TSRA Content:
Authors: Mona Kakavand, MD, and Lin Chen, BA
This is a revision and update from the previous edition of the TSRA Primer in Cardiothoracic Surgery written by Helen Mari Merritt and Chad N. Stasik.
Disease of the aortic valve is the most common of all cardiac valves making aortic valve replacement (AVR) the most common valvular procedure performed around the world. The aim of this chapter is to provide a basic overview covering the basic aortic valve anatomy, the cardiac cycle as it relates to the left ventricle and the aortic valve, indications for intervention in aortic stenosis (AS) and aortic regurgitation (AR), types of valve prosthesis, and implantation techniques.
Anatomy
The aortic valve, one of the 2 semilunar valves, consists of three leaflets (right coronary cusp, left coronary cusp, non-coronary cusp). The leaflets coapt at the free edge of the ventricular surface with three zones of apposition and appear to form a “Mercedes-Benz sign” or “peace sign” when looking straight at the valve. The commissures are the points of contact between the leaflets on the aortic wall. The sinuses of Valsalva are the small outpouchings of aortic tissue just above the valve leaflets within the aortic root.
Valvular Apparatus During Diastole and Systole
During diastole, the aortic valve is closed because the pressure in the aortic root has increased above that of the left ventricle after the ventricle empties in systole, and thus the valve cusps collect the blood in their central portion and this fills the cusps, causing them to coapt. In late diastole, blood fills the ventricle, increasing the pressure as the ventricle contracts and the left ventricular pressure increases above the aortic root pressure, causing the aortic valve to open and the blood to exit the left ventricle.
According to the vortex theory, small vortices created in the sinuses of Valsalva are integral to both valve opening and closure. As ejection occurs, small eddy currents are created by decelerating blood in the sinuses of Valsalva between the edges of the leaflets and the aortic wall. These currents move down along the aortic wall to the top of the leaflets and contribute to their complete opening. As flow across the valve decreases towards the end of systole, and the pressure gradient decreases, the vortices move toward the center of the aorta. Eventually the pressures in the ventricle and aorta equalize and a small flow reversal actually occurs, which opens and coapts the cusps. This flow reversal is responsible for the rapid closure of the aortic valve leaflets. The subsequent stretch and recoil of the leaflets produces the second heart sound and the dicrotic notch on an arterial pressure waveform.
Aortic Stenosis - Indications for Intervention
Aortic stenosis (AS) is a progressive disease with varying degrees of calcific degeneration, fibrosis and thickening depending on the etiology. For patients <70 years old, bicuspid valves account for 50% of cases, while calcific degeneration accounts for >48% of cases in patients >70 years old. No matter what the cause, when considering a patient for AVR with AS, it is usually easiest to divide patients into either symptomatic or asymptomatic groups. Usually patients with severe AS (defined by AHA guidelines aortic valve area (AVA) ≤ 1 cm2 (or AVA index to body surface area ≤ 0.6 cm2/m2), mean pressure gradient ≥ 40 mmHg, peak velocity ≥4.0 m/s) require surgery. Life expectancy is <5 years once symptoms develop.
The AHA class I guidelines for AVR in symptomatic patients: Benefit>>> Risk:
1) Severe AS with symptoms (dyspnea, angina, syncope)
2) Severe AS with low flow (<4.0 m/s) and low gradient (<40 mmHg) when stroke volume index is <35 ml/m2
with preserved EF (EF>50%)
3) Severe AS with low-flow and low gradient with left ventricular systolic dysfunction (EF <50%)
The AHA guidelines for AVR in asymptomatic patients are not so straightforward. Firstly, it is important to determine by thorough questioning that the patient does not have any symptoms and confirm this by exercise testing to determine if they are truly asymptomatic. Also be sure to look for other AHA criteria for AVR:
1) LV dysfunction (LVEF<50%) with Vmax
2) Rapid progression of stenosis or V max≥ 5m/s or BNP> 3x normal in patients with low surgical risk
3) If the patient is undergoing other cardiac surgery
If the patient is truly asymptomatic, then they should be followed by serial echocardiograms since aortic stenosis is a progressive disease and symptoms will eventually develop.
Choice of Intervention for Severe Aortic Valve Stenosis: Transcatheter Aortic Valve Intervention vs. Surgical Aortic Valve Replacement
According to AHA guideline, if surgical risk assessment is high or prohibitive and life expectancy with acceptable quality of life is greater than one year in a patient with suitable valve and vascular anatomy, transfemoral transcatheter aortic valve intervention (TAVI/TAVR) is the intervention of choice; otherwise, surgical aortic valve replacement is the preferred intervention. TAVI is now approved for low risk patients as well.
Aortic Regurgitation - Indications for Intervention
Aortic regurgitation/insufficiency (AR or AI) is also a progressive disease that can be divided according to anatomical location: leaflets, annulus, aortic root or ascending aorta. Signs and symptoms include those associated with CHF.
