55. Aortic Stenosis- Review of CT Surgery

Christopher Heid and Michael Jessen

This chapter is a revision and update of that included in the previous editions of the TSRA Review written by Amjad Syed (2^nd edition), Ravi K. Ghanta (1^st edition), and Bryan M. Burt (1^st edition). 

Introduction

Aortic stenosis (AS) is a condition whereby narrowing of the aortic valve leads to outflow obstruction of the left ventricle. The incidence increases with age and can be seen in 2-10% of people over the age of 65. The etiology of AS varies by age and geographic cohorts. Calcific degeneration of the valve is the most common cause in patients over 70 years of age, whereas those younger than 70 are more likely to have stenosis related to bicuspid aortic valve. In the developing world, rheumatic valve disease remains a common cause of AS. The prognosis of untreated AS is poor. Historically, survival for AS with symptoms of angina or syncope averages around 3-5 years, compared to 1-2 years for AS with symptoms of congestive heart failure. Treatment options include optimal medical therapy, surgical aortic valve replacement (SAVR), and transcatheter aortic valve replacement (TAVR). All patients with severe valvular heart disease being considered for intervention should be evaluated by a Multidisciplinary Heart Valve Team. Consultation with or referral to a Primary Valve Center or a Comprehensive Valve Center is reasonable for the discussion of treatment options in the setting of asymptomatic patients with severe valve disease, patients who might benefit from valve repair rather than valve replacement, and among patients with multiple comorbidities.

Anatomy of the Aortic Valve

The aortic valve lies between the left ventricular outflow tract and the aorta, allowing for unidirectional flow of blood out of the heart and into the systemic circulation. The aortic valve has three leaflets: the left coronary leaflet, right coronary leaflet, and non-coronary leaflet. The three leaflets attach to the annulus, which is the plane separating the left ventricular cavity from the aorta. The annular junctions between leaflets are termed commissures. Cephalad to each leaflet is the corresponding sinus of Valsalva, an area of aortic dilation which contains the coronary ostia. The sinotubular junction marks the transition from aortic root structures to the ascending aorta. There are two anatomic landmarks that are critical for aortic valve surgery: the anterior mitral leaflet found inferior to the left-non coronary commissure, and the conduction system tissue inferior to the non-right coronary commissure. Injury to these structures during annular suture placement can cause mitral insufficiency and heart block, respectively. 

Bicuspid aortic valve is a congenital variant that occurs in 1-2% of the population. The Sievers classification describes bicuspid valves based on the number of raphes: 0, 1, and 2. Type 0 has no raphe and can be oriented either laterally or anterior-posteriorly. Type 1 has a single raphe with fusion of either the right/left cusps or the right/non cusps most commonly. Type 2 occurs when two raphes are fused. 

Pathophysiology

AS leads to progressive left ventricular outflow obstruction. The increase in afterload against the left ventricle results in increased wall stress causing hypertrophic remodeling of the myocardium. In addition, systolic and diastolic pressures increase within the left ventricle and aortic pressure decreases, resulting in an increased pressure gradient across the valve. Systolic ejection times are prolonged as well. Symptoms from aortic stenosis are due to two mechanisms. First, these changes result in increased myocardial oxygen demand and decreased perfusion. Secondly, over time the compensatory left ventricular hypertrophy is overcome, and the ventricle dilates, resulting in symptoms of congestive heart failure. 

Clinical Features

Classic symptoms of aortic stenosis include angina and exertional syncope. When left untreated these progress to symptoms of congestive heart failure such as dyspnea on exertion, orthopnea, and paroxysmal nocturnal dyspnea. On physical exam, a crescendo-decrescendo systolic ejection murmur with radiation to the carotids may be present. The murmur is best heard over the right second intercostal space. Pulses parvus et tardus (weak and delayed peripheral pulses) may also be present. 

Diagnosis

Transthoracic echocardiography is routinely used to diagnose aortic stenosis. Echocardiography with Doppler evaluation will give both anatomic and hemodynamic information regarding the severity of AS. Other means of diagnosis include transesophageal echocardiography (TEE), cardiac magnetic resonance imaging, cardiac computed tomography, and left heart catheterization. Regardless of the method used, the variables of interest include mean gradient (MG), peak velocity (PV), and aortic valve area (AVA). Other important features to identify include the presence of aortic insufficiency, annular/leaflet calcification, aortic root dimensions, ejection fraction, and left ventricular dimensions. 

