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Aortic
Stenosis
 The
following describes changes that occur in the left ventricular
pressure-volume loop when there is
aortic stenosis. In
aortic stenosis (red loop in figure), left ventricular emptying is impaired because of high outflow
resistance caused by a reduction in the valve orifice area when it opens. This
high outflow resistance causes a large pressure gradient to occur across the
aortic valve during ejection, such that the peak systolic pressure within the
ventricle is greatly increased. This leads to an increase in ventricular afterload, a
decrease in stroke volume, and an increase in end-systolic volume. Stroke
volume (width of pressure-volume loop) decreases because the velocity of fiber shortening is decreased by the
increased afterload (see force-velocity relationship). Because
end-systolic volume is elevated, the excess residual volume added to the
incoming venous return causes the end-diastolic volume to increase. This
increases preload and activates the Frank-Starling mechanism
to increase the force of contraction to help the ventricle overcome, in part,
the increased outflow resistance. In mild aortic stenosis, this can be
adequate to maintain normal stroke volume, but in moderate stenosis (as shown in
the figure) or severe stenosis, the stroke volume may fall considerably because
the end-systolic volume increases substantially more than the end-diastolic
volume increases. The fall in stroke volume can lead to a reduction in arterial
pressure.
Stroke volume falls even further if the ventricle begins to exhibit
systolic and diastolic dysfunction.
Compensatory increases in end-diastolic volume will be limited by ventricular
hypertrophy that occurs due to the chronic increase in afterload. This
hypertrophy can lead to large increases in end-diastolic pressure.
The changes described above and shown in the figure do not include cardiac and
systemic compensatory mechanisms (e.g., systemic vasoconstriction, increased
blood volume, and increased heart rate and inotropy) that attempt
to maintain cardiac output and arterial pressure, nor do they include the
ventricular hypertrophy (remodeling)
that decreases ventricular compliance.
RK Revised
04/05/07
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Cardiovascular Physiology Concepts
