Residency · Residency · Cardiothoracic Surgery

Aortic Stenosis: Pathophysiology and Surgical Management

Overview

Aortic stenosis (AS) is the most common valvular heart disease requiring intervention in the developed world. Understanding the pathophysiology of pressure overload, the natural history of the disease, and the indications for intervention is essential for every cardiothoracic surgery trainee. This chapter covers the etiologies of AS, hemodynamic assessment, surgical aortic valve replacement (SAVR) technique, prosthesis selection, and the evolving role of transcatheter aortic valve replacement (TAVR).

Etiology

Degenerative Calcific AS (Most Common)

Degenerative calcific AS involves progressive calcification and fibrosis of a trileaflet valve, driven by risk factors similar to atherosclerosis: age, hypertension, hyperlipidemia, diabetes, smoking, and renal failure. The pathology consists of lipid deposition, inflammatory infiltration, and dystrophic calcification of the leaflets. It typically presents in the seventh to eighth decade of life.

Bicuspid Aortic Valve (BAV)

Bicuspid aortic valve is present in 1-2% of the population and is the most common congenital cardiac anomaly. Abnormal leaflet stress causes accelerated calcification, leading to presentation 10 to 20 years earlier than trileaflet disease. Associated ascending aortopathy (dilatation) occurs in 40-60% of patients and requires surveillance and possible concomitant repair. The Sievers classification categorizes BAV by raphe pattern: Type 0 (no raphe, true bicuspid), Type 1 (one raphe, with right-left cusp fusion being most common), and Type 2 (two raphes).

Rheumatic AS

Rheumatic AS results from chronic rheumatic heart disease with commissural fusion. It is almost always accompanied by mitral valve disease and is more common in developing countries. The pathology involves commissural fusion with leaflet retraction and calcification.

Pathophysiology

Pressure Overload and LV Response

Progressive obstruction to LV outflow increases LV systolic pressure. The ventricle compensates with concentric hypertrophy, which normalizes wall stress according to LaPlace's law. Systolic function is initially preserved during this compensated phase. However, progressive diastolic dysfunction develops due to decreased compliance. Eventually, decompensation occurs with LV dilatation, systolic dysfunction, and heart failure.

Hemodynamic Progression

Valve area decreases approximately 0.1 cm2 per year (range 0.03-0.3 cm2/year), and mean gradient increases approximately 7 mmHg per year. The rate of progression correlates with calcification burden.

Natural History

Asymptomatic severe AS carries a 2-year event rate of approximately 15-20%. Once symptoms develop, the prognosis without intervention is grim: angina carries approximately 50% 5-year survival, syncope approximately 50% 3-year survival, and heart failure approximately 50% 2-year survival. Sudden death in asymptomatic patients is rare (less than 1% per year) but devastating.

Hemodynamic Assessment

Echocardiographic Grading

Special Hemodynamic Scenarios

Low-flow, low-gradient AS with reduced EF presents a diagnostic challenge. The AVA is below 1.0 cm2 and mean gradient is below 40 mmHg despite the valve area suggesting severe disease, because the weakened ventricle (EF below 50%) cannot generate sufficient flow to open a severely stenotic valve fully. This may represent true severe AS or pseudo-severe AS (where the valve does not open fully simply because of low output). Dobutamine stress echocardiography differentiates the two: in true severe AS, the gradient rises with minimal change in AVA, while in pseudo-severe AS, the AVA increases above 1.0 cm2. Contractile reserve (a stroke volume increase above 20%) predicts better surgical outcomes.

Paradoxical low-flow, low-gradient AS with preserved EF is another challenging entity. Despite a normal EF, stroke volume index is low (below 35 mL/m2) due to a small, hypertrophied LV cavity with restrictive physiology. CT calcium scoring helps confirm severity: values above 2,000 Agatston units in men or above 1,200 in women indicate severe stenosis.

