* They are connected to the heart by one or two electrodes and are powered by long-lasting (5-10 years) solid-state lithium batteries. Most pacemakers are designed to pace and sense the ventricles - called the VVI pacemakers because they pace the ventricle (V), sense the ventricle (V) and are inhibited (I) by the ventricular signal. They are inserted under local anaesthesia and fluoroscopic guidance, subcutaneously under the pectoral muscles.
• In symptomatic sinus tachycardia, an atrial pacemaker may sometimes be implanted (AAI).
• In sick sinus syndrome, a dual-chamber pacemaker DDD (because it paces two or dual chambers, senses both (D) and reacts in two (D) ways, i.e. pacing in the same chamber is inhibited by spontaneous atrial and ventricular signals, and ventricular pacing is triggered by spontaneous atrial events) is implanted.
• Rate-responsive pacemakers measure activity, respiration, biochemical and electrical indicators, and change their pacing rate so that it is suitable for that level of exertion.