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The electrocardiogram (ECG)
Written by Tim Sheppard MBBS BSc. Last updated 9/11/10

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What is the conducting system of the heart?

Essentially the conducting system of the heart involves pretty much every part of the heart. All of the muscle tissue (the myocardium) can conduct electricity, and therefore the signal is carried across all of the heart muscle. Even the specialised parts of the heart designed especially to carry the electrical signal are only modified muscle cells. However, the arrangement is exceptionally clever in making sure each part of the heart beats at the right time, and except for those unfortunately people with certain heart conditions, it works.

The sino-atrial node is the big one - the important one, the one that we all take a great interest in. It sets the beat for everything else to follow. The sino-atrial node produces an impulse which is carried across the atria, stimulating the cardiac muscle (myocardium) to contract. However, only the atria will contract, because there is an insulating barrier between the atria and the ventricles; the impulse can only travel as far as this, so only the atria contract.

Of course, this isn't the end of the story; the impulse does have to get through to the ventricles. It does this via the atrio-ventricular node, found just before you get to the ventricular septum (the bit between the two ventricles).

The atrio-ventricular node slows the impulse down, to make sure that the ventricles don't contract at the same time as the atria. The blood needs to be pumped into the ventricles, and then out into the lungs or the rest of the body. Therefore the atria must contract, then the ventricles. By slowing the impulse down enormously, this is cleverly achieved, so the impulse does not reach the ventricles immediately. Eventually it passes the atrio-ventricular node and passes down a bundle of specialised fibres (made up of specialised muscle cells) called the Bundle of His (pronounced Hiss). Then it splits at the bottom, and stimulated contraction of the ventricles from the bottom upwards, forcing the blood to leave through the top of the heart (which is fortunate, because that is where the escape route lies!) and to its relevant destination.

So from the intial impulse stimulated by the sinoatrial node, the electrical signal is carried across the atria, causing them to contract. The insulating barrier stops the whole of the heart contracting at once, and the atrio-ventricular node, after slowing down the signal, allows it to pass through the Bundle of His and Purkinje fibres to stimulate ventricular contraction.

It is, of course, quite a complicated system but it is also very effective because it ensures the right parts of the heart beat at the right time. The heart muscle will only beat when it is being stimulated by the electrical signal. When the signal has passed, it will relax again; it is for this reason that the heart beat is able to happen so quickly. Importantly, the rate of the heart beat can simply increase by increasing the frequency of electrical impulses.


What is the cardiac action potential?

A normal action potential (AP) results from the opening and closing of sodium and potassium channels. There are only two channels involved, and the action potential produced is incredibly swift as a result - the effect of one immediately follows the other.

In the case of the cardiac action potential (i.e. the action potential that occurs in cells of the heart), there are three channels involved. As with a normal AP, the depolarisation to threshold leads to opening of sodium channels which cause a fast upstroke. At a membrane potential of about -30 to -40mV, calcium channels open. The entrance of calcium through these channels maintains the cell's depolarisation, so instead of lasting just a couple of milliseconds, the cardiac action potential lasts about 300 milliseconds. Eventually the potassium channels open sufficiently to cause repolarisation, and the membrane potential returns to RMP.

The important thing about the cardiac action potential is quite obviously the involvement of calcium. It is the calcium that makes the action potential last longer, and this makes sure that the heart only beats once every time the sino-atrial node fires. If the action potential were shorter, the refractory period which stops another action potential happening would be finished too early, and another action potential would fire, leading to too many heart beats per minute. This may indicate a more powerful heart, but a heart that beats too many times is inefficient, and won't pump as much blood as it's supposed to.

Note that the animations are not to scale. The cardiac action potential on the right lasts 300 msec, while the normal action potential on the left above is only 1-2 msec.


What is the sino-atrial node?

As described in the section on the heart, the sino-atrial node is found in the right atrium, and it is the pacemaker of the heart, responsible for stimulating every heart beat. It is a collection of specialised myocardial (heart muscle) cells which have an action potential dependant upon calcium and potassium currents. The action potential lasts 300 milliseconds like the normal cardiac AP, but this time involves calcium influx to stimulate the upstroke (instead of sodium), with potassium prompting the repolarisation and hyperpolarization as usual. Like all cardiac muscle cells, the sino-atrial node is capable of reaching threshold without another stimulus - it automatically depolarises; however, the sino-atrial node is the fastest, and therefore its impulse overrides the other cells of the heart.

It is said that there is a decay in the sino-atrial node membrane potential, which when it reaches threshold prompts the action potential that is carried across the heart as an electrical impulse. When the membrane potential returns to 'normal', it decays again and prompts another action potential.

