Hemodynamics of Single and Multiple Arterial Stenotic Lesions
Single Arterial Stenosis
A single arterial stenotic lesion located in the femoral artery distal to the deep
femoral branch, for
example (see figure to right), can decrease resting blood flow and maximal flow
capacity to the tissues supplied by the femoral artery (e.g., calf
musculature). A lesion at this site will result in a fall in pressure
distal to the lesion because the increased
resistance
will increase in the
pressure drop along the
length of the stenotic segment. If the pressure drop is relatively small
at resting flows (e.g., 10-30 mmHg),
autoregulation and
collateralization
can reduce the resistance in the distal vascular beds sufficiently to maintain
resting blood flow. For this reason, measuring blood flow (e.g., by Doppler techniques) under resting conditions in a limb will not necessarily
detect stenotic lesions because flow may be normal. However, measuring
pressures at different sites in the limb (i.e., measuring segmental pressures)
or determining the
ankle-brachial pressure index will detect the
presence of stenotic lesions.
Because resting blood flow with mild-to-moderate
stenosis is not necessarily compromised, ischemic pain may not be present at
rest. With more severe stenosis of the femoral artery, the
pressure drop will be much greater and the autoregulatory capacity of the distal
vascular beds may be exceeded leading to a fall in resting blood flow. When this occurs, the patient will experience ischemic pain (and other
pathologies associated with ischemia) at rest.
During limb exercise (see figure above), a patient with a stenotic lesion in the
femoral artery distal to the deep femoral branch will very likely experience ischemic pain in the calf
musculature supplied by the femoral artery. This occurs because the
maximal blood flow capacity to the calf is limited by the increased resistance
in the femoral artery. This distribution artery is in-series
with the calf vasculature, therefore, the resistances are additive and total
resistance to blood flow is higher than in the absence of the proximal stenosis when the circulation in the calf musculature
is maximally dilated. With normal vessels, the ankle-brachial pressure index does
not fall during the phase of active hyperemia because
the resistance is very low in the distributing arteries. When resistance
is increased by stenosis, the resting ankle-brachial pressure index is reduced, and
the index falls even further when the distal beds dilate during
exercise. This distal dilation increases flow across the stenotic segment,
thereby further increasing the pressure drop across the lesion. This
reduced pressure effectively reduces the perfusion pressure to the distal
vascular beds, which decreases maximal blood flow to the calf during exercise.
It should be noted that a lesion located in the
femoral artery distal to the deep femoral branch will not alter blood flow to proximal vascular beds
(e.g., those supplied by the deep femoral or internal iliac arteries).
The hemodynamics associated with a single stenotic lesion are significantly
compounded by the presence of a second lesion. Consider, for example,
stenotic lesions located in both the femoral artery distal to the deep femoral and
in the external iliac
artery as shown to the right. Under resting, non-exercise conditions,
blood flow may or may not be reduced in the deep femoral and tibial circulations
depending upon the extent of autoregulation and collateralization. The
calf circulation will more likely be compromised at rest because there are two
proximal (and in-series) stenotic lesions. In contrast, the deep femoral circulation to the
thigh is distal to only one stenotic lesion and is therefore less likely to be
compromised at rest. Regardless of the resting
blood flows, the pressures distal to the lesions will be reduced. Because
there are two in-series stenoses proximal to the ankle, the ankle-brachial
pressure index may be very reduced even under resting conditions. If it is
assumed that the perfusion pressure to the calf is reduced to the point where
the microvasculature is maximally dilated (i.e., maximal autoregulation has
occurred), then exercising the limb will actually decrease blood flow to the
calf (termed
vascular steal) as shown in the figure. This can
happen because as the vessels dilate in the thigh during exercise (assuming they
have not already dilated to their maximal autoregulatory capacity), the pressure
distal to the external iliac lesion will fall (the increased flow increases the
pressure drop across the lesion) thereby further reducing the perfusion pressure
to the lower leg and causing calf blood flow to fall. After exercise, the
microcirculation in the thigh regains its vascular tone, causing the
perfusion pressure to the calf to rise and calf blood flow to increase. The fall in calf blood flow during limb exercise occurs at the time the calf
musculature is contracting and for several minutes thereafter. This leads
to a large imbalance in the
oxygen supply/demand ratio
particularly during contraction and ischemic pain (i.e.,
intermittent claudication) in the calf will result.
Revised 11/08/07
