The incidence of heart disease has been steadily increasing as our lifestyles become more sedentary and our diets have changed. There are many causes of heart disease but the condition which most people associate with this disease is the sudden onset of a heart attack. This section will examine the cause of these myocardial infarctions (Doctor speak for heart attack).
The heart muscle is supplied with freshly oxygenated blood from the coronary arteries. Any disruption to this supply will mean that the heart muscle cells do not have enough oxygen and glucose to contract efficiently and can lead to death of the muscle tissue. The severity of a heart attack will depend on the size of the area of muscle cells affected - blockages in larger coronary arteries will affect larger areas of cells.
A recent TV advert showed what a heart attack might feel like.
What can cause these blockages?
The most common cause is a blood clot. A blood clot is called a thrombus and the process of forming a blood clot, thrombosis. If a clot forms in the coronary arteries it is called coronary thrombosis.
The clot blocking the coronary artery may not have formed actually within the heart itself, it may have travelled in the bloodstream from another site. In this situation the clot is called an embolus and the blockage of the coronary artery an embolism.
A process known as atherosclerosis increases the tendency of thrombosis within arteries. Essentially it is a build up of fatty deposits within and a thickening of the wall of the arteries, narrowing the lumen, making the wall less elastic and preventing blood flow thus encouraging the formation of blood clots. Your course requires you to understand the mechanism by which one of these atherosclerotic plaques forms:
- Cholesterol is transported around the body in lipoproteins (LDLs).Excess cholesterol leaks from lipoproteins (LDLs)
- Deposited on arterial walls
- Macrophages (white blood cells) are trapped within cholesterol
- Release free radicals which damage the arterial wall
- Activates blood platelets which stick to damaged areas releasing clotting factors (thromboxanes)
- Forms a plaque which may rupture to produce a thrombus
- Circulating thrombus is called an embolus
- Embolus may lodge elsewhere in the circulation (brain - cause a stroke, coronary arteries - cause a heart attack, lungs - cause a pulmonary embolism)
- NB: healthy arteries produce anti-clotting factors (prostaglandins) → don't form clots
Once again I would recommend looking at an animation of how this process happens or any of the other superb examples you’ll find there.
Prevention of atherosclerosis is therefore essential in prevention of heart disease. Diets rich in cholesterol, chronically high blood pressure or smoking all raise the risk of the formation of atherosclerotic plaques and therefore are risk factors in the development of atherosclerosis - as is being male!
It is worth reminding you that atherosclerosis can happen anywhere in the body and can manifest in many different symptoms. Another effect of atherosclerosis is that it can weaken the artery wall. If this happens then the artery could, when placed under pressure burst or deform. This is called an aneurysm. If this happens in the brain then it will also result in a stroke. Again, it’s worth looking online for images or animations showing the formation of these aneurysms. Some are truly spectacular. Just as the severity of an embolism depends on its location, the severity of an aneurysm is location dependent. An aortic aneurysm would obviously be life threatening! If this weakening is discovered by a doctor, then there are procedures which can help. Stents can be used to support the blood vessel, ensuring that they are not harmed by mechanical stress.
Coronary Heart Disease refers to a condition which develops when the coronary arteries themselves become damaged and narrowed by atherosclerotic plaques. Atherosclerosis causes arteries to become narrowed. This means that:
- More force required to move blood through narrowed vessels
- Blood pressure increases to compensate for this
- Increased blood pressure was an initial cause of atherosclerosis so it could result in further damage.
Development of atherosclerosis can lead to the development of angina:
- ↑exercise leads to ↑oxygen requirements by heart
- Narrowed arteries prevent more blood to pass through
- Shortage of blood to heart muscle causes chest pain
- Cells do not die as some blood can still pass through
- Pain only occurs during activity but not at rest
Heart failure is caused by the prolonged blockage of a coronary artery which causes damage to heart muscle. Frequent heart attacks will cause this damage, but there are many other causes of heart failure. The damage to the heart muscle results in a decrease in the number of contractions / a reduced cardiac output / lower pressure generated / less blood leaves heart. This means that more blood is stored on the right side of the heart and in the veins leading to swelling and enlargement of the heart and liver. At this point, a patient’s only option would be a heart transplant.
QUESTION: Why would healthcare professionals think it a good idea for people to take aspirin regularly? Note: you may have to do some research to find out the effect of aspirin on the circulatory system.
Prevention of Heart Disease
Heart disease, as mentioned is a condition which has a strong link to the way we chose to lead our lives. Diet, exercise and stress levels have all been shown to have a direct impact on the development of heart disease.
Cholesterol is needed for many things in the human body including:
- Vitamin D production in skin
- Sex hormone production in gonads and adrenal glands
- Making cell membranes
- Produce bile acid (salts)
It has properties similar to fats → soft, waxy, and insoluble (difficult to remove if deposits form). As mentioned before, it is transported in blood from liver to tissues. It is carried by lipoproteins (soluble fatty proteins which wrap themselves around cholesterol).
There are two different forms of lipoprotein, LDL (Low Density Lipoprotein) and HDL (High Density Lipoprotein) which have slightly differing roles.
- Carries cholesterol from liver to tissues
- Normally, some cholesterol 'leaks' from the lipoprotein and is absorbed to build cell membranes
- Excess LDL/cholesterol → too much cholesterol leaks out and causes atherosclerosis
- High density lipoprotein
- Picks up cholesterol from arterial walls and carries it away from tissues
- Travels to liver where cholesterol is removed with bile
- A diet low in cholesterol is therefore recommended for anyone concerned about heart disease.
How Can Smoking Lead to Heart Disease?
- Nicotine constricts arteries causing platelets to stick together → vasoconstriction → heart must work harder to force blood through → increases BP
- ↑BP causes damage to blood vessel lining / endothelium / collagen
- Leads to rise on blood platelets and makes them more sticky / form a plug / adhere to collagen fibres
- Release of thromboplastin/thrombokinase
- Fibrinogen converted to insoluble fibrin
- Platelet plug trapped by fibrin mesh
- Raises conc. of fibrinogen (in blood) → increased risk of clotting
- ↑LDL causes more cholesterol to leak out in blood
- Carbon monoxide reduces the efficiency of the blood in terms of carrying oxygen
- Haemoglobin combines with CO more readily than with oxygen → forms carboxyheamoglobin
- Associated with plaque formation
- Principle CHD = heart muscle receives inadequate amount of blood or oxygen/(coronary) blood supply reduced
Treatment of Heart Disease
- Beta blockers reduce heart rate and reduce oxygen required by heart
- Aspirin prevents blood clotting and thrombosis formation
- ACE inhibitors stabilize plaques → prevent thrombus to break off
- Statins reduce LDL and increase HDL
- Deflated balloon-like device is passed up to the heart via the aorta
- Guided into damaged coronary artery and inflated to stretch the artery
- Heart by-pass graft
- Leg veins and arteries from chest are used to by-pass the blocked region of the coronary artery
- Involves open heart surgery
- Reperfusion therapy after a myocardial infarction
- Angioplasty done within 90 minutes of onset of chest pain
- May prevent irreversible damage to the heart muscle