Flashcards in Lecture 42: Pathophysiology of Myocardial Ischemia and Infarction Deck (73):
What are the general principles of myocardial ischemia?
Imbalance between oxygen supply and demand due to impaired or inadequate perfusion
ii. accumulation of waste products
Not the same as anoxia, which is oxygen deprivation with normal perfusion (so byproducts can be removed for anaerobic metabolism)
Oxygen demands of the myocardium are dynamic
What happens when you decrease metabolic activity?
It is possible to have complete interruption of blood flow for extended periods of time without ischemia
That’s why heart is dynamic (because you would get ischemia instantly upon vigorous exercise)
What is anoxia?
When there is enough blood being supplied to muscle but there is no oxygen in that blood!
Less detrimental because at least the toxic byproducts are washed out lol
What dictates myocardial oxygen demand?
1. heart rate
2. systolic wall tension
3. myocardial contractility
What are anti-pyretics?
Drugs that reduce fever such as ibuprofen
What is wall tension determined by?
Tension = pressure x radius/wall thickness
How can wall tension be raised?
Disease states that elevate LVEDVolumes
And increase afterload like HTN and aortic stenosis
What dictates myocardial oxygen supply?
1. Coronary blood flow
2. diastolic perfusion pressure
3. coronary vascular resistance
4. Oxygen carrying capacity of blood
5. hemoglobin concentration and oxygen saturation
What is the significance of diastole to coronary blood flow?
This is when there is the greatest pressure gradient towards perfusing the coronaries
What can alter coronary blood flow?
When there is a change in the diastolic phase of the cardiac cycle
Decreasing diastole through tachycardia or AR = less CBF
Atherosclerosis that leads to stenotic coronary arteries (>80%)
Coronary spasm or platelet aggregation occurs at stenosis
How does an imbalance between demand and supply come about? Significance?
Significance = myocardial ischemia
What is the relationship between CBF and myocardial O2 consumption?
CBF increases in proportion to myocardial oxygen consumption
What is reactive hyperemia? Significance?
An adaptation of the blood vessels to reduce resistance of vascular bed by releasing substances like prostacyclin, adenosine, NO and acetylcholine
This effect is reduced in those with moderate CAD
What is an impairment of coronary flow rate?
When the difference between max CBF and rest CBF starts to fall off
Less reactive hyperemia = less ability to maintain luminal area
What are the effects of ischemia on glycolysis?
FFAs account for 60-90% of myocardial energy
Toxic TG builds up during anaeraobic metabolism since it has detergent like properties
Lactic acid builds up to inhibit glycolysis
Without enough ATP (<30%), irreversible injury occurs to sarcolemma, results in cell death, sodium accumulation and calcium depletion
What layer is most vulnerable to ischemia?
Intramural compressive forces increase the resistance in subendocardium
-more resistance = less flow
Autoregulation is better in epicardium
What are the electrophysiologic effects of ischemia?
i. resting membrane potential (extracellular K)
ii. action 4 upstroke
iii. AP amplitude and duration
iv. conduction velocity
and reduction in production of ATP and creatine phosphate (notes pg 3)
protein synthesis and protein degradation is also suppressed
Arrhythmias because ion channels and transport is fucked up
What is the mechanism for sarcolemma disruption in ischemia?
Ca accumulation lead to worsening of contractility and excitability
What metabolites are not washed out during myocardial ischemia? Significance?
Long-chain acyl carnitine
They cause arrhythmogenesis and myocardial depression
Acyl carnitine and lysophospholipids are detergent like and can render sarcolemmal ion channels dysfunciotnal (top of pg 5)
What does myocardial ischemia do to the FFA pool for oxidation/energy?
It prevents FFA from being esterified so they can leave the cell
Less FFA pool means that after an ischemic insult, there is prolonged recovery because so much of the energy of the heart is derived from FFA
What are the effects of ischemia on contractile function?
1. Impairs active relaxation in early diastole
2. causes regional stiffness (decreased compliance) shifting P-V relation up/sleft
3. very sensitive early measure of ischemia
Curve is shifted up
Fucks with both systolic contraction and diastolic relaxation
Can lead to heart failure and cardiogenic shock if can contract and relax sufficiently
What are the effects of ischemia on systolic function?
Contraction decreases proportionately to the decrease in flow
Mechanism involves interference with Ca release, binding to troponin or impaired actin/myosin interaction
Effects = dyskinesis in central zone and hypokinesis/akinesis in adjacent areas
Compensatory mechanisms = hyperkinesis through adrenergic stimulation and Starling mechanism
What is the mechanism by which ischemia causes contractile dysfunction?
