Blood Pressure, Cholesterol, Heart Disease and RISK
Introduction
I was moved to write this piece because increasingly those of you who visit us at the surgery will be aware of the fact that nowadays we frequently use a risk analysis to decide how to treat you - especially in relation to prescribing cholesterol lowering drugs such as "statins". Sometimes the decisions that we make on the basis of this assessment may seem a little mysterious especially for those who despite a fairly high cholesterol level may be told that their level of risk is too low to treat with drugs.
Aims
What are we aiming to do? I think ideally we would like to be able to divide the population into two distinct groups. One group would definitely benefit from treatment and the other would definitely not. We could then concentrate our efforts on the first group and save the second group from all the consequences of unnecessary treatment, the nation some tax and free up resources to be used in other areas of need, and save everybody a lot of time which would otherwise be wasted in attending unnecessarily at the surgery. Of course such a world does not exist and even in relation to very simple conditions such as a sore throat we cannot reliably predict who will benefit from an antibiotic because they will be saved a rare complication, who will get better anyway and who will suffer a side-effect unnecessarily. In relation to the national epidemic of coronary heart disease we know that there are interventions that will make a difference. However they are costly in simple money terms as well as in terms of the inconvenience to the individual. I think that we all know fairly intuitively that there are those "that are heading for a heart attack" because they smoke, do no exercise, are overweight stressed and have parents who died young from coronaries and those who are seemingly at low risk because they live a healthy lifestyle. We also know that occasionally we are surprised. We perhaps know, or know of, the superfit 40 year old who drops dead suddenly from a heart attack on the squash court and at the other end of the scale the 90 year old rascal who has apparently broken every health promotion rule in the book and still goes out to the shop to buy his packet of cigarettes every day. The science, inexact though it is, of risk factor analysis is designed to help us decide when our interventions are more likely to do good than harm.
Risk factors
What is a risk factor? Well it is anything that determines your likelihood of suffering something unpleasant such as a heart attack, a stroke or premature death. We all know that if you run across the road with your eyes shut you are more likely to be run over than if you keep them open. However we also know that it is not quite as simple as that because if the road is busy we are much more likely to be flattened than if it carries only one car a week. Similarly, if we listen before we run across with our eyes shut or if we ask someone else if the road is clear then we can substantially reduce the risk. So this introduces the concept of a number of factors combining to produce what is the total level of risk. In relation to heart disease, which is the area at the moment where risk analysis is most commonly used, there are essentially two types of risk: risks that you can do something about and those that you cannot change. The latter includes such things as age, gender, who your relatives are and existing disease (such as diabetes - although in this case we do believe that good diabetic control reduces that extra risk). The former include diet,cholesterol level, blood pressure, exercise, stress and especially smoking.
Levels of risk
When we are working out a level of risk what we are trying to do is put the patient into a group with roughly similar levels of risk and then we can develop a policy of how to treat each group. As stated earlier, we would ideally like to have two groups, a "pull all the stops out "group and a "leave well alone" group. Unfortunately in relation to heart disease we have many groups all with a risk label on. No group contains people who could be guaranteed to have a heart attack in a ten year period and no group contains people who could be guaranteed not to. The fact that we don't have such groups is probably indicative of our current level of knowledge and the imperfections of our investigative techniques because it is likely that such groups exist if only we knew how to find them. However, the bulk of the population would be in neither group even if we could detect them because, as always, chance plays a part in most cases.
Combining risks
We know that risk factors work together in heart disease because of a very large study that was done looking at risk in a group of patients in the United States called the Framingham study (1). We also have some additional data that appertains to diabetic patients, who form a special risk group, from an important study done in the UK called the UKPDS study(2). From the data produced by these studies we now have quite sophisticated calculators that give us a risk value. Sometimes the calculator is in the form of a chart but increasingly this is incorporated in the software that our desktop computers use. My desktop computer automatically calculates a risk level for every patient who is in the appropriate age group that we have good research data for (30 - 74) and for whom there is no current diagnosis of heart disease (because if there is such a diagnosis the patient automatically goes into a higher risk group). It uses recorded data and makes assumptions based on population averages where the data is incomplete. It also prompts me to collect data where it is missing or if it is old.
