Читать книгу A Statin Nation - Dr Malcolm Kendrick - Страница 6
ОглавлениеBefore starting with more exciting topics, such as what causes heart attacks and strokes – and what doesn’t – there is the unfortunate requirement of attempting to make clear what I am talking about. A recurring theme in this book is that medical terminology is often hopelessly confusing. It can act as a barrier rather than an aid to understanding. So, here is my initial attempt to attain some form of clarity.
This book is primarily focused on two often fatal conditions – heart attacks and strokes. Whilst there are many different diseases that can damage the heart and the brain, in the majority of cases the problem is a disease of the arteries supplying blood to these organs.
With heart attacks, the arteries affected are the coronary arteries. The disease is often referred to as coronary artery disease (CAD). Other common terms regularly, confusingly and interchangeably used are coronary heart disease (CHD) or ischaemic heart disease (IHD) (ischaemia is a general term that means lack of blood supply, leading to lack of oxygen supply). CAD, CHD and IHD are different terms for describing the same thing.
With strokes, the main problem is with the carotid arteries that supply blood to the brain. These arteries branch from the aorta, the single artery that leaves the heart. The carotid arteries separate from the aorta around the base of the neck. One carotid artery goes up the left side of the neck, the other goes up the right. (There are also vertebral arteries going into the back of your brain, through the spinal column, but they are less likely to cause problems, though they can.)
Whilst the underlying cause of a stroke, as with a heart attack, is disease in the carotid arteries, you will never come across the term carotid artery disease, carotid brain disease or even ischaemic brain disease. Why? That is just the way it is. Equally, I have never heard anyone refer to a stroke as a form of ‘brain disease’. It is funny how thinking and terminology, even in closely related areas, can develop in completely different ways.
The actual disease in the blood vessels that leads to most heart attacks and strokes is usually referred to as atherosclerosis. This is the development of lumps, or thickenings, in the artery wall. These thickenings are most often called atherosclerotic plaques.
You may have come across the term arteriosclerosis. I have never worked out what this is, and I don’t think anyone else has either. I think it is the same thing as atherosclerosis, even though some people say it is not. So, I will just leave this to one side.
Atherosclerosis, or atherosclerotic plaques, do not just occur in the coronary and carotid arteries. You can suffer from atherosclerosis in arteries almost anywhere else in the body. Arteries that supply the kidneys, the bowel, liver, adrenal glands … I recently admitted a lady with severe abdominal pain who had suffered a ‘bowel attack’. An artery supplying blood to a section of her large bowel had blocked off completely and her bowel infarcted (infarction is the sudden loss of blood supply due to a blockage, usually a blood clot, leading to death of the downstream tissue). About 0.6 metres of bowel was later removed surgically. I hate to admit it, but I got the diagnosis completely wrong. On the plus side, at least I sent her to hospital, recognising that she wasn’t very well. I thought she had bowel cancer and had obstructed.
In short, the underlying disease that we are looking at with heart attacks and strokes is actually vascular disease, which can manifest itself in almost all organs of the body. Vascular disease, due to atherosclerosis, often comes under the umbrella of CVD. This can be shortened to CV, as in CV (cardiovascular) mortality.
I think I also need to explain a bit more about the vascular system. Essentially, this consists of the heart, the arteries and the veins. The heart begins life where an artery and vein merge together. This area then enlarges, develops valves and an electrical conduction system, etc., transforming itself into the fully formed and highly complex organ that we call the heart.
It is the first organ in the body to function, and it starts pushing blood around the body twenty days after conception. As with most aspects of foetal development it is just unbelievable how it happens, and that it happens, almost perfectly. Mind-boggling.
The heart pumps blood into the arteries at relatively high pressure. Veins bring blood back to the heart at much lower pressure. Arteries have much thicker and more muscular walls than veins, primarily because they need to withstand much higher blood pressure. The other main difference between arteries and veins is that the bigger veins have valves in them to stop the blood simply dropping back down to your ankles. If a valve in your leg gives way, you can end up with a bulging varicose vein because of gravity, and increased pressure acting on the vein beneath the failed valve.
