Introduction

One of the two semilunar valves of the heart, the aortic valve lies between the aorta and the left ventricle, at the end of the left ventricular outflow tract. Efficient functioning of the aortic valve plays a vital role in maintaining adequate cardiac function. This chapter explores the anatomy and physiology of the aortic valve, as well as describing the pathological processes responsible for diseases of the aortic valve. Investigations and treatment of aortic valve disease are subsequently discussed, to provide the reader with a broad overview of the management of aortic valve disease.

Surgical anatomy of the aortic valve

The aortic valve sits at the level of the opening of the left ventricle into the ascending aorta. The valve consists of three cusps, referred to as leaflets, which are semilunar in shape and attach to the annulus of the valve. At the central part of the valve, where the leaflets meet (coapt), the edges of the leaflets are slightly thickened. There is a nodule on the tip of each leaflet, known as the Nodule of Arantius.

While the normal aortic valve consists of three leaflets (known as tricuspid), a small proportion of the population (around 1–2%) have a bicuspid aortic valve, which is made up of only two cusps. Between the leaflets and the wall of the ascending aorta are three distinct pocketlike dilatations, known as the sinuses of Valsalva. The leaflets of the aortic valve are attached to the annulus, just under these sinuses. The left and right coronary arteries arise from two of the sinuses; the three sinuses of Valsalva are therefore individually known as the left, right and non-coronary sinuses. Accordingly, the three leaflets of the aortic valve are also referred to as the left, right and non-coronary cusps, depending on their relationship with their respective sinuses of Valsalva (Hinton & Yutzey 2011; Moorjani, Viola & Ohri 2011).

Aetiology and pathogenesis

Aortic stenosis

Aortic stenosis is the most common heart valve disease in adults, and the third most common cause of cardiovascular disease in adults after hypertension and coronary artery disease. The three most common causes of aortic stenosis are calcific degeneration of a tricuspid valve, calcific degeneration of a congenitally abnormal valve (unicuspid or bicuspid) and rheumatic valve disease. There are several other less common causes of aortic stenosis, including Lupus, Paget’s disease and hyperuricemia (Olszowska 2011).

While calcification of the aortic valve is predominantly seen as a degenerative condition which progresses with advanced age, there are multiple other risk factors for the development of aortic valve stenosis. These include hypertension, hyperlipidaemia, smoking, male gender and diabetes mellitus. A genetic component has also been postulated, following the discovery of associations between aortic stenosis and certain specific chromosomes. As these risk factors broadly resemble the risk factors for atherosclerosis, aortic stenosis is also recognised as an independent risk factor for myocardial infarction and cardiovascular mortality.

Calcific aortic valve disease manifests as thickening of the aortic valve leaflets and subsequent narrowing of the valve orifice. While previously thought to represent two separate entities, aortic valvular sclerosis and aortic valve stenosis are now recognised as being individual stages of the overall spectrum of calcific aortic valve disease. Initial calcific degeneration of the aortic valve is labelled as aortic valve sclerosis and is characterised by microscopic calcium deposits and leaflet thickening with no macroscopic evidence of disease and no change to the function of the valve leaflets (Thaden, Nkomo & Enriquez-Sarano 2014).

There are three simultaneously occurring pathological processes responsible for ongoing degeneration of the already sclerosed aortic valve. Mechanical stress and subsequent endothelial dysfunction lead to lipid accumulation, inflammation and calcification (due to alteration of the extracellular matrix). As the valve becomes increasingly diseased, the leaflets become less functional and the central orifice is further narrowed.

The degenerative aetiology in aortic stenosis is by far the most common, making up almost 90% of cases. Around 80–85% are tricuspid valves and the remaining 5–10% are the congenitally abnormal (predominantly bicuspid) valves (Roberts 1970, 1992). Bicuspid aortic valve is found in 1–2% of the general population and is most commonly caused by a congenital fusion of the left and right leaflets (Shabana 2014).

The ventricular response to pressure overload is compensatory hypertrophy of the muscle of the LV wall. This aims to preserve systolic function by maintaining normal wall stress. However, over time, the LV will become less compliant. Myocardial oxygen demand will increase, while coronary perfusion decreases; eventually the ventricle will fail, leading to the clinical manifestation of heart failure.

Aortic regurgitation

Aortic regurgitation is caused by the leaflets of the aortic valve failing to coapt appropriately. This lack of apposition of the valve leaflets causes the valve to become incompetent, allowing blood to regurgitate back into the left ventricular cavity. The underlying cause of the regurgitation can be either a primary leaflet abnormality, or abnormality of the aortic root and/or ascending aorta (Moorjani, Viola & Ohri 2011).

