[By heart disease I mean, the development of atherosclerotic plaques in large arteries. Mainly the coronary arteries (supplying blood to the heart) and carotid arteries (supplying blood to the brain). This, I will refer to as Cardiovascular Disease CVD. Not entirely accurate, but language never is.]
At this point, I am going to start at the end. What kills people? (Or what causes myocardial and cerebral infarctions). You might think that this was clear cut, but of course it is not, far from it. For example, there is a major cause of death from ischaemic strokes that has nothing whatsoever to do with CVD. This is Atrial Fibrillation.
Atrial fibrillation (AF), is a condition where the upper chambers in the heart (atria) do not contract in a regular and co-ordinated fashion. Instead, they fibrillate – AF definition : ‘(of a muscle, especially in the heart) make a quivering movement due to uncoordinated contraction of the individual fibrils.’ AF is pretty common.
People who have AF tend to develop blood clots in the atria. These can break loose, and are then ejected from the heart into other parts of the body. Quite commonly these clots travel into the brain. As the artery the clot is traveling down narrows, the clot gets stuck, and blood supply is cut off, leading to an area of ‘ischaemia’ (no oxygen) and a stroke. A cerebral infarction.
Which means that it is perfectly possible to have strokes (cerebral infarctions) that are unrelated to CVD. If, that is, you define CVD as the development of atherosclerotic plaques. You can also have strokes where an artery in the brain bursts, causing bleeding into brain tissue, which is called a haemorrhagic stroke. This is clinically indistinguishable from an ischaemic stroke. You need a brain scan to see what type of stroke has happened.
Ergo, whist death from a stroke is clearly a form of cardiovascular disease, many strokes have nothing whatsoever to do with atherosclerotic plaques – which is what I am calling CVD.
Equally you can die from something commonly defined as a ‘heart attack’ which has nothing to do with atherosclerotic plaque development either. You can, for example, develop a fatal arrhythmia. This is where the conduction system in the heart goes wonky, the heart stops contracting regularly, and you die. [Of course, quite often this happens as part of a myocardial infarction].
If we put aside these forms of dying of strokes and heart attacks, we can then focus more clearly on the event that kills you with CVD? Which is, in general, a clot forming on a vulnerable plaque, and blocking an artery, leading to a myocardial infarction (heart attack).
Looking at strokes. If a clot forms in a carotid artery, it does not tend to block the artery completely. Instead, a part of the clot breaks off, and travels up into the brain where is gets stuck – causing a stroke (as per AF).
But… there are those who disagree with this simple model. Mainly with regard to heart attacks. I am fully aware of a growing movement which states that the myocardial infarction (heart attack) happens first, then the clot forms afterwards. (Incidentally, this concept is not new; it was first proposed over eighty years ago. You may think that is seems completely mad. However, there is strong evidence that would appear to support this ‘reverse’ hypothesis’ (infarction first, then the blood clot forming in the artery).
For example, in many cases after a confirmed and accurately diagnosed myocardial infarction, you cannot find any blood clot in the artery leading to the infarcted area. In other cases, you can find a blood clot that is several days, or weeks old. This age of the clot can be established because of the ‘evolved’ state of the thrombus. In short, there is no ‘temporal’ connection between the blood clot forming and the heart attack occurring.
On the other hand, you can find an acute blockage of a coronary artery that has not caused any symptoms, let alone a myocardial infarction.
Anyway, if you try to bring these facts together you find that:
- Myocardial infarctions can occur without any clot being found in an artery
- A clot can form in coronary artery days, or weeks, before any symptoms of an MI
- A clot can fully obstruct the coronary artery, without causing a myocardial infarction
Given these facts (facts which, incidentally, are not in dispute), you can make a pretty strong case that there is no causal association between a blood clot blocking a coronary artery, and an MI taking place. Instead people can, and indeed do, argue that the process is the other way around. Infarction first, then clot.
Perhaps, now, you can begin to understand why it has taken me thirty years to try and work out the underlying process of CVD. At times I have thought that there isn’t any… but that is another story, for another place
The reverse hypothesis – why it is not correct
I have studied the ‘reverse hypothesis’ – if that is a reasonable term for it – for many years, and I believe that it is wrong. The primary cause of a heart attack is simply a blood clot blocking a coronary artery. However, there are two major complications that lead to the apparent contradictions listed above. In no particular order they are the following:
- An infarction does not mean that heart muscle dies
- Collateral circulation develops
What is an infarction?
Cardiology is, unfortunately, dominated by highly simplistic thinking. Namely, plaque develops, clot forms, infarction occurs. The infarction occurring within minutes of the clot formation.
But this is nothing like the reality of what actually happens. Heart muscle, like all tissues in the body, is enormously complicated. If you suddenly reduce the blood supply, it can do several different things. It can infarct, it can hibernate, or it can do nothing much.
