cardiovascular disorders
Cardiac Pacemakers
Types of cardiac pacemakers
Types of pacemakers are designated by 3 to 5 letters (see table Examples of Pacemaker Codes* ), representing which cardiac chambers are paced, which chambers are sensed, how the pacemaker responds to a sensed event (inhibits or triggers pacing), whether it can increase heart rate during exercise (rate-modulating), and whether pacing is multisite (in both atria, both ventricles, or more than one pacing lead in a single chamber). For example, a VVIR pacemaker paces (V) and senses (V) events in the ventricle, inhibits pacing in response to sensed event (I), and can increase its rate during exercise (R). VVI and DDD pacemakers are the devices most commonly used. They offer equivalent survival benefits. Compared with VVI pacemakers, physiologic pacemakers (AAI, DDD, VDD) appear to reduce risk of atrial fibrillation (AF) and heart failure and slightly improve quality of life. Advances in pacemaker design include lower-energy circuitry, new battery designs, and corticosteroid-eluting leads (which reduce pacing threshold), all of which increase pacemaker longevity. Mode switching refers to an automatic change in the mode of pacing in response to sensed events (eg, from DDDR to VVIR during AF). Leadless ventricular pacemakers that consist of a combined impulse generator and lead that are entirely contained within the right ventricle have recently been introduced. They are placed transvenously using specially designed delivery systems and are retained in the right ventricle by screws or tines. The leadless pacemakers currently in use are about 1 mL in size, 2 grams in weight, and are of VVI or VVIR configuration. 