Using risk to decide when and how to treat
We know that most people would get some reduction in the likelihood of a having heart attack or developing angina if they took a type of cholesterol -lowering drug called a statin. In most cases the reduction of risk would be in the order of 30% over a ten year period and this is almost irrespective of cholesterol level because no matter how low your cholesterol is in most cases you would be better off if it were still lower. So why don't we treat almost everyone? Well, these drugs aren't cheap, if they were given to almost everyone then a very large slice of all the money spent on the health service would go on them and in addition there would be substantial resources needed to carry out the monitoring that would be necessary, because we have to do regular blood tests on people taking them and check that they are well from time to time. Secondly, they aren't without problems because infrequently they can cause such significant side-effects as inflammation of the muscles which means that they have to be stopped. So whilst someone who has, say, a 60% risk of having heart disease, and who reduces that to 40% by taking one of these drugs has reduced their risk by one in five over ten years, someone who starts out with a risk of 3% and reduces that to 2% would be in a group in which out of a hundred people, each taking in excess of 3650 doses over a ten year period only one would benefit - 97 would never have had a heart disease anyway and 2 would develop it despite the drug. Of course for that one person it would be definitely worthwhile but at present we have no way of knowing which that person is. That is the crunch for risk analysis - we are dealing in likelihood, possibility and probability and not certainties. The same arguments apply in respect of lowering blood pressure. We know that except at very low blood pressures no matter how low your blood pressure is in most instances your risk of heart disease and stroke is lower if your blood pressure were lower still. However, there is a law of diminishing returns here and it would be necessary to give some people a very large amount of medication and perhaps inflict unacceptable side-effects on them in order to get their blood pressure to an ideal level and in some patients, with the drugs that we have available at the present time you might still not get there.
Blood pressure
Contrary to what a lot of patients tend to think a raised blood pressure is not a disease in itself. It's not even a symptom like a cough or a temperature because without a machine to measure it you would have no way of knowing that you had it. It is no more and no less a measurement than height or weight. As with weight, in particular, the more you have of it above a certain amount the more likely you are to get sick and die young. Despite that you could go through a very long life with a lot of it and know nothing about it and in the old days many people did and nowadays there still are those rare people who keep themselves away from the doctor until the day they die and only when the post mortem is done are there the tell-tale signs of chronically raised blood pressure. So what does it do? Well the blood inside the arteries, the blood vessels that lead away from the heart, has to be pressurised to some extent in order to find its way to your head and your brain. Beyond the pressure that is necessary to make sure that you don't faint when you stand up any excess is superfluous. What is more it has two major effects. One is on the heart, which if it has to pump against a high pressure becomes like a weight lifters biceps, big. The other is on the blood vessels themselves which may become thickened (by which time reducing the blood pressure becomes almost impossible), lined with fatty deposits (atheroma) or they may burst causing a heamorrhagic stroke. Those who think that it might be good to be big-hearted are unfortunately mistaken. If you have an enlarged heart (called left ventricular hypertrophy to us medics), and raised blood pressure is a major cause of this, then there is a very serious increase in the likelihood that you well suffer a heart related death and even if you don't it would tend to indicate that your blood pressure is raised in a sustained way to do damage to other organs as well. Sometimes the heart just gets worn out pumping against a high blood pressure and goes into what we call "heart failure" which means that the patient accumulates fluid in the bits of the body that hang down - mainly the legs if you are upright or the bottom if you are confined to bed but worst of all it may accumulate in the lungs and if this happens suddenly, as it sometimes does, it can pose one of the most urgent medical emergencies.