Evaluating the severity of AS can be complex. Narrowing of the aortic valve causes blood flow to accelerate across the valve (increase in PV) and pressure to decrease across the valve (increase in MG). Aortic valve area (AVA) as calculated by standard techniques will be lower. In patients with good LV function and normal cardiac output, greater degrees of aortic stenosis lead to higher velocity and higher gradients.

Patients with severe AS by valve area in the setting of left ventricular (LV) dysfunction and low cardiac output may present with low gradients (termed low flow low gradient AS). To distinguish between aortic stenosis and LV dysfunction as the etiology of symptoms, low dose dobutamine stress studies (echocardiography or catheterization) may be obtained. In patients with true AS, the valve area will not change significantly during dobutamine infusion. In contrast, patients with only moderate AS but severe LV dysfunction will exhibit and increase in AVA and only modest change in velocity/gradient with dobutamine. These patients are unlikely to improve with aortic valve interventions. Finally, some patients can have AS in the setting where LVEF is normal but cardiac output is decreased. Resulting pressure gradients are low even though severe AS may be present (termed paradoxical low flow low gradient AS). The diagnosis can be challenging, but patients with this condition benefit from surgical or transcatheter valve replacement. Cardiac CT scans can play a role in the evaluation of AS. Aortic valve calcification correlates with anatomic severity and values greater than 1650 arbitrary units (AU) measured by CT, suggest anatomically severe AS.

Grading of aortic stenosis is as follows:

	AVA (cm2)	MG (mmHg)	PV (m/s)
Mild	1.6-2.5	<20	2.0-2.9
Moderate	1.1-1.5	20-39	3.0-3.9
Severe	≤1	≥40	≥4

Stages of AS according to the 2020 AHA/ACC guidelines:

1. At risk (bicuspid or sclerotic valve with normal hemodynamics)
2. Progressive (mild-moderate AS with normal LVEF, possible early diastolic dysfunction)

C1) Asymptomatic severe with normal LVEF

C2) Asymptomatic severe with LV dysfunction (LVEF <50%)

D1) Symptomatic severe high gradient (MG >40 mmHg)

D2) Symptomatic severe low flow low gradient with reduced LVEF (LVEF <50%)

D3) Symptomatic severe low gradient with normal LVEF or paradoxical low-flow severe AS

Surgical Indications for Aortic Stenosis

Summary of Recommendations for AVR from the 2020 AHA/ACC valve guidelines:

1. AVR is indicated for symptomatic severe high gradient AS (Stage D1)

Class of recommendation I, Level of Evidence A

• AVR is indicated for asymptomatic severe AS with LVEF <50% (Stage C2)

Class of recommendation I, Level of Evidence B

• AVR is indicated for asymptomatic severe AS (Stage C1) when undergoing cardiac surgery for another indication

Class of recommendation I, Level of Evidence B

• AVR is recommended for low-flow, low gradient severe AS with reduced LVEF (stage D2) 

Class of recommendation I, Level of Evidence B

• AVR is recommended for symptomatic low-flow, low gradient severe AS with normal LVEF (Stage D3), if AS is the most likely cause of symptoms

Class of recommendation I, Level of Evidence B

• AVR is reasonable in apparently asymptomatic patients with severe AS (Stage C1) and low surgical risk when exercise testing demonstrates decreased exercise tolerance or a fall in systolic blood pressure ≥10 mm Hg from baseline

Class of recommendation IIa, Level of Evidence B

• AVR is reasonable for asymptomatic patients with very severe AS (AV ≥5m/s) and low surgical risk

Class of recommendation IIa, Level of Evidence B

• AVR is reasonable in apparently asymptomatic patients with severe AS (Stage C1) and low surgical risk when the serum BNP is >3 times normal

Class of recommendation IIa, Level of Evidence B

• In asymptomatic patients with high gradient severe AS (Stage C1) and low surgical risk when aortic velocity increases ≥0.3 m/s per year

Class of recommendation IIa, Level of Evidence B

1. AVR may be considered for patients with severe asymptomatic AS (Stage C1) and a progressive decrease in LVEF (to at least <60%) on at least 3 imaging studies

