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==Commonly Used Pacemakers==
The most often used codes are:
* '''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 in ventricular 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 synchronized 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 ventricles. They include echocardiography, finding the narrowest QRS, and invasive hemodynamic measurements with pressure and flow wires.