2D imaging for the assessment of PHV pathology as it provides additional information due to better image quality., A combined 2D and 3D TEE examination is the best approach in assessing PHV dysfunction.,
There is a large variety of PHV. Attention is paid to determine the type of prosthesis.
There are three main types of mechanical valves, which differ primarily by the construction and functioning of occluders: monoleaflet tilting disc, bileaflet, and ball and cage. Monoleaflet tilting disc valves consist of a single circular occluder, which usually opens to the angle 60°–80°. Bileaflet valves have two semilunar discs, opening of which forms three orifices: the smallest one at the center and two larger on either side of it. Ball and cage valves consisting of a ball with a circular metal ring and a cage formed by two or three metal arches are no longer implanted. However, many patients still have caged ball valves who require follow-up. Bioprosthetic valves consist of three flexible leaflets. After their opening, a single orifice is formed with a larger area than mechanical valves.
Valve should be inspected with particular attention to (a) opening and closing motion of the moving parts of the prosthesis; (b) any abnormal echo-dense mass attached to the discs, sewing ring, stent or cage, and the presence of leaflet calcification in bioprosthesis; (c) the appearance of the sewing ring with careful inspection of the region of separation from the native annulus. Utmost importance is given to visualize the full excursion of the discs/leaflets. Limited opening or complete immobility of discs/leaflets is a sign of prosthetic valve thrombosis whereas their normal excursion points more toward other causes including PPM. However, it is often difficult to see mobility of the discs clearly by TTE or TEE because of the echogenic shadowing of mechanical PHV. In such situation, fluoroscopy or computed tomography is indicated to confirm normal excursion of the disc [Figure 1] and [Figure 2].
|Figure 1: Two-dimensional transesophageal echocardiography showing one disc is fully opened whereas the other disc is stuck in closed position during ventricular diastole in a bileaflet mitral PHV|
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|Figure 2: Three-dimensional transesophageal echocardiography showing one disc is fully opened whereas the other disc is stuck in closed position during ventricular diastole in a bileaflet mitral PHV|
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Thrombus formation is the most common cause of obstruction of mechanical PHV. Risk and possibility of thrombus formation is high in patients with subtherapeutic INR. Special attention should be given both during TTE and TEE to look for a thrombus in such patients with high prosthetic valve gradient. A thrombus should always be distinguished from pannus because they have different etiology of formation and different line of management. Pannus is formed as a result of chronic ingrowth of fibrous tissue at the interface between prosthetic material and native tissue. Thrombus looks soft, less echodense, and more mobile whereas pannus, which is confined to the area of sewing ring, looks more echodense and less mobile [Table 1].
The combination of finding of a soft, echodense mass on the PHV and a subclinical INR has a positive and negative predictive value of 87% and 89%, respectively, for thrombus formation. If a thrombus is visualized, its size should be measured and location should also be identified to determine the adverse hemodynamic consequences of thrombus. Obstruction due to thrombus in a PHV is associated with catastrophic hemodynamic consequences and needs urgent management with thrombolytic therapy or surgical intervention. A large thrombus extending beyond the valve is a risk factor for the complication of thrombolytic therapy. A thrombus area <0.85 cm2 on TEE anticipates a low risk for embolism or death associated with thrombolytic therapy.
A thrombus or a pannus on a prosthetic valve is difficult to visualize during TTE due to the presence of intense reverberation. TEE including 3D TEE is better imaging modality for their detection. Large vegetation in prosthetic valve endocarditis may impair the opening of the valve resulting in high gradient. It can be detected by as irregularly shaped, independently mobile mass of relatively low echogenicity in the valve ring, leaflet, stent, or occluder of the prosthesis.
Value of TTE is often limited in the detection of vegetation, but TEE, on the other hand, demonstrates high sensitivity (86%–94%) and specificity (88%–100%) for the detection of vegetations.