May involve interference with systolic calcium binding to troponin
What is the mechanism by which ischemia causes diastolic dysfunction?
Early part of diastole is isovolumic relaxation, an ACTIVE energy requiring pahse
But if not enough ATP then you are less compliant and you have greater pressure in the ventricle
What is the definition of infarction?
Prolonged ischemia that leads to irreversible contractile dysfunction
What is the definition of stunning?
When acute ischemia with reperfusion causes prolonged contractile dysfunction
Transient instead of permanent
i. Accumulation of phosphate and hydrogen which depress myocardial contractility (because these are the accumulated toxins)
ii. altered calcium uptake
iii. increased neutrophil-derived free radicals
What is hibernation?
Chronic hypoperfusion causing contractile dysfunction
Still reversible with reperfusion
Chronic state of stunning
Occurs with severe CAD impairing resting CBF
What is angina?
Chest discomfort produced by ischemia
Not known why but thought to be due to
-lactate, bradykinin byproducts on cervicothoracic receptors
Chest tightness radiating to left arm, neck and jaw
What is stable angina?
Chronic, transient, demand-related
Reproducible at a greater workload/when your body performs vigorous exercise
Resolves after stressor is removed
What is unstable angina?
1. new onset angina
2. angina occurring shortly after MI
3. angina increasing in frequency, duration or severity
4. angina occurring at lower workload
5. angina at rest
What is variant or Printzmetal’s angina?
When vasospasm causes abrupt reduction to myocardial blood flow
Occurs in the morning and at rest (conditions when perfusion should be normal)
What is silent angina?
Increased in transplant, diabetes
Detected with ECG and stress test
How is ischemia diagnosed?
When there is angina an consistent changes in ST and T waves for a patient (downward ST segment)
What are the symptoms of myocardial ischemia?
What are the signs of myocardial ischemia?
What are the lab tests for myocardial ischemia?
4. Stress testing
What is the significance of the ST segment in diagnosing myocard ischemia?
ST segment reflects period of relative inactivity between systole and repolarization
Ischemic cells have decreased resting membrane potential with current flowing from normal myocardium into ischemic zone
Results in ST segment DEPRESSION in leads opposite the area of ischemia
ST elevation is seen in myocardial infarction rather htan ischemia
What are the principles to treating myocardial ischemia?
1. Reduce demand
2. Improve supply
How do you reduce myocardial demand?
1. nitrates (reduce LVEDP so decrease wall tension, vasodilate coronaries)
2. Beta blockers (reduce HR, contractility and BP, prolongation of diastole) … firstline treatment for ischemia
3. Ca channel blockers (reduce preload, afterload, HR, BP, contractility)
Reducing preload = less contractility because of FrankStarling relationship
What is the first-line treatment for ischemia?
Lowering contractility = less demand
Lowering HR = more diastolic filling = more CBF
How do you reduce demand in the heart?
Calcium channel blockers (verapamil, diltiazem)
All three reduce contractility
Nitrates reduce contractility because it reduces LVEDP or preload
How do you improve supply in heart (opening up the lumen)?
1. anti-platelet agents like aspirin or clopidogrel
2. anti-coagulants like heparin
3. Percutaneous coronary intervention (PCI)
4. Stent implantation (bare-metal or drug0eluting)
5. coronary artery bypass graft surgery (CABG)
What are the consequences of plaque rupture?
1. unstable angina
3. ischemic stroke
What is the MoA of heparin?
Binds to antithrombin III (AT)
AT then inactivates thrombin
And factor Xa
Also binds thrombin into a
Complex that inactivates the mofo
Effect is SIZE dependent
What is the pathophysiology of MI?
Often proceeded by unstable angina (aka acute coronary syndrome)
Plaque rupture due to macrophage derived metalloproteinases degrading the fibrous cap
What is the role of metalloproteinases to plaque rupture?
Degrades the fibrous cap
Released by foam cells (according to notes pg 10)
Can MI occur in absence of atherosclerosis?
Yes 10% of time
Can be due to coronary embolism, severe vasospasm trauma or extreme increases in myocardial O2 demand
How many patients progress to MI with unstable angina?
What is acute coronary syndrome?
An umbrella term that refers to situations where blood supplied to the heart is suddenly blocked
Examples: unstable angina and MI
What is MI?