Class of recommendation IIb, Level of Evidence B

1. AVR may be considered in patients with moderate AS (Stage B) at the time of cardiac surgery for another indication

Class of recommendation IIb, Level of Evidence C

The 2020 AHA/ACC guidelines state that a mong patients in whom a bioprosthesis is appropriate, decisions between surgical aortic valve replacement (SAVR) and transcatheter aortic valve implantation (TAVI) should include the presence of symptoms, patient age and anticipated life expectancy, the indication for intervention, predicted surgical risk, and anatomy or other factors referable to transfemoral (TF) TAVI feasibility (all Class 1):

• SAVR is preferred among patients <65 years of age or with life expectancy >20 years.
• SAVR is preferred if vascular anatomy or other factors preclude TF TAVI.
• SAVR is preferred among asymptomatic patients with a Class 2a indication for intervention, such as an abnormal exercise test, very severe AS, rapid progression, or elevated BNP.
• If feasible, TF TAVI is preferred among patients >80 years of age or with life expectancy <10 years.
• SAVR or TF TAVI is recommended after shared decision making among symptomatic patients ages 65-80 years with no contraindication to TF TAVI.
• TAVI is preferred among symptomatic patients of any age with high or prohibitive surgical risk, if predicted survival after intervention is >12 months with an acceptable quality of life.
• After shared decision making, palliative care is recommended among symptomatic patients with predicted post-TAVI survival <12 months or for whom minimal improvement in quality of life is expected.

Among patients with BAV, transthoracic echocardiography is recommended to assess valve morphology, assess AS and AI, assess the aortic root and ascending aorta, and evaluate for the presence of aortic coarctation. If the aortic sinuses, sinotubular junction, and ascending aorta cannot be accurately or fully assessed on echocardiography, then cardiac magnetic resonance angiography or computed tomography angiography is indicated. Lifelong serial imaging is indicated if the aorta diameter is ≥4.0 cm.

Among patients with BAV, indications for replacement of the aorta: aortic diameter >5.5 cm (Class 1), aortic diameter 5.0-5.5 cm plus an additional risk factor for dissection (family history of dissection, aortic growth >0.5 cm per year, aortic coarctation; Class 2a), or aortic diameter ≥4.5 cm with an indication for SAVR (Class 2a).

Treatment Considerations

The treatment of AS is continuously evolving as technology advances. The tenet to successful management of AS is the heart team approach, whereby a multidisciplinary committee reviews cases and a consensus treatment is derived. Generally speaking, outcomes from optimal medical therapy for AS are abysmal, and this strategy should only be used in patients unfit for any sort of procedural intervention. Balloon valvuloplasty can be used as a palliative procedure for even the most critical of patients. The decision to pursue SAVR vs TAVR is more complex. Traditionally, TAVR was reserved for patients with prohibitive surgical risk (STS PROM >8%). The PARTNER trial published in the New England Journal of Medicine (NEJM) in 2010 randomized patients with severe AS who were deemed unfit for surgery into TAVR vs best medical therapy (including balloon valvuloplasty). This study showed improved mortality and readmission rates in the TAVR cohort, paving the way for widespread use of transcatheter aortic valve therapies for high-risk patients. As TAVR became more commonplace, it’s use was broadened to lower risk patients. The PARTNER 2 trial, published in NEJM in 2016, randomized intermediate risk (STS PROM 3-8%) to SAVR vs TAVR. In this study, rates of death or disabling stroke were similar between SAVR and TAVR, prompting widespread implementation of TAVR for intermediate risk patients. The PARTNER 3 trial, published in 2019, observed improved rates of the composite outcome of death, stroke, or rehospitalization in low risk (STS PROM <3%) patients treated with TAVR. These studies have been met with some criticisms, which are beyond the scope of this chapter. One significant limitation in our understanding of TAVR is the lack of long-term durability data. SAVR, on the other hand, has been extensively studied with respect to long-term durability. The 10-year freedom from structural valve degeneration of bioprosthetic SAVR is 85-99%, depending on the valve used, patient factors, and definitions of valve degeneration. Mechanical aortic valve prostheses are even more durable than their bioprosthetic counterparts. Reintervention for mechanical valve degeneration is exceedingly rare. When comparing survival between bioprosthetic and mechanical valves, a survival advantage for mechanical valves is seen around 12.5 years post implant. The late survival advantage and long-term durability may make mechanical prosthesis a preferred option for younger patients. The primary disadvantage of mechanical valves is the risk of valve thrombosis necessitating lifelong anticoagulation. However, newer mechanical valves are FDA approved for less intensive anticoagulation (to an INR of 1.5-2, compared to the previously required INR of 2-3). 

