Pacemakers and pacing

There are separate articles on Pacemaker Syndrome and Pacemaker Complications, Inserting Temporary Pacemakers, and Implantable Cardioverter Defibrillators.

• Pacemakers provide electrical stimuli to cause cardiac contraction during periods when intrinsic cardiac electrical activity is inappropriately slow or absent.

• Pacing systems consist of a pulse generator and pacing leads.

• Pacemaker output generally stimulates the cavity of the right atrium and/or right ventricle (endocardial pacing). Alternatively, epicardial leads can be implanted surgically on to the heart's surface.

• Battery life varies depending on the type of pacemaker and indication. Figures quoted by the manufacturers therefore range widely from 5-13 years¹.

Pulse generators

• The pulse generator is internal in permanent pacemakers (subcutaneously or submuscularly) and external in temporary pacing.

• It can be set to a fixed-rate (asynchronous) or demand (synchronous) mode.

• In the fixed-rate mode, there is a small risk of producing dangerous dysrhythmias if the impulse coincides with the vulnerable period of the T wave.

• On-demand pacemakers detect spontaneous ventricular activity and the output of the pacemaker is either suppressed or discharged in order to make the impulse fall within the safe period of the QRS complex.

Types of pacemaker²

Unipolar and bipolar pacemakers³

• Permanent leads are either unipolar (where a single contact is made with the heart) or bipolar.

• Unipolar systems (ventricular) are used in cases where atrioventricular (AV) conduction is likely to return.

• When there is normal AV conduction and a sinoatrial (SA) disorder then the pacing wire is situated in the right atrium.

• Quadripolar leads have recently been developed which offer more targeted cardiac resynchronisation.

Dual-chamber pacemakers

• These have pacing electrodes in both the right atrium and the right ventricle.

• They allow maintenance of the physiological relationship between atrial and ventricular contraction and also allow the paced heart to follow the increase in sinus rate that occurs during exercise.

Dual-site atrial pacing

• Newer pacing systems have two atrial leads, one in the right atrial appendage and the other either in the coronary sinus or at the os of the coronary sinus.

• The ventricular lead is in the right ventricle, either at the apex or at the outflow tract.

• One study found that dual-site pacing improved survival and rate control in patients with atrial fibrillation and heart failure with and without systolic left ventricular dysfunction.

Biventricular pacemakers

• Pacemaker leads are placed in the right atrium, right ventricle and left ventricle.

• Useful in the management of patients with heart failure who have evidence of abnormal intraventricular conduction (most often evident as left bundle branch block (LBBB) on ECG) which causes deranged ventricular contraction or dyssynchrony. Recently, His bundle pacing has been found to produce superior results to biventricular pacing⁴

Implantable cardioverter defibrillators (ICDs combined with internal defibrillator)

• Designed to treat a cardiac tachyarrhythmia directly.

• If a patient has a ventricular defibrillator and the device senses a ventricular rate that exceeds the programmed cut-off rate of the defibrillator, the device performs cardioversion/defibrillation.

• Alternatively, the device, if so programmed, may attempt to pace rapidly for a number of pulses, usually around 10, to attempt pace-termination of a ventricular tachycardia.

Novel developments³
The last few years have seen numerous advances in the field of pacemaker development. These have principally addressed weaknesses in current pacemaker design. Small, completely intracardiac units without transvenous leads have been developed to overcome many of the problems related to hardware complications.

Direct pacing via the His bundle results in a more physiological approach to pacing. The use of flexible sheets of piezoelectric wires to convert cardiac motion to energy in order to power pacemaker devices offer a promising alternative to energy-limited batteries. Efforts are also being directed towards creating biological pacemakers using gene therapy to increase automaticity of existing non-pacemaker cardiac myocytes.

Pacemaker codes

The North American Society of Pacing and Electrophysiology and the British Pacing and Electrophysiology Group have developed a code to describe various pacing modes. It usually consists of three letters, but some systems use four or five:

• Letter 1: chamber that is paced (A = atria, V = ventricles, D = dual-chamber).

• Letter 2: chamber that is sensed (A = atria, V = ventricles, D = dual-chamber, 0 = none).

• Letter 3: response to a sensed event (T = triggered, I = inhibited, D = dual - T and I, R = reverse).

• Letter 4: rate-responsive features; an activity sensor (eg, an accelerometer in the pulse generator) in single or dual-chamber pacemakers detects bodily movement and increases the pacing rate according to a programmable algorithm (R = rate-responsive pacemaker).

A pacemaker in VVI mode denotes that it paces and senses the ventricle and is inhibited by a sensed ventricular event. The DDD mode denotes that both chambers are capable of being sensed and paced.

Indications for permanent pacing⁵

• Persisting symptomatic bradycardia.

• Complete AV block (Stokes-Adams attacks, asymptomatic, congenital), Mobitz types I and II AV block, persistent AV block post anterior myocardial infarction.

