Diseases Of The Aortic Valve
The aortic valve links the left ventricle (LV) and the aorta. It is normally tricuspid. Impaired aortic valve opening, due to its narrowing, is known as aortic stenosis (AS). It impedes outflow of blood from the LV into the aorta and imposes a pressure load on the LV. Deficient valve closure (aortic regurgitation, AR, incompetence) allows blood to flow back into the LV and thus imposes a volume load on the LV.
Senile calcification This is the most common cause. Calcium deposits occur at the base of the cusp, without involvement of the commissures. This is most likely related to prolonged mechanical stress, and is more common in people with congenital bicuspid valves. About 50% of patients aged under 70 with significant AS have bicuspid valves, whereas most older patients with AS have tricuspid valves.
Rheumatic AS as a result of rheumatic heart disease is unusual without coexisting mitral valve disease. Male sex, diabetes and hypercholesterolaemia are also risk factors for AS.
Congenital A unicuspid aortic valve is usually fatal within 1 year of birth. Bicuspid aortic valves develop progressive fusion of the commissures, and symptoms usually present after 40 years. Infants with atherosclerosis due to lipid disorders may develop AS in conjunction with coronary artery disease (CAD).
A slow reduction in aortic valve area requires the LV to pump harder to expel blood into the aorta, which causes left ventricular hypertrophy and eventual myocardial dysfunction, arrhythmias and heart failure (see Chapter 46). ‘Critical’ AS occurs when there is greater than a 75% reduction of valve area, to <0.5 cm2/m2 body surface area, and a >50 mmHg gradient between peak systolic LV and aortic pressure at a normal cardiac output. With worsening AS, cardiac output cannot increase adequately during exercise and eventually becomes insufficient at rest. As AS progresses the left ventricle dilates, and LV end-diastolic pressure (EDP) increases to the point where overt LV failure ensues.
AS is typically associated with a triad of symptoms: angina, syncope and breathlessness. Patients present usually between the ages of 50 and 70 years, most commonly with angina either due to reduced cardiac output secondary to AS reducing coronary artery perfusion relative to myocardial demand or concurrent CAD (50% have concurrent CAD). In AS the hypertrophied LV has an elevated oxygen demand and inadequate cardiac output for this demand occurs during exercise. Exercise tolerance is decreased, and if cerebral blood flow is insufficient patients may develop exercise-associated syncope. Once patients with AS develop angina, syncope or LV failure, their median survival is less than 3 years.
Patients with mild AS have a normal blood pressure and pulse. In moderate to severe AS the pulse is slow-rising and has a narrow pulse pressure. There may be a demonstrable thrill (vibration) felt on palpation over the precordium. The apex beat is heaving due to LV hypertrophy. Initially, the apex is non-displaced; however, once the LV starts dilating in late-stage AS then it will displace. Auscultation reveals a normal S1, a quiet S2 and an ejection systolic murmur (Figure 53a; see Chapter 32), heard best in the second intercostal space on the right and which classically radiates to the carotids. It is louder with squatting and softer with standing or during the Valsalva manoeuvre (forced expiration against a closed glottis). With worsening AS and a fall in cardiac output, the murmur may become softer (silent AS).
The ECG shows LV hypertrophy with strain (ST depression, T-wave inversion). Atrial fibrillation and ventricular arrhythmias are often seen when LV function has deteriorated. Echocardiography shows reduced valve opening and calcification of cusps, and permits calculation of valve area. Doppler imaging allows calculation of the pressure gradient between the LV and aorta.
Many patients with AS are old and have comorbidities – in some cases their symptoms can be conservatively managed as the risks of valve intervention outweigh the benefits. When valve interven- tion is planned it is important that it is done before the LV starts to dilate. Risk factors for CAD, such as hypertension, and angina symptoms can be treated medically. However, it is important that systemic hypotension and arterial vasodilatation are avoided, so β-blockers and other negative inotropes should be stopped. Cardiac catheterization with coronary angiography must be performed prior to valve replacement, and coronary artery bypass performed if significant CAD is present. Several types of mechanical valve are available, including those of a ‘ball and cage’ variety or tilting disc. These will always require anticoagulant therapy (see Chapter 8). Valves can also be obtained from pigs or human cadavers and these have the advantage that anticoagulants are not generally required, hence these can be used in women of childbearing age, because warfarin is teratogenic. Balloon valvuloplasty can be performed in children with non-calcified valves, but is of little value in adults.
AR occurs when the valve cannot close firmly at the end of ven- tricular systole and as a result blood flows back into the ventricle from the aorta at the start of diastole.
Causes of AR include rheumatic disease, where fibrous retraction of the valve cusps prevents apposition, infective endocarditis causing valve damage and congenital malformations (e.g. bicuspid valve) (Figure 53b).
AR imposes a volume load on the LV because of flow back into the ventricle. Acute AR (trauma, infective endocarditis, aortic dissection) is usually catastrophic. Here the LV cannot accommodate the acute increase in volume and LV EDP rises. The early increase in LV EDP causes premature closure of the mitral valve and inadequate forward LV filling, resulting in cardiovascular collapse and acute respiratory failure.
In chronic AR, volume load and LV EDP increase gradually, and LV hypertrophy allows adequate output to be maintained. As the aortic valve never completely closes, there is no LV isovolumetric relaxation phase (see Chapter 16) and the pulse pressure is wide.
Patients usually do not present with symptoms until LV failure develops. Signs include a wide pulse pressure (caused by reduction in diastolic pressure) and a collapsing pulse (see Chapter 16). The LV apex is displaced laterally and is hyperdynamic. Auscultation reveals a high-pitched early diastolic murmur at the left sternal edge, and often a systolic flow murmur across the aortic valve. AR is associated with several eponymous signs. While these are rare in clinical practice, they are favoured by some finals examiners. Quincke’s sign is visible nail bed pulsation; Corrigan’s sign denotes visible pulsations in the carotids; de Musset’s sign is pulsatile head bobbing; Traube’s sign is a ‘pistol shot’ heard on auscultation of the femoral arteries; the Austin Flint murmur is a rumbling late diastolic murmur caused by premature closure of the mitral valve; it denotes severe AR.
Echocardiography can determine the aetiology and severity of AR by imaging the valve leaflets and LV dimensions, aortic root diameter and diastolic closure or fluttering of the mitral valve. Doppler imaging quantifies the amount of regurgitation.
Acute severe AR requires urgent valve replacement. Chronic AR has a generally good prognosis until symptoms develop. Patients with moderate AR should undergo echocardiography every 6–12 months. Valve replacement should be considered in symptomatic patients, or in asymptomatic patients with worsening LV dimensions, LV function or aortic root diameter. Valve replacement is similar to that for AS, except that replacement of the aortic root may also be required in patients with a severely dilated ascending aorta.