Pacemaker
Pacemakers (PMs) are small computer devices incapsulated in titanium and epoxy which are used to treat bradyarrhythmias and sinus arrest. These devices lie in a subcutaneous “pocket” on the chest and has electrode leads going into the heart. There is always an electrode to the right ventricle; electrode to the right atrium is optional depending on the indication and pacemaker "mode", and if cardiac resynchronisation therapy (CRT) is desired, an extra electrode is implanted in a vein outside the left ventricle. The pacemaker delivers electrical pulses to the cardiac muscle, stimulating a contraction.
The device either continuously paces the heart or paces the heart when it detects that the heart rate is going below a certain threshold, depending on the settings.
The most important indications for pacemakers and the modes used are:
- Sick sinus syndrome (SSS), especially with tachy-brady syndrome – DDD mode (AAI mode was previously used)
- AV block, mostly 2nd degree type 2 and 3rd degree – VDD or DDD mode
- If there is concurrent atrial fibrillation in addition to an indication for pacemaker – VVI mode
Not all pacemakers are permanent; temporary pacemakers exist as well. These can pace the heart transcutaneously or transvenously. Temporary pacemakers may be used while the patient is waiting to get a permanent one implanted, or if there is a reversible cause of the pacemaker indication.
How it works
A pacemaker delivers a small negatively charged electrical impulse to the myocardium of approximately 2.5 V lasting 0,40 milliseconds. This is called "pacing". To avoid pacing the myocardium only when it's needed and not at the wrong time in the cardiac cycle, the pacemaker must also have a "sense" function, where it detects the depolarisation of the myocardium.
Pacemakers are programmed with a lower rate limit (LRL), which is the heart rate below which the pacemaker will take over. The lower rate limit corresponds to a lower rate interval (LRI), which is the time between each beat for a given LRL. When the LRL is 60 bpm, the LRI is 1000 ms because there is 1000 ms between each beat at 60 beats per minute. In other words, when the LRL is 60 beats per minute, the pacemaker will pace whenever 1000 ms has passed since the last beat, this ensuring that the (paced) heart rate never falls below 60 bpm.
Modes
The pacemaker can be put into a variety of different modes, each described by a three or four-letter code describing the function of that mode. The first letter shows which chamber is paced, V is for (right) ventricle, A is for (right) atrium, and D is for both ("double"). The second letter shows which chamber is being sensed with the same nomenclature as above (V, A, or D). The third letter shows what the pacemaker does with the information from sensing, I is for inhibiting, T is for triggering, and D is for both. If the pacemaker is programmed to increase its pacing rate on demand, there will be a fourth "R", for rate response.
VVI mode means that the pacemaker senses the depolarisation of the (right) ventricle and paces (depolarises) the ventricle. If the pacemaker senses a depolarisation in the ventricle the pacemaker will be inhibited (hence the "I") from pacing for that cardiac cycle. VVI mode can be accomplished with only one lead (in the right ventricle), and so may be achieved with a single-chamber pacemaker. VVI mode and AAI mode effectively "replace" a failing sinus node.
DDD mode means that the pacemaker senses the depolarisation in both the right ventricle and atrium, and it senses depolarisation in both the ventricle and atrium. The third "D" means that a sensed depolarisation may either trigger or inhibit the pacemaker for that cardiac cycle. This means in practice that it will pace the atrium if the lower rate interval is exceeded, and it will pace the ventricle if too much time has passed from the atrial depolarisation (P wave) and a ventricular depolarisation (QRS complex) hasn't occured yet. For DDD mode, one electrode lead in the atrium and one in the ventricle are needed, only being possible with a so-called dual-chamber pacemaker. DDD mode effectively replace both a failing sinus node and a failing AV node.
Implantation
Pacemakers are implanted under local anaesthesia in the pectoral region. The pacemaker leads are inserted into the subclavian vein or axillary vein and led back into the heart with the help of fluoroscopy.
Complications
The rate of complications increase with the duration the pacemaker is left in the body. As such, it's only rarely indicated for people younger than 40. Endocarditis with infection of the electrodes is the worst complication. The pocket in which the pacemaker is implanted may also become infected, which is also serious.
Conduction system pacing
Conduction system pacing (CPS) is a newer method of pacing where the cardiac conduction system is paced, rather than the cardiac muscle. Usually the bundle of His is paced. It's more technically demanding but may give better results as the resulting electrical function more closely mimics the physiological.
Intracardiac pacemakers
The weak point of pacemakers are the electrode leads. Intracardiac pacemakers are tiny devices that are implanted in the right ventricle and acts as both the pacemaker and the leads. This reduces the risk of endocarditis, but they are expensive, are difficult to exchange, and have a battery life of only 10 years.