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.
Aortic regurgitation
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.
Investigations
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.
