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Cardiac Rhythms: A New Way to Dance?

By Kate Horner

Photo credit: Pixabay

As well as being an enthusiastic scientist, in my spare time I also do a lot of frontline ambulance work. This means that if you live near me and need to ring 999 one day for an ambulance, you might get me turning up at the door! You poor thing. We see a huge variety of injuries and illnesses, but one thing that has always really intrigued me is the heart. Not necessarily the blood flow through it, although that's interesting in its own right, but more the electrical conduction of the heart and how it actually beats. On the ambulance, the way to see the electrical activity in your heart is to run an ECG (electrocardiogram- also sometimes called an EKG). This involves sticking some little dots on various places on the body and attaching wires to each one. The ECG machine can then see what's happening and you get the little bumpy line travelling across the screen, just like you see on all those TV programmes.

Image credit: Pixabay

But the question is, what is it that your heart is doing to make that beating green line? And what happens when it goes wrong?

The Heart and Normal Sinus Rhythm

When your heart is beating as it should (which is hopefully what is happening for most of you right now), we say that it is in Normal Sinus Rhythm (NSR). This is the default rhythm that gives the characteristic ECG you see above and in most of the TV programmes (you can find more about NSR in Reference [1]). But what is your heart doing that creates this?

Well, the first thing to remember is that your heart is split into four chambers- two upper chambers, called atria, and two lower chambers, called ventricles. Blood enters the atria first, gets squeezed down into the bottom ventricles and then finally pushed up and out to wherever it needs to go in the body. This means that the muscular walls of the various chambers have to contract in order to push the blood around (like squeezing toothpaste out of the tube). These muscle contractions are controlled by electrical impulses which travel around the heart in carefully timed waves. And it's these waves which create the ECG trace.

Image credit: www.lifeinthefastlane.com

In NSR, the rhythm is quite simple. The Sino-Atrial Node, who shall play the conductor of this cardiac orchestra, lives near the top of the heart and is charge of starting each impulse. Once it starts, the electrical wave passes across the top two heart chambers (the atria), like a Mexican wave, causing them to contract, which pushes blood down into the ventricles. The impulse then travels down special fibres in the very centre of the heart right to the apex. It then spreads out and up the outside of the heart. These lower chambers (the ventricles) contract upwards along with the impulse forcing the blood up and out to the body. Once this is complete, the heart can have a moment to relax before the same thing happens all over again. And again...and again.

Unfortunately, just like anything else in the body, this synchronised dance can go wrong, with varying effects. Here are just a few examples.

Some Common Arrhythmias

Around 2 million people in the UK experience heart rhythm problems (arrhythmias) every year. [2] Naturally, some arrhythmias are more common than others. The most common sustained arrhythmia is Atrial Fibrillation (AF) with a risk of approximately 25 % for those aged over 40. [3] There are many things that can cause AF, including various heart-related diseases, certain drugs and high blood pressure. An example of an ECG trace of someone suffering from AF can be seen below.

Image credit: www.lifeinthefastlane.com

Fibrillation is a word you'll often hear when talking about heart rhythms. It is a general term to mean that the contractions in the heart muscle have become uncoordinated. Like an orchestra when they have stopped listening to the conductor and are all doing their own thing. When a heart is experiencing AF, this uncoordinated contraction, or jiggling, occurs only in the atria (hence the name). It results in an abnormally fast pulse rate. A normal pulse rate is usually between 60 and 100 beats per minute (bpm) during rest, but in AF this can significantly exceed 100 bpm. Because the atria aren't contracting when they should, the heart beat is inefficient and the heart will lack that useful resting period. Sometimes someone with AF has no symptoms at all, but usually problems include dizziness, feeling short of breath or having unpleasant palpitations. Luckily, AF isn't usually life-threatening, though it is uncomfortable and can have some complications. For these reasons, it is often treated with medication, or, in more severe cases, with electrical or surgical intervention.

Another form of fibrillation involves the other heart chambers, so is aptly named Ventricular Fibrillation (VF). This arrhythmia, however, is far less friendly. An example of an ECG showing VF is shown below.

Image credit: www.lifeinthefastlane.com

VF is perhaps the most important arrhythmia for emergency medical professionals to know. You've probably heard, "They're in VF!" shouted on many TV medical dramas for just this reason. Without quick and effective intervention, VF will be fatal. In AF, it was the atria which were contracting wildly and without rhythm, but this time it's the ventricles at the bottom of the heart. During VF these chambers can attempt to contract at rates of up to 500 bpm! This loss of effective beating results in a loss of blood coming out of the heart to the body. Clearly, this is very serious as blood is no longer travelling around as it should. Essentially, the heart muscle is just wiggling around like a bag of worms rather than beating.

