Editors

PekkaRaatikainen

JarkkoKarvonen

Cardiac Pacemakers and Monitoring Their Function

Essentials

The indications for pacemaker therapy should be known and the principles of pacemaker function understood.
The most common pacemaker malfunctions, signs indicating the end of battery life and complications associated with the insertion of a pacemaker should be recognised.
It is important to understand the significance of arrhythmias (e.g. atrial fibrillation) on pacemaker function.

Principles of pacemaker function

A pacemaker system consists of a pulse generator, which is implanted subcutaneously below the clavicle under local anaesthesia, and one or more pacing leads, i.e. electrodes, which are inserted through a vein into the heart (picture 1).
A modern pacemaker continuously monitors the heart rhythm and will pace only when the heart rate falls below a set limit. A pacemaker identification card carries standardised codes in order to provide detailed information about the operation of the device (picture 2).
The pacing lead is nowadays always bipolar, but it can be programmed to function as unipolar.
- In unipolar pacing, the distal tip of the lead functions as one electric pole whilst the case of the pulse generator houses the other electric pacing pole. This system is more prone to be affected by external interference and may sometimes cause twitching of the muscle tissue in the chest. The pacing spike is clearly visible on an ECG, which will assist the evaluation of the pacemaker function.
- In bipolar pacing, the lead has both poles close to each other at the distal tip. With no current flow at the pulse generator casing, bipolar pacemakers are less prone to cause muscle twitching. Bipolar leads are superior to unipolar ones in sensing the heart rhythm despite external or myopotential interference. The pacing spike is small and sometimes difficult to see on an ECG.
Rate-responsive pacemakers contain a sensor (usually for monitoring body movement or breathing or both) and respond by changing the pacing rate (R=rate) according to physical activity.

Indications for pacemaker therapy

The main indication for pacemaker therapy is symptomatic bradycardia.
There must be documented evidence of the relationship between the symptoms and bradycardia. Before a permanent pacemaker is inserted, all treatable and transient causes must be excluded.
- In milder cases, withdrawal of bradycardia-inducing medication may be sufficient treatment.
- If bradycardia is caused by an acute illness (e.g. myocardial infarction), the need for pacing should be assessed only after the underlying condition has been treated.
In sinus node dysfunction pacemaker insertion is indicated if
- persistent bradycardia or occasional sinus arrest causes symptoms (e.g. dizziness, fainting or cardiac insufficiency)
- the patient's exercise tolerance is reduced due to significantly diminished heart rate response to exercise (chronotropic incompetence).
In atrioventricular block (AV block) the indications for pacemaker insertion are
- persistent third degree atrioventricular (AV) block
- Mobitz type II AV block (distal AV block).
- In symptomatic patients (syncope), also trifascicular block or Mobitz type I (Wenckebach) AV block is an indication for pacemaker insertion.
- The pacemaker is usually not needed in first degree AV block or in such intermittent Mobitz type I second degree AV block that is detected during rest.
Other indications for a pacemaker insertion include for example:
- severe cardiac insufficiency (biventricular pacing)
- reflex bradycardia.
In addition to a carefully taken history, a 12-lead ECG is the most important tool when evaluating the need for a pacemaker insertion. Other investigations often needed are continuous ambulatory ECG monitoring and clinical exercise testing. Invasive electrophysiological studies are seldom necessary.