• Pacemakers may have a role in the suppression of resistant tachyarrhythmias.

• Prevention of atrial fibrillation⁶.

• Pacemakers have a role in the management of some patients with dilated cardiomyopathy or hypertrophic obstructive cardiomyopathy .

The National Institute for Health and Care Excellence (NICE) recommends that dual-chamber pacemakers can be used to treat symptomatic bradycardia in people with sick sinus syndrome, AV block, or both, but identified a number of circumstances where dual-chamber pacemakers should not be used for symptomatic bradycardia⁷. In the light of subsequent trials, NICE amended their guidance to include symptomatic bradycardia due to sick sinus syndrome without atrioventricular block as an indication for a dual‑chamber pacemaker⁸.

Precautions for patients

Driving⁹

The patient must inform the Driver and Vehicle Licensing Agency (DVLA) that they have a pacemaker.

For an ordinary driving licence

• The patient can start driving again after one week as long as:
  

  • There are no symptoms such as dizziness or fainting which may affect driving.

  • The patient attends regular check-ups in the pacemaker clinic.

  • The patient has not recently had a heart attack or heart surgery.

For a large goods vehicle (LGV) or passenger-carrying vehicle (PCV) licence

• The patient cannot drive these vehicles for six weeks after the pacemaker is fitted.

• The patient can apply for another licence when they no longer have any symptoms that would affect driving - eg, dizziness or fainting.

• The current licence is replaced with a three-year licence and the patient will have to go to a pacemaker clinic regularly.

Sports

• Any strenuous activity should be avoided for about three to four weeks after the pacemaker has been fitted. After that, the patient can continue or start most activities and sports.

• For contact sports, care should be taken to avoid collisions that may damage the pacemaker, and a protective pad should be considered.
  

Hospitals and medical treatment

• A doctor or technician should be informed that the patient has a pacemaker before any investigations or treatment.

• Always show the pacemaker registration card to any doctor or dentist providing treatment.

• Most pacemaker generators have an X-ray code that can be seen on a standard CXR.

• Some hospital equipment, including equipment used in surgery, may interfere with pacemakers. The pacemaker may need to be protected during any operation and reprogrammed afterwards.

• Radiotherapy may damage the pacemaker's circuits. The degree of damage is unpredictable and may vary with different systems. But the risk is significant and builds up as the radiation dose increases. The pacemaker should be shielded as much as possible and moved if it lies directly in the radiation field.

• MRI scans can be dangerous with a pacemaker and the patient should not have an MRI scan. If an MRI scan is absolutely necessary, the pacemaker output in some models can be reprogrammed.

• Short-wave or microwave diathermy may bypass the pacemaker's noise protection and interfere with or permanently damage the pulse generator.

• Transcutaneous electrical nerve stimulation (TENS) may sometimes briefly inhibit unipolar pacing, which then requires reprogramming of the pulse generator.

Outside interference

• Most pacemakers are very resistant to outside interference and the pacemaker has special circuits to detect and remove unwanted electrical activity.

• However, devices with risk include anti-theft systems in shops and other business premises, and metal detectors. They are unlikely to cause clinically significant symptoms in most patients but patients should not stay nearby for longer than is necessary.

• Any hand-held metal detector should not be held near the pacemaker for any longer than is necessary.

• Household devices such as shavers, hairdryers and microwave ovens are not a problem, as long as they are well maintained.

• The following items can be used when they are kept 6 inches away from your pacemaker (it is usually the motor that may cause an electromagnetic field): hand-held hair dryers and older shavers with an electrical cord; pagers; sewing machines and servers (sewing machines that overcast edges to prevent fraying); electric toothbrush and the base charger of an ultrasonic toothbrush; large stereo speakers which often have large magnets. Do not lift large stereo speakers close to your pacemaker; when using an induction range for cooking keep your pacemaker 2 feet from the range.

• Household tools such as drills, mowers and electric screwdrivers can be used normally.

• A mobile phone or a cordless phone can be used safely, but the phone should be kept more than 6 inches away from the pacemaker. The ear on the opposite side to the pacemaker should always be used, and the phone should not be put in a pocket over the pacemaker.

Travelling and security systems

• Airport screening systems and anti-theft systems in shops and libraries may (rarely) cause problems and there is also a small chance that the pacemaker may trigger the alarms.

• The pacemaker registration card should always be carried by the patient.

• If a patient with a pacemaker has to go through a security gateway, they should go through quickly and not stand close to the gateway for too long.

At work

• Some workplaces have strong electromagnetic fields which can interfere with the pacemaker - eg, arc welding.

• Power-generating equipment, arc welding equipment and powerful magnets (as in medical devices, heavy equipment or motors) can inhibit pulse generators and there is a risk that the pacemaker may not work properly for patients who work closely with or near such equipment.