The key issue is how quickly the membrane potential decays; some nerves release a chemical which will cause a steeper decay, so threshold will be reached sooner, and the action potentials will occur more frequently; the heart rate will increase. Other nerves can release a chemical which causes the membrane potential to sink lower (hyperpolarization) and have a slower decay, leading to less frequent action potentials and a slower heart rate.


What is the electrocardiogram (or ECG)?

The electrocardiogram or ECG is a way of measuring the electrical activity of the heart. However, is is unfortunately a very complicated issue which requires a relatively good understanding of everything that has been said about the heart so far. It is a very useful tool indeed for analysing the activity of the heart - especially since many problems can be diagnosed by looking at the conducting system - but it's also very hard to get your head round, and not something to be taken lightly.

There are different forms of the ECG, because the readings can be taken from different parts using different leads, but for the purposes of simplicity, this page will concentrate on the simplest ECG involving leads appropriately named lead I, lead II and lead III.

The three different leads produce specific shapes which are put together for the common shape produced in the images to the left and below. Although easily confused with the action potential shape, the shape of the ECG has nothing to do with the action potential. The only real similarity is that they're both related to electrical activity.

The characteristic ECG shape is produced for each and every heart beat, and should be as close to the 'normal' shape as possible. If it differs, this could indicate some kind of illness, which is why the ECG is really helpful for doctors investigating heart disease.

Because association with the general shape is so important, it is separated into particular shapes which can be identified on the ECG of any real person. These are called the P wave, the QRS complex, and the T wave.

The P wave shows atrial depolarisation and will effectively show where the atria contract. The QRS complex is the same, but for the ventricles, and is higher because the ventricles are stronger and bigger. There are more cells in the ventricles because they are bigger, so the electrical activity will produce a greater response in the ECG. The T wave shows the repolarisation of the ventricles, so where everything is relaxing again.

Measuring the electrical activity in the heart requires measurement of the voltage. However, it's not very easy to stick probes into the heart; so instead, we measure the electrical activity at the skin. Of course, you can't feel the electric impulse travelling through your skin, but it is detectable there!

Electricity is the movement of charge. When the sino-atrial node fires, the electrical impulse is carried throughout the cells of the heart, with charges moving through special channels called gap junctions. These gap junctions are found in special connections between heart cells called intercalated discs. So, using special channels in these intercalated discs, charges move between heart (or 'cardiac') cells, and the impulse is carried throughout the heart.

As mentioned in the section on membranes, the resting membrane potential is usually negative, and when an action potential occurs, the membrane potential becomes more positive. So, relative to resting membrane, the inside of a cell with an excited membrane (that is, a membrane that has an action potential travelling across it) will be positive. This will attract negative charges to it.

Since the substance which fills out the spaces between cells conducts electricity, then the negative charge which results from the positive charge inside the cell will be detected at the surface. Indeed, due to the positive inside of heart cells, special sensors can sense the negativity as far away as the arms and legs.

However, this doesn't give the characteristic ECG shape. Not yet, anyway. All we're sensing is a negative charge. However, it's not like the whole of the heart will cause a negative charge to be detected. When the atrial cells are depolarised, there will be a negative charge detected there, but still a positive charge around the ventricular cells.

So we have two poles or a dipole. One side of the heart will be sensed as negative, the other positive. The state of this depends on the state of the heart beat.

We're going to concentrate on how you get the QRS complex, so we're looking at the electrical activity of the ventricles. To begin with, the interventricular septum (the wall between the two ventricles) is excited from the left side of the heart. This curves round to excite the whole of the septum, and then the whole of the ventricles. Then everything repolarises again, but the cells at the bottom of the heart repolarise quicker than the cells at the top - so, as shown in the animation on the left, although to begin with the depolarisation is heading towards the bottom, it curves upwards and heads for the top at the end. The cells at the bottom return to the resting state first, so the depolarisation must be heading towards the top, where there are still depolarised cells.

Understanding that the direction of the dipole changes is one of the most important parts of the ECG. Remember that to begin with, the left hand side of the septum is excited, then the whole of the septum. So to begin with the dipole is pointing left to right, then it's pointing 'top' to 'bottom'. Then, as everything repolarises, it heads back upwards and ends up going 'bottom' to 'top'.

To sense this we put sensors on the limbs - the right arm (RA), left arm (LA), and left leg (LL), with leads travelling between each sensor. Lead I is between the two arms (RA to LA), Lead II is between RA and LL, and Lead III is between LA and LL. It is considered that the triangle formed by these leads is the extention of a triangle formed around the heart, and it is called Einthoven's Triangle.