Selection of pacing mode

The pacing mode is chosen according to the patient's symptoms and the cause of bradycardia (table T1). The aim is to prevent asystole and to restore both the normal variation in heart rate and atrioventricular synchrony.
Atrial pacamaker (AAIR) can, in principle, be used in diseases that are limited to the sinus node. A plain AAIR pacemaker has been abandonded almost completely. A physiological pacemaker is nearly always used also in diseases of the sinus node, because some patients will later develop also an atrioventricular block.
The most important indication for ventricular pacemaker (VVIR) is slow atrial fibrillation. It is also used in situations where there is no possibility or need for dual chamber pacing (e.g. infrequent episodes of bradycardia in an elderly patient).
- In special cases a leadless pacemaker that is inserted in the heart with a catheter may be used for ventricular pacing.
Dual chamber pacemaker (physiological pacemaker, DDD or more rarely VDD) is the first-line choice for the treatment of AV block and nowadays also for defects of the sinus node.
- It can also be used for the treatment of reflex bradycardia (DDD).
Heart failure pacemaker (biventricular pacing) is used in the treatment of severe cardiac insufficiency in patients with left bundle branch block (LBBB). By improving the contractility of the left ventricle (resynchronisation), it alleviates symptoms and reduces both mortality and the need for hospitalisation.
Conduction system pacing (CSP) (His bundle pacing or left bundle branch pacing) can be used instead of regular ventricular pacing or as an alternative to biventricular pacing. The initial results are promising and this pacing method most likely becomes more common in the future.
Rate-responsive pacemaker (R mode) is used if heart rate response to exercise is diminished.
The pacing mode must be changed if the patient's intrinsic heart rate or clinical situation changes.

The most common pacing modes and their indications

	Pacing mode	Indications	Function	Disadvantages
AAI	Atrial pacing	Sinus node dysfunction	Senses and paces the atrium	Requires normal atrioventricular conduction
VVI	Ventricular pacing	Slow atrial fibrillation Rarely occurring bradycardia	Senses and paces only the ventricle	Does not restore atrioventricular synchrony Pacemaker syndrome
VDD	Single-lead physiological pacing	Atrioventricular block	Senses the atria and ventricle, paces only the ventricle	Does not pace the atria Inadequate sensing of atrial activation
DDI	Atrioventricular sequential pacing	Atrioventricular block + atrial arrhythmia	Senses and paces the atria and, if necessary, the ventricle	Does not use atrial tracking to pace the ventricle
DDD	Dual chamber pacing	Atrioventricular block Sinus node dysfunction*	Senses and paces the atria and the ventricle Changes the heart rate	Pacemaker-mediated tachycardia
_ _ _ R	Rate-responsive pacings	Sinus node disease (AAIR/DDDR) Slow atrial fibrillation (VVIR)	Changes the heart rate according to sensor activity, may be connected to all modes except VDD	The kinetic sensor may erroneously react to vibration (e.g. when using a chain saw)

^* When DDD pacing is used in the treatment of sinus node dysfunction, the device must be programmed to minimise ventricular pacing.

Follow-up of a patient with an implanted pacemaker

A specialist physician will implant the pacemaker and also be in charge of the follow-up.
The first follow-up appointment is usually 1-3 months after the implantation followed by appointments every 1-2 years and when needed. When the battery starts to show signs of depletion, the patient should be seen more frequently.
Nowadays, the majority of the visits to a pacemaker clinic may be replaced by remote pacemaker follow-up.

Investigations at a pacemaker clinic

Information stored in the pulse generator's memory
- Rate histograms and other data automatically stored in the device's memory yield important information about the pacemaker function and, for example, of the occurrence of atrial fibrillation.
Battery life
- The normal battery life of modern pacemakers is usually at least 10 years.
- Signs suggestive of low battery life include the pacing mode becoming more simple (e.g. DDD→VVI, R function no longer operational) and pacing rate falling below the set limit.
- The patient must be instructed to contact the pacemaker clinic if his/her pulse is consistently below the programmed lower limit.
Pacing lead performance
- A significant change in the impedance is suggestive of lead damage. If damage is suspected, an x-ray should be requested.
Pacing threshold
- The pacing (stimulation) threshold is the minimum amount of energy needed to generate activation of the paced cardiac chamber(s) (atrium, ventricle or both).
- The voltage (usually 1.0-5.0 V) and duration (0.3-1.0 ms) of the output stimulus are set at least 2 times higher than the threshold in order to provide an adequate safety margin.
- An increased threshold may indicate migration of the tip of the lead, development of fibrosis around the tip or lead fracture.
Detection of intrinsic cardiac activity
- The amplitude of the atrial complex (P wave) is usually 1.5-5 mV and that of the ventricular complex (R wave) 5-20 mV.
- The device is usually programmed so that it senses one's own cardiac rhythm but does not sense other signals (oversensing).
Underlying intrinsic rhythm and atrioventricular conduction
- Dependence on the pacemaker
- Interaction between the intrinsic rhythm and the paced rhythm
Checking rate-responsive pacing (R mode) and other specialist features
Modern pacemakers perform the majority of the above-mentioned tests automatically at regular intervals, and the measurement results can be retrieved from the memory of the pacemaker device either at the pacemaker clinic or through remote monitoring.
Additionally, it is generally recommended to check 12-lead ECG.