Irreversible heart muscle necrosis resulting from prolonged ischemia
What are the factors that affect the extent of MI damage?
Location of plaque
Size of vasc bed
Collaterals, local tissue factors
How do you diagnose MI?
ECG abnormalities (ST elevation, loss of R wave and new Q wave)
Elevation in myocardial proteins such as:
i. creatine kinase
iv. troponin T and I
What are enzymes that indicate MI?
4. Troponin T and I
What is the significance of creatine kinase in blood?
Shows patient just had MI
What is the difference between myocardia ischemia and myocardial infarction presentation?
Ischemia = ST depression
Infarction = ST elevation
-current is flowing out of the infarct
What are the ECG changes of MI?
What are the arrhythmic complications of MI?
1. sinus tachycardia (due to pain, anxiety or shock)
2. sinus bradycardia (when there is RCA involvement)
-brady due to vagal efferents near RCA
3. atrial flutter and fibrillation
4. isolated ventricular premature beats
5. conduction block (all the types of bundle branch block lol)
What are the LV dysfunction complications of MI?
Acute diastolic dysfunction which leads to pulmonary edema
Systolic dysfunction (systemic hypoperfusion)
-extensive myonecrosis can cause shock, positive feedback loop with lower BP and then Lower CBF which then leads to more myonecrosis, etc.
Pericarditis due to transmural infarct or immulogic response (Dressler’s syndrome)
Ventricular free wall rupture
Ventricular septum rupture (VSD)
Rupture of papillary muscle
What is oliguria?
Low output of urine
What is the relationship of pericarditis and MI?
Can occur in 25% of patients following MI
Commonly follow anterior infarcts and majority are reabsorbed spontaneous
Friction rubs are heard 48 hours since infarction
What is Dressler’s syndrome?
An immunologic cause of post-infarction pericarditis
Manifest as a localized fibrinous pericarditis
Occurs 2-10 weeks psot MI
Distinct from early pericarditis without immunologic cause
Treated with corticosteroids and aspirin
What are the mechanical complications of MI?
1. VSD (due to anterior MI)
2. free wall rupture (anterior/inferior MI)
-highest mortality >95%
3. papillary muscle rupture (inferior MI)
-you see a V wave
May be predisposed by use of NSAIDs and corticosteroids
What are the characteristics of free wall rupture?
Occurs in elderly female or hypertensive patients
Transmural infarction in LAD is risk factor
Site is usually border between normal and infarcted myocardium
Causes hemopericardium and tamponade
May form a pseudoaneurysm if rupture is subacute
What are the characteristics of VSD?
Less catastrophic than free wall rupture
Can result in a serpiginous tract
Murmur is heard
What is a serpiginous tract?
A descriptive term for a twisted, vermiform radiolumency surrounded by a sclerotic rim, seen in bones or infarction
Accompanied by intramedullary calcification
What are the characteristics of papillary muscle rupture?
Results in acute MR
Occurs in relatively small infarcts (compared to the other mechanical failures from bigger infarcts)
Murmur is heard
How do aneurysms form in heart after MI?
These are areas that are dyskinetic (because already infarcted)
Started to bulge out due to pressure increase
But cannot prevent bulge because there is not contractile function!
Other areas of muscle are hyperkinetic, which push more pressure to that area of dyskinesis
What are the key characteristics of LV remodeling?
Structural alterations during healing
-consists of expansion in infarcted region due to loss of myocytes and aneurysm
-expansion of infarcted region can lead to aneurysm
-compensatory dilation/hypertrophy elsewhere
LV hypertrophy is inadequate which leads to progressive dilation, wall stress elevation, CHF and ultimately sudden death lol
ACE inhibitors favorably alter the remodeling process
What is the treatment of MI?
PCI or stent
Beta blockers to reduce infarct size (reducing demand)
Oxygen and morphine
Same shit, reduce demand and increase supply (as before)
What does streptokinase do?
Lyses clots by converting plasminogen to plasmin, something that lyses thrombin in clots
What is used to treat POST MI patient?
1. aspirin, clopidogrel and warfarin
2. beta-blockers (decrease demand)
3. ACE inhibitors and implantable cardioverter defibrillator
5. smoking cessation
6. nutrition counseling
8. control of HTN and diabetes
What are the 4 thrombolytic agents approved for MI use?
1. anisoylated plasminogen streptokinase activator complex (APSAC)
2. recombinant tissue-type plasminogen activater (tPA and rPA)