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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 . 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 an '''atrial paced rhythm'''. Accordingly the ventricular beat complex is delayed until the atrial signal has passed through the AV node. In the second image the ventricles are paced directly, resulting in a '''ventricular paced rhythm'''. As ventricular pacing occurs exclusively in the right ventricle the ECG shows a left bundle branch block pattern. An exception to this rule is left ventricular pacing in patients with congenital anomalies and patients with an a surgically placed epicardial pacemaker that has been placed during surgery. Another exception is septal or RVOT placement of the pacinglead pacing lead, which results in a less widened to normal QRS complex.
* '''AAI''': 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 en in ventriculair events, but can only pace the ventricle. This type of pacemaker is used in patients with a reliable sinus node, but suffering from the results of 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 They include echocardiography, finding the narrowest QRS finding , 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 seperate separate category and usually not put in 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 an , 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.
==Pacemaker Indications==
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 Mobitz I or II) atrioventricular block. Syncope , syncope with sinus node disease. Alternating , alternating bundle branch block. Persisting , and persisting AV block after surgery.
===Atrial-sensed ventricular-paced rhythm===

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