Current Approach to Mechanical Valve Obstruction

Although mechanical valve obstruction is rare. It is a rather fatal complication. Pannus and thrombus are common in etiopathogenesis. Multibl imaging methods such as transthoracic echocardiography, transesophageal echocardiography, synefl uoroscopy and computed tomography are used for diagnosis. Surgical treatment is performed in the pannus. In thrombosis, fi brinolytic or surgical treatment can be performed. In this article, we presented the current approach to mechanical valve obstruction. Review Article Current Approach to Mechanical Valve Obstruction Hüsnü Değirmenci1*, Eftal Murat Bakırcı1, Hikmet Hamur1, Arif Arısoy2 and Mücahit Tan1 1Erzincan Binali Yıldırım University, Faculty of Medicine, Department of Cardiology, Turkey 2Gazi Osman Paşa University, Faculty of Medicine, Department of Cardiology, Turkey Received: 17 December, 2020 Accepted: 28 December, 2020 Published: 29 December, 2020 *Corresponding author: Hüsnü Değirmenci, Erzincan Binali Yıldırım University Faculty of Medicine, Department of Cardiology, Erzincan/Turkey, E-mail: ORCID: https://orcid.org/0000-0003-0618-7227


Introduction
Rheumatic valve disease continues to be the most common cause of valve pathologies in developing countries, and mechanical valve replacement continues to be the most commonly used treatment method in these valve pathologies [1]. Mechanical Valve Obstruction (MVO) is one of the most important complications of mechanical valve replacement. MVO is rare but is a feared complication of valve replacement. Mortality risk plays an important role in the fact that MVR is a feared complication and the diffi culty in making an etiological diagnosis also plays an important role [2]. In this context, it is the main cause of thrombosis and pannus obstruction. The diagnosis and treatment of these causes are completely different from each other, and misdiagnosis and wrong treatment will lead to life-threatening clinical pictures.
For this reason, we decided to write a review called "Current approach to mechanical valve obstruction".

Pathophysiology
Thrombus, overgrowth of the pannus, or both may be responsible for the pathophysiological process. The incidence of prosthetic valve obstruction has been reported to be 0.3-1.3% [3]. In a study conducted by Deviri et al., Thrombusrelated obstruction was observed with a rate of 78%, pannusrelated obstruction with 10.7%, and obstruction due to both of them 11.6% [2]. While pannus is more responsible for aortic valve obstruction, thrombus is more responsible for mitral valve obstruction.

Mechanical valve thrombosis
Prosthetic Valve Thrombosis (PVT) is a highly fatal complication. Annual PVT incidence has been reported to be 0.03%-5.7% [2]. Thrombus formation is observed in the mechanical valve rather than the bioprosthetic valve. There are obstructive and nonobstructive types. PVT is observed especially in the fi rst postoperative year. Compared to the pannus, thrombus formation appears earlier and as larger masses in imaging methods. Endothelialization begins to develop within a few weeks or months after mechanical valve replacement. During this endothelialization process, clot formation may develop on the endothelium. The thrombus, which is small initially, starts to grow over time. In addition to the endothelialization process, turbulent fl ow and stasis in the mechanical valve also contribute to thrombus formation. This Citation: Değirmenci  relative stasis and turbulent fl ow explains why tricuspid valve thrombosis is more than left-sided valve thrombosis and why mitral valve thrombosis is more than aortic valve thrombosis.
In addition, as the mechanical valve surface area increases, the size of the thrombosis and pannus also increases. The most important reason for this complication is subtherapeutic anticoagulation [4].

Differences of pannus and thrombus
Less common than thrombus formation is a complication located on mechanical valves. Although the exact mechanism is not known, biological reaction is blamed. However, thrombin production, fi brin deposition, fi broblast proliferation, collagen deposition and neoangiogenesis are blamed in the pathogenesis of pannus. The trigger of the Pannus formation is not clearly understood. It is for this reason that steps towards treatment have been incomplete. Tranforming growth factor beta levels were found to be elevated. Pannus formation can also be a trigger of thrombus formulation [5][6][7]. The differences between pannus and thrombus are shown in Table 1 [2,4].
is important in treatment selection. However, this distinction is not easy at all. In the initial diagnostic evaluation, valvular movements, obstruction and clot burden should be evaluated with TTE. In addition, patient prosthesis mismatch should also be considered.

Transthoracic echocardiography
While evaluating aortic mechanical valve obstruction [8], we look at certain parameters in Doppler echocardiography.  Table 2 shows the evaluation of the aortic mechanical valve by Doppler echocardiography [6]. In case of concurrent aortic insuffi ciency, PV and MV are signifi cantly affected. If the aortic mechanical valve is to be evaluated with Doppler echocardiography parameters, the stroke volume should be normal or close to normal (50-70 ml).
When evaluating mitral valve obstruction [8], we look at PV, MG, ratio of prosthetic valve PV to LVOT velocity, EOA and Pressure Half-Time (PHT). These values may be affected by conditions such as increased heart rate, increased fl ow, patient prosthesis incompatibility. Table 3 shows the evaluation of the mitral mechanical valve by Doppler echocardiography [6].

