Path 11a Flashcards

Compared with corresponding veins at the same branching level, arterial walls are generally:

A. thinner
B. less elastic
C. thicker
D. more porous

The main reason arterial walls are thicker than comparable veins is to accommodate:

A. lower flow velocities
B. pulsatile flow and higher pressure
C. valve leaflet tension
D. reduced oxygen content

The basic constituents of blood vessel walls are endothelial cells, smooth muscle cells, and:

A. neural crest cells
B. extracellular matrix
C. fibrocartilage plates
D. lymphoid aggregates

Which set best represents major extracellular matrix components of vessel walls?

A. keratin, fibrin, actin
B. myosin, titin, desmin
C. elastin, collagen, glycosaminoglycans
D. albumin, globulin, fibrinogen

The tunica intima normally consists primarily of a single layer of:

A. mesothelial cells
B. fibroblasts
C. endothelial cells
D. smooth muscle cells

Beneath the endothelial cells of the intima lies a:

A. thick muscular coat
B. basement membrane
C. pericyte ring
D. lymphatic plexus

The intima is separated from the media by the:

A. external elastic lamina
B. tunica adventitia
C. vasa vasorum
D. internal elastic lamina

Compared with arteries, the smooth muscle cells in veins are arranged more:

A. concentrically
B. densely
C. haphazardly
D. circumferentially

Which vessel type has several well-organized concentric layers of smooth muscle cells?

A. veins
B. capillaries
C. venules
D. arteries

The media of elastic arteries such as the aorta contains abundant:

A. collagen
B. fibrin
C. keratin
D. elastin

The high elastin content of the aortic media allows the vessel to:

A. resist all dilation
B. expand and recoil
C. block pulse transmission
D. collapse in diastole

Expansion during systole and recoil during diastole in elastic arteries primarily helps:

A. filter plasma proteins
B. propel blood forward
C. reduce venous return
D. increase capillary leakage

In older individuals, arteries often become progressively tortuous and dilated, a change termed:

A. stenotic
B. aneurysmal
C. ectatic
D. thrombosed

The principal sites of physiologic resistance to blood flow are the:

A. elastic arteries
B. capillaries
C. venules
D. arterioles

The vessel layer external to the media is the:

A. intima
B. adventitia
C. endothelium
D. glycocalyx

In many arteries, the adventitia is separated from the media by the:

A. internal elastic lamina
B. basement membrane
C. external elastic lamina
D. endothelial junction

The adventitia is composed mainly of loose:

A. epithelial tissue
B. connective tissue
C. skeletal muscle
D. lymphoid tissue

Which structures are characteristically found within the adventitia?

A. glomeruli and podocytes
B. chordae and valves
C. bile ducts and nerves
D. nerve fibers and vasa vasorum

The vasa vasorum are best described as small vessels that supply the:

A. middle media of large arteries
B. inner media of large arteries
C. outer media of large arteries
D. endothelial glycocalyx

A pathology specimen from a large artery shows ischemic injury in the outer portion of the media. Dysfunction of which structure is most directly implicated?

A. internal elastic lamina
B. vasa vasorum
C. capillary pericytes
D. venous valves

Capillaries are approximately the diameter of a:

A. platelet
B. neutrophil nucleus
C. red blood cell
D. smooth muscle cell

The typical capillary diameter is closest to:

A. 2 to 3 μm
B. 7 to 8 μm
C. 12 to 15 μm
D. 20 to 25 μm

Capillaries possess an endothelial lining but lack a true:

A. intima
B. basement membrane
C. glycocalyx
D. media

Cells lying just deep to capillary endothelium that resemble smooth muscle are called:

A. fibroblasts
B. pericytes
C. mesangial cells
D. myofibroblasts

Veins have less rigid walls than arteries, making them especially prone to:

A. calcification and compression
B. vasospasm and compression
C. dilation and compression
D. elastin hyperplasia and compression

Because of their less rigid walls, veins are also more susceptible to:

A. tumor and inflammatory infiltration
B. fibrinoid necrosis and inflammatory infiltration
C. pulse-pressure injury and inflammatory infiltration
D. medial hypertrophy and inflammatory infiltration

Which statement about vascular layers is most accurate?

