V. PERMANENT PACEMAKERS
A. Types of Pacemakers
A cardiac pacemaker is an electronic device that senses cardiac electrical activity and delivers electrical stimuli to the heart when needed. If the natural pacemaker (AV sinus node) is no longer working well or the electric current is blocked by disease of the conducting bundles, a pacemaker can be implanted. A pacemaker lead (electrode) is inserted into the right ventricle and the pacemaker (pulse gene¬rator) is inserted just under the skin (see Fig. 3). The lead is then attached to the pacemaker. A small electrical current is passed through the lead at the rate set by the cardiologist. When the impulse reaches the heart muscle the muscle contracts in the same way it would as if the electrical impulse occurred naturally.
Permanent pacemakers can be classified on the basis of five characteristics:
1. The cardiac chamber paced by the device, ventricle (V),or atrium (A)
2. The chamber sensed by the device (V or A)
3. Device response to sensing
4. Device programmability
5. Additional functions.
A five-position North American Society of Pacing and Electrophysiology/British Pacing and Electrophysiol-ogy Group generic pacemaker code is used to describe pacemakers on the basis of the above features. VVI pacing has a ventricular lead placed in the ventricle which paces and senses only the ventricle. It looks for ventricular electrical activity and delivers its stimuli when necessary; so that it paces only the ventricle, senses only the ventricle, and responds to sensing by inhibition of its stimulus. Thus the output of the pacemaker is inhibited by the sensed normal ventricular signal as shown in Fig. 4. Generally the first three or four positions are used; for example, a VVIR pacemaker implies a pacemaker that paces and senses the ventricle, is inhibited by a sensed event, and has rate response function. Figure 5 shows AOO, VDD, and DDD pacing modes.