by Oliver Moras
Pacemakers are devices which are used in order to treat arrhythmia (irregular heartbeat). There are 2 types of arrhythmia: bradycardia, which is a slow heartbeat (less than 60 bpm) and tachycardia, which is a fast heartbeat (more than 100 bpm). Irregular heartbeat can be caused by congenital heart disease (heart conditions present since birth), certain heart medications, natural aging, sick sinus syndrome and heart block.
Sick sinus syndrome is the condition where the sinus, the natural pacemaker of the heart, does not function properly. The sinus node sends signals which cause the atria (upper chambers of the heart) to contract, pumping blood to the ventricles (lower chambers of the heart). This syndrome could cause heartbeats to be missed (as no signal is sent) or the heart rate to be slowed (as signals are being sent slower). Furthermore, it may mean that the heart is unable to increase its heart rate which is problematic as during exercise, insufficient blood would be circulated so insufficient oxygen would be provided to body cells for respiration (the process where energy is produced). When this occurs, other tissues like the atrioventricular node take over; however, the atrioventricular node tends to be too slow, fast or unable to maintain a consistent heart rate therefore a pacemaker is required.
On the other hand, the heart block is where the atrioventricular node, which passes the electric signal from the right atrium to ventricles, becomes slower or irregular (signals are blocked). Heart block can be described as 1st, 2nd or 3rd degree, where each degree relates to the extent of how slow the heart rate is.
There are 3 main types of pacemakers. Pacemakers used to treat bradycardia have 2 main functions: to pace and to sense. Pacing is where an electric impulse is sent to the heart when it's rhythm is slow or interrupted, starting a heartbeat. Sensing is the monitoring of the natural heart beat and if it is present, the pacemaker will not send an electric impulse.
Currently, the 2 main types of this pacemaker used are signal chamber pacemakers, pacemakers with 1 lead which connects to the right atrium or ventricle, or dual chamber pacemakers, pacemakers with 1 lead which connects to the right atrium and 1 lead which connects to the right ventricle. While both types of pacemaker are used in modern medicine, nowadays, dual chamber pacemakers are used for the vast majority of cases in the UK as the expense and risks associated with both pacemakers and procedures to implant them are virtually identical. Dual chamber pacemakers are implanted into patients who only need a single chamber pacemaker but 1 lead is left inactive and powered off. The benefit is that incase 2 leads are required later in the patients life, another operation is not necessary, significantly removing potential risks and complications related to future operations. Also, pacemakers are able to monitor and record your heartbeat and this data is sent to a small device which the patient keeps at home that is able to relay the information to the hospital, cancelling the need for frequent hospital trips.
The second type of pacemakers are implantable cardioverter defibrillators. In order to deal with life-threatening arrhythmias such as arrhythmias which can cause cardiac arrest, they are able to first sense the arrhythmia and then send a much larger electric impulse which shocks the heart in order to correct its rhythm and stop fibrillations (uncoordinated twitching of muscle fibres)—hence the term, defibrillator. Most of these devices act as both a pacemaker and an implantable cardioverter defibrillator however they are extremely expensive (the standard devices used In Queen Alexandra Hospital are around £10 000 per device) but they are implanted as a preventative treatment for cardiac arrest susceptible patients.
Lastly are cardiac resynchronisation therapy (CRT) devices also known as biventricular devices. They are used to synchronise the pumping of the ventricles in patients who have hearts with chambers that contract in an incorrect sequence. Just like the other pacemakers, they are able to sense the natural beating of the heart and send electric implies when needed but now to many more areas of the heart. The regular device is a CRT pacemaker but another device called the CRT defibrillator that has a defibrillator feature, which similar to implantable cardioverter defibrillators, is able to send a much larger impulse as a preventable treatment for cardiac arrest, etc. Both CRT devices use three leads: 1 lead which connects to the right atrium, 1 lead which connects to the right ventricle and 1 lead which connects inside a vein on the left atrium, in order to be able to control the contracting of the whole heart.
The main caution with pacemakers and other implanted devices is the risk of electromagnetic interference which can be caused by strong electromagnetic fields. This stops the sensing of the pacemakers; although, there are electronic filters and physical adaptations to reduce interference. Regardless, lots of activities and situations must be avoided in order to not damage the pacemakers (such as having an MRI).
While these are currently the most circulated pacemakers in hospitals, medicine is a constantly and rapidly developing field. Many more devices and even alternative routes of treatment for heart related conditions are being continuously developed thus a description of pacemakers in the future may look very different to what it looks like now. Already, leadless, pill-size pacemakers are being tested and could become a new standard soon and so the development of pacemakers, much like with every other aspect of medicine, is only beginning.
Sources:
https://www.medtronic.com/uk-en/patients/treatments-therapies/crt-devices.html
https://www.nhs.uk/conditions/pacemaker-implantation/
https://medlineplus.gov/pacemakersandimplantabledefibrillators.html
Medtronic: FOR YOUR PACEMAKER
Medtronic: LIFE WITH A PACEMAKER
Biotronik: What You Need to Know about Your Pacemaker - Patient’s manual
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