A. intima contains multiple muscle layers
B. adventitia lies internal to media
C. intima is endothelial-based
D. capillaries contain external elastic lamina

A histology slide shows a vessel with a single endothelial layer, basement membrane, no media, and nearby pericytes. This vessel is a:

A. muscular artery
B. vein
C. arteriole
D. capillary

A cerebral vascular lesion creates a direct communication between an artery and a vein, bypassing the capillary bed. This lesion is a:

A. Arteriovenous fistula
B. Saccular aneurysm
C. Capillary hemangioma
D. Venous thrombosis

Some arteriovenous fistulas arise most commonly as:

A. Atherosclerotic plaques
B. Developmental defects
C. Septic emboli
D. Autoimmune vasculitis

Fibromuscular dysplasia is best described as focal irregular:

A. Dilation
B. Calcification
C. Thickening
D. Necrosis

Fibromuscular dysplasia classically affects:

A. Medium and large arteries
B. Capillaries and venules
C. Small veins only
D. Lymphatic channels

Which vessel is a classic site of fibromuscular dysplasia?

A. Renal artery
B. Coronary sinus
C. Pulmonary vein
D. Inferior vena cava

Which additional vascular bed is commonly involved in fibromuscular dysplasia?

A. Carotid artery
B. Coronary vein
C. Hepatic vein
D. Superior vena cava

Fibromuscular dysplasia is seen most frequently in:

A. Older men
B. Young women
C. Neonates
D. Elderly women

The specialized lining of blood vessels is formed by:

A. Smooth muscle cells
B. Fibroblasts
C. Endothelial cells
D. Pericytes

One major physiologic property of intact endothelium is a:

A. Prothrombotic surface
B. Nonthrombogenic surface
C. Highly calcified surface
D. Contractile surface

The endothelial surface normally helps maintain blood in a:

A. Coagulated state
B. Hyperviscous state
C. Fluid state
D. Pressurized state

In acute inflammation, histamine most directly causes rapid egress of fluid by acting as a:

A. Vasoactive agent
B. Anticoagulant protein
C. Basement membrane enzyme
D. Platelet inhibitor

In response to vasoactive agents such as histamine, endothelial junctions allow rapid escape of:

A. Erythrocytes, electrolytes, protein
B. Fluid, electrolytes, protein
C. Platelets, electrolytes, protein
D. Fibrin, electrolytes, protein

During inflammation, which cells can slip between adjacent endothelial cells?

A. Myocytes
B. Hepatocytes
C. Leukocytes
D. Megakaryocytes

Endothelial cells influence vascular smooth muscle relaxation by producing:

A. Endothelin
B. Nitric oxide
C. Angiotensin II
D. Thromboxane A2

Endothelial cells influence vascular smooth muscle contraction by producing:

A. Nitric oxide
B. Prostacyclin
C. Endothelin
D. Bradykinin

Low blood pressure causes tissue injury chiefly because it produces inadequate organ:

A. Compliance
B. Filtration
C. Perfusion
D. Oxygen extraction

Sustained systolic pressure above which value is associated with increased atherosclerotic risk?

A. 119 mm Hg
B. 129 mm Hg
C. 139 mm Hg
D. 159 mm Hg

Sustained diastolic pressure above which value is associated with increased atherosclerotic risk?

A. 79 mm Hg
B. 89 mm Hg
C. 99 mm Hg
D. 109 mm Hg

A patient’s hypertension is traced to primary aldosteronism. This is classified as:

A. Essential hypertension
B. Malignant hypertension
C. Borderline hypertension
D. Secondary hypertension

Which condition is a classic cause of secondary hypertension?

A. Pheochromocytoma
B. Mitral stenosis
C. Atrial fibrillation
D. Pulmonary fibrosis

Renal artery stenosis most classically causes:

A. Essential hypotension
B. Secondary hypertension
C. Malignant hypotension
D. Primary vasculitis

Approximately what proportion of hypertension is essential?

