7 Ischemia & Acute Coronary Syndromes

ischemia results from reduction of blood flow from a combination of

fixed vessel narrowing

abnormal vascular tone

what is poiseuille’s law?

Q (flow) = deltaP(pi)(r^4) / 8nL

P : pressure

r : radius

n : viscosity

L : length

how does pressure affect flow?

greater pressure difference = greater flow

how does radius affect flow?

larger radius = larger flow ; really affects flow because it is to the 4th power

how is viscosity related to flow?

inversely related; more viscosity less flow

how is vessel length related to flow?

inversely related; increased vessel length less flow

how do vessels physiologically respond to decreased blood flow?

compensatory vasodilation of the distal vessels

what occurs in stenosis of less than 60%?

blood flow is not affected because of compensatory vasodilation

what occurs in stenosis of more than 70%

normal flow at rest; maximal blood flow reduced (so symptoms can appear when going to gym or going up stairs)

what occurs in stenosis of more than 90%

ischemia can develop at rest

how does endothelial cell dysfunction contribute to ischemia?

inappropriate vasoconstriction of coronary arteries

loss of normal antithrombotic properties (prone to thrombosis)

how does atherosclerosis affect vasoconstriction?

normally with physical activity and mentral stress → vasodilation (overtakes sympathetic influence)

in atherosclerosis there is impaired release of vasodilators → sympathetics are now unopposed and there is vasoconstriction

besides atherosclerosis what are other causes of myocardial ischemia?

hypotension (low blood pressure - not enough blood volume)

decreased blood oxygen content (anemia)

massive internal bleeding (less blood supply going to heart)

what is stunned myocardium?

transient (temp) ischemia without necrosis; systolic dysfunction

effects of ischemia reversible; contractile function recovers

what is hibernating myocardium?

persistent reduction in blood flow resulting in CHRONIC ventricular contractile dysfunction

can recover with revascularization

hibernating myocardium is usually from

multivessel CAD

what is the levine sign?

clenched fist

what are the signs of ischemia?

tightening/ increased pressure

levine sign (clenched fist)

what are the symptoms of ischemia?

tachycardia

diaphoresis

nausea

SOB

what is the gold standard in diagnosing CAD?

coronary angiography

what is nuclear imaging stress test?

radioactive dye injected that causes blood flow to be highlighted; decreased brightness when stressed

what is the first line of treatment for ischemia?

decrease risk factors such s smoking, alcohol, diet, exercise

how is acute angina treated medically?

sublingual nitroglycerin

what does nitroglycerin do?

promotes venodilation → reduces venous return (lower pressure in veins) → decreases cardiac output → decreases strain to pump large volumes of blood → decreases wall stress → decreases myocardial oxygen demand

dilates coronary arteries → increases coronary blood flow

what medical treatments are used to prevent recurrent ischemic episodes?

organic nitrates

Beta blockers

calcium channel blockers

what do beta blockers do?

reduce myocardial oxygen demand by decreasing force in ventricular contraction and heart rate

what medical treatments are used to prevent acute cardiac events?

anti-platelet therapy → antithrombotic properties

lipid regulating therapy (statins) → decreases lipid LDL to decrease vascular inflammation

angiotensin-converting enzyme (ACE) inhibitors → treats hypertension

what happens if medicine does not work to treat ischemia?

mechanical revascularization methods such as

percutaneous coronary intervention (PCI) → balloon tipped catheter

coronary artery bypass graft (CABG)

what is acute coronary syndrome?

ACS is an umbrella term for situatiosn where there is sudden decreased blood supply to the heart

type of ACS depends on

degree of coronary obstruction associated with ischemia (transient or permanent)

ACS is usually diagnosed as

partially occlusive thrombus

complete obstruction

what is partially occlusive thrombus? aka?

unstable angina of angina pectoris

aka non-ST elevation myocardial infarction (NSTEMI)

what is complete obstruction called?

ST-elevation myocardial infarction (STEMI)

what kind of thrombus is seen in unstable angina? myocyte necrosis?

partially occlusive thrombus

no necrosis

what kind of thrombus is seen in NSTEMI? myocyte necrosis?

partially occlusive thrombus

yes necrosis

what kind of thrombus is seen in STEMI? myocyte necrosis?

completely occlusive thrombus

yes necrosis

know

what is the difference between a STEMI and NSTEMI?

STEMI : vessel is completely blocked

NSTEMI : vessel is partially blocked

how can NSTEMI show up on EKG?

ST depression

T inversion

what are the causes of acute coronary syndrome?

90% of cases are secondary to plaque rupture and subsequent thrombosis

what are the effects of ACS?

ST changes on EKG

angina/unstable angina

+/- biomarkers detected in blood

what are the biomarkers used to detect ACS?

cardiac specific troponin

creatine kinase MB isoenzyme

myoglobulin

what is the most commonly used biomarker in diagnosis ACS? why

cardiac specific troponin because up to 80% of pts with acute MI will have elevated troponin within 2-3 hours of ED arrival

what are other causes of elevated troponin?

renal failure/insufficiency

muscular trauma

pulmonary embolism

congestive heart failure

severe septicemia

KNOW THIS

what are nonatherosclerotic causes of acute MI?

coronary emboli from mechanical/infected cardiac valves

inflammation from acute vasculitis

connective tissue disordes

cocaine abuse

what are the functional changes secondary to ACS?

systolic dysfunction (impaired ventricular contraction)

diastolic dysfunction

ventricular remodeling

what happens with systolic dysfunction?

• decreased ejection fraction and stroke volume

• increases systolic diameter and volume

• increased wall stress

• increased myocardial demand

what happens with diastolic dysfunction

ventricular relaxartion is an active process and is energy dependent so when ischemia/infarction occurs there is a reduction in myocardial compliance

what happens with decreased compliance?

increased diastolic pressure

decreased myocardial perfusion/supply

what is ventricular remodeling?

changes in size, shape, structure and physiology of heart after injury to mycoardium

loss of effective contractile myocytes causes

wall motion abnormalities

define hypokinetic, akinetic and dyskinetic

hypokinetic : segment of decreased contraction

akinetic : segment that does not contract

dyskinetic : segment that bulges outward during contraction