Synfl uoroscopy
Synfl uoroscopy plays an important role in imaging the radiopaque valve discs, comparing the opening and closing angles of the valves and detecting the dehiscence [9]. Compared to synfl uoroscopy, valve movements, valve structure and hemodynamic parameters are better evaluated in TOE.

Computed tomography
Although it has a limited diagnostic role in prosthetic valve obstruction due to its inadequate evaluation of valve movements and valve hemodynamics, it is a superior examination to echocardiography, especially in the evaluation of pannus in the atrial position. CT is not used in the initial evaluation and is an examination that provides additional information when synfl uoroscopy and TOE are insuffi cient [9][10][11]. However, multislice CT plays an important role in detecting abnormalities after valve replacement. Current ECG-gated multislice CT technology, with its superior spatial and adequate temporal resolutions, minimizes beam-hardening and motion artifacts and thus is superior to echocardiography and cardiac Magnetic Resonanse Imaging in terms of spatial resolution in examinations of prosthetic valves [13].

Treatment
In a suspected mechanical valve thrombosis, evaluation is made primarily by TTE, TOE and fl uoroscopy. With these diagnostic methods, it is primarily checked whether thrombosis causes obstruction or not. When evaluated according to the 2017 European valve guideline [14], emergency valve replacement is recommended with a class 1 indication in clinically critical obstructive mechanical thrombosis patients without serious comorbidities. In patients with non-obstructive mechanical   valve thrombus> 10 mm, which is complicated by an embolic event, surgical intervention with a class 2a indication is recommended.
Fibrinolytic infusion (10 mg bolus with unfractionated heparin; 90 mg tissue Plasminogen Activator (tPA) as infusion in 90 minutes or 1500000 units of streptokinase infusion in 60 minutes) can be administered with a class 2a indication in right-sided mechanical valve thrombosis, in cases where surgery is at high risk or when surgery cannot be performed. In obstructive mechanical valve thrombosis, if the patient is not clinically critical and anticoagulation is insuffi cient, intravenous unfractionated heparin is administered with aspirin (100 mg and above / daily) or alone and followed. If successful, it is followed up with anticoagulants. If obstructive thrombus has developed in non-critical patients although anticoagulation is suffi cient, priority surgery; If surgery is high risk, fi brinolytic is used.
In non-obstructive mechanical valve thrombosis, anticoagulation is optimized and the patient is followed clinically and echocardiographically. If thromboembolism is detected in the follow-up with clinical, cerebral and imaging methods, the size of the thrombus is checked. Priority surgery if the thrombus is 10 mm or more; If surgery cannot be performed, thrombolytics are applied. If the thrombus is below 10 mm, anticoagulation is optimized and the patient is followed up. If the thrombus persists and recurrent thromboembolism develops, priority surgery; If surgery cannot be performed, fi brinolytic is applied. If the thrombus persists and there is no recurrent thromboembolism, medical monitoring is followed. If there is no thromboembolism in non-obstructive mechanical valve thrombosis and the thrombus size is 10 mm or more, the anticoagulant is optimized and the patient is followed up. If thromboembolism develops or thrombus persists, fi rst of all surgery; If surgery cannot be performed, fi brinolytic is applied. If the thrombus does not persist or thromboembolism does not develop, medical monitoring is followed. If there is no thromboembolism in non-obstructive mechanical valve thrombosis and the valve thrombus is less than 10 mm, anticoagulant is optimized. If the anticoagulant cannot be optimized, if thromboembolism develops in the followup or thrombus persists, surgery fi rst; If surgery cannot be performed, fi brinolytic is applied.
Let's now consider the 2017 American guide [15]. In the presence of symptoms of valve obstruction, emergency surgery or low-dose fi brinolytic infusion is recommended as a class 1b indication in patients with left-sided mechanical valve thrombosis. Our recommendations are based on multiple case reports, single-center studies, multi-center studies, metaanalyzes, and registry reports, rather than a randomized controlled study comparing fi brinolytic therapy and surgical intervention. While the 30-day mortality rate is 10-15% with surgical treatment, this rate decreases to 5% in NYHA 1-2 patients. In fi brinolytic therapy, the 30-day mortality rate is 7%, while the thromboembolism rate is 18%, the bleeding rate is 6%, and the hemodynamic success rate is 75%.  [16] (182 patients) reported in the literature, and low dose and slow infusion tPA treatment (25 mg 6 hour infusion) was found to be as successful as other regimens. Although no death was observed in this regimen, PROMETEE study [17] in which an ultra-slow fi brinolytic regimen was applied (25 mg tPA 25 hour infusion) was performed because it is similar to other regimens in terms of major non-fatal complications. In this study, the ultra-slow dose slow infusion regimen was found to be very reliable without losing its effectiveness in patients except for the NYHA class 3-4 patient group. Intracranial bleeding was detected as 0.8% in the TROIA study, whereas no cases were observed in the PROMETEE study [17].

Conclusion
MVO is a complication with high morbidity and mortality rates. Multidisciplinary team approach is required in this complication. In addition, early diagnosis and early treatment are extremely important in this complication. In these patients, MVO should be suspected in case of shortness of breath, signs of embolism and hemodynamic deterioration. Today, low dose thrombolytic regimens are used frequently and successful results are obtained. Therefore, we recommend the use of lowdose fi brinolytic regimens in patients with high surgical risk and suitable for fi brinolytic regimen.