In a previous article, we covered the basic structure of the heart, its conduction system and the cardiac cycle. Here, we are going to discuss, on a basic level, what an artificial cardiac pacemaker is and give an overview of why a patient may need one inserted.
What is an artificial cardiac pacemaker?
A pacemaker is a small electrical device that is surgically implanted into a patient's chest. It generates electrical impulses and delivers them, through electrodes, to the heart muscle chambers, causing them to contract.
The pacemaker weighs between 20 and 50 grams and is about the size of a matchbox. It consists of a pulse generator (made up of a computer circuit and battery) and one or more leads which carry the impulse to the heart.
Almost all pacemakers work on demand. This means that they can be programmed to adjust the discharge rate in response to the body's needs. For instance, if the pacemaker detects that the heart is beating abnormally, it will emit electrical pulses to remedy this. It will do nothing if the heart is beating correctly. Many modern pacemakers also have sensors that recognise body movement and breathing rate.
What is an implantable cardioverter defibrillator?
This is a device similar to a pacemaker. It can detect when the heart is beating dangerously abnormally, then deliver a large electric shock to "reboot" it and prevent the body going into cardiac arrest. It reestablishes a normal heart rhythm.
Some devices now include both a pacemaker and an ICD in one unit. A conventional ICD has a pacing lead that is implanted along a vein (transvenously). However, newer ICD's have pacing leads implanted below the skin (subcutaneously).
Why might you need a pacemaker to be implanted?
If the electrical signals which control the heart become disrupted, it can lead to several potentially dangerous heart conditions such as:
- Bradycardia (an abnormally slow heartbeat)
- Tachycardia (an abnormally fast heartbeat)
- Heart block (where the heart beats irregularly because the electrical signals are not transmitted properly)
- Cardiac arrest (where issues with the electrical signals cause the heart to stop beating)
To prevent and/or remedy these conditions, a pacemaker made need to be implanted.
Specific Conditions
The following diseases can cause the dangerous conditions mentioned above and affected patients made need to be fitted with a pacemaker.
1) Sick Sinus Syndrome - This is a condition where the SA node does not work correctly. This can lead to bradycardia, tachycardia or a combination of both. Most cases are related to age, where the SA node tissue becomes hardened or scarred. This can disrupt the electrical impulses that it delivers. Some medications (like beta blockers) can also cause this condition.
2) Atrial Fibrillation - A condition that cause the heart to beat abnormally fast (often over 140 beats per minute). Most affected people respond to medication, but those who do not may require the implantation of a pacemaker.
3) Heart Block - This is when a pulse that needs to be sent from the SA node to the AV node is either delayed or absent. This can be caused when the heart is damaged (acquired heart block) or when a baby is born with 1 or more heart conditions (congenital heart block).
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