Cholesterol
Cholesterol is a fat. Most of the cholesterol in the blood is made in the liver although we do take cholesterol in with our food it is not generally thought that that in itself is significant. What is much more significant is the intake of saturated fat, such as butter, lard, hard margarine or other solid animal fats which is used to make the cholesterol. Cholesterol is not all bad. It is a necessary building block for steroid hormones which include the cortisone type hormones necessary for maintaining the salt balance of the blood and an adequate blood pressure to prevent you from collapsing and the sex hormones which apart from their obvious functions are necessary for building up muscle etc. So clearly we can't do without cholesterol entirely. So what harm does it do? Well, like blood pressure, a raised cholesterol is simply a measurement and without the appropriate tests you would have no way of knowing that you had it. The degree to which it is harmful is dependant on other factors (unless you have one of the rare inherited raised cholesterol conditions associated with very very early heart disease - such as in the twenties and thirties or even earlier in extreme cases). Basically what happens is that cholesterol gets deposited in the lining of the blood vessels forming what are known as atheromatous plaques. This is more likely to happen if the blood pressure is raised or if the patient smokes. These plaques have two effects. One is simple - just as a deposit of limescale can block the hot water pipes in the house so the plaques can cut down the flow of blood in the affected vessels. Most commonly affected are the blood vessels around the heart (coronary arteries) which can lead to angina when if the heart speeds up because of exercise or stress the flow is not then sufficient to provide adequate oxygen and the muscle complains by causing a pain felt in the middle of the chest rather like a cramp pain. Also affected are the blood vessels leading to the legs which if they become blocked can lead to "intermittent claudication" where the patient gets a cramp-like pain on walking, pain in bed at night (rest pain) or in the worst cases actual gangrene which may require amputation. Also there is a risk that one of these plaques will ulcerate and as with a cut you get the formation of clot on the surface. This clot, in the initial stages, is particularly liable to break off and float downstream where the blood vessels are narrower and so block them completely. If the vessel is a coronary artery this can cause a heart attack or "myocardial infarction" if the result is that some of the muscle is totally deprived of oxygen and nutrition thus dieing - similarly if the blood vessel is supplying the leg then such a blockage can cause the blood supply there to be interrupted leading to intense pain and the risk of muscle death and gangrene.
Good cholesterol
When you have your cholesterol checked we don't just get one number. We do get the total cholesterol level (which should ideally be under 5.0 mmol/litre) but we also get three other numbers. One is the LDL (Low Density Lipoprotein) level which is pretty much the one that does the harm and is associated with a risk of atheroma, another is the triglyceride level, which is not quite so clearly related to heart disease risk but which we know is not really a "good thing" to have and is associated with a risk of pancreatitis and may be caused by poorly controlled diabetes or an excess intake of alcohol in some cases and the third is HDL (High Density Lipoprotein) cholesterol - so called because it is a more dense fraction of the fatty component of the blood. This cholesterol is GOOD NEWS. So far as we know, the more the better. We think it is associated with the scavenging of atheroma. It seems to be promoted by exercise and a healthy lifestyle and is elevated by cholesterol lowering drugs. Apart from the fact that the more HDL you have then the less of your total cholesterol is any other type, it does seem to be a positive and INDEPENDANT predictor of coronary risk.
Summary
Analysing your level of risk to develop a particular illness depends on a number of separate factors. Currently we use this technique in deciding who to treat with cholesterol lowering drugs.
In heart disease there are two types of risk that we can recognise. Those that we can do something about and those that we can't.
Of the risk factors that we can do something about we recognise cholesterol level, exercise, diet, smoking and blood pressure as the main ones.
Some risk factors require medical intervention but others such as exercise, smoking and diet are down to the patient.
As our techniques improve it is to be hoped that we will be able to give each patient a very accurate prediction of what is likely to happen to them in respect of conditions such as heart disease and diabetes to which risk analysis is appropriate but at the moment the best we can do is place people into low high and intermediate risk groups. Even the highest risk group does not reach a 100% certainty nor the lowest risk group quite get to zero.
No matter whether you are in a high or a low or intermediate risk group for heart disease there are certain interventions that are unequivocally beneficial. These are:
References:
1) Framingham Study - http://www.framingham.com/heart/index.htm
2) UKPDS - http://www.dtu.ox.ac.uk/ukpds/