The most common causes of leaflet abnormalities resulting in aortic regurgitation include senile calcification, bicuspid aortic valve, rheumatic fever and infective endocarditis. Causes of aortic regurgitation due to aortic pathology include aortic dissection, syphilis, connective tissue disorders and annulo-aortic ectasia, which is defined as idiopathic root dilatation (Maurer 2006). In the western world, degenerative AR (either bicuspid or tricuspid aortic valve) is the most commonly encountered cause of aortic regurgitation, accounting for around two-thirds of all cases. As the aortic annulus dilates, the leaflets fail to coapt and can also begin to prolapse. In bicuspid aortic valves, which comprise approximately 15% of aortic regurgitation cases, it is the fused cusp which begins to prolapse. This combination of leaflet prolapses, and annular dilatation leads to significant regurgitation (Robicsek et al. 2004).

The development of aortic regurgitation can either be acute or chronic. In acute aortic regurgitation (which usually occurs secondary to infective endocarditis or acute aortic dissection), there is a sudden increase in end-diastolic pressure secondary to volume overload. This increased pressure leads to a decrease in cardiac output.

Symptoms and diagnosis

Aortic stenosis

Patients with aortic stenosis may remain asymptomatic for many years despite continuing subclinical deterioration of the valve, although the duration of this asymptomatic phase varies. During this period, there is a risk of sudden cardiac death of only about 1% per year (Taniguchi et al. 2018). However, in symptomatic patients with AS, the incidence of sudden death is very high (Taniguchi et al. 2018). As the state of the valve continues to deteriorate, patients begin to develop symptoms. The principal symptoms of aortic stenosis are related to cerebral ischaemia, myocardial ischaemia and failure of the LV.

Cerebral ischaemia due to aortic stenosis manifests as dizziness or light-headedness, classified as pre-syncopal episodes. In severe cases true syncope is observed and it is well recognised as a poor prognostic factor. As myocardial hypertrophy continues, and coronary perfusion of the myocardium worsens, myocardial demand outstrips supply, leading to myocardial ischaemia and classical symptoms of angina pectoris. As the left ventricle starts to fail, the patient eventually develops exertional dyspnoea. The development of symptoms in aortic stenosis correlates with a reduced life expectancy. The average life expectancy of a symptomatic patient with severe aortic stenosis is around two to three years (Baumgartner et al. 2017).

Clinical examination will reveal an ejection systolic murmur, best heard at the level of the 2nd intercostal space, at the right parasternal edge with radiation of the murmur to the carotid arteries. A small volume and slow rising pulse (pulsus parvus et tardus) may be felt when assessing peripheral and central pulses.

Any clinical suspicion of valvular pathology must be investigated with a transthoracic echocardiogram (TTE). Once the diagnosis of aortic stenosis has been established, the echocardiogram will estimate the peak and mean gradients across the valve (measured in mmHg), the forward velocity across the valve (measured in m/s) and the valve orifice area (measured in cm²). This allows the degree of aortic stenosis to be classified as mild, moderate or severe. Assessment of the ventricular function and the dimensions of the aortic root and ascending aorta should also be part of the TTE study. If the TTE fails to provide definitive results, additional forms of imaging such as transoesophageal echocardiogram or cardiac magnetic resonance imaging scanning should be considered (Baumgartner et al. 2017).

• Any history of cardiovascular disease

• Any risk factors for cardiovascular disease

• Suspected myocardial ischaemia

• LV systolic dysfunction

• Men over 40 years of age

• Post-menopausal women.

The criteria for diagnosing severe aortic stenosis are listed in Table 7.1 (Moorjani, Viola & Ohri 2011).

Table 7.1: Criteria for diagnosis of severe aortic stenosis

Aortic regurgitation

Due to the compensatory mechanisms outlined above, patients with chronic aortic regurgitation often remain asymptomatic for many years. As the LV starts to decompensate and the end-diastolic pressure rises, symptoms of exertional dyspnoea develop. Decreased aortic diastolic pressure (due to regurgitation) leads to decreased coronary perfusion in the presence of increased oxygen demand, due to myocardial wall hypertrophy. This manifests clinically as angina pectoris. In the case of acute aortic regurgitation, the sudden onset of high end-diastolic pressure in a non-dilated and non-compliant ventricle leads swiftly to pulmonary oedema and significant dyspnoea (Maurer 2006; Baumgartner et al. 2017).

Clinical examination will reveal a diastolic murmur, best heard at the level of the 4th intercostal space, at the left parasternal edge. The increased forward stroke volume and diastolic flow reversal leads to several clinical signs that can be elicited on examination (Maurer 2006; Moorjani, Viola & Ohri 2011). These include:

• Corrigan’s pulse (collapsing pulse): high amplitude and abruptly collapsing pulse

• Widened pulse pressure

• Corrigan’s sign: visible pulsation of the carotid arteries

• De Musset’s sign: visible head bobbing with each heartbeat

• Quincke’s sign: pulsation of the capillary nail beds

• Duroziez’s sign: femoral artery bruit.

Coronary angiography prior to aortic valve surgery should also be undertaken in the following scenarios (class I evidence) (Baumgartner et al. 2017):

• Any history of cardiovascular disease

• Any risk factors for cardiovascular disease

• Suspected myocardial ischaemia

• LV systolic dysfunction

• Men over 40 years of age

• Post-menopausal women.