Luckily, there is some hope! To stop the fibrillation, you need a defibrillator. These are the "jump-start" machines that you will also have seen on TV- the ones where they very dramatically shout, "Clear!" and have paddles that shock the patient who jumps into the air. In reality, this isn't quite what happens, it tends to be less dramatic than depicted, but then that wouldn't make for a thrilling programme. But the defibrillator (also called an AED- Automated External Defibrillator) is a fantastic and life-saving invention! Basically, it fires a couple of electrical shock waves through the heart muscles making them stop wiggling around and, hopefully, giving the conductor time to take over. The sooner the AED is used after VF starts, the more likely it will succeed. But it needs to be used in conjunction with good, effective CPR (cardio-pulmonary resuscitation) to really work. (Check out the British Heart Foundation website for easy instructions on Hands-only CPR. [4]) In fact, these AED machines are so amazing that you will find them in all sorts of public places nowadays- supermarkets, train stations, airports, town centres etc. Next time you're out and about, have a look and see how many you can spot! They're really easy to use, don't need special training and carefully talk you through every step. Super!

Unfortunately, if VF is not treated successfully, the result will be another arrhythmia- asystole. Actually, not sure if you can really class asystole as an arrhythmia since realistically it's an absence of any electrical activity in the heart, but I'll leave it in this section anyway. Asystole is sometimes called "flatline" because that's what the ECG looks like. It is one of the things a clinician will look for when deciding whether to pronounce death. Interestingly, TV doctors sometimes shock someone in asystole, but in reality, you cannot shock a flatlined patient- it will just never work. So now you can join my outrage when you're enjoying a medical drama and that happens.

More Obscure Cardiac Dance Moves

The field of cardiac arrhythmias is vast and fascinating, and we don't have time to explore them all right now, but let me try to give you a little flavour.

Wolff-Parkinson-White (WPW) syndrome, named after the three people who first described the condition, is a fairly rare congenital defect which results in an extra electrical connection in the heart. This surplus pathway, like a spare wire in an electronic device, can upset the normal conduction of the heart.

Image credit: www.lifeinthefastlane.com

People with WPW syndrome can often suffer with episodes of supraventricular tachycardia (SVT) as well, shown in the ECG above. The term "tachycardia" simply means a heart rate that is too fast i.e. above 100 bpm. (The term for beating too slowly, i.e. less than 60 bpm, is bradycardia.) SVT can occur in people without WPW too and is usually harmless but uncomfortable. Symptoms include dizziness, shortness of breath and palpitations (where have we heard those before?). It can be treated in a similar way to AF if needed.

We've already seen that VF is a shockable rhythm, but it's not the only one. There is another, called Ventricular Tachycardia (VT). As the name suggests, it involves the ventricles beating too quickly. This is also a serious condition since if the ventricles pump too quickly, they don't have time to fill with blood properly which means the cardiac output is impaired. It can also lead to VF. Not good.

The last condition I will mention here are bundle branch blocks. You can have a left bundle branch block (LBBB) or a right bundle branch block (RBBB), but anatomically, each is the same sort of thing but on a different side. In a nutshell, there is some kind of obstruction along the pathway that the electrical impulses travel. These blockages can occur due to injury e.g. heart disease or a heart attack, or after cardiac surgery. This could lead to one part of the heart being slightly out of sync with the rest. Sometimes a bundle branch block can have no real impact on a person's lifestyle, but sometimes they can be more serious and require treatment such as a pacemaker.

Will You Try the Cardiac Dance Moves?

I think the heart is an amazingly clever organ, and we've only looked at one aspect. Learning to read ECGs is a true art form, taking years of practice to get just right. But once you can understand them, it's like a exciting puzzle with huge amounts of information at your fingertips. Just from a wobbly line!

If you want to learn about other heart rhythms and have a giggle at the same time, lots of cardiac enthusiasts have created heart beat dances. A quick online search will provide hours of physiology entertainment for all! One of my favourites is shown below- think I might try out Atrial Flutter next time I'm on a dance floor. And Sinus Rhythm with Intermittent Unifocal PVC looks fun. Which is your favourite?


References

  1. ECG Library- NSR, http://lifeinthefastlane.com/ecg-library/normal-sinus-rhythm/ [Accessed 11/09/15]
  2. Arrhythmia, http://www.nhs.uk/conditions/arrhythmia/Pages/arrhythmia.aspx [Accessed 11/09/15]
  3. Atrial Fibrillation, http://lifeinthefastlane.com/ecg-library/atrial-fibrillation/ [Accessed 11/09/15]
  4. Hands-only CPR, https://www.bhf.org.uk/heart-health/nation-of-lifesavers/hands-only-cpr [Accessed 11/09/15]

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