to in the adult population Pacemaker Function and Follow-up A basic understanding of the pacemaker system and the continuous interactions with the intrinsic action of the patient's rhythm is necessary Knowing the type of pacemaker, the programmed parameters, as well as the underlying rhythm is vital to be able to accurately interpret the ECG and internal electrograms The purpose of this review is to familiarize the reader with the pacemaker nomenclature and the various pacemaker systems utilized for symptomatic or irreversible bradyarrhythmias To standardize and facilitate the understanding of pacemakers, the North American Society of Pacing and Electrophysiology and the British Pacing and Electrophysiology Group (BPEG) developed the NBG coding system, last revised in 2002.195 This three- to five-letter coding system determines the pacemaker's function The first letter indicates the chamber or chambers paced Abbreviations are the following: A for atrium; V for ventricle; D for dual, meaning atrium and ventricle; and O, which indicates that therapies are disabled or asynchronous Position II indicates the chamber that will be sensed, with the same abbreviations as already mentioned The third letter describes the response to a sensed event The letter I indicates that the pacemaker will inhibit a sensed event or T that a pacing stimulus was triggered by a sensed event, or D having dual capabilities to both inhibit or trigger in response to a sensed event Position IV is a rate-modulation mode Rate modulation R utilizes an accelerometer that responds to the patient's physiologic need and is based on parameters such as motion or minute ventilation Position V indicates the presence or absence of multisite pacing Some common programmed modes to review are the following: ▪ The DDD or DDD(R) system has the ability to pace and sense in either and or both chambers Functions include pacing in both chambers or inhibition of pacing by a sensed event in the respective chamber and atrioventricular coordinated pacing ▪ The VVI or VVI(R) system is a ventricular demand pacing mode (Fig 22.35) The ventricle is paced and sensed, and the pulse generator inhibits pacing output in response to a sensed ventricular event FIG 22.35 Tracing of a single VVI pacemaker with ventricular capture and appropriate ventricular sensing Note that with the appropriately sensed intrinsic ventricular beat the device resets the timer and the next ventricular paced beat occurs at the lower rate limit ▪ The AAI or AAI(R) system is an atrial demand pacing mode The atrium is paced and sensed, and the generator inhibits pacing output in response to a sensed atrial event ▪ The VOO or DOO modes are asynchronous in that the generator will deliver a pacing stimulus at a fixed rate without the ability to sense any intrinsic event Timing Cycles Timing cycles represent the beat-to-beat behavior of implantable devices in response to the patient's intrinsic rhythm (Fig 22.36A–B) An understanding of the basic concepts of pacemaker timing cycles is fundamental in optimizing care of the patient's rhythm Pacemaker timing cycles consist of all the possible