The 2020 ACC/AHA valve guidelines offer recommendations for valve selection as well as for the decision between SAVR and TAVR. In all cases, decision-making is a shared responsibility between the patient and providers, considering patient preferences, anatomy, ability to tolerate anticoagulation, and life expectancy. The guidelines consider mechanical valves to be reasonable in patients under 50 years of age and biologic valves a preferred option in those over 65 years. For patients between 50-65, an individualized approach is recommended. In experienced centers, patients under 50 with suitable anatomy may pursue a Ross procedure (pulmonary autograft). SAVR is recommended over TAVR in patients under 65 years or with greater than 20-year life expectancy, taking into account appropriate surgical risk assessment.  TAVR is recommended over SAVR in patients over 80 years of age, with less than 10-year life expectancy, or in patients with prohibitive surgical risk (e.g. STS PROM > 8%). An individualized approach is taken for those between 65 and 80 years. In patients with less than 12 months life expectancy post TAVR, palliative care should be considered. It should be noted that TAVR in the ACC/AHA guidelines implies transfemoral TAVR, as mortality rates for TAVR via alternate (non-femoral) access are higher. It must also be emphasized that the guidelines for valve selection and SAVR versus TAVR are highly complex and a heart team should evaluate each patient individually. The final decision should be a shared process with the patient and surgeon. 

Technical Considerations in AVR

SAVR is performed by conventional sternotomy, hemi-sternotomy, or right mini-thoracotomy using full cardiopulmonary bypass. The sternotomy approach is described here. After median sternotomy the pericardium is incised longitudinally and the heart is cradled by securing the pericardium to the skin and subcutaneous tissue with stay sutures. Systemic heparin is administered at 400u/kg and ACT is maintained >420s. The aorta is dissected from the pulmonary trunk to allow for aortic cannulation and cross clamping. A multistage venous cannula is placed in the IVC via the right atrium and an aortic cannula is placed in the proximal aortic arch. An LV vent is placed via the right superior pulmonary vein. Standard antegrade and retrograde cardioplegia catheters are placed. The patient is placed on full cardiopulmonary bypass and the aorta is cross-clamped. Cold blood cardioplegia is utilized for myocardial protection via both antegrade and retrograde infusion. Systemic cooling to 32 degrees Celsius is standard. A transverse aortotomy is performed 1-2cm above the coronary ostia. If a root enlargement procedure is to be performed, the aortotomy can be fashioned obliquely. The calcified annulus and leaflets are debrided using sharp dissection. A crushing clamp can also be used to debride calcification. After valve excision, the LV is copiously irrigated to prevent embolization of any debrided tissue. Valve sizers are used to measure the annulus. An appropriately sized valve should be used to avoid patient prosthesis mismatch. If necessary, a root enlarging procedure can be performed to allow for adequate size valve implantation. The three most common techniques are the Nicks procedure (extending the aortotomy through the middle of the non-coronary sinus with patch reconstruction), the Manouguian procedure (extending the aortotomy through the commissure of the left and non-coronary sinuses with patch reconstruction) and the Konno procedure (a vertical aortotomy through the right sinus of Valsalva, immediately to the left of the RCA orifice onto the RVOT and ventricular septum, witch patch closure of the septal and RVOT defects). A new “Y” root enlargement technique to enlarge the valve by 2-3 sizes has been described involving extension of the aortotomy through the commissure of the left and non-coronary sinuses onto the aorto-mitral curtain, then in a “Y” shape underneath the anatomic aortic annulus toward the fibrous trigones of the mitral valve but not violating the mitral annulus or leaflets. Interrupted horizontal mattress sutures placed from the ventricular side through the annulus to the aortic side are used to secure the valve in the supra-annular position. These sutures are placed through the valve sewing ring and the valve is seated and secured. The aortotomy is closed using a two layered closure technique. The aorta is de-aired and the cross clamp is removed. The patient is separate from cardiopulmonary bypass.