Criteria for diagnosis of severe aortic regurgitation are listed in Table 7.2 below (Moorjani, Viola & Ohri 2011):

Table 7.2: Criteria for diagnosis of severe aortic regurgitation

Indications for surgery

According to the 2017 ESC/EACTS Guidelines for the management of valvular heart disease, there is evidence for intervention for aortic valve disease in the following scenarios (Baumgartner et al. 2017).

In aortic stenosis:

• Symptomatic patients with severe aortic stenosis (class I)

• Asymptomatic patients with severe aortic stenosis and left ventricular ejection fraction <50% not due to another cause (class I)

• Asymptomatic patients with severe aortic stenosis and an abnormal exercise test clearly related to aortic stenosis (class I)

• Asymptomatic patients with moderate aortic stenosis undergoing additional cardiac surgery (class IIa)

• Asymptomatic patients with severe aortic stenosis and an abnormal exercise test demonstrating a decrease in baseline blood pressure (class IIa)

• Asymptomatic patients with severe aortic stenosis, preserved LVEF and a normal exercise test should be considered if the surgical risk is low and one of the following additional factors is present (class IIa):

• Peak velocity >5.5m/s

• Peak velocity progression >0.3m/s/year in the presence of severe valve calcification

• Markedly elevated BNP levels (>3 times corrected normal range)

• Severe pulmonary hypertension (systolic pulmonary artery pressure >60mmHg).

In aortic regurgitation:

• Symptomatic patients with severe aortic regurgitation (class I)

• Asymptomatic patients with severe aortic regurgitation and left ventricular ejection fraction (LVEF) <50% not due to another cause (class I)

• Asymptomatic patients with severe aortic regurgitation undergoing additional cardiac surgery (class I)

• Asymptomatic patients with severe aortic regurgitation and left ventricular ejection fraction (LVEF) >50% with severe LV dilatation (left ventricular end-diastolic diameter >70mm or left ventricular end-systolic diameters >50mm) (class IIa).

Surgical approaches and considerations

Selection of prosthetic valve

Most aortic valve surgery currently undertaken in the United Kingdom is replacement of the aortic valve. A small number of centres perform aortic valve repair surgery, which avoids the need for any form of prosthetic valve but this is a specialised technique only suitable in cases where both the anatomy and pathology of the aortic valve disease are favourable. Hence aortic valve repair is reserved for cases where the surgeon is confident that a durable and high-quality repair can be performed. Early failure of aortic valve repair often leads to redo cardiac surgery, negating any initial benefit.

Prior to aortic valve surgery, the surgeon and patient should engage in shared decision-making regarding the most appropriate type of prosthetic valve. The main factors when considering valve choice are the durability of the prosthesis and the need for anticoagulation. The two main types of valve currently available are bioprosthetic (tissue) or mechanical.

A mechanical valve has a much longer lifespan and can therefore be implanted into younger patients with minimal risk of additional surgery being required later in life. However, to minimise the risk of significant clot formation on the leaflets of the valve, lifelong anticoagulation therapy is required. Poor compliance with this anticoagulation therapy can lead to greater risks of thrombosis or haemorrhage.

Lifestyle is affected by anticoagulation therapy, due to the need to abstain from contact sports and avoid certain foodstuffs and excessive alcohol intake. Anticoagulants are also teratogenic. This means that women of childbearing age who wish to become pregnant cannot safely be given a mechanical valve. They must either opt for a bioprosthesis (accepting the need for redo surgery) or receive a mechanical valve and agree to comply with anticoagulation, accepting that they will no longer be able to conceive a child (Jaffer & Whitlock 2016; Vause et al. 2016).

Operative techniques

Traditional aortic valve surgery is performed through a median sternotomy. Once the pericardium has been incised and total body heparin has been given, the heart is cannulated, and cardiopulmonary bypass is commenced. Cardioplegia cannulas are then inserted so that the heart can be adequately arrested and protected once the cross-clamp has been applied. Cardioplegia strategy depends on surgeon preference, anatomy, surgical access and the specific pathology of the individual patient’s aortic valve disease.

Following application of the cross-clamp, initial cardioplegia is delivered through the antegrade cannula into the aortic root. If the predominant pathology of the aortic valve is regurgitation, a significant proportion of the cardioplegia will regurgitate into the heart, causing LV distension. The amount of cardioplegia directed down the coronary ostia will therefore also be reduced. To adequately arrest and protect the heart in this situation, retrograde cardioplegia can be given, delivered through a cannula inserted into the coronary sinus. Alternatively, an aortotomy can be performed, allowing for direct cannulation of the individual coronary ostia and selective delivery of cardioplegia down the left and right ostia as required.

Once the heart is adequately arrested, protected and vented, the aortic valve can be properly exposed, usually by using stay sutures and hand-held retractors as required. The native valve is excised, and the annulus thoroughly decalcified. The surgeon is then able to estimate the size of the annulus using specific valve sizers. Having settled on the correct size, the prosthetic valve can be opened and prepared by the scrub practitioner.