A. 20% to 30%
B. 40% to 50%
C. 60% to 70%
D. 90% to 95%

Essential hypertension is best described as:

A. Idiopathic hypertension
B. Renal failure hypertension
C. Adrenal tumor hypertension
D. Pregnancy-induced hypertension

Left untreated, about half of hypertensive patients die from:

A. Pulmonary embolism
B. Liver failure
C. IHD or heart failure
D. Intracerebral abscess

Another major cause of death in untreated hypertension, accounting for about one-third, is:

A. Stroke
B. Sepsis
C. Arrhythmia
D. Aortic rupture

A patient has rapidly rising blood pressure, renal failure, retinal hemorrhages, and papilledema. The best diagnosis is:

A. White coat hypertension
B. Essential hypertension
C. Secondary hypotension
D. Malignant hypertension

Malignant hypertension, if untreated, may lead to death within:

A. 1 to 2 months
B. 1 to 2 years
C. 5 to 10 years
D. 20 years

Which pressure profile best matches malignant hypertension?

A. >180/>120 mm Hg
B. >200/>100 mm Hg
C. >200/>120 mm Hg
D. >210/>110 mm Hg

Retinal findings classically seen in malignant hypertension include:

A. Cotton wool absence and exudates
B. Hemorrhages and exudates
C. Retinal detachment and exudates
D. Venous thrombosis and exudates

Papilledema in malignant hypertension may be:

A. Always absent
B. Always present
C. Present or absent
D. Limited to children

Cardiac output is determined by:

A. Preload and afterload
B. Stroke volume and heart rate
C. Pressure and resistance
D. Systole and diastole

A patient’s stroke volume falls, but heart rate is unchanged. Cardiac output will:

A. Increase
B. Decrease
C. Stay identical
D. Become pressure-independent

Renin is released by the:

A. juxtaglomerular cells
B. mesangial cells
C. macula densa cells
D. proximal tubular cells

Renin release is increased by:

A. high distal sodium delivery
B. increased afferent pressure
C. reduced catecholamine levels
D. low afferent arteriolar pressure

A patient with marked sympathetic activation develops RAAS stimulation. Which trigger can directly promote renin release?

A. increased distal sodium
B. elevated catecholamines
C. increased glomerular pressure
D. increased natriuretic peptides

Low sodium at the distal convoluted tubule most directly promotes:

A. endothelin release
B. CRP synthesis
C. renin secretion
D. nitric oxide release

Renin converts plasma angiotensinogen into:

A. angiotensin II
B. angiotensin I
C. aldosterone
D. bradykinin

Angiotensin I is converted to angiotensin II by:

A. angiotensin-converting enzyme
B. renin
C. aldosterone synthase
D. catechol-O-methyltransferase

ACE is produced mainly by vascular:

A. smooth muscle
B. fibroblasts
C. pericytes
D. endothelium

Angiotensin II raises blood pressure directly by inducing vascular:

A. dilation
B. leakage
C. contraction
D. calcification

Angiotensin II also raises blood pressure by stimulating adrenal secretion of:

A. aldosterone
B. cortisol
C. catecholamines
D. vasopressin

Another blood pressure-raising effect of angiotensin II is increased tubular:

A. potassium secretion
B. calcium reabsorption
C. chloride filtration
D. sodium resorption

Which hormones are released from atrial and ventricular myocardium during volume expansion?

A. catecholamines
B. myocardial natriuretic peptides
C. glucocorticoids
D. endothelins

Myocardial natriuretic peptide release is most strongly triggered by:

A. hypoglycemia
B. hyperkalemia
C. volume expansion
D. acidosis

In renovascular hypertension, renal artery stenosis first causes decreased:

A. renal venous return
B. distal potassium loss
C. adrenal perfusion
D. glomerular flow

The pressure sensed as low in renovascular hypertension is in the glomerular:

A. afferent arteriole
B. efferent arteriole
C. venule
D. basement membrane

Liddle syndrome is caused by a gain-of-function mutation in an epithelial:

A. potassium channel
B. chloride channel
C. sodium channel
D. calcium channel

The sodium channel abnormality in Liddle syndrome increases distal tubular sodium reabsorption in response to:

A. renin
B. aldosterone
C. angiotensin I
D. nitric oxide

Hypertension is associated with which two small-vessel lesions?

