For the sake of your
have seen how the heart works relentlessly and tirelessly day and
night for which it needs oxygen and energy in the form of glucose and
fatty acids supplied by the blood. The blood is delivered to the
cardiac tissue by the coronary arteries. If for some reason the oxygen
supplied is not enough and the perfusion is not adequate, this results
in imbalance between oxygen supply and demand. This is known as
ischemia (derived from the combination of Greek words ischeia
(deficiency) and haima (blood). The commonest cause of ischemia
of the heart is the disease process — atherosclerosis of the
coronary arteries. This is also the process which results in strokes,
other ischemic injuries of the brain, claudication or gangrene of the
limbs and can damage other organs like kidneys, intestines, eyes etc.
Arteries are clogged by this process. We will devote some time to
understanding, how this process develops and the various factors
responsible for this. Although in majority of cases this process is
responsible for producing ischemic heart disease or coronary artery
disease, but it needs to be mentioned that this can result from other
rare causes. These causes include the following situations if a small
clout gets detached somewhere and lodges in one of the coronary
arteries (emboli), if the muscles of the coronary arteries go in spasm
producing narrowing of the lumen or congenital abnormalities of the
coronary arteries producing ischemia and even infarction right in
infancy. Myocardial ischemia may also occur when the cause may not lie
in the coronary arteries but when the demand of oxygen by the heart is
disproportionately high. This can occur in the case of severe anemia
or abnormal variety of haemoglobin, low oxygen in the atmosphere as in
the case of very high altitude and abnormal thickness of the cardiac
muscles resulting from various diseases like high blood pressure or
narrowing of aortic valve of the heart.
Scientists have been busy during the last 50 years trying to unravel the mysteries of this process which kills and disables people in the West and as mentioned earlier is expected to be No. 1 killer at the global level in the next 10 years. The process develops in the human body over a period of many years, usually many decades. It perhaps is not a continuous process but occurs in sporadic manner both in space as well as in time. It does not involve the entire length of arteries uniformally but is focal in character and usually has a predilection for certain areas and less for others. The origin of the arteries and point of bifurcation or any other point where there is likelihood of turbulence in the flow of the blood are more prone to atherosclerosis. As mentioned, time and age are important factors in the development of atherosclerosis. Therefore, it was not clear whether it was entirely a function of time and thus a part of ageing process. We are now quite clear that this is not so although we know this would certainly increase with the age and is also more common in men. Post-mortem studies in young people in traumatic deaths, specially during Korean and Vietnamese wars showed that atherosclerotic streaks were present in young people as young as 20 years old. And now we know that given predisposing circumstances, atherosclerosis manifests as early as the second or third decades of one’s life.
Effects of ischemia
Lack of appropriate perfusion to a part of the heart can produce biochemical, mechanical and electrical disturbances. Once the oxygen supply falls, the heart is unable to get the fuel supply and the energy it requires to function. That part of the heart muscle undergoes necrosis or the death of the cell and such an area is known as an infarct. Ischemia may be reversible if the obstruction is incomplete and the time is less than 15-20 minutes. If the obstruction of the blood is complete or is more than 20 minutes, the damage will be irreversible. Once certain portion of the muscle gets damaged, the mechanical effect leads to abnormal contraction of the wall of the heart and abnormal functions of the valves of the heart. It can be still more dangerous if the electrical function of the heart are affected resulting in irregular or more dangerous kind of heart rhythm which can kill instantly.
As mentioned above there are certain risk factors which predispose the production of atherosclerosis and therein lies the reasons for understanding process of this disease with the aim of modifying the risk factors. The knowledge about the risk factors is based on the very systematic and long-standing studies carried out in experiments on animals and observations and systematic studies in various communities. In well known studies carried out in a small town, Framingham near Boston, USA, started in 1948, have yielded wealth of material on this subject. The basic risk factors can be divided into those which are beyond control and those which can be modified. The unmodifiable risk factors include age, male gender and genetic predisposition. There is not much we can do about these three unless the growth of knowledge in human genome enables future parents to select a custom made off-spring. But the risk factors which can be modified by lifestyle changes are smoking, obesity, physical activity and emotional imbalance. Risk factors which can be modified by pharmaco-therapy include lipid (fat) disorders, hypertension, diabetes mellitus and insulin resistance.
