Implantable pacemakers and defibrillators are highly sophisticated microelectronic devices, which monitor the heart's activity.
They diagnose harmful rhythms and provide corrective therapy. This is done by either directly stimulating the patient's heart muscle with small electrical impulses, or by shocking it with a high voltage energy pulse. They are software controlled and can be programmed using an external programmer. This is similar to a laptop computer. Both pacemakers and defibrillators restore the natural rhythm of the heart.
Both types of device are fully implanted in the patient in the upper chest region and mostly just below the left collar bone. The majority of heart leads are passed through a vein and positioned inside the heart.
Implantable pacemakers
Implantable pacemakers have mainly been used in the treatment of bradycardia (slow heart rat/heart block). However, advances in technology have now enabled other heart conditions to be treated such as atrial fibrillation and tachycardia. Some pacemakers have also been developed to assist heart failure patients, especially those who's left and right heart chambers are not naturally synchronised.
A pacemaker contains microelectronics, telemetry communications systems and a battery. It works by sensing the electrical activity of the heart and deciding what type of corrective therapy is required. When, for example a slow heart needs correcting, it will generate a tiny electrical impulse and send it to the heart via insulated heart wires (pacing lead).
A small electrode at the tip of the heart lead (in contact with the heart muscle), allows the pulse of energy to stimulate the muscle to produce a more regular heart beat. The pacing pulses are timed so that the heart beats in a manner very similar to a naturally occurring heart rhythm. When natural heart beats occur, the pacemaker senses these through the same heart lead and stops delivering electrical pulses.
Implantable defibrillators
Implantable defibrillators contain special arrhythmia detection algorithms, which sense when harmful ventricular tachycardia (VT) or ventricular fibrillation (VF) takes place. Like pacemakers, they also contain electronics, software and a battery and use heart leads. They are similar to a pacemaker in that they are designed to correct abnormal heart rhythms. A defibrillator does this by either pacing the heart quickly with small electrical pulses, or by delivering larger high voltage shocks to the heart muscle to 'neutralise' harmfully fast heart rhythms.
Adjusting implantable pacemakers and defibrillators
Implantable pacemakers and defibrillators are adjusted (programmed) according to individual patient needs. Settings are adjusted non-invasively using an external programmer (rather like a lap-top computer) which uses a form of telemetry communication. Bi-directional data communication takes place harmlessly through the skin and device parameters are set. Pacemakers and defibrillators have substantial electronic data processing capabilities and electronic memories that can store a whole range of information about a patient's condition as well as data about how the device is performing. Clinicians use programmers to access important information about how the implant is functioning (battery condition, condition of heart leads etc) and how often certain types of therapy are delivered.
