Diseases Of The Mitral Valve
The mitral valve is normally bicuspid and separates the left atrium
(LA) and left ventricle (LV). The valve may narrow (mitral stenosis) or
leak (mitral regurgitation).
Causes
Mitral stenosis (MS) is usually caused
by prior episodes, often during childhood, of acute rheumatic fever.
This causes thickening and fusion of the mitral commissures, cusps or
chordae tendineae, making the cusps less flexible and narrowing the
orifice. Symptoms from MS usually develop more than 10 years after the acute
attack of rheumatic fever, which patients may not recall. The normal area of a
mitral valve is 6 cm2; critical MS occurs when this area falls to 1 cm2. Other
less common causes include congenital mitral stenosis and carcinoid tumours,
which are very rare.
Pathophysiology
MS prevents the free flow of blood
from the LA to the LV, and slows ventricular filling during diastole. The left
atrial pressure rises to maintain cardiac output, and there is atrial
hypertrophy and dilatation. The elevated left atrial pressure causes
pulmonary congestion and can result in pulmonary hypertension and oedema,
and right heart failure (see Chapter 46). Patients with MS rely on
atrial systole (the so-called ‘atrial kick’) for ventricular filling, and atrial
fibrillation (caused by atrial enlargement) significantly reduces cardiac
output. The LV is usually normal in MS, but may be abnormal due to either
chronic underfeeding of the LV or rheumatic scarring.
Clinical features
Patients present in their thirties to
forties with dyspnoea, either on exertion or during situations that
raise cardiac output (e.g. fever, anaemia, pregnancy). This is a result of pulmonary
congestion, which causes the lungs to become stiffer. Patients may present
with palpitations, chest pain, stroke (via embolization of thrombi) or haemoptysis
(coughing up of blood). Hoarseness may be present as a result of the
enlarged LA compressing the left recurrent laryngeal nerve. Symptoms may be
precipitated by atrial fibrillation. The patient’s cheeks may appear pinkish –
‘malar flush’or ‘mitral facies’, due to slight arterial hypoxia
resulting from the reduced cardiac output. The apex beat is described as
tapping and the first heart sound is loud. Auscultation reveals an opening
snap soon after S2 that is best heard at the apex, and by a rumbling
middiastolic murmur leading to a loud S1. The duration of the
murmur is related to the severity of the MS. It is brief in mild MS and pandiastolic
(i.e. lasts for the whole of diastole) in severe MS. When auscultating for
the diastolic murmur of MS, ask the patient to lean towards their left side and
to hold their breath in expiration. This manoeuvre helps accentuate the murmur
as it brings the heart against the
thoracic wall and briefly increases left-sided cardiac output. Patients in sinus
rhythm may have presystolic accentuation of the murmur due to atrial
contraction, and a large venous ‘a’ wave (see Chapter 16). If the mitral valve
is completely immobile there may be no opening snap or a loud S1. As
MS becomes more severe, the pulse becomes less prominent, crackles are
heard on auscultation of the lung bases because of developing pulmonary oedema,
and the jugular venous pressure becomes elevated.
The ECG may show signs consistent with
LA enlargement only, although many patients are in atrial fibrillation. The
chest X-ray may show left atrial enlargement with normal left ventricular size,
but with increasing severity of MS there may be pulmonary venous congestion,
enlarged pulmonary arteries, denoting pulmonary hypertension, and right
ventricular enlargement.
Management
Mild MS may require little treatment,
although management should include measures to avoid anaemia and tachyarrhythmias
as these may precipitate decompensation and cardiac failure (see Chapter
46). If the patient is in atrial fibrillation, rate control with a
β-blocker or rate-limiting Ca2+ channel blocker is crucial. Anticoagulation
must be given to prevent stroke resulting from an embolus arising from the
fibrillating atrium. Patients with MS can remain minimally symptomatic for many
years, but deteriorate quickly once symptoms worsen. Therefore, valve
replacement with a mechanical valve, valvotomy (surgical separation
of commissures) or balloon valvuloplasty (the use of a balloon catheter
to force cusps open) should be performed in moderately symptomatic patients.
Causes
Acute mitral regurgitation (MR) is usually
a result of infective endocarditis, ruptured chordae tendineae or
ischaemic papillary muscle rupture. Chronic MR arises from myxomatous
degeneration of the mitral leaflets, mitral valve prolapse (reversal
into atrium) and chronic MR may also develop in any disease causing LV
dilatation, so preventing apposition (coming together) of the mitral leaflets,
or because of ischaemic dysfunction of the papillary muscles. As MR causes LV
dilatation, mitral regurgitation begets further mitral regurgitation.
Pathophysiology
In acute MR the LV ejects blood
back into the LA, imposing a sudden volume load on the LA during ventricular
systole. Left atrial pressure rises suddenly and this is rapidly followed by a
rise in pulmonary venous pressure and capillary pressure. This leads to fluid
entering the lung interstitium, causing stiffness and dyspnoea, or into the
alveoli, causing pulmonary oedema.
Chronic MR is characterized by LV dilatation and hypertrophy,
and dilatation of the LA. The latter protects the pulmonary circulation from
the effects of the regurgitant volume. This form of MR is called chronic
compensated. However, LA dilatation leads to atrial fibrillation. The
fibrillating atrium is liable to develop thrombi that may be embolized
(dislodge and move freely in the blood) causing stroke (see Chapter 8).
MR imposes a diastolic volume load on
the LV that causes dilatation, because
each systolic stroke volume is composed of a portion that enters the aorta (LV
output) and an ineffective portion that re-enters the LA (LV regurgitant
volume) and adds to the venous return. The regurgitant volume increases when LV
emptying is impaired, such as with aortic stenosis or hypertension.
Clinical features
Patients with mild chronic MR are
usually asymptomatic. As MR worsens, patients develop fatigue, dyspnoea on
exertion, orthopnoea and pulmonary oedema as a result of progressive
LV failure and elevation of pulmonary capillary pressure (see Chapter 46). The
development of atrial fibrillation is common because of dilatation of the LA.
Chronic MR is associated with a pansystolic murmur, which is heard best
at the apex, and which radiates classically to the axilla. S1 is
soft and S2 is widely split because of an early aortic component.
Echocardiography can detect a prolapsing or rheumatic valve, and determine LV
size and function. Doppler imaging of the regurgitant jet can assess the
severity of MR.
Management
Management is focused on promoting LV
emptying into the aorta. Reduction of afterload with angiotensin-converting
enzyme inhibitors is beneficial (see Chapter 47). Patients with atrial
fibrillation receive anticoagulants to prevent stroke. A prolapsing
valve may sometimes be repaired. Dilatation of the mitral valve ring may be
corrected by implantation of an artificial ring. Rheumatic valves and those
damaged by endocarditis often need replacement with an artificial valve. Valve
replacement is best performed prior to the development of LV dysfunction or pulmonary
hypertension, and should always be performed in patients with symptomatic MR
despite medical therapy. The risks of surgery are higher in acute MR; however,
valve replacement should be performed in patients with uncontrollable heart
failure or endorgan failure, even in cases of acute infective endocarditis.