Indications for pacemaker clinic consultation

The pacemaker clinic should be consulted, or an extra visit arranged, should any of the following signs and symptoms occur:
- bleeding (large haematoma) at the insertion site
- pneumothorax
- diaphragmatic twitching or twitching around the pulse generator casing
- continuous pain around the pulse generator
- skin erosion or signs of infection around the pulse generator (an infected pacemaker and leads will almost always need to be replaced)
- recurrence of episodes of syncope or other symptoms
- ECG findings suggestive of pacemaker malfunction (table T2).

The most common types of pacemaker malfunction and associated ECG findings

Malfunction	ECG finding	Cause
Failure to output	No pacing spikes despite the underlying heart rate being slower than the programmed pacing rate.	Oversensing Faulty pacing lead or pulse generator Erroneous ECG interpretation
Failure to capture	The pacing spike is not followed by either a QRS complex or a P wave (exit block)	Lead dislodgement or fracture Certain drugs, electrolyte imbalance, myocardial ischaemia, fibrosis Electrical cardioversion
Undersensing	The pacemaker paces despite adequate intrinsic activity	The sensitivity of the pacemaker has been incorrectly programmed Faulty lead or pulse generator
Oversensing	Incorrect pacing rate and/or pauses in pacing Arrhythmia initiation (R-on-T)	Myopotential interference External interference Far-field sensing
Pacing rate faster than programmed	The pacemaker paces faster than the set lower pacing limit. Broad-complex tachycardia with pacing spikes	Pacing is guided by a sensor (R mode) Pacemaker tachycardia
Pacing rate slower than programmed	Pacing rate is slower than programmed. Intrinsic heart rate slower than the pacing rate programmed Pacing with a fixed rate irrespective of intrinsic cardiac activity (asynchronous pacing mode)*	The battery is nearing the end of its life The hysteresis feature or night time slowing of pacing rate is activated, oversensing Interference* caused by diathermy, defibrillation, MRI or other factor

* The frequency of fixed-rate pacing (asynchronous mode) triggered by interference is dependent on the device. The device will return to normal function on the disappearance of the disturbance. Should interference switch the pacemaker to the safety mode, the programming unit is needed to return normal function.

Innocent phenomena

Innocent phenomena that are part of the normal operation of the pacemaker but are often thought to be signs of malfunction are described below.
- Hysteresis feature: the pacemaker is set to sense down to a programmed hysteresis rate (e.g. 50/min). If heart rate falls below the hysteresis limit, the device will then begin to pace at the programmed pacing rate (e.g. 60/min). Some pacemakers allow circadian variation characterised by a slower night time pacing rate (night time setting).
- R mode: the actual pacing rate may be higher than the programmed lower pacing rate.
- A fusion beat can occur due to the pacemaker firing on an intrinsically occurring complex. This phenomenon is harmless.

Pacemaker malfunction

The most common types of pacemaker malfunction and associated ECG findings have been presented in table T2.
If the pacemaker fails to capture, a pacing spike is not followed by a contraction of the heart (exit block).
- If the intrinsic rhythm is faster than the programmed pacing rate, it is possible to make the device pace momentarily at a fixed rate by placing a magnet over the pulse generator.
Pacemaker malfunction is unpleasant for the patient, and may be dangerous.
- If the pacemaker is oversensing it presumes that no pacing is needed. This may lead to long pauses.
- If the pacemaker is undersensing it delivers paced beats despite an adequate intrinsic rate. It is therefore possible that the timing of a paced ventricular beat is such that it triggers ventricular tachycardia or fibrillation (R-on-T).
If pacemaker malfunction is suspected, a 12-lead ECG must be studied and an assessment made whether the sensing and pacing occur in accordance with the data presented in the pacemaker identification card. If the ECG does not solve the problem, the pacemaker clinic personnel must be consulted regarding follow-up investigations and treatment.