The AHA defines severity of AR as the following:
1) Jet width ≥65 % LVOT diameter
2) Vena contracta > 0.6 cm
3) Regurgitant volume ≥ 60 ml/beat
4) Regurgitation fraction ≥50%
5) Effective regurgitant orifice area≥0.3 cm2
Like in aortic stenosis, aortic valve replacement or repair should be offered to all symptomatic patients with severe AR
The indications for surgery in patients with severe AR according to AHA class I indications:
1) Severe AR, with symptoms
2) Severe AR in asymptomatic patients undergoing cardiac surgery for other reasons
3) Severe AR in asymptomatic patients with LV dysfunction (≤55%)
For asymptomatic patients with severe AR and normal LV function (EF ≥55%), AVR is reasonable when there is severe LV dilation ( LVESD >50 mm). If there is evidence of progressive decrease in LVEF to < 55%- 60%) or increase in LVEDD to >65 on at least 3 studies, then AVR may be considered. If the patient is truly asymptomatic and there are no other surgical criteria, the patient should be followed with serial echocardiograms.
More frequently, severe AR is treated with surgical AVR, but TAVI can be used as well. Importantly, antegrade cardioplegia cannot be used in patients with severe AR, as it will go straight into the ventricle instead of the coronaries, so retrograde cardioplegia into the coronary sinus should be used in this situation.
Types of Aortic Valve Prostheses and Their Applications
Available prosthetic options include mechanical valves, stented and stentless bioprosthetic valves, homografts, and autografts. Mechanical valves are the most durable option and therefore used for younger patients (<50 years of age) because of their expected benefit to last the lifetime of the patient. Unfortunately, the valves are thrombogenic and require lifetime anticoagulation. This carries with it an inherent risk of bleeding. An ideal candidate would be a patient already on warfarin for chronic atrial fibrillation and <50 years old with symptomatic severe AS or AR.
Stented bioprosthetic valves are manufactured from fixed porcine aortic valves or bovine pericardium. The stent reduces the effective orifice area (EOA) creating a higher transvalvular gradient than a size-matched mechanical valve or stentless valve. This can be overcome by placing a larger valve and/or performing annuloplasty or root enlargement. With modern stented bioprosthetic valves, > 90% of patients are expected to be free from reoperation at 12 years.
Stentless porcine valves, such as the Medtronic Freestyle® valve and St. Jude Toronto SPV® valve, provide a larger orifice area and lower gradient than a similarly sized stented bioprosthetic valve. However, implantation is technically more difficult than a stented valve. Stentless valves are ideal for small aortic annuluses that are not amenable to root enlargement.
In general, biologic stented or stentless valves should be implanted in older patients (>50 years of age), because the risk of structural deterioration and reoperation is less than the risk of bleeding from anticoagulation. Younger patients with contraindications or objections to anticoagulation can also receive bioprosthetic valves. Keep in mind that the major limitation of bioprosthetic valves is durability and the potential need for reoperation for structural deterioration. Bioprosthetic valves fail extremely quickly in young patients, necessitating earlier reoperation. There are also bioprosthetic valve options that are manufactured with future valve-in-valve TAVI intervention in mind.
Aortic homografts, such as the CryoValve@ allograft by CryoLife, offer a hemodynamic profile similar to the native valve. The allograft includes the aortic root, ascending aorta and arch, as well as a portion of the anterior leaflet of the mitral valve- which makes this an excellent choice for patching/filling a defect left by an annular abscess from endocarditis or in a patient with a small aortic root. Compared to other valve options, availability is limited. Pulmonary autografts (the patient’s own pulmonary valve and root) are used for AVR in the Ross procedure. The advantages of a pulmonary autograft are similar to a stentless valve with the additional benefit of being able to grow along with the patient. This is an ideal and durable option in children and young adults if there is no connective tissue disorder, but patients will often require pulmonary conduit replacement in the future.
Aortic Annular Suturing
There are many techniques for suturing the prosthetic valve to the aortic annulus: pledgeted and nonpledgeted, everting and noneverting, simple and horizontal mattress, and continuous and interrupted.
Pledgeted sutures are thought to decrease the risk of paravalvular leak by distributing the pressure more evenly along the suture line, especially when the annulus is heavily calcified and friable. The data supporting this are conflicting. Pledgeted horizontal mattress sutures can be placed with the pledgets above the annulus in an everting manner, which is technically easier, but decreases the effective orifice area. Most stented bioprosthetic valves are designed to sit above the annulus, so your pledgets will have to be placed on the ventricular side of the annulus (non-everting) and allow a larger prosthesis to be placed.
Proponents of nonpledgeted sutures cite ease of placement, decreased thrombogenicity, and elimination of another source of infection. Nonpledgeted sutures can be placed as interrupted, simple, horizontal mattress sutures, or in a continuous running fashion.
Excellent results with low incidences of paravalvular leaks have been published using any variety of suture techniques. You can use any method successfully as long as you take evenly spaced solid bites all the way around the annulus.