Lecture 9: Myocardial infarction and angina Flashcards
What is the coronary circulation?
- The circulation of blood that supplies the heart muscle (myocardium) with O2 and nutrients
- Coronary arteries: arise from root of aorta, supply O2-rich blood to heart muscle, main arteries on the surface smaller penetrate into muscle
- Coronary veins: collects deoxygenated blood after utilised by the myocardium, drain into the coronary sinus which empties into the right atrium
- Only 1/10 of mm of endocardial surface can obtain nutrients from blood in chambers
Why is there phasic blood flow through the coronary circulation?
- Little flow of blood during systole to the heart
- During exercise, heart is in systole
- When you need more blood supply to the heart, you are effectively reducing it → getting more time n vessels are compressed
How is venous drainage primarily facilitated in the heart?
- Venous drainage in the heart is primarily accomplished via the coronary sinus, which empties into the right atrium.
- The return of blood from the heart muscle is aided by small anterior cardiac veins.
- Major venous vessels are located within the epicardial fat layer, lying superficially to their arterial counterparts.
- Blood pathway: aorta → right coronary artery → cardiac veins → coronary sinus → right atrium
- While the heart contracts, it can pose a challenge as the squeezing action may compress or shut blood vessels.
What factors can lead to a reduction in coronary blood flow?
- Reduction in the diastolic interval, particularly during exercise when the heart rate is increased.
- Increase in ventricular end-diastolic pressure, which occurs when the ventricles are filled with blood from the left atrium. This increased pressure makes it more challenging for blood to be pushed through the coronary arteries.
- Fall in arterial pressure, which can further compromise coronary blood flow.
What is the resting coronary blood flow, and how does it relate to cardiac output?
The resting coronary blood flow is approximately 225 ml/min, which constitutes about 4-5% of the cardiac output.
What percentage of oxygen is typically extracted from the blood as it flows through the heart?
Around 70% of the oxygen carried by blood is removed as it flows through the heart.
How does the body respond to increased oxygen requirements in the heart?
Coronary blood flow is augmented by dilating the coronary arteries.
What is the major cause of ischemic heart disease?
Atherosclerosis
What are some factors contributing to atherosclerosis?
Genetic predisposition
Excessive cholesterol intake
Sedentary lifestyle
What happens during atherosclerosis?
- Cholesterol is deposited beneath the endothelium of arteries
- Arteries are invaded by fibrous tissue and calcified
- Atherosclerotic plaques form, protruding into the arterial lumen and partially or completely blocking blood flow.
What is a myocardial infarction?
- Occlusive thrombus
- Complete occlusion of coronary artery
- Blood clot attached to plaque completely occludes the blood vessel, resulting in a heart attack.
What is stable angina?
Caused by plaque causing partial occlusion of a coronary artery.
What is unstable angina?
- Caused by a combination of plaque and thrombus
- Partially occlusive thrombus allowing some blood flow
- Can occur periodically.
What is a myocardial infarction?
- Also known as a heart attack.
- Occurs due to the sudden blockage of blood flow to a part of the myocardium (heart muscle).
- Complete occlusion of a blood vessel leads to ischemia (loss of blood supply and nutrients) and subsequent necrosis (cell tissue death).
- It is the most common cause of morbidity and mortality, with approximately 123,000 heart attacks occurring per year.
What are the mechanisms that can lead to plaque rupture and subsequent occlusion in coronary artery disease?
- Plaque ruptures are more likely to occur when there is a thin fibrous cap.
- Plaque rupture can be triggered by factors such as stress, exertion, or a large meal.
- Hemorrhage within the plaque → blockage of the vessel lumen and subsequent occlusion.
- Release of tissue factor from the plaque → activate the coagulation cascade → occlusion.
- Exposure of collagen underneath the plaque, combined with turbulent blood flow, can activate platelets and the coagulation cascade, resulting in occlusion.
- Dysfunction of the endothelium can lead to impaired vasodilation, increased vasoconstriction, and ultimately, occlusion.
What are the consequences of blood vessel occlusion in myocardial infarction?
- Occlusion of blood vessels leads to a cessation of blood flow.
- Collateral blood flow may develop to compensate for the occlusion.
