Download PACEMAKERS

What your doctor said about…
PACEMAKERS
The heart is an engine. A muscle pump. A double pump, serving the lungs and the
body. Blue blood from the head and the feet enter the right side of the heart, and are
directed (through valves) to the lung circuit. In the lungs, oxygen is added, and the
bright red blood then travels through the left heart and on to the aorta. Fresh blood is
then supplied to all organs, giving up its oxygen, and returning through capillaries and
veins to the heart.
This muscle is very like the engine of your car. It has fuel lines (coronary arteries),
cylinders (the atria and ventricles) and an electrical system.
The Electrical System of the Heart
At the entrance to the heart, in the ATRIUM, there are a collection of cells called the
SINUS NODE. These cells are responsible for generating a spark, about one a second.
This spark travels down nerve fibres to a junction box, the ATRIOVENTRICULAR
NODE. This AV node acts like a junction box, splitting the signal, one half for the right
heart, one half for the left.
The signal then travels downwards towards the main pump muscle in the VENTRICLES.
As the signal reaches the muscle, a contraction occurs. This contraction of muscle
shrinks the size of the blood chamber, ejecting blood out to the lungs or body.
The heart’s natural pacemaker, the sinus node, is therefore a very important little item.
Without proper functioning of this spark plug, the heart stops beating, and within a very
short time, other organs stop too.
We do have some backups, however. These backups in various places in the heart lie
silent for most of your life, throwing in the odd spark now and again, really not too useful
in an emergency. In fact, they can be a problem, causing “ARRHYTHMIA” or
palpitations. At least something is better than nothing!
It is amazing that, for an entire lifetime, this spark plug and its wiring diagram work
ceaselessly, stimulating heart muscle to contract, and muscle to pump nourishing blood
around your body. However, age and illness can damage the electrics.
Disease of the Electrical System
The master spark plug (the sinus node) usually has a frequency of 60 heart signals a
minute. The brain can tell it to run faster if you are active, say 80-120 heart signals a
minute. But it is unusual to have a heartbeat exceed 160 beats a minute, unless you are
really overexerting yourself. The pulse is regular, of course, and settles down to a
resting pulse of 60-80 a minute.
The numbers can be changed with drugs. For example, betablockers (like
METOPROLOL, ATENOLOL and others) deliberately slow the pulse. In a way, you
can think of them as “governors” on the engine, a way of preventing the heart from
speeding up. These are very helpful when you want the heart to rest, for example in
ANGINA or HEART FAILURE. Patients on betablockers have a heart rate of 60 at
rest. Other pills like DILTIAZEM and DIGOXIN also slow the electrics.
On the other hand, some drugs like ATROPINE can cause the heartbeat to speed up. We
rarely use these.
So what can go wrong? Well, the master spark plug can slow down, go to sleep
temporarily, or pack it in altogether. Most patients might function very well with a
pulse in the 50’s, a few would tolerate the 40’s, but once the pulse is in the 30’s,
FATIGUE and maybe shortness of breath would be evident. If you haven’t got a reason
to have such a slow pulse, you need a new master spark plug – an ARTIFICIAL
PACEMAKER.
On the other hand your spark plug might be just fine, but the electrical wires in your heart
might get rusty and break. Whether this is between the spark plug (SN) and the junction
box (AVN), or further down the wires, something needs to be done. Suppose only half
of the heartbeats from the master spark plug get down to the muscle – that’s not enough!
Or maybe none do, and you ventricles “backup” emergency system is called into play.
That doesn’t sound good… These HEART BLOCKS can be identified on a cardiogram
or 24hour heart monitor, and rectified by a new electrical wiring system.
As mentioned, age and heart disease (like heart attacks, longstanding high blood pressure,
valve malfunction and heart failure) can all challenge the electrics. The good news is – it
isn’t too difficult to figure out, and there are PACEMAKERS to fix it.
Symptoms
A failure of the electrical system may give rise to a slow pulse and WEAKNESS. A
sudden failure might cause you to FAINT for no reason. Of course, there are other
reasons for these symptoms, but if you have heart disease, electrical malfunction has to
be considered. A 24 hour EKG (“a Holter monitor”) can be done very easily.
Artificial Pacemakers
An artificial pacemaker is a device implanted in the chest designed to replace the electrics
of the heart. IT DOES NOT TREAT EXTRA HEARTBEATS, drugs do that. It raises
the pulse from 30 to 60, ie TREATS SLOW PULSE.
