Usage
Pacemakers are implanted in the chest of patients and help to control ones heartbeat. They make the heartbeat of a patient more regular. They are often implanted following a medical emergency such as a heart attack or overdose but can also be permanently implanted to correct slow and/or irregular heartbeats. They only work when needed. For example, if your heart rate is too slow it will send pulses to speed it up. Newer pacemakers can also detect body motion or breathing rate, and increase heart rate during exercise. There are some newer pacemakers being developed that are implanted directly into the heart, and thus do not require leads, minimizing some risks and increasing speed of recovery.
Types
- Single Chamber - Usually carries electrical impulses to the right ventricle
- Dual Chamber - Carries impulses to right ventricle and right atrium to help control the timing of contractions between the two chambers
- Biventricular Pacemaker – Also called cardiac resynchronization therapy, this is for people with heart failure and abnormal electrical systems. Stimulates both ventricles to make ones heart beat more efficient.
stability. Some leads are also coated with polyurethane, which increases the ability of the lead to glide.
Pacing Codes
Pacemakers are assigned a code that consists of 3-5 letters, denoting the function of the pacemaker in question. They are assigned these codes use a programmer, which is also used to save the data from a device, as well as trouble shoot those which are malfunctioning.
First Letter
- Indicates the chamber(s) paced: A = Atrial Pacing, V = Ventricular Pacing, D = Dual-chamber
Second Letter
- Indicates the chamber in which electrical activity is sensed: A, V and D = Same as before, O = pacemaker discharge is not dependent on sensing electrical activity
Third Letter
– Response to a sensed electrical signal: T = Triggering of pacing function, I = inhibition of pacing function, D = dual response (ex. Spontaneous atrial and ventricular activity inhibits pacing, but lone atrial activity triggers paced ventricular response), O = No response to underlying electrical signal (normally present when sensing function, second letter, is O as well)
Fourth Letter
– Represents rate modulation: R = rate-response (“physiologic”) pacing, O = no programmability
Fifth Letter
– Represents multisite pacing: A = Atrial, V = Ventricular, D = Dual (pacing + shock)
The simplest settings are VVI and AAI. VVI mode senses and paces the ventricle and is inhibited by a sensed ventricular event. AAI does the same, but in the atrium. The most common setting, on the other hand, is DDD which means that both chambers are capable of being sensed and paced.
Magnet Inhibition
Pacemakers include a feature where a magnet can be placed over the chest in the region the device is implanted. This temporarily makes the pacemaker go into asynchronous mode, in which the device begins pulsing at a rate that is determined by the battery life remaining. This feature thus allows medical professionals to determine when a pacemaker should be replaced, and also allows them to make a pacemaker enter a generalized pattern if there are issues, such a patient getting shocked from their implant. Some devices can be programmed to get rid of this magnetic response and will this require a device programmer to change their parameters. Show on the right is the style of magnet used, which is placed on the chest of a patient, over their pacemaker, when inhibition is needed.
Replacement
The battery in a pacemaker is sealed inside, and thus when the battery dies the pacemaker will need to be replaced. Battery levels are checked by doctors and burses, and pacemakers themselves can check their batteries to see when they are getting low. When medical professionals check the battery levels, they do so through magnet inhibition. Pacemakers are coded to adjust their asynchronous rates based off of their battery levels. A common rate is 100 bpm for a fully charged pacemaker, and this drops to 85 bpm when replacement is recommended. These rates do vary between manufacturers, so knowledge of the model implanted is important to ensure patient safety.
Common Malfunctions
1. Failure to Output – Occurs when the pacemaker fails to provide an output signal. Can be caused by battery failure, lead fracture, oversensing, poor connection, and many other reasons. Can be assisted using medication or a replacement pacer. A chest radiograph is necessary to check what is truly wrong.
2. Failure to Capture – Occurs when pacing signals are not followed by contraction of the heart. There are many reasons this can occur, including lead dislodgement, drugs, and fractured insulation. Management of these complications is very similar to that of output failure, with temporary pacing being used until a technician can take a closer look
3. Oversensing – Occurs when pacing incorrectly senses electrical activity that is not cardiac and inhibits pacing when it shouldn’t. This may lead to a lower heart rate than desired, and can be caused by muscular activity, electromagnetic interference, or fractured lead insulation. This is diagnosable and treatable with magnet inhibition, that places the pacemaker in asynchronous mode.
4. Undersensing – Occurs when pacemaker incorrectly misses intrinsic depolarization (when the heart beats on its own) and sends a signal regardless. This means that it is essentially operating in asynchronous mode, for reasons that match those in the error types listed above.
5. Pacemaker-mediated Tachycardia – If a premature ventricular contraction (PVC) is transmitted backwards to the atrioventricular node, it can depolarize the atria. This will be detected by the atrial sensor, which will stimulate the ventricular leads to fire, creating an endless loop. A maximum rate is limited by the pacemakers hard coded upper limit, but maintaining this maximum rate for extended periods of time can be extremely unhealthy, and lead to ischemia. Magnet inhibition can be used to slow the pacemaker by forcing it into asynchronous mode.
6. Runaway Pacemaker – Malfunction in the pacemaker generator resulting in life threatening tachycardia (up to 200 bpm). Most commonly caused by battery failure or external damage. Requires immediate medical action. Magnet inhibition can help, but commonly damage to the device prevents it from responding to the magnet. Surgery to quickly cut the leads of the pacemaker may be necessary.
7. Pacemaker Syndrome – Phenomenon in which a patient feels symptomatically worse after getting a pacemaker and continues to get worse symptoms of congestive heart failure. Mainly due to lost of atrioventricular synchronization. This can reverse the pathway to have a ventricular origin, making the atrial contribution to the pumping severely decrease, bring cardiac output and blood pressure down with it. This can be fixed by altering the pacemaker to improve the synchronization, possibly requiring a double-chamber pacemaker instead of single.
8. Twiddler Syndrome – Some patients persistently disturb the pacemaker generator, resulting in malfunction. Radiographs of the chest often reveal twisting, coiling, lead fracture, dislodgement, or migration. Surgical correction is necessary and the patient may need to be educated on what they have done to affect the pacemaker in such a way.
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