- Local dilation and collateral flow can result in overfilling with stagnant blood.
- Depletion of oxygen leads to deoxygenated hemoglobin, causing affected areas to appear blue or brown.
- Highly permeable vessel walls lead to fluid leakage, resulting in edema in the affected area.
- Muscle cells swell due to diminished cellular metabolism.
- Without a blood supply, cardiac muscle cells can die within approximately 20 minutes.
What are the cellular consequences of ischemia during myocardial infarction?
- Ischemia results in a lack of oxygen and glucose supply to the affected tissues.
- ATP levels decrease due to the inability to undergo glycolysis and oxidative phosphorylation.
- Impaired Na, K ATPase function leads to decreased membrane potential, causing depolarization and potentially resulting in arrhythmias.
- Intracellular edema occurs due to ion imbalance, leading to cell death.
- Increased intracellular calcium levels activate proteases and lipases, contributing to cell death.
- Anaerobic metabolism leads to an increase in intracellular hydrogen ions and protein denaturation.
What is collateral circulation in the context of cardiovascular health?
- Development of new blood vessels in response to blocked or narrowed arteries.
- It occurs when there is partial occlusion in a blood vessel, leading to the growth of heart vessels around the occlusion.
- These collateral vessels slowly narrow due to conditions like atherosclerosis but provide an alternative pathway for blood to reach tissues.
- Collateral vessels may reroute blood flow around obstructions, ensuring some level of blood supply to the affected area.
- This process involves the formation of branches between veins and arteries, gradually building up around atherosclerotic plaques.
Causes of death in myocardial infarction
○ Decreased in cardiac output (cardiac shock)
§ Can’t supply enough blood to the organs
○ Pulmonary oedema
§ Fluid in the lungs
○ Ventricular fibrillation
§ Completely chaotic rhythm in the heart -> no ejection of blood
* Occasionally
○ Heart rupture
Wall thins -> blood flows out [hole in heart]
What is cardiac shock, and what are its potential causes and consequences?
- Cardiac shock occurs when there is insufficient force to pump blood into the peripheral arterial tree.
- CAUSE: Systolic stretch, where the heart wall bulges out due to thinness or damage, impairing its ability to contract effectively.
- EFFECT: severe hypotension, with systolic blood pressure dropping below 90 mm Hg, resulting in the death of peripheral tissues.
- Decreased cardiac perfusion → increased ischemia → perfusion of the heart itself down the coronary artery.
- In severe cases, blood may accumulate in the pericardial space, leading to cardiac tamponade.
- The mortality rate for cardiac shock is high, estimated at around 70%.
What is pulmonary oedema?
- Buildup of fluid in the lungs
- Reduced systemic blood circulation -> blood pools in the atria n in blood vessels of lungs -> can’t breathe
- Less reabsorption n more filtration
- Increased capillary pressure in the lungs
- Little difficulty in first few
- But reduced circulation to kidney -> reduced urine output -> increased total blood volume
- Congestive symptoms
What is cardiac arrhythmia, and why is it particularly dangerous following a myocardial infarction?
- Cardiac arrhythmia refers to abnormal heart rhythms, which can be dangerous, especially after a myocardial infarction.
- It can cause changes in the ECG and electrical properties of the heart, potentially leading to ventricular fibrillation.
- Several factors contribute to arrhythmias post-myocardial infarction:
- Potassium depletion [inadequate perfusion]
- Injury current generated by dying cells → depolarization.
- Sympathetic reflexes triggered by a fall in blood pressure → activating the sympathetic nervous system and altering the heart’s electrical properties.
- Muscle weakness and ventricular dilation → abnormal conduction pathways around the infarcted area.
How is myocardial infarction diagnosed?
- History
○ Heavy, crushing, chest, pain radiating down arm but some Mis are silent- Unrelated to exercise
○ Pain exists after exertion - Associated w nausea, vomiting, sweating
- ECG changes
Biochemical markers
- Unrelated to exercise
We cannot normally feel our heart, so why do we feel pain with a MI?
- Ischemia of cardiac muscle can cause severe pain
- Causes unclear, probably release of acidic substances lactic acid or pain mediators
- Histamine, kinins, proteolytic enzymes
- RESULT: Stimulate nerve endings in cardiac muscle