Heart valve disease: is it dangerous?
Valves are like one-way doors that only open if you push them in the correct direction. There are four heart valves, one at the exit of each chamber (ie atrium or ventricle) in your heart, which stop blood flowing backwards into the heart chamber that it just flowed out of.
However, just like any other bit of the human body, heart valves have their fair share of ways to go wrong. There are two main heart valve diseases, described in the image below, called stenosis and regurgitation.
Valves with stenosis do not open properly – like a “push” door that is really heavy – so it is harder for blood to flow forwards through them. Less blood flows through stenotic valves than through normal valves, for a given heart contraction strength. As a result, there is higher blood pressure behind a stenotic valve, and lower blood pressure in front of it, than there should be.
Valves with regurgitation do not close properly – like “push” doors with broken hinges so that you can also open them by pulling. Blood can flow backwards through regurgitating valves. Blood should NOT flow backwards! There is more blood volume behind a regurgitating valve, and less blood volume in front of it, than there should be.
Uhhhh sorry, what’s the difference between pressure and volume?
Pressure is how forcefully fluid molecules and the walls of their (stretchy) container push against each other. Volume is how much space is taken up by the fluid in a container. In the circulatory system, the ‘fluid’ is blood and the ‘containers’ (heart chambers and blood vessels) are separated by valves.
In an elastic container (for example, a vein), its pressure will increase when its volume increases – imagine a stretchy water balloon getting harder to fill up (due to high pressure) as it gets fuller (with more volume of water).
In a muscular container (for example, an artery or heart chamber), muscle contraction and relaxation will respectively increase and decrease blood pressure, independently of changes in blood volume. Muscles are like elastic bands that can regulate how stretchy they are.
Muscles in the heart chambers contract and relax in order to squeeze or suck blood forwards through the circulatory system because blood moves from areas of high pressure to low pressure. Muscles in the walls of arteries contract and relax in order to keep blood pressure as constant as possible despite any changes in blood volume.
Right, now let’s press on with the article by looking at the actual diseases!
Look at the image of the circulatory system below. Focus your eyes on each valve individually. Ask yourself, for each valve, which parts of the circulatory system would have increased or decreased blood pressure or volume (or both!), if that valve developed stenosis or regurgitation. Remember that stenotic valves cause higher blood pressure behind them and lower pressure in-front of them; remember that regurgitating valves cause higher blood volume behind them and lower blood volume in front of them.
So how do these pressure and volume changes from heart valve diseases cause symptoms?
In the pulmonary and systemic circuits, low blood pressure leads to insufficient oxygen supply to the body and brain which causes breathlessness and fainting, whereas high blood pressure damages fragile capillaries (mini-blood vessels that are thinner than hairs) and causes oedema (when water gets squeezed out of blood vessels into the surrounding organs).
In a heart chamber (ventricle or atrium), high pressure causes muscle hypertrophy (thickening) because every time the heart muscle contracts to force blood out, it has to work harder than usual. Heart muscles are like arm muscles – if they train hard, they get chunky. By contrast, high volume in a heart chamber causes enlargement (more space inside the chamber) because every time the chamber relaxes, blood flows both forwards and backwards into it – hence the chamber must make room for more blood than usual.
However, this ‘heart gets bigger and stronger’ scenario is very dangerous for the following two reasons:
- Coronary arteries travel around the outside of the heart giving oxygen to heart muscles.
Therefore, if the heart muscle is too thick, oxygen from the coronary arteries won’t reach the inside of the heart muscle because it’s too far away. Similar to how you won’t get warm if you’re too far away from the heater. Heart muscles die quickly without oxygen.
- Heart muscles only contract once they have received an electrical signal from their neighbours.
All heart muscle in each chamber must contract together. This can only happen if the spread of electrical signal through the whole heart muscle is very fast compared to the speed of muscle contraction. If the chamber is too big, the spread of electrical signal takes too long, so different bits of heart chamber contract at different times, creating a wobbly movement called fibrillation that is too weak to push the blood out of the chamber. For an analogy, imagine a big cross-country race – the runner nearest the referee will hear the “GO” before the runners further away because sound takes time to travel. This system will be unfair if the runners are standing very far apart – it takes sound one second to travel 330 metres – and the runners start at different times.
So now we know why heart valve diseases are dangerous: in the heart, cardiac muscle hypertrophy leads to ischemia (lack of oxygen) and heart chamber enlargement leads to fibrillation. Outside the heart, high blood pressure causes oedema and capillary damage, while low blood pressure causes fainting or dizziness. Below is a final image to summarise that. Thank you for reading this article.