Factors Affecting The Development Of Coronary Heart Disease

The seriousness of smoking is shown by the fact that 18% of all deaths from CHD are related to smoking. This is because smoking creates and encourages the development of lesions, therefore creating a site susceptible to blockage. Also long-term smoking encourages the build up of plaque which can cause atheroma, and parts of the plaque can also break off and block the artery as a thrombus would. Both these effects would greatly increase the risk of blockage and therefore an MI. Smoking also decreases the amount of oxygen available due to the carbon monoxide in tobacco smoke which binds to the haemoglobin in the red blood cells which means that the ability of the red blood cell to carry oxygen is reduced. This means that the heart has to work harder to pump the required amount of blood around the body, increasing the amount of pressure on the heart and increasing the risk of CHD. However, just because the patient doesn’t smoke doesn’t mean that they are immune to the effects of smoking. Passive smoking or breathing in other people’s smoke, can also reduce the amount of oxygen delivered and the efficiency of use of oxygen in the myocardium. The graph below shows this data:

                                                                                           

 Another major ‘risk factor’ is diet. A healthy diet will decrease the risk of CHD while an unhealthy diet will increase the risk. This is due to the substances which are in the foods which are deemed to be unhealthy, for example saturated fats and cholesterol. High levels of cholesterol in the blood greatly increase the risk of CHD. Diets rich in saturated fat increase the level of cholesterol in the blood and hence the risk of CHD. Lipids, cholesterol and triglycerides, are transported in the blood bound to proteins. These proteins are called lipoproteins and are classified on the basis of size. The largest lipoproteins are chylomicrons and very low-density lipoproteins (VLDL), which are generally increased after a meal. The smaller lipoproteins are known as low density (LDL) and high density (HDL). LDL’s encourage the development of atherosclerosis when they become modified by the process of oxidation. These oxidised LDL’s are more negatively charged and can therefore enter the artery wall and can contribute their lipid to the fatty build up causing or increasing the chance of atherosclerosis. The effects of cholesterol can be reduced by maintaining a low number of LDL’s to a high number of HDL’s. HDL’s have properties which allow them to remove cholesterol from the circulation and therefore protect against CHD. The best way to achieve this ratio of a high level of HDL’s to a low level of LDL’s is to consume food which contains monounsaturated fats as they lower LDL levels but at the same time do not decrease HDL levels. These fats can be found in olive oil, walnut oil, rapeseed oil and avocado.

 

 An unhealthy diet, such as the one which is high in calories, salt and cholesterol, carries more ‘risk factors’ than just high cholesterol. An unhealthy diet can also cause obesity, which can cause or greatly increase the risk of, CHD. In 1979, Bray conducted a study in which a ratio of weight to height was established as the best way of showing obesity in relation to the risk of CHD. The relationship shows a ‘J shape’ where those who are underweight show a greater risk than those who are ‘normal.’ However, once the weight of ‘normal’ has passed, the risk of CHD increases with the level of obesity. This is because obesity causes a rise in blood pressure and a greater strain on the heart.

 An unhealthy diet and therefore obesity can also cause the level of physical activity to decline. This introduces yet another ‘risk factor’ into the situation. Physical activity can effect all controllable ‘risk factors’ therefore physically inactivity can greatly increase the risk of CHD for many reasons. Firstly, physical activity is seen to increase the amount of HDL’s, therefore protecting the body from CHD. However, physical inactivity will cause the amount of HDL’s to fall as only with regular exercise will these HDL’s benefits maintain themselves. Physical inactivity causes the instance of blood clotting to increase. Heart attacks are sometimes caused by blood clotting around atheroma, physical activity can prevent blood clotting. Physical inactivity can also cause obesity as the calories consumed are not being burnt off. This then turns into fat and more pressure is put onto the heart increasing hypertension. Physical activity will burn off this fat, lowering hypertension and therefore reducing the risk of CHD.

 Unhealthy diet can also cause another ‘risk factor,’ namely non-insulin-dependant diabetes mellitus (NIDDM or Type 2 Diabetes) to arise. NIDDM is caused by an insufficient production of insulin from the pancreas as the cells of the body, especially fat and muscle cells, develop a resistance to the action of insulin. This shows that in obese people, many more fat cells will be present, therefore the resistance to insulin will become more pronounced. Diabetes affects the development of coronary heart disease in many ways. Firstly, diabetes increases the levels of LDL’s in the blood stream while also increasing the transport of cholesterol into smooth muscle cells. These 2 factors therefore increase the level of fatty build up in the arteries and therefore encourage the development of atherosclerosis. Diabetes can also occur from an earlier period of life this is known as Insulin-dependant-diabetes mellitus (IDDM or type 1 diabetes), where there is usually a complete absence of insulin, which will also affect and therefore increase the development of CHD. IDDM affects the development of CHD in the same way as NIDDM however, IDDM is not caused by obesity and is a ‘risk factor’ in its own right. Paradoxically most cases of IDDM are probably caused by infection in early childhood whilst with later onset NIDDM there is a strong genetic component.

 Another Genetic ‘risk factor’ is gender. As Fig 2 shows, men are more inclined to develop CHD than women. However, once women reach the postmenopausal years the risk of CHD becomes similar for both men and women. Fig 2 also shows the importance of age in the development of CHD.

 The effect of cholesterol on CHD also carries with it a genetic component. This component is called familial hypercholesterolemia (FH). In its homozygous form, patients who suffer from FH develop high cholesterol from an early age and normally die before the age of twenty. This is caused by a defect in the receptor for the LDL molecule. This means that LDL’s build up in the blood stream which causes rapid formation of Plaque, causing the development of CHD to take place. Although, FH in its homozygous form is rare, 1 in 500 suffer from FH in its heterozygous form, making it a relatively common disorder. In heterozygous FH, the number of LDL receptors are reduced compared to the amount in people who do not suffer from the disorder. This means that the cholesterol builds up slowly causing the patient to die from CHD in their 30th of 40th year. Other genetic disorders which effect the development of CHD by means of increased cholesterol include apoB apoprotein abnormalities. These disorders are clinically identical to FH but here, the receptor in these cases, is healthy, however the protein is abnormal. Other disorders do exist but they are extremely rare.

 Other ‘risk factors’ apart from the development of cholesterol also have genetic components of them. Some patients have a greater tendency to form clots and /or are predisposed to hypertension. One ‘risk factor’ in which environmental and genetic factors are hard to separate is obesity. This is because a child may become obese due to his parents or due to a genetic problem. The validity of the genetic problem had been doubted until animal models showed genetic obesity (the ob ob mice).

 Recent work has shown that the mother’s nutritional status during pregnancy can have a profound effect on the development of CHD in later life. Malnourishment increasing the risk of CHD.

 In conclusion, CHD is a major cause of death the serious illness throughout the world. The causes, environmental and genetic, are now well understood and a concerted effort to tackle such causes as high blood pressure, hypercholesterolemia, obesity and smoking are underway and hopefully a decrease in deaths and illness from CHD will be observed in the next 10-20 years.

By Alex Lawson

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