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Valvular Stenosis
Stenosis of either
atrioventricular
valves (tricuspid, mitral) or outflow tract valves (pulmonic, aortic) leads to a
pressure gradient across the valve during the time blood is flowing through the
valve opening. This increased pressure gradient is expressed as an increase in
the pressure proximal to the valve and a small fall in pressure distal to the
valve. The magnitude of the pressure gradient depends on the severity of the stenosis and the flow rate across the
valve.
 Mitral
valve stenosis results from a narrowing of the mitral valve orifice
when the valve is open. The high resistance across the stenotic
mitral valve causes blood to back up into the left atrium thereby increasing LA
pressure. This results in the left atrial (LA) pressure
being much greater than left ventricular (LV) pressure during diastolic
filling (shaded gray in figure). The gradient is highest during early diastole
when the the flow across the valve is highest. Normally, the
pressure gradient across the valve is
very small (a few mmHg); however, the pressure gradient can become quite high
during severe stenosis (10-30 mmHg). Despite this elevated LA pressure, filling
(end-diastolic volume) of the left ventricle may be significantly reduced. (The effects of mitral stenosis on ventricular filling can be
appreciated better by examining the changes in the LV pressure-volume
loop). The reduced ventricular filling (decreased preload)
decreases ventricular stroke volume by the Frank-Starling mechanism.
If stroke volume falls significantly, the reduced cardiac output may result in a
reduction in aortic
pressure. The increase in LA pressure can cause pulmonary
congestion and edema because of increased pulmonary capillary
hydrostatic pressure. Mitral valve stenosis is associated with a diastolic
murmur because of turbulence that occurs as blood
flows across the stenotic
valve.
Tricuspid valve stenosis is similar to mitral
valve stenosis except that the pressure and volume changes occur on the right
side of the heart.
 Aortic valve
stenosis is characterized by the left ventricular pressure being
much greater than aortic pressure during left ventricular ejection
(shaded gray in figure). LV pressure is greatly elevated and the aortic
pressure is slightly reduced in this example. Normally, the
pressure gradient across the aortic valve
is very small (a few mmHg); however, the pressure gradient can become quite high
during severe stenosis (>100 mmHg). The pressure gradient
across the stenotic lesion results from both increased resistance
(related to narrowing of the valve opening) and turbulence
distal to the valve. The magnitude of the pressure gradient is determined by the
severity of the stenosis and the flow rate across the valve. Severe aortic
stenosis results in 1) reduced ventricular stroke volume due to increased afterload
(which decreases ejection velocity),
2)
increased end-systolic volume,
and 3) a compensatory increase in end-diastolic volume and pressure. (These
changes in ventricular pressures and volumes are best depicted using
pressure-volume loops).
Long-term consequences include left ventricular hypertrophy
and heart failure.
Aortic valve stenosis is associated with a mid-systolic
systolic murmur because of turbulence that occurs as blood flows across the
stenotic valve.
Pulmonic valve stenosis is analogous to
aortic valve stenosis except that the changes in pressure are on the right side
of the heart. A pressure gradient occurs across the pulmonic valve during
right ventricular ejection. Compensatory increases in right ventricular
end-diastolic pressure as well as right atrial pressure and volume occur.
RK Revised
04/05/07
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Cardiovascular Physiology Concepts
