Download Lecture 17: Cardiovascular System Electrical Activity and EKG The

Lecture 17: Cardiovascular System
Electrical Activity and EKG
The Cardiac Action Potential Has a Prolonged Refractory Period
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Heart action potential has a prolonged spike (depolarized)
Membrane is refractory for a long time
This prevents summation and gives the heart time to fill
All Parts of the Heart Beat Spontaneously
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Heart muscle does not require stimulation by a nerve
Nerves usually inhibit the heart beat; cutting the nerves -> heart speeds up
Beat originates as a depolarization in the heart muscle cell itself (self stimulation)
All parts of the heart can beat spontaneously
Advantage: if one part of the heart is damaged another part can still produce a
beat
Risk: beats originating outside of the pacemaker can produce life-threatening
arrhythmias
Normally the Heart Beat Originates in the SA Node
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The heart beat originates from the part of the heart with the fastest beat
Normally this is the SA (sinoatrial) node of the right atrium
The SA node is called the pacemaker
Ectopic beats are those originating outside of the normal pacemaker
The Atria Are Electrically Insulated From the Ventricles
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The upper part of the heart (the 2 atria) is insulated from the lower part
Electrical excitation can pass from the atria to the ventricles only at the AV node
The Heart Has Special Electrical Conducting Tissue
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Electrical excitation is passed through special conducting tissue from the AV
node to the ventricles
Route: bundle of His -> bundle branches -> Purkinje fibers
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Excitation is Delayed in the AV Node
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The excitation starts in the SA node and spreads across the atria
When the excitation reaches the AV node there is a delay of about 0.1 seconds
before it passes into the bundle of His
The delay allows the atria to contract before the ventricles are stimulated
This results in better filling of the ventricles
From the AV node impulses enter the Bundle of His and then travel along the
right and left bundle branches in the septum between the right and left ventricles
In the ventricles the impulse spreads through the Purkinje fibers
Gap Junctions in the Intercalated Discs Spread Electrical Excitation Between Heart
Cells
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Gap junctions are low resistance connections between 2 cells
When the impulse reaches cardiac muscle cells it is rapidly passed from one
muscle cell to the next because of gap junctions in the intercalated discs
Excitation of the Heart Can be Followed From the Body Surface With the EKG
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EKGs are not measured across the membranes of cardiac cells
EKGs are measured by electrodes attached to the skin of the body surface
These electrodes record the average activity of millions of heart cells
EKG voltages are very small because the electrodes are far from the heart and
most of the electrical activity cancels out
The most common set of connections is with electrodes connected to both arms
and the left leg (Einthoven's triangle).
This produces the 3 limb leads (I, II, III).
Alternate connections give 3 more limb leads (AVL, AVR, AVF)- the 6 limb
leads give electrical views of the heart in the frontal plane
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6 chest leads are also used (V1, V2, V3, V4, V5, V6)- these give views of the
heart in a transverse plane
A Typical EKG Record Contains P, QRS and T Waves
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The P wave is caused by depolarization (excitation) of the atria
The QRS is produced by depolarization (excitation) of the ventricles
The T wave represents repolarization (recovery) of the ventricles
The small atrial recovery wave cannot be seen- it is swamped out by the large
QRS wave
Different electrodes give different views of these waves- the wave below is
typical for lead II (lead II has the same direction as the axis of the normal heart)
The EKG Gives Information on Heart Rate, Rhythm, Orientation and Pathology
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If 2 QRS waves are close together the heart is beating at a fast rate; if they are far
apart the heart has a slow rate
If the heart is functioning properly each P wave is followed by a QRS wave
If electrical conduction between the atria and ventricles is partially or completely
blocked there will be a disturbance of the heart rhythm- the atria and ventricles
may beat independently of each other
The orientation of the heart can be determined from the sizes of the waves coming
from different electrodes
Damage to the heart caused by poor coronary circulation can cause the waves to
widen (slower conduction). There are also other changes in wave shape, such as
depression of the ST segment
The systemic circulation provides the
functional blood supply to all body tissue.
It carries oxygen and nutrients to the cells
and picks up carbon dioxide and waste
products. Systemic circulation carries
oxygenated blood from the left ventricle,
through the arteries, to the capillaries in
the tissues of the body. From the tissue
capillaries, the deoxygenated blood returns
through a system of veins to the right
atrium of the heart.
The coronary arteries are the only vessels
that branch from the ascending aorta. The
brachiocephalic, left common carotid, and
left subclavian arteries branch from the
aortic arch. Blood supply for the brain is
provided by the internal carotid and
vertebral arteries. The subclavian arteries
provide the blood supply for the upper
extremity. The celiac, superior mesenteric,
suprarenal, renal, gonadal, and inferior
mesenteric arteries branch from the
abdominal aorta to supply the abdominal
viscera. Lumbar
arteries provide blood for the muscles and spinal cord. Branches of the external iliac
artery provide the blood supply for the lower extremity. The internal iliac artery
supplies the pelvic viscera.
Major Systemic Arteries
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All systemic arteries are branches, either directly or indirectly, from the aorta. The aorta
ascends from the left ventricle, curves posteriorly and to the left, then descends through
the thorax and abdomen. This geography divides the aorta into three portions: ascending
aorta, arotic arch, and descending aorta. The descending aorta is further subdivided into
the thoracic arota and abdominal aorta.
Major Systemic Veins
After blood delivers oxygen to the tissues and picks up carbon dioxide, it returns to the
heart through a system of veins. The capillaries, where the gaseous exchange occurs,
merge into venules and these converge to form larger and larger veins until the blood
reaches either the superior vena cava or inferior vena cava, which drain into the right
atrium.
Fetal Circulation
Most circulatory pathways in a fetus are like those in the adult but there are some notable
differences because the lungs, the gastrointestinal tract, and the kidneys are not
functioning before birth. The fetus obtains its oxygen and nutrients from the mother and
also depends on maternal circulation to carry away the carbon dioxide and waste
products.
The umbilical cord contains two umbilical arteries to carry fetal blood to the placenta and
one umbilical vein to carry oxygen-and-nutrient-rich blood from the placenta to the fetus.
The ductus venosus allows blood to bypass the immature liver in fetal circulation. The
foramen ovale and ductus arteriosus are modifications that permit blood to bypass the
lungs in fetal circulation.
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