The administration of dipyridamole, a vasodilator, in patients with significant narrowing of a coronary artery can fully dilate normal vessel branches that run parallel to the narrowed segment. This reduces the pressure on the partially occluded vessel. However, the reduced pressure on the narrowed vessel further compromises blood flow to the ischemic myocardium. This phenomenon, known as coronary steal, occurs because dipyridamole acts by blocking the cellular uptake and metabolism of endogenous adenosine. It is important to note that dipyridamole interferes with coronary autoregulation.

The skin contains two types of resistance vessels: arterioles and arteriovenous anastomoses. Arteriovenous anastomoses are present in certain areas such as fingertips, palms of the hands, toes, soles of the feet, ears, nose, and lips. These anastomoses bypass the capillary bed by shunting blood from the arterioles to the venules and venous plexuses. Morphologically, AV anastomoses differ from arterioles and are either short and straight or long coiled vessels with thick muscular walls. They are primarily under sympathetic neural control and dilate maximally when their nerve supply is interrupted. Conversely, stimulation of the sympathetic fibers to these vessels can constrict them. AV anastomoses are highly sensitive to vasoconstrictor agents like epinephrine and norepinephrine. They are not under metabolic control and do not exhibit reactive hyperemia or autoregulation of blood flow. The regulation of blood flow through AV anastomotic channels is mainly governed by the nervous system in response to reflex activation by temperature receptors or from higher centers of the central nervous system.

Most resistance vessels in the skin have some basal tone and are under dual control of the sympathetic nervous system and local regulatory factors, although neural control predominates. Stimulation of sympathetic nerve fibers induces vasoconstriction, while cutting the sympathetic nerves induces vasodilation. After chronic denervation of cutaneous blood vessels, the initial degree of tone gradually returns over a few weeks through an enhancement of basal tone. Denervation of skin vessels leads to increased sensitivity to catecholamines in circulation, known as denervation hypersensitivity.

Raynaud's disease is a condition characterized by the fingers and toes being very sensitive to cold. Exposure to cold causes constriction of arterioles in the fingers and toes, resulting in localized blanching of the skin accompanied by tingling, numbness, and pain. This blanching is followed by cyanosis (a dark blue color of the skin), which subsides as the arterial spasm resolves, leading to redness. The exact cause of Raynaud's disease is unknown.
Inpatientswithmarkednarrowingofacoronaryartery,administrationofdipyridamole,avasodilator,canfullydilatenormalvesselbranchesthatareparalleltothenarrowedsegmentandtherebyreducethepressureonthepartiallyoccludedvessel.Thereducedpressureonthenarrowedvesselfurthercompromisesbloodflowtotheischemicmyocardium.Thisphenomenon,knownascoronary steal, occursbecausedipyridamoleactsbyblockingthecellularuptakeandmetabolismofendogenousadenosine.Ofnoteisthatdipyridamoleinterfereswithcoronaryautoregulation.
Regulation of Skin Blood Flow
The skin contains essentially two types of resistance vessels: arterioles and arteriovenous anastomoses. AV anastomoses shunt blood from the arterioles to the venules and venous plexuses; hence, they bypass the capillary bed. Such anastomoses are found in the fingertips, palms of the hands, toes, soles of the feet, ears, nose, and lips. AV anastomoses differ morphologically from arterioles; the anastomoses are either short and straight or long coiled vessels, approximately 20 to 40 µm in luminal diameter, and they have thick muscular walls richly supplied with nerve fibers (
Fig. 17.37 ). These vessels are almost exclusively under sympathetic neural control, and they dilate maximally when their nerve
Fig. 17.36 A, Angiogram(withintracoronaryradiopaquedye)ofmarkednarrowingoftheleftanteriordescendingbranchoftheleftcoronaryartery(white arrow). B, Thesamesegmentofthecoronaryartery(black arrow) afterangioplastyandinsertionofadrug-elutingstent.(CourtesyofDr.MichaelAzrin.)
Fig. 17.37 Arteriovenous (AV) Anastomosis in the Ear Injected With Berlin Blue Dye. A,artery;V,vein;arrowpointstoanAVanasto-mosis.ThewallsoftheAVanastomosisinthefingertipsarethickerandmorecellular.(FromPritchardMML,DanielPM.J Anat. 1956;90:309.) supply is interrupted. Conversely, reflex stimulation of the sympathetic fibers to these vessels may constrict them and obliterate the vascular lumen. Although AV anastomoses do not exhibit basal tone, they are highly sensitive to vasoconstrictor agents such as epinephrine and norepinephrine. Furthermore, AV anastomoses are not under metabolic control, and they do not show reactive hyperemia or auto-regulation of blood flow. Thus regulation of blood flow through these anastomotic channels is governed principally by the nervous system in response to reflex activation by temperature receptors or from higher centers of the central nervous system.
Most of the resistance vessels in the skin exhibit some basal tone and are under dual control of the sympathetic nervous system and local regulatory factors. However, neural control predominates. Stimulation of sympathetic nerve fibers
Thefingersandtoesofsomeindividualsareverysensitivetocold.Onexposuretocold,thearteriolestothefingersandtoesconstrict.Theconsequentischemiaresultsinlocalizedblanchingoftheskinassociatedwithtingling,numbness,andpain.Theblanchingisfollowedbycyanosis(adarkbluecoloroftheskin)andlaterbyrednessasthearterialspasmsubsides.Thecauseofthiscondition,calledRaynaud’sdisease,isunknown.
induces vasoconstriction, and cutting of the sympathetic nerves induces vasodilation. After chronic denervation of the cutaneous blood vessels, the degree of tone that existed before denervation is gradually regained over a period of several weeks. This restoration of tone is accomplished by an enhancement of basal tone. Denervation of the skin vessels results in enhanced sensitivity to catecholamines in circulation (denervation hypersensitivity).