Download EKG

The heart has a natural pacemaker that regulates the pace or rate of the heart. It sits in the upper portion
of the right atrium (RA) and is a collection of specializes electrical cells known as the SINUS or SINOATRIAL (SA) node.
Like the spark-plug of an automobile it generates a number of "sparks" per minute. Each "spark"
travels across a specialized electrical pathway and stimulates the muscle wall of the four chambers of
the heart to contract (and thus empty) in a certain sequence or pattern. The upper chambers or atria are
first stimulated. This is followed by a slight delay to allow the two atria (atria is plural for atrium and
pronounced ay-tree-ya) to empty. Finally, the two ventricles are electrically stimulated.
In an automobile, the number of sparks per minute generated by a spark plug is increased when you
press the gas pedal or accelerator. This revs up the motor. In case of the heart, adrenaline acts as a gas
pedal and causes the sinus node to increase the number of sparks per minute, which in turn increases
the heart rate. The release of adrenaline is controlled by the nervous system. The heart normally beats
at around 72 times per minute and the sinus node speeds up during exertion, emotional stress, fever,
etc., or whenever our body needs an extra boost of blood supply. In contrast, it and slows down during
rest or under the influence of certain medications. Well trained athletes also tend to have a slower heart
beat.
The sequence of electrical activity within the heart is displayed in the diagrams above and occurs as
follows:
As the SA node fires, each electrical impulse travels through the right and left atrium. This electrical
activity causes the two upper chambers of the heart to contract. This electrical activity and can be
recorded from the surface of the body as a "P" wave" on the patient's EKG or ECG
(electrocardiogram).
The electrical impulse then moves to an area known as the AV (atrio-ventricular) node. This node sits
just above the ventricles. Here, the electrical impulse is held up for a brief period. This delay allows the
right and left atrium to continue emptying it's blood contents into the two ventricles. This delay is
recorded as a "PR interval." The AV node thus acts as a "relay station" delaying stimulation of the
ventricles long enough to allow the two atria to finish emptying.
Following the delay, the electrical impulse travels through both ventricles (via special electrical
pathways known as the right and left bundle branches). The electrically stimulated ventricles contract
and blood is pumped into the pulmonary artery and aorta. This electrical activity is recorded from the
surface of the body as a "QRS complex". The ventricles then recover from this electrical stimulation
and generates an "ST segment" and T wave on the EKG.
In summary, the heart constantly generates a sequence of electrical activity with every single heart
beat. This can be recorded on paper or displayed on a monitor by attaching special electrodes to a
machine that can amplify and record an EKG or ECG (electrocardiogram). The animation (above)
shows the sequence of electrical activity throughout the heart. Note how the chambers of the heart
contract when they are electrically stimulated. This in turn makes the heart valves open and shut.
Monitoring Heart Electrical Activity (EKG)
The heart contains special tissue that produces and sends electrical impulses to the heart muscle.
It is these impulses that trigger the heart to contract. Each time the heart beats, it sends out an
electric-like signal. The heart's electrical signals can be measured with a special machine called an
electrocardiogram (EKG or ECG).
To record the ECG, small patches or stickers called electrodes are placed on different parts of the
body. One is put on each arm and leg and six across the chest. Each of these leads monitors
distinct areas of the heart. Using combinations of these electrodes, different tracings of the heart's
electrical activity can be made and permanently recorded on paper or in a computer. The tracing
at the top is how a given lead of an EKG tracing might appear .
Three major waves of electric signals appear on the ECG. Each one shows a different part of the
heartbeat. The first wave is called the P wave. It records the electrical activity of the heart's two
upper chambers (atria).The second and largest wave, the QRS wave, records the electrical activity
of the heart's two lower chambers (ventricles).The third wave is the T wave. It records the heart's
return to the resting state.
By studying the shape and size of the waves, the time between waves, and the rate and regularity
of beating, a doctor can learn a lot about the heart and its rhythm.
Heart Conduction System
The sinoatrial node (SAN), located within the wall of the right atrium (RA), normally generates
electrical impulses that are carried by special conducting tissue to the atrioventricular node
(AVN).
Upon reaching the AVN, located between the atria and ventricles, the electrical impulse is relayed
down conducting tissue (Bundle of HIS) that branches into pathways that supply the right and left
ventricles. These paths are called the right bundle branch (RBBB) and left bundle branch (LBBB)
respectively. The left bundle branch further divides into two sub branches (called fascicles).
Electrical impulses generated in the SAN cause the right and left atria to contract first.
Depolarization (heart muscle contraction caused by electrical stimulation) occurs nearly
simultaneously in the right and left ventricles 1-2 tenths of a second after atrial depolarization.
The entire sequence of depolarization, from beginning to end (for one heart beat), takes 2-3
tenths of a second.
All heart cells, muscle and conducting tissue, are capable of generating electrical impulses that
can trigger the heart to beat. Under normal circumstances all parts of the heart conducting
system can conduct over 140-200 signals (and corresponding heart beats) per minute.
The SAN is known as the "heart's pacemaker" because electrical impulses are normally generated
here. At rest the SAN usually produces 60-70 signals a minute. It is the SAN that increases its'
rate due to stimuli such as exercise, stimulant drugs, or fever.
Should the SAN fail to produce impulses the AVN can take over. The resting rate of the AVN is
slower, generating 40-60 beats a minute. The AVN and remaining parts of the conducting system
are less capable of increasing heart rate due to stimuli previously mentioned than the SAN.
The Bundle of HIS can generate 30-40 signals a minute. Ventricular muscle cells may generate
20-30 signals a minute.
Heart rates below 35-40 beats a minute for a prolonged period usually cause problems due to not
enough blood flow to vital organs.
Problems with signal conduction, due to disease or abnormalities of the conducting system, can
occur anyplace along the heart's conduction pathway. Abnormally conducted signals , resulting in
alterations of the heart's normal beating, are called arrhythmias or dysrrythmia.
By analyzing an EKG a doctor is often able to tell if there are problems with specific parts of the
conducting system or if certain areas of heart muscle may be injured.
Heart Conducting Questions
List the electric pathway through the heart and explain how they work together to make the heart beat.
Using numbers, show the route of the electrical impulse/nerve impulse
Label the EKG wave