Take for example lead II. When the dipole is following down the septum from 'top' to 'bottom', the 'top' of the heart will be excited, and a negative charge will be detected by the sensor closest to it - on the RA. The LL sensor will detect a positive charge because that end of the heart is at rest; a dipole is therefore felt across the heart, and sensed by these two sensors.

What if the lead is not following the same direction as the dipole? Well, it comes down to vectors. If you want to describe the direction of something travelling diagonally on a graph, you could either say 'bottom right', or you could be a bit more accurate and say 'three squares to the right, two squares down'. This is effectively what you do with a vector; so the vector V shown in the image on the left could be split into horizontal and vertical parts - a certain distance to the right, and a certain distance downwards.

Lets take the example of Lead II again. As the excitation is spreading across the septum, you can see that the dipole is not quite in the same direction as lead II - it is facing in almost the same direction, but it's a bit more to the right - coming from the left, towards the right, so not quite 'top' to 'bottom'. The RA sensor still reads negative, the LL sensor still reads positive, but it's not quite so strong because the vector isn't quite in that direction; the reading would be lower.

And what if the dipole is at right-angles to the lead? Well, lets take Lead II again. The RA sensor and LL sensor are going to get the same readings, because they'll both be as negative or positive as each other. The reading is therefore given as 0.

This is the same with all the leads. The image to the right shows the lines of isopotential around each of the poles, and a line at right angles to the dipole. Any reading taken across this line will give a value of 0; so if the lead measures at right angles to the lines of isopotential, the result is a value of 0.

That is all very complicated indeed, but if it makes sense, this should all be easy:

Lead I

Lead I follows from Right Arm to Left Arm. As the dipole is directed from left to ight across the septum, it is facing the opposite direction of lead I and gives a negative value; then it grows as it increases in amplitude (i.e. how far it travels in a particular direction) and faces in a more similar direction to Lead I. Eventually it reaches the same direction as lead I, but it has decreased in amplitude. Finally it dwindles as it faces the opposite direction to Lead I briefly before 'dying' and returning to 0.

Notice that as it crosses the point between going in the opposite direction to Lead I and the same direction at Lead I, it has a brief moment of being at right angles to Lead I. At this point, the value is given as 0 because both the RA and LA sensors will gives the same result. A value of 0 is also given when there is no electrical impulse at all, because obviously 0 means there's nothing going on.

Lead II

Lead II follows from Right Arm to Left Leg as previously discussed. As the dipole is directed from left to right across the septum, it is facing briefly the opposite direction, and then at right angles to the direction of lead II, so a value of 0 is obtained. Then the dipole moves round so that when the amplitude is greatest (i.e. the dipole is across the greatest distance), it is also pointing in virtually the same direction as Lead II, so a very high peak is obtained, just as is found in the high peak of the QRS complex. This slowly dwindles to return to facing the opposite direction, and then back to 0 as everything is repolarised again.

Notice again that the value of 0 is obtained as the direction swaps from being the opposite to being the same as the lead, and also that negative values are obtained when the leads is in the opposite direction to the dipole. Notice especially for this lead that it is closest to the QRS complex. This gives the most typical result because it is pointing in the same direction as the 'general' direction of the dipole. Ultimately it is clear that the electrical impulse has to get from the top of the heart to the bottom, so anything pointing in the same direction will give a typical result.

Lead III

Lead III follows from Left Arm to Left Leg. There are no real surprises here, except that it gives quite a different result. There is no negative to begin with because the dipole starts facing towards the same direction as Lead III. It curves round to be at right angles to Lead III hence, the return to a value of 0, and because the dipole continues curving round, Lead III's trace falls into negative values. The dipole continues to swing round, and eventually falls to 0 again as everything is repolarised and at rest once again.

So the results from an ECG depend upon the direction and distance of the electrical impulse. The ECG is often a twelve lead, so it would be possible to get the characteristic ECG shape shown earlier; however, if the direction of the dipole changes slightly due to heart cells that have gone wrong, or if the distance is less because cells aren't conducting as well as they should be, the shape would change - the bit between S and T could be raised (known as a raised ST interval) or the QRS complex could be disturbed.

If something happens to the atrioventricular node, there might be a block, and suddenly the atrial and ventricular contractions could go out of sink. You wouldn't be able to see this from outside the person, but on their ECG, the P wave and the QRS complex would be out of sink. This shows how the ECG is a brilliant tool - and, if you understand the explanation given, you'll see how sensitive to the function of the heart the ECG is.


Further Reading