Cardiac Pacemaker.pdf

The Heart

The Anatomy of the Heart

The heart is a muscle, about the size of your fist that is encased in a sac called the

pericardium. The pericardium helps to keep the heart in position and protects it from getting

hurt. The pericardium and the heart are separated by a layer of lubricating fluid, which allows

the heart to pump freely inside the walls of the chest. The heart is made up of three layers of

muscle, the endocardium, myocardium and epicardium. The myocardium makes up about

seventy five percent of the heart tissue. The epicardium is a thin lining that covers the

myocardium. There is a layer called the endocardium that is between the myocardium and the

inside of the heart. The endocardium acts as the inner covering of the heart and protects the

myocardium. The heart functions as a pump that circulates nourishment and oxygen to, and

carbon dioxide and waste away from, tissues and organs of the body. The heart is separated

into four different chambers through which blood is pumped. A thick wall of muscle called the

septum, which divides the heart into two halves, separates the heart. Each half is then

separated into an upper and lower chamber by valves. The upper chambers are called the

atria and are the inputs to the heart. The lower chambers are called the ventricles and are the

outputs of the heart. The valves that separate the upper and lower chambers are called the

atrioventriclular valves. The valve that separates the right atrium from the right ventricle is

called the tricuspid valve and the valve that separates the left atrium from the left ventricle is

called the mitral valve. A different set of valves controls the flow of blood from each ventricle

to the main arteries. The valve that separates the right ventricle from the pulmonary artery,

the artery that carries blood to the lungs, is called the pulmonary valve. The aortic valve is the

valve that separates the left ventricle from the aorta, the main artery that carries blood to the

rest of the body's organs and tissues .

What Is the Electrical System in Your Heart?

The electrical system in your heart controls the

speed of your heartbeat. The system includes a

network of electrical pathways, similar to the

electrical wiring in your home. The pathways

carry electrical signals through your heart. The

movement of the signals is what makes your

heart beat.

When working properly, your heart's electrical

system automatically responds to your body's

changing need for oxygen. It speeds up your

heart rate as you climb stairs, for example, and

slows it down when you sleep. When your heart

rate speeds up, it means your heart pumps

faster and your body gets more oxygen-rich

blood.

Your heart's electrical system is also called the cardiac conduction

system .

Parts of the Electrical System

Your heart's electrical system includes three important parts

• S-A node (sinoatrial node)

• A-V node (atrioventricular node)

• His-Purkinjesystem

Normally, as the electrical impulse moves through the heart, the heart contracts about 60 to

100 times a minute. Each contraction represents one heartbeat. The atria contract a fraction

of a second before the ventricles so their blood empties into the ventricles before the

ventricles contract. Arrhythmias are abnormal beats that can be detected on the ECG. [3] Two

of the arrhythmias that people with heart problems encounter are tachycardia and

bradycardia. Tachycardia is a problem in which the heart beats at a rate faster than the

normal human heart rate. Treatment for tachycardia consists of cardio version or the delivery

of a broad depolarizing shock to a restricted region of the heart. Rapid bursts of pacemaker

impulses timed and placed at the proper time can often stop the tachycardia. Bradycardia is a

problem in which the heart beats at a rate slower than the normal human heart rate. An

implanted pacemaker can restore the lower heart rate to a more physiological value that will

improve cardiovascular function. Fibrillation is another major problem that affects the heart.

[3]

Under some conditions, almost all heart tissue is capable of starting a heartbeat, or becoming

the pacemaker. An arrhythmia occurs when:

• The heart's natural pacemaker develops an abnormal rate or rhythm

• The normal conduction pathway is interrupted

• Another part of the heart takes over as pacemaker

In any of these situations, the body may not receive enough blood because the heart cannot

pump out an adequate amount with each beat as a result of the arrhythmia's effects on the

heart rate. The effects on the body are often the same, however, whether the heartbeat is too

fast, too slow, or too irregular. Some symptoms of arrhythmias include, but are not limited to:

• Weakness

• Fatigue

• Palpitations

• Low blood pressure

• Dizziness

• Fainting

The S-A Node: Your Heart's Natural Pacemaker

The S-A node is a bundle of specialized cells in your right atrium. The S-A node cells are

special because they create the electricity that makes your heart beat. The S-A node normally

produces 60-100 electrical signals per minute — this is your heart rate HR, or pulse.

The S-A node is called the "natural pacemaker" of your heart because it controls your heart

rate.

What Is a Pacemaker?

Although the heart has its own natural pacemaker that sets its rhythm, the term pacemaker

most commonly refers to an artificial electronic device that is implanted in the chest to

regulate the heart's rhythm. Generally. Several different pacemakers exist including internal and

external pacemakers. Internal pacemakers may be permanent or temporary whereas external

pacemakers are temporary .

Pacemakers correct an abnormally slow heartbeat by sending electrical impulses to one or

more chambers of the heart. These signals make the heart contract in a more regular rhythm

than the chamber would otherwise.

Every pacemaker system has two parts:

The pulse generator, which produces the pacing impulses, and

The lead or leads, which deliver the impulses to the heart.

The same leads also carry signals back from the heart. By "reading" these signals, the pulse

generator is able to monitor the heart's activity and respond appropriately. Pacemakers are

prescribed for people of all ages whose hearts beat too slowly. Pacemakers detect the slow heart rate

and send electrical impulses to the heart to stimulate the heart muscle to beat faster. Well over 2 million

pacemakers have been implanted worldwide since 1960.

When people refer to a pacemaker, they are actually discussing a pacing system: a

pacemaker, a pacing lead, and programmer. Two parts are placed inside the body:

The pacemaker is a small metal case that contains electronic circuitry and a battery. The

pacemaker sends a tiny electrical pulse at a specific time.