A. hyaline and hyperplastic arteriolosclerosis
B. medial calcification and phlebitis
C. vasculitis and aneurysm
D. thrombosis and fibrosis

Hyperplastic arteriolosclerosis occurs classically in:

A. chronic hypotension
B. mild hypertension
C. isolated hyperlipidemia
D. severe hypertension

The wall change in hyperplastic arteriolosclerosis is concentric laminated:

A. calcification
B. ulceration
C. thickening
D. hemorrhage

The classic appearance of hyperplastic arteriolosclerosis is:

A. fatty streaking
B. onion-skinning
C. fibrin splitting
D. lipid vacuolation

The concentric laminations consist mainly of:

A. foam cells
B. endothelial cells
C. fibrin thrombi
D. smooth muscle cells

These laminations are accompanied by thickened reduplicated:

A. basement membrane
B. elastic cartilage
C. intimal collagen
D. lymphatic channels

In malignant hypertension, hyperplastic arteriolosclerosis is accompanied by:

A. cholesterol clefts
B. fibrinoid deposits and necrosis
C. granulomatous inflammation
D. mucin pools

Necrotizing arteriolitis in malignant hypertension is especially prominent in the:

A. liver
B. spleen
C. kidney
D. pancreas

Arteriosclerosis literally means:

A. hardening of arteries
B. arterial inflammation
C. narrowed arterial lumen
D. loss of elasticity

Mönckeberg medial sclerosis is characterized by:

A. intimal cholesterol deposition
B. fibrinoid necrosis
C. medial smooth muscle loss
D. calcification of muscular arteries

Mönckeberg medial sclerosis typically involves the:

A. external elastic lamina
B. tunica adventitia
C. internal elastic membrane
D. venous intima

Atheromas are also called:

A. fibrous nodules
B. atherosclerotic plaques
C. calcific thrombi
D. dissecting hematomas

Atheromas characteristically:

A. protrude into vessel lumens
B. spare large arteries
C. arise in veins
D. remain extramural

Between ages 40 and 60, myocardial infarction incidence increases:

A. twofold
B. threefold
C. fivefold
D. tenfold

Statins lower circulating cholesterol by inhibiting:

A. acyl-CoA oxidase
B. lipoprotein lipase
C. lecithin acyltransferase
D. HMG-CoA reductase

HMG-CoA reductase is the rate-limiting enzyme in hepatic:

A. triglyceride breakdown
B. cholesterol biosynthesis
C. bile acid secretion
D. ketone oxidation

Which process is present throughout all stages of atherogenesis?

A. inflammation
B. calcification
C. vasospasm
D. thrombocytosis

This process is closely linked to plaque formation and rupture in atherosclerosis:

A. fibrosis
B. hyalinosis
C. ectasia
D. inflammation

C-reactive protein is synthesized primarily by the:

A. spleen
B. liver
C. bone marrow
D. vascular endothelium

CRP expression is increased by inflammatory mediators, especially:

A. IL-2
B. IL-10
C. IL-6
D. interferon-γ

Which circulating mediator can directly stimulate vasoconstriction at an atheroma site?

A. Bradykinin
B. Histamine
C. Adrenergic agonists
D. Natriuretic peptides

A ruptured plaque is followed by local vasospasm. Which source can directly provoke vasoconstriction near the atheroma?

A. Platelet contents
B. Red cell membranes
C. Vasa vasorum rupture
D. Venous smooth muscle

Endothelial dysfunction over an atherosclerotic plaque promotes vasoconstriction mainly by reducing secretion of:

A. Endothelin
B. Aldosterone
C. Angiotensin II
D. Nitric oxide

Perivascular inflammatory cells can worsen vasoconstriction at an atheroma by releasing:

A. Surfactant proteins
B. Vasoactive mediators
C. Basement membrane fragments
D. Fibrin degradation products

An aneurysm is best defined as a localized abnormal:

A. Thrombosis
B. Dilation
C. Dissection
D. Calcification

An aneurysm involving an attenuated but intact arterial wall is called a:

A. False aneurysm
B. Dissecting aneurysm
C. Fusiform aneurysm
D. True aneurysm

A vascular wall defect allows blood to collect outside the vessel while still communicating with the lumen. This is a:

A. True aneurysm
B. Pseudo-aneurysm
C. Saccular aneurysm
D. Dissection

Blood enters a defect in the arterial wall and tunnels between wall layers. This process is an arterial:

A. Ectasia
B. Stenosis
C. Fistula
D. Dissection

A spherical outpouching involving only part of a vessel wall is termed:

A. Fusiform aneurysm
B. Saccular aneurysm
C. False aneurysm
D. Traction aneurysm

Which aneurysm type often contains thrombus and involves only a portion of the vessel wall?