Pacemaker syndrome

In pacemaker syndrome, a paced ventricular beat is followed by retrograde ventriculo-atrial conduction which causes almost simultaneous contraction of both the atria and ventricles leading to reflex hypotension, lethargy and syncope.
The treatment consists of reducing the rate of ventricular pacing or converting to a different pacing mode.

Pacemaker-mediated tachycardia

Pacemaker-mediated tachycardia is a form of reentrant tachycardia that can occur during dual chamber pacing (DDD). A paced ventricular beat is conducted retrograde to the atria and sensed by the pacemaker to generate another paced ventricular beat.
The rate of tachycardia is the upper pacing rate programmed to the device. Treatment consists of reprogramming. A magnet placed over the pulse generator may be used as a first aid measure.
Atrial fibrillation or flutter may also cause DDD pacing at the upper rate when the fast atrial rate triggers fast ventricular pacing. This can be avoided by switching to the DDI mode. Modern pacemakers are able to do this automatically (automode switch).

External interference

Pacemakers are equipped with an internal protection system against external interference. The system is activated when the device senses strong external interference. Pacemaker inactivation is thus avoided, and the pacemaker will continue to pace, irrespective of intrinsic cardiac activity, delivering a fixed rate (asynchronous mode) until the interference has disappeared.
Ordinary domestic appliances and office devices as well as mobile phones are mostly harmless relative to a pacemaker.
- Malfunctioning devices may sometimes interfere with pacemaker's functioning.
- TENS (Transcutaneous Electrical Nervous Stimulation) used to treat pain may cause pacemaker malfunction, and it should not be used without consulting the pacemaker clinic.
Many hospital investigations and procedures may affect pacemaker operation.
- When diathermy is used to control bleeding during surgical operations, brief bursts of energy and primarily bipolar diathermy should be used, and the neutral electrode should be placed as far from the pacemaker as possible.
- The operation of the pacemaker must be verified after the procedure if the risk of electromagnetic interference is high (distance of diathermy from the pulse generator less than 15 cm), if abnormal pacemaker functioning was observed during the procedure (oversensing), or if any changes to the pacemaker functioning were made for the duration of the procedure.
- Before radiotherapy, the dose that the pacemaker will receive should be assessed. In the majority of cases, radiotherapy can be carried out without problems. If the pacemaker gets an exceptionally high dose (the pacemaker is within the radiotherapy field), it may be necessary to remove the pulse generator and to place a new pacemaker system to the other side before radiotherapy is started.
- Magnetic resonance imaging (MRI) has traditionally been contraindicated, but in recent years the industry has introduced pacemakers and leads that allow MRI to be performed.
Pacemaker clinic personnel should be consulted regarding any occupational restrictions.

Arrhythmias in patients with an implanted pacemaker

Arrhythmias are common and may cause problems suggestive of disturbed pacing or sensing.
- During atrial fibrillation, extra pacing spikes may be evident on an ECG as the device is not always able to sense the fine fibrillation waves.
- If an extrasystole occurs during the pacemaker's refractory period, it may erroneously appear that the pacemaker is not working.
The guidelines for drug treatment in arrhythmia are similar to those given for other patients, but particular caution must be exercised during electrical cardioversion.
- A pacemaker does not remove the need for anticoagulation in atrial fibrillation.
- The paddles (or pads) of the defibrillator must be placed as far from the pulse generator as possible in such a way that the electric field runs perpendicular to the course of the pacemaker leads (the use of anterior and posterior pads is recommended).
- After electrical cardioversion, the operation of the pacemaker must be verified at the pacemaker clinic or through remote monitoring.
The data stored in the memory of the pulse generator may be used in the diagnosis of atrial fibrillation and other arrhythmias.
- Modern models record abnormal rhythms in the device's memory in ECG format.

Removal of a pacemaker

Pacemakers are usually not reused, but they should be removed from deceased persons before burial and appropriately destroyed because the battery of the device is hazardous waste. The removal is particularly important before cremation because the battery may explode when heated.
- A leadless pacemaker is not removed and, due to the small size of its battery, it does not prevent cremation.
In some cases, the data stored in the memory may be used to determine the cause of death.

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