Just to look at infarction. The dictionary definition is… ‘obstruction of the blood supply to an organ or region of tissue, typically by a thrombus or embolus, causing local death of the tissue.’ Well, I’ve got news for you, heart muscle does not die (not unless the organism surrounding it dies).
Infarction, in the case of myocardial infarction, does not mean death of the tissue. Instead, the heart muscle undergoes a complex transformation into a different cell type. One that needs far less oxygen to survive and one that cannot do the contracting thing. But it is not dead. Because dead cells become necrotic and necrotic cells disintegrate. After a heart attack do you see disintegrated areas of the heart? No, you most certainly do not. You see a form of scar tissue developing.
There is also a halfway house that lies between infarction, and nothing happening. This is ‘hibernation’, a state whereby heart muscle simply decides to stop contracting/beating (to save oxygen use). If you do MRI scans on the heart, in those with known heart disease, such ‘silent’ regions are a relatively common finding. Sometimes these regions wake up and start beating again, sometimes they do not. Sometimes they go on to infarct.
Adding further to the complication, if a coronary artery starts to narrow, the heart will create small ‘collateral’ blood vessels to get round the narrowing, and keep the blood supply up. When, and if, the artery fully blocks, the collateral circulation will maintain the blood flow, and so nothing very much will happen, even if the coronary artery is fully blocked. Many people survive on collateral circulation alone.
All of which means that, after an artery blocks, one of three things can happen:
- There is a sudden infarction (could be large enough to be fatal)
- The heart muscle decides to hibernate, if there is sufficient collateral circulation to keep things ticking along
- Nothing much happens. If there is high level of collateral circulation, the heart just carries on much as before.
Only in the first case will the obstructive blood clot closely precede the heart attack. In case two the hibernating heart muscle may later decide to infarct, if the circulation does not improve. This can happen under periods of high physical or psychological stress. Thus the formation of the blood clot can precede the infarction by days, weeks or months. Indeed, the clot may have been fully cleared away by the time infarction occurs.
In short, the apparent contradictions to the: clot → blockage → infarct hypothesis can be explained, reasonably easily. So long as you realise that what happens inside the heart is not a case of simple pipe-work, where there a fixed number of tubes (arteries) supply oxygen to a pump (the heart). If you block one tube, the pump is immediately damaged.
Heart attacks and strokes
However, my main reason for disbelieving the ‘reverse hypothesis’ is that the standard model works for both heart attacks and (most) ischaemic strokes.
In ischaemic strokes, as mentioned before, a blood clot forms over a plaque in a carotid artery. This rarely blocks the artery; as carotid arteries are much wider bore than coronary arteries. However, what happens next – after a variable time period – is that the clot breaks off and travels into the brain.
This is, essentially, exactly the same process as a heart attack – except the clot lodges further down the vascular tree. Not only are the processes of stroke and heart attack virtually identical, the risk factors for both are virtually identical. Perhaps most telling is the fact that people with plaques in their coronary arteries almost always have plaque in their carotid arteries, and vice-versa. For example, here is the title of a paper on this topc: ‘Tight relations between coronary calcification and atherosclerotic lesions in the carotid artery in chronic dialysis patients.’1
This is further supported by a study that has just come out, and published in BMJ open. Key points were:
- We studied the risk of heart disease following a stroke in those patients with no cardiac history. This study is the largest of its kind and, by bringing together multiple data sets, robustly quantifies the risk of heart disease following stroke. As with all meta-analyses, the main limitation of this work relates to publication bias.
- Most patients with stroke die of heart disease.
- One in three patients with ischaemic stroke with no cardiac history have more than 50% coronary stenosis.
- 3% are at risk of developing myocardial infarction within a year of their stroke.
- Patients with stroke need to be screened for silent heart disease and appropriate and aggressive management of total cardiovascular risk factors is required2.
In short, the two conditions are the same. It is beyond any reasonable doubt that with ischaemic stroke, and myocardial infarctions, we are looking at the same underlying disease. To be frank I don’t think may people would disagree with this. But it does lead to a critical point. Namely has anyone, ever, argued that cerebral infarctions (strokes) happen before the blood clot blocks an artery in the brain?
No they have not. Because to do so would, quite frankly, be bonkers. We can be absolutely certain that blood clots cause infarctions in the brain. Yet many people quite strongly argue that with myocardial infarction it is the other way around. For the reasons outlined above I do not, and cannot, believe this. There is only one process, and no need to start searching for another.
You may wonder why I have gone off in such a wide detour here? There are two reasons. First to make it clear that this area is gigantically complex. Secondly, to reinforce the point that wherever and however you look at CVD, alternative hypotheses have been proposed. Until you have tracked them down, and examined them fully, you cannot really move on.
Next: Some answers.