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The Science of Pacemakers

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A pacemaker is a device implanted into the body to regulate the rhythm of the heart.

Physically, the pacemaker is made up of an electronic circuit (a miniature computer) and a battery sealed in a metal box with one, two or three electrode leads attached to it. These leads go into the heart and the electrical impulses made by the pacemaker are conducted down the electrode lead into the heart.

A pacemaker is usually about 40mm × 40mm (1.5ins × 1.5ins) in diameter and 8mm (1/4in) thick. It weighs about 20 to 60 grams (1 to 2oz). Normally the device is surgically inserted in the body just below the collarbone; it may be visible as a slight bump in the skin. Sometimes temporary pacemakers are kept outside the body, for example beside a hospital bed, with the electrode leads going from there into the patient's body.

What Do They Do?

A pacemaker performs two functions: pacing and monitoring.

When pacing, the pacemaker sends an electrical impulse to the heart through the electrode lead. This impulse (or pacing pulse) starts a heartbeat. The pacemaker sends out an impulse if the heart's own rhythm becomes abnormal, for example if it is interrupted (stops and starts), irregular or too slow.

In order to pace the heart effectively, the pacemaker will also constantly monitor the heart's natural electrical activity. If the pacemaker senses an abnormal rhythm, it will then emit a pulse though the electrode leads to help regulate the heart beat. If the heart is beating normally (regularly) the pacemaker will not emit a pulse. In this case, the pacemaker is only working on demand.

The 'Normal' Heart and Its Rhythm

Before exploring why a person might need to have a pacemaker fitted, it's useful to have a look at how a 'normal' heart works.

The heart is divided into four chambers, two upper chambers called the atria (the right atrium and the left atrium) and two lower chambers called the ventricles (the right ventricle and the left ventricle). The right and left sides of the heart are separated by a wall of muscle, called the septum. A valve is situated between the right atrium and the right ventricle and between the left atrium and the left ventricle to ensure that blood only flows one way.

Blood comes into the heart through the atria (upper chambers of the heart) and the atria pump blood through the valves and into the ventricles. The ventricles pump blood into the rest of the body when the heart beats.

A normal healthy adult heart beats regularly at between 60 and 100 beats per minute. When exercising, a person's heart rate can go up to 150 beats per minute, depending on how old and how fit the person is. The heart pumps oxygenated blood throughout the body. The faster the heart beats, the more blood is pumped through the body.

The heart normally has its own natural pacemaker called the sinus or sino-atrial (SA) node. This is a group of cells in the right atrium, which send out regular electrical impulses across the two atria, making them contract. It also sends electrical impulses to another node, the atrio-ventricular (AV) node. The AV node is situated where the atria (upper chambers) meet the ventricles (lower chambers) on the right hand side of the heart. The AV node transmits the electrical impulse through conduction pathways in the ventricles, which makes the ventricles contract – a heartbeat.

Why Might Someone Need a Pacemaker?

There are various reasons why the heart may not beat regularly, leading someone to need to have a pacemaker fitted.

Heart Block

Heart block means that the electrical impulses in the heart slow down or become irregular or even stop altogether, because the electrical impulse is blocked from getting from the atria to the ventricles. This is usually caused by heart disease or aging of the heart, but it may also happen after heart surgery. A heart rate that is too slow is called brachycardia (also referred to as bradycardia).

The atria and the ventricles beat independently of each other, so although the atria may continue to beat normally the ventricles may beat more slowly if they do not receive the correct electrical impulses from the atria. The slower heart rate reduces blood flow to the body and may cause breathlessness, fainting, blackouts and fatigue.

It may be that a person feels all right when walking around on flat ground and doing gentle activities, but that the heart is unable to beat fast enough when the person is doing more strenuous exercise. This will cause the person to feel breathless and tired and unable to continue exercising.

Irregular Heart Rate or Heart Rhythm

If a person's heart rate is too fast and/or irregular, he or she may be given a pacemaker to regulate the heart rhythm. A too-fast heart beat is called tachycardia.