A. Saccular aneurysm
B. Fusiform aneurysm
C. Dissecting aneurysm
D. Mycotic aneurysm

A diffuse circumferential dilation of a long vascular segment is called a:

A. Saccular aneurysm
B. False aneurysm
C. Berry aneurysm
D. Fusiform aneurysm

Which aneurysm pattern can involve extensive portions of the aortic arch, abdominal aorta, or iliac arteries?

A. Pseudo-aneurysm
B. Saccular aneurysm
C. Fusiform aneurysm
D. Traumatic aneurysm

A tall young patient with lens problems and aortic root dilation has a connective tissue disorder caused by defective synthesis of:

A. Fibrillin
B. Elastin
C. Type II collagen
D. Fibronectin

In Marfan syndrome, abnormal aortic dilation is linked to aberrant activity of:

A. VEGF
B. TGF-β
C. PDGF
D. FGF

Which syndrome causes aneurysms through mutations in TGF-β receptors?

A. Marfan syndrome
B. Turner syndrome
C. Williams syndrome
D. Loeys-Dietz syndrome

In Loeys-Dietz syndrome, defective vessel support includes impaired synthesis of:

A. Fibrin and laminin
B. Type IV collagen
C. Elastin and collagens I/III
D. Actin and myosin

Weak vessel walls from defective type III collagen are characteristic of vascular:

A. Osteogenesis imperfecta
B. Ehlers-Danlos syndrome
C. Alport syndrome
D. Marfan syndrome

In the vascular form of Ehlers-Danlos syndrome, the defective collagen type is:

A. Type III
B. Type I
C. Type II
D. Type IV

Atherosclerotic thickening of the intima can cause ischemia of the inner media because it:

A. Blocks venous drainage
B. Compresses the adventitia
C. Decreases luminal turbulence
D. Increases diffusion distance

In a large artery with severe intimal plaque, the inner media becomes ischemic primarily because oxygen and nutrients must diffuse a:

A. Lower concentration gradient
B. Shorter radial distance
C. Greater radial distance
D. More alkaline environment

A rare infectious cause of aortic aneurysm in late disease is:

A. Tertiary syphilis
B. Rheumatic fever
C. Lyme disease
D. Tuberculosis

The vascular lesion underlying syphilitic aortic aneurysm is an obliterative:

A. Vasculitis
B. Endarteritis
C. Phlebitis
D. Lymphangitis

Late-stage syphilitic vascular injury shows a predilection for:

A. Large veins
B. Capillaries only
C. Coronary sinusoids
D. Small vessels

The two most important causes of aortic aneurysms are:

A. Diabetes and smoking
B. Syphilis and Marfan syndrome
C. Atherosclerosis and hypertension
D. Vasculitis and thrombosis

A patient with infective endocarditis develops a bacterial aneurysm after septic material lodges in an arterial wall. This aneurysm is best termed:

A. fusiform aneurysm
B. true aneurysm
C. mycotic aneurysm
D. dissecting aneurysm

Mycotic aneurysms can arise from all of the following except:

A. adjacent suppurative spread
B. chronic medial calcification
C. direct arterial infection
D. septic embolization

A vascular infection spreads from a nearby abscess into the arterial wall, producing aneurysmal dilation. This is a classic mechanism for:

A. fusiform aneurysm
B. atherosclerotic AAA
C. false aneurysm
D. mycotic aneurysm

Aneurysms caused by atherosclerosis form most commonly in the:

A. thoracic arch, common iliacs
B. abdominal aorta, common iliacs
C. carotid bifurcations, common iliacs
D. pulmonary trunk, branches

Atherosclerotic abdominal aortic aneurysms are usually located:

A. above renals, above bifurcation
B. below renals, above bifurcation
C. at aortic root
D. below aortic bifurcation

An abdominal aortic aneurysm may take which gross form?

A. saccular or longitudinal
B. fusiform or longitudinal
C. dissecting or fusiform
D. saccular or fusiform

A large AAA may reach a maximum diameter of approximately:

A. 15 cm
B. 5 cm
C. 25 cm
D. 60 cm

The maximal length of an abdominal aortic aneurysm can be approximately:

A. 10 cm
B. 15 cm
C. 25 cm
D. 5 cm

AAA may occasionally involve which arteries by extension or ostial thrombus?