Apart from these small differences, arteries and veins have an identical structure. There is a single layer of cells that lines the inner surface. This layer is usually referred to as the endothelium, made of single endothelial cells, which are wide and flat, a bit like miniature wall tiles, although the endothelium can be several cells thick in certain places. Underneath the endothelium there is a muscular/elastic layer, the media. Surrounding the media is a tough outer layer, the adventitia, which holds everything together.
When arteries branch and become smaller and smaller, they are called arterioles. Smaller veins are the venules and the smallest blood vessels are capillaries, which join the arterioles to the venules. These are so narrow that red blood cells must be squashed and distorted in order to squeeze through.
A little-known fact (little known to most doctors I have spoken to, anyway), is that the larger arteries and veins have their own blood vessels to supply them. They are the vasa vasorum, which means blood vessel of the blood vessels. Yes, blood vessels need their own blood vessels to get the nutrients they need. Something I never learned at medical school, for sure; or maybe I was asleep during that lecture.
DIAGRAM 1
As capillaries are far too small to see with the naked eye, for many hundreds of years the circulatory system was a bit of a mystery to everyone. Blood flowed out of the heart, and blood flowed back into the heart, but how could this happen? Clearly it could not be the same blood flowing in as flowed out. As far as doctors could then establish, the blood leaving the heart simply disappeared into various organs around the body, never to be seen again. No one believed the blood could actually be going around and round.
Luckily, there was a solution to the amazing disappearing blood problem. Galen, a Roman physician in the second century AD, and the most influential doctor of all time, proposed that the circulatory system consisted of two separate one-way systems, rather than a single, unified system of circulation. He thought that venous blood was generated in the liver, and was then distributed and consumed by all organs of the body. He further decreed that arterial blood originated in the heart, from where it was also distributed and consumed by all organs of the body. The blood was then regenerated in either the liver or the heart, completing the cycle. Clear? As mud.
This was a ridiculous model. However, for centuries after his death Galen’s teachings remained so influential that to question him represented a terminal career move, both metaphorically and literally. William Harvey (1578–1657), who is credited with establishing how the circulatory system works, was acutely aware of the dangers of questioning Galen. To quote from the brilliant Paul Rosch.
It is impossible to overestimate the power that Galen had over medicine at the time. He was such an unquestioned authority that he was referred to as ‘The Medical Pope of the Middle Ages’. Although Harvey announced his discovery in 1615, he waited 13 years before publishing his results, since it was considered sacrilegious to challenge Galen.
Any contrary opinions were considered to be heretical, and would not only quickly end your career, but could even cause you to be burned at the stake. Harvey’s hesitation to openly defy Galen proved to be justified.
Most physicians rejected his 1628 book because he could not explain how the arteries and veins met. If organs did not consume blood, how did different part of the body obtain nourishment? If the liver did not make blood from food, where did blood originate? Why was blood blue in veins, but red in arteries? It took two decades for Harvey’s colleagues to acknowledge his achievements.1
It’s a great pleasure of mine, almost a secret vice, to read about the history of influential medical ideas. The passage of time has the great benefit of allowing you to see exactly how and why ideas of the greatest stupidity were so widely believed, and then defended with vigour and venom by the great and the good. At which point, you can draw parallels with the thinking and actions of today. Mentioning no names – yet.
Anyway, it turns out that Galen was wrong and Harvey was right, surprise, surprise. Blood does circulate around the body, travelling from the heart, in arteries, down through arterioles and capillaries, and then back again in venules and veins. How simple everything seems when you know the answer.
One point I need to add here is that the heart also pumps blood through the lungs, where it picks up oxygen and gets rid of carbon dioxide. This means that, in the lungs, blood vessels containing high levels of carbon dioxide are called arteries. On the other hand, blood vessels full of oxygen are called veins. And that’s the exact opposite in the rest of the body, just to add to the general confusion.
The arteries and veins in the lungs (pulmonary blood vessels) also have the same basic structure as the blood vessels elsewhere in the body. However, both the arteries and veins here have thin walls, as the blood pressure in the lungs is relatively low.