Thrombus formation is rare in bioprosthetic valves when compared to mechanical PHV. However, the possibility of thrombus formation in early postoperative period when endothelialization of the suture zone is not yet complete cannot be ruled out. Hence, anticoagulation is recommended for the first 3 months after implantation of a bioprosthetic valve. Calcification, thickening, and reduced mobility of the leaflets may be noted in bioprosthetic valves due to structural valve degeneration (SVD) resulting in high gradient across the valve.
| Step 2: Quantitative Parameters by Doppler Echocardiography in Prosthetic Heart Valve|| |
Velocity and gradient across prosthetic heart valve
While assessing pressure gradient across PHV, the following few important points should be remembered:
- High cardiac output state gives rise to high gradient across PHV
- Flow through the smaller central orifice of bileaflet mechanical prostheses may give a high gradient due to a localized high velocity. This phenomenon may lead to an overestimation of gradient and a false suspicion of prosthetic valve dysfunction. It is difficult to confirm this phenomenon by echocardiography. If there is a doubt, valve leaflet mobility should be evaluated with TEE and fluoroscopy. Continuous-wave (CW) Doppler image should be examined carefully for the superimposition of two envelopes – one with higher velocity is from the central orifice jet
- The mean pressure gradient should be measured instead of peak gradient. The peak pressure gradient through PHV is unreliable indicator of hemodynamics as high velocities are commonly observed immediately after valve opening. This applies to both mitral and aortic PHV
- Pressure recovery phenomenon leads to overestimation of pressure gradient.
Prosthetic valve gradient is said to be raised when there is increased mean gradient of more than 15–20 mmHg in aortic prosthesis and more than 5–7 mmHg in mitral prosthesis.
A meticulous search for the maximal flow velocity signal is essential to measure the maximum gradient through the high-velocity turbulent flow across the valve.
Shape of flow velocity spectrum
Shape of the transprosthetic flow velocity in normal aortic prosthesis is triangular with early peaking and short acceleration time (<80 ms). Rounded velocity contour with the peaking in mid-ejection and prolonged acceleration time are noted in stenosis of aortic PHV [Figure 3] and [Figure 4].
|Figure 3: Normal triangular shape of forward flow with early peaking across a normally functioning aortic prosthetic heart valve|
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|Figure 4: Abnormal rounded, symmetrical shaped forward flow with peaking at mid ejection with a gradient of 52 mmHg (mean) across a stuck aortic prosthetic heart valve with severe stenosis|
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Once a high mean gradient is obtained across the prosthesis, the next step in Doppler echocardiography is to measure Doppler velocity index (DVI) to differentiate whether the high gradient is due to high-flow state or due to obstruction resulting from PPM or prosthetic valve dysfunction.
Doppler velocity index
The DVI is a dimensionless index, which compares velocity flow across the left ventricular outflow tract (LVOT) and the prosthesis. It is calculated as the ratio between the velocity time integral (VTI) of the LVOT flow and the VTI of flow across prosthesis.
- DVI = VTI LVOT/VTI prosthetic valve flow.
VTI LVOT is obtained by measuring pulsed-wave (PW) Doppler at LVOT after placing the sample volume at LVOT in apical 5-chamber (ap5ch) view. VTI of aortic prosthesis is obtained from CW Doppler through aortic prosthesis. VTI of mitral prosthesis is obtained from CW Doppler through mitral prosthesis.
DVI of aortic prosthesis is calculated by dividing VTI of LVOT by VTI of aortic flow. DVI of mitral prosthesis is calculated by dividing VTI of LVOT by VTI of mitral flow.
- DVI of aortic PHV = VTI LVOT/VTI aortic valve
- DVI of mitral PHV = VTI LVOT/VTI mitral valve.
For a normally functioning aortic prosthesis, the DVI is typically above 0.30 and for a normally functioning mitral prosthesis DVI should be <2.2. DVI calculation is not recommended for tricuspid valve and pulmonary valve [Table 2].
|Table 2: Normal and abnormal values of Doppler velocity index in aortic and mitral prosthesis|
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DVI <0.25 in aortic prosthesis is suggestive of significant aortic prosthesis stenosis whereas DVI >2.5 of mitral prosthesis is suggestive of mitral prosthesis stenosis.
If there is a normal DVI, high prosthetic valve gradient is possibly due to high flow state, localized high gradient in the central orifice flow of bileaflet PHV, or technical error.
Conversely, the combination of high gradient and a low DVI in aortic prosthesis or high gradient and a high DVI in mitral prosthesis suggests prosthesis obstruction either due to PPM or due to intrinsic prosthetic valve dysfunction, the distinction of which can be done by calculating effective orifice area (EOA) of the PHV implanted.
