Why Do People Need Pacemakers?

Mr. Delos has some answers...

Dear Ashley:

1. What causes you to have a pacemaker? Do you know that all living animals, including humans, have natural pacemakers in their hearts? These natural pacemakers are a special type of heart tissue that send out tiny electric shocks between once and twice a second in most people. When the other type of heart muscle receives this tiny shock it causes the muscle to contract (or get shorter) for a brief period of time, this is what causes the heart to "beat."

Can you find for me the name of the special tissue that regulates the heart beat? I'll give you a hint, it is called the _____ node. When this special type of heart tissue cannot do it's job, a person may require an "artificial pacemaker." So, why would this tissue not be able to do its job? Sometimes the heart muscle around the area gets "sick" (we are not always sure why) and prevents the electricity from passing. When this happens, a condition known as "heart block" occurs. There are several different types of heart block, see if Mrs. Weeg can help you find them.

2. Why does your heart get slower? Think of the special tissue that makes the heart beat at a regular rate as a clock ticking. Most of the time, clocks are very reliable. Sometimes, though, the clock may need winding or new batteries and will begin to be not so reliable.

The heart may slow down for one of two basic reasons. The node that regulates the heart beat may get tired (why is not always easy to find out) and the heart will slow down; or, the pathway between the node and the rest of the heart muscle may put up "road blocks" and prevent the electricity from passing. When this happens the regulating (this is not the answer to my question above) node may continue to want the heart to beat at 70 beats per minute but the signal may only be able to get around the road blocks at 35 beats per minute.

Do you know what the normal heart rate is for a child? An adult? A horse? An elephant? A hummingbird? Look them up, you might be surprised.

3. Is it fat? Fat does not usually cause the heart to slow down. Some people who have "heart attacks" will get a slow heart rate and may require a pacemaker, however. If you can find a book with pictures of the heart and the arteries around the heart, look for something called the "Right Coronary Artery." This artery supplies the blood (and, more especially, the oxygen) to the node that regulates the heart beat. If it is blocked by fat, the person *may* end up needing a pacemaker.

4. Are there different sized pacemakers? Yes. But most pacemakers are about the size on the one that I sent to Mrs. Weeg. Because of computer technology and the space program, pacemakers seem to get smaller every year.

5. How do the pacemakers know what to do? Thank you, Mrs. Weeg. Pacemakers have special electrical circuits in them that regulate how the pacemaker operates. If a circuit called the "sensing circuit" detects the electrical signal from a natural heart beat, it sends a message to the "output circuit" and tells it _not_ to send out a pacemaker beat.

The sensing circuit only looks for a certain amount of time; for instance, if the pacemaker is set to pace at 60 beats per minute (this equals 1 beat per second) the sensing circuit only looks for 1 second. If it does not see a natural heart beat, it allows the pacemaker to send out a small electric shock to cause the heart to beat.

These days all of this looking and waiting is controlled by a small computer in the pacemaker and something called a "crystal oscillator" which is the timekeeper (same as in an electronic wrist watch or clock). You may want to ask your science teacher about what kinds of material are used as "crystal oscillators" in watches.

I hope my answers have not been too confusing. Please write me back if you need for me to clarify anything. I may also be able to "send over" some pictures if you think that will be helpful.

Which one are you in the picture? I'm the one on the outside looking in.

Keep the questions coming,
Your Key Pal, too,
Delos Johnson


Search The Global Classroom
Patricia A. Weeg
Return to Global Classroom