A pacing lead is an insulated wire that carries the tiny electrical pulse to the heart so a

heartbeat can begin.

The third part, the Programmer , is kept in a hospital or clinic. A nurse or doctor uses this

specialized computer to see how the pacemaker is working and if necessary, to adjust the

settings of a pacemaker.

The three parts of a pacing system work together to relieve symptoms of bradycardia .

A pacing system increases the heart rate to meet the oxygen needs of the body. By

increasing the heart rate, the symptoms of bradycardia are often eliminated. This often means

patients have more energy and less shortness of breath. However, a pacing system is not a

cure, but rather a treatment for a slow heart rhythm. A pacing system will not prevent or stop

heart disease ( coronary artery disease ), nor will it prevent heart attacks .

As of today, the only treatment for bradycardia is a pacemaker. This remarkable invention has

been benefiting more than 2 million people for over 50 years. Because of a pacemaker,

people with bradycardia can expect to lead normal lives.

History of Pacemakers

Early History

The history of cardiac research has grown in a stair-step manner. Early ideas and inventions

led to the development of more complicated instruments and machines. The use of electricity

for stimulation of the heart in the late eighteenth and early nineteenth centuries stirred many

ideas. There was much controversy over who was the first to invent the artificial pacemaker.

Dr. Albert S. Hyman is believed to be the founder of the artificial pacemaker, but there

appears to be evidence that he was not the first. An Australian physician by the name of Mark

C. Lidwill, along with physicist Major Edgar Booth, built a portable pace making unit. It was

demonstrated in 1931, while Hyman developed the device in 1930-31. Lidwill's apparatus had

one pole applied to the skin and another in the appropriate cardiac chamber.

The First Implantation

As far back as 1954, cardiac pacemakers were used to stimulate the heart. Stimulation was

made by skin electrodes, which left uncomfortable burns when used for a couple of days or

more. Later, electrode wires leading through the skin were tried, but infections along the wires

were an unsolved problem. So the solution to this problem, according to Dr. Ake Senning,

was the implantation of the entire pacemaker. Prompted by Sennings, Dr. Rune Elmqvist

designed the world's first implantable pacemaker

The First Pacemaker

The first pacemaker included a pulse generator, which delivered about two volts with an

impulse period of two milliseconds. The original transistors showed large leakage currents

and were found not suitable. Therefore, two newly developed transistors were used in its

place. The charging current came from a line-connected vacuum tube radio-frequency

generator with a frequency of 150 kHz. Theoretically, one charging which was done overnight,

was enough for about four months,

but it only lasted for one month.

Earl Bakken's first wearable, battery-

powered, transistorized cardiac

pacemaker, shown above on the left

with Medtronic, stamped on it, can be

seen on at The Bakken exhibit The

electrical schematic in the background

is Earl's original handwritten

schematic design for the pacemaker

that made history and saved lives.

The First Implant

Arn Larsson was the lucky recipient of

the first implanted pacemaker. He was

a forty-three year old patient who

suffered from life-threatening Adams-Stokes seizures. His condition was so bad that it

required thirty resuscitations per day. Dr. Senning implanted this pacemaker on October 8,

1958. Since this operation, Mr. Larson has had no complications and still leads an active life.

History of Pacemakers: Technology Development Through

the Decades

The Early Years

Early External Pacemakers - The first pacemakers of the 1950s were not totally implanted in the body.

One end of a small wire, called a "lead," was implanted into the heart. The other end of the lead was

connected to an external pacemaker that was AC powered. One serious drawback--patients could go

only as far as their extension cord and a power failure was a constant concern.

First Battery-Powered External Pacemaker - In 1957 the world's first transistorized, battery-powered,

wearable pacemaker was developed. This gave patients mobility and eliminated concerns about a

power failure.

1960's

First Human Implant of a Totally Implantable Pacemaker - The first human implant of a totally

implantable pacemaker was in 1960. Its battery life was approximately 12-18 months.

Advances in Pacing Leads - In the mid-1960s, "transvenous leads," leads that could be inserted

through a vein leading to the heart, replaced earlier leads that were attached to the outer surface of the

heart. Pacemaker and lead implants could now be done without opening the chest cavity or using

general anesthesia.

World's First "Demand" Pacemaker - "Demand" pacemakers, introduced in the mid-1960s, sense

when the heart is beating on its own and provide pacing only when necessary. Earlier pacemakers

continuously paced the heart at a set "fixed" rate. All new pacemakers today are "demand" models.

1970's

Further Advances in the 1970s - New lead designs were developed to replace earlier "smooth tip"

leads. Still used today, these new "tined" (pronged) leads and "active fixation" (screw-in type) leads

provide a more secure attachment to the heart tissue and help prevent the lead from slipping out of

place.

Extended Battery Life and New Casing - The introduction of a lithium iodine battery in

1975 greatly extended the pacemaker battery life (10+ years for some models) and replaced

the mercury-zinc battery.

Titanium casing was developed to enclose the battery and circuitry. Epoxy resin with silicone

rubber previously encased the inner components. The new titanium casing (along with special

filters) helps shield the components and greatly reduces outside electromagnetic interference.

Patients with these newly designed pacemakers could now safely use microwave ovens and

other appliances and equipment found in the home and office.

First Programmable Pacemakers - With the introduction of programmable pacemakers

in the mid-1970s, pacemaker settings could be programmed using radio-frequency signals.

This eliminated the need for surgery when/if any pacemaker programming adjustments were

necessary.

First Dual-chamber Pacing - The first programmable pacemaker that could sense and

pace the upper (atrium) and lower (ventricle) chambers of the heart was introduced in the late

1970s. Using two leads, dual-chamber pacemakers maintain synchronized timing between

the upper and lower chambers of the heart to ensure efficient blood flow.

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