Effective orifice area
EOA is a better estimation of valve function than gradient because it reflects the true prosthetic lumen and is the most validated parameter to identify PPM. PPM occurs when the EOA of a normally functioning prosthesis is too small in relation to the patient's body size. This consequently results in high transvalvular gradient, as the cardiac output required by the patient needs higher flow. In other words, gradient across a valve is normal only when the EOA is proportionate to flow requirement.
EOA of a PHV is calculated in the same way as is done for measuring native aortic valve area using continuity equation.
The cross-sectional area of the LVOT is obtained by measuring LVOT diameter from parasternal long-axis (PLAX) view. Methods of measuring VTI of LVOT, aortic prosthesis, and mitral prosthesis are already described under the section of calculation of DVI.
Calculation of EOA of aortic PHV using [Figure 5], [Figure 6], [Figure 7].
|Figure 5: Left ventricular outflow tract diameter in zoomed view of parasternal long axis|
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|Figure 6: Pulsed wave at left ventricular outflow tract in ap5ch view for left ventricular outflow tract velocity time integral|
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|Figure 7: Continuous wave Doppler at aortic prosthetic valve in ap5ch view for AV VTI|
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The measured EOA value should be compared with normal reference values of EOA provided by the manufacturer and the American Society of Echocardiography.
Indexed EOA, obtained by dividing EOA by the patient's BSA, is an important parameter for identifying PPM.
In a normally functioning PHV, the measured EOA should be close to the reference value for the same model and size of prosthesis.
Markedly low measured value of EOA indicates intrinsic prosthesis dysfunction.
If the measured EOA is same as the reference value ± 1 standard deviation, intrinsic prosthesis dysfunction is unlikely and indicates the presence of PPM. The indexed EOA is then used to confirm the presence and severity of PPM. Indexed EOA ≤0.85 cm2/m2 in aortic PHV and ≤1.2 cm2/m2 in mitral PHV indicates mild/moderate PPM whereas indexed EOA ≤0.65 cm2/m2 in aortic PHV and ≤0.9 cm2/m2 in mitral PHV indicates severe PPM [Table 3].,,,,,
|Table 3: Severity of Prosthesis–patient mismatch as per indexed effective orifice area|
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If the indexed EOA is >0.85 cm2/m2 in the aortic prosthesis or >1.2 cm2/m2 in the mitral prosthesis, PPM is unlikely, and high flow state, technical error, and the possibility of localized high gradient through central jet of bileaflet PHV are to be considered as contributing factors.
EOA calculation is not recommended for tricuspid valve and pulmonary valve.
Pibarot et al. have proposed the algorithm in a flowchart for the approach to a patient with raised gradient across PHV [Flowchart 1].
EOA measurement is invalid in the presence of concomitant moderate-to-severe aortic or mitral regurgitation. Pressure half-time (PHT) method should not be used to measure the orifice area of mitral prosthesis unlike native mitral valve stenosis., However, significantly prolonged PHT (>200 ms) is suggestive of stenosis of mitral prosthesis.
Structural valve degeneration (SVD) in bioprosthetic heart valves may result in valve dysfunction with high gradient across the valve. Clinically relevant SVD in aortic bioprosthetic valve is defined as an increase in the mean transvalvular gradient >20 mmHg with decrease in the EOA >0.6 cm2 (and/or decrease in DVI > 0.15), whereas possible SVD is defined as an increase in the mean transvalvular gradient of >10 mmHg with a concomitant decrease in EOA >0.3 cm2 (and/or decrease in DVI > 0.08). However, there is variability in the definition of SVD, with a universal definition for SVD still lacking.
| Conclusion|| |
High gradient across PHV does not always mean prosthetic valve dysfunction. There may be various causes. A multiparametric stepwise approach is necessary to find out the cause. 2D echocardiography does not provide enough information. TEE is particularly valuable and sensitive method allowing the visualization of thrombus, pannus, and vegetation. Doppler examination allowing calculation of various parameters is the most important aspect of assessment of high gradient across PHV. 3D TEE is extremely valuable as it allows an accurate assessment of all the parts of prosthetic valve.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2], [Table 3]