Indications for Intervention

ACC/AHA 2020 Guidelines

Symptomatic severe AS with high gradients is a Class I indication for AVR. Severe AS with LVEF below 50% warrants AVR regardless of symptoms (Class I). Severe AS in patients undergoing other cardiac surgery should receive concomitant AVR (Class I). Asymptomatic severe AS with very severe criteria (peak velocity at or above 5.0 m/s or rapid progression) is a Class IIa indication. Asymptomatic severe AS with exercise-induced symptoms or hemodynamic compromise is also Class IIa. Moderate AS in patients undergoing other cardiac surgery should be considered for concomitant AVR (Class IIa).

Surgical Aortic Valve Replacement (SAVR) Technique

Approach

Standard median sternotomy is the most common approach. Ministernotomy (upper hemi-sternotomy) for isolated AVR offers reduced pain and faster recovery with equivalent outcomes. Right anterior mini-thoracotomy is performed at selected centers.

Conduct of Operation

After cannulation, cardiopulmonary bypass is initiated with systemic cooling to 32-34 degrees Celsius. An aortic cross-clamp is applied and cardioplegia delivered antegrade via the coronary ostia after aortotomy. The aortotomy is made as a transverse or hockey-stick incision 1 to 2 cm above the right coronary ostium. All three leaflets are excised, and annular calcification is meticulously debrided while protecting the coronary ostia, membranous septum (conduction system), and anterior mitral leaflet. The annulus is sized, and annular sutures (typically 12-16 interrupted pledgeted mattress sutures) are placed circumferentially. The prosthesis is seated, sutures are tied, and the aortotomy is closed with running polypropylene suture.

Critical Anatomic Relationships

The right coronary ostium, located above the right-noncoronary commissure, can be occluded by an oversized prosthesis or low-riding sutures. The left coronary ostium sits above the left-right commissure. The membranous septum and AV node lie beneath the noncoronary-right coronary commissure — sutures placed too deep here cause complete heart block. The anterior mitral leaflet is in continuity with the aortic valve via the aortic-mitral curtain, and aggressive debridement can cause mitral injury.

<image>Surgeon's view looking down into the aortic root after aortotomy and excision of the native aortic valve. The three sinuses of Valsalva are labeled (right coronary, left coronary, and noncoronary). The left and right coronary ostia are visible. The location of the membranous septum and AV node is highlighted beneath the junction of the right and noncoronary sinuses. Pledgeted annular sutures are shown placed circumferentially around the annulus, ready for prosthesis seating. The aortic-mitral curtain continuity is labeled between the left and noncoronary sinuses.</image>

Prosthesis Selection

Mechanical Valves

Mechanical valves offer essentially unlimited lifespan (30+ years) but require lifelong warfarin anticoagulation (target INR 2.0-3.0 for the aortic position). Risks include bleeding complications (1-2% per year), thromboembolism (1% per year), and valve thrombosis. They are preferred in younger patients (below 50-55 years), those already on anticoagulation, and those who accept lifelong warfarin. The On-X valve is unique in being FDA-approved for a lower INR target (1.5-2.0) after 3 months with aspirin.

Bioprosthetic Valves

Bioprosthetic valves have limited durability (10-20 years, lasting longer in older patients due to less calcification) but require no long-term anticoagulation (aspirin alone). Their main risk is structural valve deterioration (SVD) requiring reintervention. They are preferred in older patients (above 65 years), those with contraindications to anticoagulation, and women of childbearing age (since warfarin is teratogenic). Valve-in-valve TAVR is an emerging option for failed bioprosthetic valves, extending the useful life of the initial prosthesis.

Factors in Decision-Making

The decision between mechanical and bioprosthetic valves involves patient age, life expectancy, lifestyle, anticoagulation compliance, comorbidities, desire for pregnancy, and the availability of valve-in-valve TAVR. Shared decision-making with the patient is a Class I guideline recommendation.

Sutureless and Rapid-Deployment Valves

Devices such as the Perceval (LivaNova) and Intuity (Edwards) reduce cross-clamp time and facilitate minimally invasive approaches. Early and mid-term hemodynamic data are favorable, though paravalvular leak rates are slightly higher than with conventional sutured valves.