Sinus Node Disease

Also known as having a slow natural pacemaker, sinus node disease occurs when the sinus node cannot send the signal to begin a heart beat or cannot increase the heart rate as required. Often, if the sinus node is not working properly, other tissues in the heart take over the job of the sinus node. However, these tissues may not be able to maintain a regular heart rate or may create a heart rate that is too fast or too slow – this is known as 'sick sinus syndrome'. If the heart's rhythm is too slow, a pacemaker can help by taking over the job of the sinus node. If the heart's rhythm is also sometimes too fast, this may be controlled with drugs.

Heart Failure

Heart failure means that the heart has become less efficient and cannot pump enough blood round the body when a person is at rest or when they are active. This may be treated by implanting a pacemaker to synchronise the ventricles as they may have stopped beating in time with each other. This is called resynchronisation therapy.

Types of Pacemaker

Single-chamber Pacemakers

This type of pacemaker has one lead that is connected to only one chamber of the heart, either the right atrium in cases of sinus node disease, or the right ventricle in some cases of heart block.

Dual-chamber Pacemakers

These pacemakers often have two leads. One lead is connected to the right atrium and the other to the right ventricle. Dual-chamber pacemakers are also used in cases of heart block where the natural pacemaker (sinus node) is working normally.

Bi-ventricular Pacemakers

A bi-ventricular pacemaker has three leads. One lead is connected to the right atrium, one to the right ventricle and one to the left ventricle. These are used in the case of heart failure.

On-Demand Pacing

Although some pacemakers work constantly, sending out regular electrical impulses, almost all work on demand. This means that the pacemaker senses if the heart is beating irregularly or too slowly and will send out electrical impulses to correct the beats and make the heart beat regularly. If there is no heartbeat at all, the pacemaker will send out continuous impulses. If the heart is beating naturally and normally by itself, the pacemaker will not send out any impulse. The pacemaker can sense when a person is exercising and will increase the rate at which it sends out electrical impulses accordingly.

Implantable Cardioverter Defibrillators (ICDs)

ICDs are not pacemakers, as such. However, like a pacemaker, an ICD monitors the heart's rhythm and can detect when it becomes abnormal. The ICD reacts appropriately, either by sending out a pacing pulse or by sending out a small electric shock to stop the abnormal heart rhythm and restore it back to normal.


Cardioversion is performed in at attempt to restore the heart's normal rhythm, but it does not involve the insertion of a pacemaker. Instead, the patient is given a general anaesthetic and a controlled electrical current is applied to the chest wall.

Temporary Pacing

In an emergency, usually after a heart attack, the body may suddenly lose the ability to regulate the heart rhythm. This could cause serious problems as the heart cannot beat fast enough to maintain the blood pressure.

The first option in such cases is to use a Temporary External Pacemaker; this is usually a function on the more expensive defibrillators. Two pads are attached to the patient. These deliver an electric shock through the patient to the heart.

The big disadvantage of this method is that an electric shock big enough to get all the way through the chest wall to the heart is actually rather painful. When carrying out the procedure the shock happens 60 times a minute, so the patient usually has to be sedated. The other problem with this method is that it is not always totally effective.

The next step in this sort of emergency is to put in a Temporary Internal Pacing Wire. Firstly, a cannula (a thin plastic tube that goes into the vein) is inserted into the carotid vein, in the neck, or the subclavian vein, under the clavical, or the femoral vein, which is in the groin. A wire is then threaded down the cannula and an X-ray is taken to see where it is. Once the wire is in the heart, it is plugged into the pacing box. The whole procedure takes place in an X-ray room, so the position of the wires can be seen throughout the procedure.

Implanting the Pacemaker

A permanent pacemaker is usually implanted under local anaesthetic, with the electrode leads being inserted transvenously (through the veins). If a patient is nervous about the procedure, he or she can be given sedation – as much as is required to make the patient comfortable. In some cases, if it is likely that the procedure will be more complicated than usual, for example because of a pre-existing heart condition, the procedure can be done under general anaesthetic. General anaesthetic can also be used if a patient is too nervous to have the pacemaker put in using just a local anaesthetic and sedation. The patient will need to talk with his or her doctor to ensure that the risks of the various methods of anaesthesia are understood and that his or her needs are met.

