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==Commonly Used Pacemakers==
The most often used codes are:
the atria are paced, when the intrinsic atrial rhythm falls below the pacemakers threshold* '''VVI''': the ventricles are paced, when the intrinsic ventricular rhythm falls below the pacemakers threshold* '''VDD''': the pacemaker senses the atrial in ventriculair events, but can only pace the ventricle. This type of pacemaker is used in patients with a reliable sinus node, but with an AV-block.* '''DDD''': the pacemaker records both the atrial and ventricular rate and can pace one of each chambers when needed.* '''DDDR''': as above, but the pacemaker has a sensor that records a demand for higher cardiac output and can adjust the heart rate accordingly.* Biventricular pacemakers ('''CRT-P'''): leads in both ventricles are present to synchronize contraction. The lead pacing the left ventricle is usually positioned in the coronary sinus. This cardiac resynchronization therapy can improve symptoms and survival in some heart failure patients. Several optimizing methods are being evaluated to find the most effective pacing delay between left and right ventricle They include echocardiography, finding the narrowest QRS, and invasive hemodynamic measurements with pressure and flow wires.* '''[[ICD]]''' (Internal Cardioversion Device): this device can detect and treat [[Ventricular Tachycardia]] and [[Ventricular Fibrillation]]. ICDs are a separate category and usually not put into 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, a 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 (Mobitz I or II) atrioventricular block, syncope with sinus node disease, alternating bundle branch block, and persisting AV block after surgery.
===Atrial-sensed ventricular-paced rhythm===
[[File:ECG000020.jpg|thumb|Failure of ventricular capture]]
appropriate inhibition, atrial========Failure of appropriate inhibition, ventricular========Failure of appropriate pacemaker firing========Retrograde atrial activation========Pacemaker mediated tachycardia====