Patient-Prosthesis Mismatch (PPM)

Definition

PPM occurs when the effective orifice area of the prosthesis is too small relative to the patient's body size. Moderate PPM is defined as indexed EOA below 0.85 cm2/m2, and severe PPM as indexed EOA below 0.65 cm2/m2.

Clinical Impact

Severe PPM is associated with higher residual gradients, persistent LV hypertrophy, reduced LV mass regression, and decreased long-term survival. It is most relevant in active, larger patients.

Prevention Strategies

PPM is predictable and preventable. Preoperative planning with annulus measurements and body surface area calculations should anticipate the risk. Aortic root enlargement procedures — Nicks (posterior enlargement through the noncoronary sinus), Manouguian (through the aortic-mitral curtain), or Konno (anterior septal enlargement) — accommodate a larger prosthesis. Stentless or sutureless valves offer superior hemodynamics in borderline cases.

<image>Cross-sectional comparison of mechanical and bioprosthetic aortic valve prostheses in the aortic position. Left panel shows a bileaflet mechanical valve (St. Jude-type) with labeled pyrolytic carbon leaflets, pivot guards, and sewing ring. Right panel shows a stented bioprosthetic valve (pericardial) with labeled pericardial tissue leaflets, stent posts, and sewing ring. Below each valve, a table lists key characteristics: expected durability, anticoagulation requirements, audible click (present/absent), and INR target. A central inset shows the concept of patient-prosthesis mismatch with a small valve in a large annulus creating high residual gradients.</image>

SAVR vs. TAVR: Current Landscape

Low-risk TAVR trials (PARTNER 3, Evolut Low Risk) showed noninferior or superior 1-to-2-year outcomes for TAVR versus SAVR. However, long-term durability data for TAVR valves remain limited. SAVR remains preferred for patients under 65 (durability concerns with TAVR), bicuspid aortic valve (off-label for most TAVR devices with higher paravalvular leak), concomitant cardiac pathology requiring surgery, and unfavorable access anatomy. Heart Team decision-making is essential.

CT surgery residents must maintain SAVR operative volume despite TAVR expansion and should gain exposure to the TAVR procedure and the Heart Team process. SAVR will remain relevant for complex patients, failed TAVR, and younger patients for the foreseeable future.

Clinical Pearls

Once symptoms of severe AS develop, the mortality curve is steep and AVR is urgent — do not delay referral. Low-flow, low-gradient AS is a diagnostic challenge requiring dobutamine stress echo and CT calcium scoring to differentiate true from pseudo-severe AS. Suture placement in the region of the noncoronary-right coronary commissure carries the highest risk of complete heart block, and sutures should be placed superficially in this zone. Patient-prosthesis mismatch is predictable and preventable: measure the annulus carefully and be prepared to perform aortic root enlargement if needed. The age threshold for bioprosthetic valve selection continues to shift younger (currently approximately 50-55 years in many guidelines) due to the availability of valve-in-valve TAVR. Bicuspid aortic valve patients must have their ascending aorta assessed: if above 4.5 cm (or 4.0-4.5 cm with risk factors), concomitant aortic replacement is indicated. Shared decision-making about valve type is a Class I guideline recommendation.

References

• Otto CM, Nishimura RA, Bonow RO, et al. 2020 ACC/AHA Guideline for the Management of Patients with Valvular Heart Disease. Circulation. 2021;143(5):e72-e227.
• Mack MJ, Leon MB, Thourani VH, et al. PARTNER 3: Transcatheter aortic-valve replacement with a balloon-expandable valve in low-risk patients. N Engl J Med. 2019;380(18):1695-1705.
• Popma JJ, Deeb GM, Yakubov SJ, et al. Evolut Low Risk Trial: Transcatheter aortic-valve replacement with a self-expanding valve in low-risk patients. N Engl J Med. 2019;380(18):1706-1715.
• Head SJ, Mokhles MM, Osnabrugge RL, et al. The impact of prosthesis-patient mismatch on long-term survival after aortic valve replacement. Eur Heart J. 2012;33(12):1515-1522.
• Cohn LH, Adams DH. Cardiac Surgery in the Adult. 5th ed. McGraw-Hill; 2017.

Read this lecture as Markdown