Alessandro Volta first discovered the ability of an electrical signal to cause heart
contraction in 1876. This was recognized as a way of influencing heart rates by Russian
physiologists Chagovetc and Vedensky. Dr Hyman devised an EXTERNAL pacemaker
device in 1927, but the ball didn’t really get rolling until John Hopps, a Canadian
electrical engineer working in Winnipeg, designed a fairly large external pacing device in
1950. In this decade, devices used electrodes place on the outer chest, and had to be
plugged into a wall outlet. Not a comfortable experience, and heaven help you if you had
a power outage…
Elmqvist and Senning implanted the first pacemaker in 1958, and after some trial and
error, Arne Larsson got his device. Battery problems were common, and replacements
were needed quite frequently. An induction coil placed on the outside of the chest
recharged the batteries. 43 years and 21 replacements later, Arne Larson died at a ripe
old age.
Today, we only use EXTERNAL PACEMAKERS for a very short period. Usually, a
patient will have a signal sent through chest pads to the heart in the Emergency Room.
Shortly thereafter, a wire can be placed through an IV in the neck, advanced into the
heart, and connected to a spark generator. This is called a TEMPORARY
TRANSVENOUS PACEMAKER, which gives us heart rate stability until your
PERMANENT device is inserted in the next day or so.
A PERMANENT PACEMAKER is a small device, a bit larger than a twoonie, that is
slipped into the tissues under the collar-bone. A special wire is passed through large
internal veins to the right heart, and secured in place with a screw or anchor. At the tip
of this wire are two ELECTRODES. A spark jumps from one to another, and causes the
heart muscle to contract. Many patients have a wire in both the upper and the lower
chamber, to mimic the ATRIA-then-VENTRICLE contraction that PRIMES-thenPUMPS the engine.
A single wire pacemaker is all you need to live. A double wire may improve the coordination of heart function, and is becoming quite popular. Patients who have an erratic
rapid pulse (called ATRIAL FIBRILLATION) usually can’t use the ATRIAL wire.
Are Pacemakers smart?
Very. Basically, a pacemaker is a LAPTOP COMPUTER shrunk to the size of a coin.
It knows when your electrics need some help, and gives a spark only when needed. It
gives just enough of a spark to cause contraction, not too much or too little. It can tell
when you are active, and need a faster heart rate. It can respond quickly to you
becoming active, or slowly. It can be programmed to work more slowly when you go to
bed, and forgive your heart if it runs rates between 50 and 60. It can record your heart
rates in memory over a year, and even diagnose some rhythm problems. Regular visits
to the Pacemaker Clinic are designed to check proper function, and REDUCE THE
DEMANDS ON THE BATTERY.
Depending on whether you use the pacemaker a lot, or just a little, the battery might last
6 years or 20. We don’t put in a new battery, however, we replace the whole pacemaker!
Your Pacemaker clinics have technologists who are skilled in pacemaker adjustment.
They use device called a PROGRAMMER to talk “through the skin” to the pacemaker.
All quite painless..
Like all mechanical devices, things can go wrong. A wire can dislodge or break, for
example.
What happens in airports?
Pacemakers are made of steel and titanium. They will set off alarms at security.
However, it is pretty difficult to re-program your pacemaker unless you are in the habit of
sitting in your microwave oven! Tell the security guards you are bionic, and they’ll
check you through safely.
Are magnets dangerous?
NO. This is a common source of confusion. A pacemaker has two jobs to do: SENSE
when your own electrics are working, and provide a PACED BEAT if not. All a magnet
does is temporarily switch off SENSING. So if I put a magnet right on top of your
pacemaker, it will go into an automatic mode, giving you a heart rate of 70 whatever you
are doing.
Do batteries suddenly fail?
NO. We have a “battery indicator” that gives us about a YEAR’s warning of when the
current is getting low. You get more frequent checkups when this happens. Then you
get a new pacemaker.
What is an ICD?
An IMPLANTABLE CARDIOVERTOR DEFIBRILLATOR is a very fancy toy indeed.
In addition to being a pacemaker, it can diagnose dangerous fast heart rhythms (usually in
people who have had several heart attacks, or have a weak heart), then deliver a shock
through the pacer wire - to restore normal rhythm. High tech, $6000. But lifesaving
for patients at high risk. Remember, most fast heart rhythms are treated with DRUGS,
not ICDs.
What is the future?
Smaller units. Longer battery life. Self-diagnosis to optimize function. We are always
striving to make our “artificial pacemakers” more like what you were born with, a
challenging task indeed!
HMB – Pacemakers: May 2010