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A pacemaker is indicated when the electrical impulse conduction or formation is dangerously disturbed. The paced '''pacemaker rhythm''' can easily be recognized on the ECG as it shows '''pacemaker spikes''': vertical signals that represent the electrical activity of the pacemaker. Usually these spikes are more visible in unipolar pacing than in bipolar pacing. In the first example image, the atria are being paced, but not the ventricles, resulting in a '''atrial paced rhythm'''. Accordingly the ventricular beat is delayed until the atrial signal has passed the AV node. In the second image the ventricles are paced directly, resulting in '''ventricular paced rhythm'''. As ventricular pacing occurs exclusively in the right ventricle the ECG shows a left bundle branch pattern. An exception to this rule is left ventricular pacing in patients with congenital anomalies and patients with an epicardial pacemaker that has been placed during surgery. Another exception is septal or RVOT placement of the pacinglead which results in a less widened to normal QRS complex.
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[[Image:picture_pacemaker.jpg|thumb|A (used) DDDr pacemaker]]
[[Image:chest_xray_pacemaker.jpg |thumb| Chest x-ray of a patient with a pacemaker]]
[[Image:Pacemaker2.jpg |thumb| Atrial sensed, ventricular paced rhythm (tracking). Note the LBBB morphology with left axis deviation indicating the pacing lead in the right ventricular apex.]]
[[Image:DVA0838.jpg|thumb|Atrial paced rhythm]]
Pacemakers can be categorized according to the NASPE coding system, that usually consists of 3-5 letters.
|+ '''The revised NASPE/BPEG generic code for antibradycardia pacing'''<cite>Bernstein</cite>
! I || II || III || IV || V
* '''[[ICD]]''' (Internal Cardioversion Device): this device can detect and treat [[Ventricular Tachycardia]] and [[Ventricular Fibrillation]]. ICDs are a seperate category and usually not put in the pacemaker category, although they do have a pacing function. Usually the first treatment is anti-tachy pacing (pacing at a rate +- 10% above the ventricular rate in ventricular tachycardia, which can convert the rhythm to sinus rhythm). If this is not effective an defibrillator shock is delivered, usually with 16-36 Joules of energy. ICDs can save lives in patients who have a high risk of ventricular arrhythmias. All ICDs have optional pacemaker activity to treat bradycardias. New biventricular ICDs have 3 leads: an atrial lead, a left ventricular lead and a right ventricular lead.
* Biventricular ICDs ('''CRT-D'''): an ICD with biventricular pacing option.
A full list of pacemaker indications can be read in the ESC guidelines on cardiac pacing <cite>Vardas</cite>. A selection of class I indications are: chronic symptomatic third- or second degree (Mobtiz I or II) atrioventricular block. Syncope with sinus node disease. Alternating bundle branch block. Persisting AV block after surgery.
====Retrograde atrial activation====
====Pacemaker mediated tachycardia====
[http://www.hrsonline.org/swPositionStatementFiles/ps101036428.asp Heart Rhytm Society]