These pacemakers contain a sensor to detect the need for increased rate. The simplest ones, Dahms says, gauge response by body movement. Some, however, respond to the person's breathing. Still others base their response on changes in blood temperature. Thus, if a person starts to walk or run, the device senses the activity and increases the heart rate.
Many pacemakers automatically provide for a slower heart rate at night or when the person rests.
A very few devices are clock-timed to slow at night. Despite such sophisticated capabilities, miniaturization techniques have allowed pacemakers to become quite tiny in size, some as small as a quarter and less than an ounce in weight. The many companies making pacemakers today offer more than a hundred models. Pacemakers last four to 12 years. Factors that shorten a pacemaker s life include the requirement to pace every beat, the size of the batteries, dual-chamber pacing, and the tightness of the electrical connection to the heart, which is not completely controllable.