Obesity: Many studies have brought out that obesity is linked to an early development of hypertension, ischemic heart disease, strokes and a host of other medical complications. The USA Centre for Disease Control and Prevention has declared this to be an epidemic in the USA where it has increased by 60 per cent in the last decade. Prevention and treatment of these is obviously desirable.
Smoking: Smoking is harmful not only to the smoker himself, but also to the people around, called passive smokers. The chemicals in the smoke, including nicotine, produce abnormalities in the function of the endothelial cells and reduce blood flow and promote formation of clots. According to the American Heart Association, the risk of heart disease increases by more than 50 per cent for a smoker and by more than 30 per cent for a passive smoker. This risk factor is eminently modifiable and should be promoted in all the communities.
Physical activity: Physical activity is important for good health. It reduces obesity and blood pressure, decreases insulin resistance as also cholesterol levels. It promotes emotional well-being and stability of the cardiac functions. A minimum of 30-40 minutes of moderate physical activity in the form of brisk walk or a game is desirable.
Hypertension: Hypertension is a risk factor in atherosclerosis. In the famous Framingham studies, mentioned above, the incidence of ischemic heart disease, strokes, as well as the ischemia of the limbs was significantly higher in persons who had hypertension as compared to those who did not have it. Hypertension is treatable by pharmacological means and lifestyle changes as well.
Diabetes mellitus and insulin resistance: Diabetes mellitus is a very serious problem and one of the very important risk factors in atherosclerosis because of the presence of abnormal lipid in the blood. Diabetes is often associated with high LDL cholesterol and tryglycerides which are atherogenic. Control of glucose level and cholesterol level by lifestyle changes and drugs is now possible.
Lipid disordes: The role of various lipids consisting of various fractions of cholesterol and its combination with proteins (lipoproteins) has been the subject of intensive ongoing research which started 50 years ago. The experiments on animals and the observations in human beings have left us in no doubt that abnormal lipid levels are very important factors in the genesis of atherosclerosis. In certain genetic diseases where lipids levels are very high, atherosclerosis and its complications can be seen as early as the second decade. Similarly, diseases like diabetes mellitus and certain kidney diseases where the cholesterol level becomes very high, complications of atherosclerosis are seen more frequently. Various fractions of cholesterol and other lipids have been analysed and their precise role in this process is now known. Low density cholesterol is the one which seems to play a very important role in the early stages of formation of atherosclerotic plaque whereas high density lipid protein helps in removing this cholesterol from the arterial wall. Lipids can be reduced by exercise, dietary precautions and now pharmacological means. There are a large number of drugs for lowering blood lipids. A group of drugs known as Statins have been fairly successful and are being used as a matter of routine by physicians to lower this cholesterol. But the role of diet and exercise cannot be over emphasised.
Male gender/menopause in women: The coronary artery disease is cetainly more common in men. In women risk factors accelerate after menopause. It is due to the female hormone, oestrogen, and relatively higher levels of HDL cholesterol which protect the women against the coronary artery disease before menopause. This perhaps is the rationale behind giving hormone replacement therapy to women after menopause so that their protection against the disease at least to some extent continues. It may be mentioned, however, that although this kind of therapy has now been accepted, there are certain risks like increased evidence of thrombosis and some controversy relating to carcinoma of the breast.
Other risk factors: Besides the major risk factors mentioned above, there are other relatively less defined and unproven risk factors. These include abnormal coagulation process and levels of fibrogen, higher level of aminoacid — homocysteine (this can be reduced by liberal intake of folic acid) and racial factors. Stress is an obvious risk factor although its exact role is difficult to define. There is no doubt that this has a great role to play in its present epidemic. Dr Dean Ornish from San Fransisco has shown that the practice of yoga and meditation and adoption of a vegetarian diet and exercise can reopen blocked coronary arteries.
(To be concluded)