A. renal and mesenteric arteries
B. coronary and bronchial arteries
C. carotid and vertebral arteries
D. femoral and popliteal arteries

AAA may compromise branch vessels because mural thrombi can:

A. rupture the intima
B. calcify the media
C. embolize veins directly
D. occlude vessel ostia

A patient with AAA is also found to have smaller aneurysms in the:

A. coronary arteries
B. iliac arteries
C. renal veins
D. pulmonary arteries

A subset of inflammatory AAA is now linked to:

A. ANCA-associated vasculitis
B. lupus vasculopathy
C. IgG4-related disease
D. giant cell arteritis

A mycotic AAA is best defined as an aneurysm that:

A. contains sterile thrombus
B. arises from hypertension alone
C. forms above renal arteries
D. becomes infected in its wall

In mycotic AAA, suppuration most directly accelerates rupture by destroying the:

A. intima
B. media
C. adventitia
D. vasa vasorum

In general, aneurysms of what size are managed aggressively?

A. 5 cm or larger
B. 2 cm or larger
C. 3 cm or larger
D. any visible aneurysm

Standard aggressive management for large aneurysms classically involves:

A. anticoagulation
B. β-blockers
C. bypass with prosthetic graft
D. venous ligation

In selected patients, AAA can also be treated with:

A. coil embolization only
B. valve replacement
C. endoluminal stent grafts
D. carotid endarterectomy

A thoracic aortic aneurysm compressing the esophagus would most likely cause:

A. difficulty swallowing
B. cough with hemoptysis
C. isolated hoarseness
D. exertional syncope

Thoracic aortic aneurysm can cause persistent cough by compressing the:

A. phrenic nerves
B. vagus trunks
C. sympathetic chain
D. recurrent laryngeal nerves

Thoracic aortic aneurysm may produce cardiac disease by causing:

A. aortic valve dilation
B. tricuspid prolapse
C. mitral stenosis
D. pulmonic atresia

Aortic dissection occurs most commonly in which patient?

A. woman aged 20, lupus
B. child with vasculitis
C. man 40 to 60, hypertension
D. elderly man, diabetes only

Younger adults with aortic dissection often have abnormalities affecting aortic:

A. platelets
B. connective tissue
C. endothelium only
D. coronary flow

More than 90% of dissections in the older major group occur in patients with antecedent:

A. hyperlipidemia
B. smoking history
C. renal infection
D. hypertension

An aortic dissection most commonly begins with which lesion?

A. Medial calcification
B. Adventitial rupture
C. Intimal tear
D. Vasa vasorum thrombosis

In a fortunate case, the dissecting hematoma passes through a second distal tear. What happens next?

A. It thromboses immediately
B. It seals the false lumen
C. It enters the pericardium
D. It reenters the lumen

Reentry of the dissecting hematoma into the aortic lumen through a distal tear creates a:

A. Fusiform aneurysm
B. Double-barreled aorta
C. True vascular shunt
D. Mycotic channel

The most serious complications occur when dissection involves the aorta between the:

A. Aortic valve, distal arch
B. Subclavian artery, bifurcation
C. Renal arteries, iliacs
D. Root, coronary sinus

The more common and more dangerous proximal lesions are called:

A. DeBakey type III
B. Distal dissections
C. False aneurysms
D. Type A dissections

Type A dissections involve:

A. Ascending aorta only
B. Ascending aorta, ± descending
C. Distal aorta ± descending aorta
D. Iliac arteries ± descending aorta

A dissection that begins distal to the subclavian artery and spares the ascending aorta is:

A. DeBakey type II
B. Proximal type A
C. Type B dissection
D. Double-barreled aorta

The classic pain of aortic dissection is best described as:

A. Sudden, excruciating, radiates back
B. Gradual, pleuritic, improves leaning
C. Exertional, pressure-like, substernal
D. Sharp, positional, localized epigastric

As an aortic dissection progresses, the pain classically:

A. Resolves after minutes
B. Remains substernal
C. Becomes pleuritic
D. Moves downward

The pain of aortic dissection may be confused with:

A. Pulmonary embolism
B. Esophageal rupture
C. Myocardial infarction
D. Acute pericarditis

The most common cause of death in aortic dissection is:

A. Renal infarction
B. Rupture into body cavities
C. Coronary thrombosis
D. Mesenteric ischemia