Page 44 - Libro 2

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PART I — INTRODUCTION TO THE VASCULAR SYSTEM
75 10
8
6
4 25
2 00
Heat
Contraction Viscous
Expansion
50
Potential Energy (Pressure)
Kinetic Energy
0123 LENGTH cm
Figure 2-11 Energy losses across a stenosis.
A critical stenosis is defined as a degree of nar- rowing at which pressure and flow begin to be af- fected. Experimentally, changes in pressure and flow do not occur until the cross-sectional area has been reduced by 75% (or a 50% diameter reduction). Be- cause energy losses also depend on the velocity of blood flow, in high-flow (low-resistance) systems, significant drops in pressures and flow occur with less severe narrowing than in low-flow systems. Therefore, a critical stenosis varies with the resis- tance of the run-off bed. In the carotid or coronary systems (low-resistance systems), a critical stenosis may be reached with less narrowing than in the rest- ing lower extremity (high resistance). In the exercis- ing leg, resistance drops, flow increases, and a steno- sis may become critical or flow limiting.
Collateral vessels are preexisting pathways that enlarge with a stenosis or occlusion. They are one of the main mechanisms to compensate for a stenosis. Collateral arteries can be divided into (1) stem arter- ies, which are the large branches; (2) midzone col- laterals, which are the small intramuscular branches; and (3) reentry arteries, which are the vessels that rejoin a major artery distal to the area of stenosis or occlusion (Fig. 2-12). The resistance of the collat- eral bed is almost fixed and will only slightly dilate gradually. Exercise, sympathectomy, and vasodilator drugs have little effect on collaterals unlike the pe- ripheral run-off bed.
Blood flow increases with exercise to at least three to five times resting flow in normal limbs. In limbs with mild-to-moderate disease, blood flow is in- creased far less. In patients with multilevel disease,
flow after exercise may change very little. At rest, blood pressure distal to an arterial lesion will de- crease with mild-to-moderate disease and even more with severe disease. Exercise will cause a further de- crease in peripheral pressure.
Stem arteries
Midzone collateral arteries
Re-entry arteries
Peripheral vascular bed (run-off)
Venules and veins
Stenosis
Figure 2-12 Collateral arteries including stem arteries, mid- zone collaterals, and reentry arteries.
mmHg
erg · cm-3 x 104