TAVR is performed via femoral, carotid, or trans-caval approach. Trans-aortic and trans-apical techniques are less commonly performed in the current era. Although there are many different approaches to TAVR, the basic principles are similar. After vascular access, a pigtail catheter is placed in the proper aortic sinus depending on the valve being implanted. An aortogram is obtained. The aortic valve is crossed with a stiff wire to gain ventricular access. A catheter is placed into the LV and simultaneous pressure measurements are obtained in the LV and aorta to confirm a hemodynamically significant gradient. A pigtail wire is placed via the pressure-monitoring catheter into the LV and the valve is advanced over the wire. The valve is then deployed (either balloon or self-expandable) under angiographic visualization. Valve deployment is often done under rapid pacing to induce hypotension and prevent valvular displacement. An aortogram is performed to assess valve competency and placement post deployment. Intraoperative echocardiography is also performed to evaluate the valve and to rule out pericardial effusion.

Postoperative Care

Routine post cardiotomy care is standard for SAVR patients. Early extubation, preferably in the operating room, has been shown to improve outcomes. Inotropes and vasopressors are weaned for a cardiac index >2.2 L/min/m² and mean arterial pressure of 65 mmHg or more. Chest tubes are removed when output and chest x-ray are acceptable. Pacing wires are removed on post-operative day 2-3 if no rhythm disturbance exists, although one must be aware of the potential for delayed heart block. An important consideration in caring for AS patients post operatively is the poor LV compliance secondary to chronic LV hypertrophy. As a result, these patients are highly dependent on adequate preload for ventricular filling. A central venous pressure of 15-18 mmHg is often required for these patients. Additionally, atrial fibrillation is poorly tolerated due to the loss of atrial contribution to ventricular filling. 

TAVR patients can be cared for in the intensive care unit or the ward depending on multiple factors. Rapid atrial pacing can be performed immediately post operatively to stratify the risk of complete heart block requiring device insertion. If there is concern for the development of heart block, a temporary transvenous pacing wire is left in place. Serial electrocardiograms are performed prior to removing the temporary wire. 

Outcomes after aortic valve replacement are variable as this is a very heterogenous cohort of patients. The STS 30-day mortality after SAVR is approximately 3% and continues to improve over time. Complications after any aortic valve replacement include stroke, dysrhythmia, heart block, bleeding, access complications, and thromboembolic events. 

Suggested Readings

1. Al-Atassi T, El Khoury G, Boodhwani M. Surgical Treatment of Aortic Valve Disease. Sabiston and Spencer Surgery of the Chest, 9^th edition. Chapter 77, Pages 1334-1349.
2. Ehsan A, Sellke FW. Aortic Valve Replacement. Atlas of Cardiac Surgical Techniques, 2^nd Edition. Chapter 9, 129-139.
3. Nishimura RA, Otto CM, Bonow RO et al. 2020 AHA/ACC Guideline for the Management of Patients with Valvular Heart Disease. J Am Coll Cardiol. 2021;77(4):e25-e197.
4. Leon MB, Smith CR, Mack M et al. Transcatheter Aortic Valve Implantation for Aortic Stenosis in Patients Who Cannot Undergo Surgery. N Engl J Med. 2010;363:1597-1607.
5. Leon MB, Smith CR, Mack M et al. Transcatheter or Surgical Aortic Valve Replacement in Intermediate-Risk Patients. N Engl J Med. 2016;374:1609-1620.
6. Mack MJ, Leon MB, Thourani VH et al. Transcatheter Aortic Valve Replacement with a Balloon-Expandable Valve in Low Risk Patients. N Engl J Med. 2019;380:1695-1705.
7. Glaser N, Jackson V, Holzmann MJ, Franco-Cereceda A, Sartipy U. Aortic valve replacement with mechanical vs. biological prostheses in patients aged 50-69 years. Eur Heart J. 2016;37(34):2658-2667.
8. Yang B and Naeem A. A Y Incision and Rectangular Patch to Enlarge the Aortic Annulus by Three Valve Sizes. Ann Thorac Surg. 2021;112(2):e139-e141.