On average, it takes one and a half hours to implant a pacemaker, but it can take as little as 30 minutes or as long as half a day, depending on the condition of the patient's heart. For example, it may take longer to implant a pacemaker in someone with a congenital heart defect than in someone with a previously normal heart. The patient will normally only need to stay in hospital for one night before and one night after the procedure.

The procedure is usually undertaken by a cardiologist specialising in electrophysiology. He or she is assisted by a team comprising nurses, radiographers and possibly another doctor from the pacing team.

Before the Procedure

The patient is given intravenous antibiotics to prevent infection during the procedure. He or she will be given a course of oral antibiotics to take after the procedure.

During the Procedure

Firstly, any anaesthesia or sedation will be given to the patient. Once the patient is ready (or asleep, if they have had a general anaesthetic), the procedure begins. The pacemaker is usually inserted near the left collarbone. The cardiologist numbs the area and makes a small incision, a few inches long, just under the patient's collarbone. The leads are then passed into the heart through a vein near the incision. The leads are guided into place by the cardiologist using a fluoroscope – a type of X-ray – so that he or she can see exactly where the leads are going. The leads are positioned on the inside wall of the heart, as appropriate to the type of pacemaker being implanted (see above). Once each lead is positioned, it is tested to make sure it is in the correct place.

Once all the leads are in place, they are connected to the pacemaker. The pacemaker is then put into position in a small pocket of flesh just under the skin near the collarbone. The pacemaker is then tested to check the amount of electrical energy needed to make the heart beat and is adjusted accordingly. The incision made in the skin is sewn up.

Sometimes a Redivac drain needs to be inserted at the site of the pacemaker, to drain away any excess blood or fluid. This is usually stitched in place, but should be removed the day after the procedure.

Epicardial Implantation

Sometimes, instead of being inserted into the heart through a vein, a pacemaker lead is attached to the outer wall of the heart (the epicardium). In this case, the pacemaker itself is placed under the skin of the abdomen.

After the Procedure

Once the patient is awake enough to respond, depending on how much sedation he or she has had, he or she will have a chest X-ray to double check the position of the pacemaker and the leads. The patient may be asked to cough while being X-rayed, so the cardiologist can make sure that the leads are secured in their positions.

The patient will be asked to keep their left arm as still as possible and not to perform any sudden movements. They may be asked not to move their left arm above their head for up to a month after the procedure. This is to make sure that the pacing wires and electrodes do not get displaced.

Programming the Pacemaker

Before the patient goes home, the pacemaker must be programmed to ensure that it will respond correctly to the needs of the patient and his or her heart. To do this, a pacing technician places a magnet on the patient's chest over the site of the pacemaker. Using a computer wirelessly connected to the magnet, the technician can programme the pacemaker to work at the most effective setting for the particular patient's condition. To test the pacemaker's response, he or she may make the patient's heart beat very fast and very slow. This can be a strange feeling for the patient!

Living With a Pacemaker

People who have had a pacemaker implanted usually notice a great improvement in their energy levels and ability to take exercise. However, it may take some time for the pacemaker to get used to the patient's body and it may have to be adjusted to work at its best. The patient should go to the pacing clinic at regular intervals for a check-up to ensure that the pacemaker is working efficiently and effectively for his or her condition.

Pacemakers can affect and be affected by magnetic equipment, for example airport security gates. People with a pacemaker are advised to carry a card with details of their pacemaker on it so that they can bypass airport security gates. Carrying a pacemaker information card may also be useful to show to medical professionals who are not familiar with the patient and may be unaware that they have a pacemaker. Some people choose to wear a MedicAlert or similar tag on a necklace, wristband or bracelet to alert medical professionals or others that they have a pacemaker.

Mobile phones and even headphones can affect pacemakers, so people with pacemakers are advised to keep these at least six inches away from the pacemaker site. Most equipment in the home, for example microwave ovens, should not interfere with the pacemaker as long as they are well maintained.

One final thing about pacemakers - they cannot be cremated, so if a person wishes to be cremated the pacemaker will need to be removed before the cremation.

Please Note: h2g2 is not a definitive medical resource. If you have any health concerns you must always seek advice from your local GP. You can also visit NHS Direct or BBC Health Conditions.

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