Download The Conduction System of the Heart

The Conduction System of the
Heart
Anatomy
The Circulatory System
Cardiac Physiology
• In a single cardiac contraction, or
heartbeat, the entire heart contracts in
series – first the atria and then the
ventricles
• Specialized muscle cells of the conducting
system control and coordinate the
heartbeat
• Contractile cells produce the powerful
contractions that propel blood
The Conduction System
• The heart is controlled by the ANS
(autonomic nervous system) – it contracts
on its own, without neural or hormonal
stimulation
• The heart has its own regulating system =
conduction system
• The conduction system is composed of
specialized muscle cells/tissue that
initiates and distributes electrical impulses
(the action potential).
Components of the conducting
system
• SA Node - Located in the right atrial wall
below the opening to the superior vena
cava
• AV Node - Located in the interatrial
septum (floor of the right atrium)
• Conducting cells – interconnect the two
nodes and distribute the stimulus
throughout the myocardium
Components of the Conduction
System
1. SA NODE
• Contains pacemaker cells, which initiates and
establishes heartbeat
• Known as the natural pacemaker
• Average = 75 beats/minute
2. AV NODE
• Receives the action potential from the SA node
• Action potential slows considerably  allows
time for the atria to empty into the ventricles
Components of the Conduction
System
3. AV BUNDLE
• Receives impulse from the AV node
• Located at the interventricular septum
• Passes impulse onto the bundle branches
4. BUNDLE BRANCHES
• Located below the AV bundle
• Passes impulse onto the conduction myofibers
5. CONDUCTION MYOFIBERS (Purkinje Fibers)
• Located in the ventricular myocardium
• Receives action potential/impulse from bundle branches
and passes it onto ventricular myocardial cells to
complete ventricle contraction
ECG or EKG
•Impulses can be detected on the body surface and recorded with an
electrocardiograph.
•The recording that is made, the electrocardiogram (ECG/EKG), traces the
flow of current through the heart.
•Each time the heart beats, a wave of depolarization radiates through the
atria, pauses at the AV node, then travels down the interventricular septum
to the apex, turns, and spreads through the ventricular myocardium toward
the base
•An ECG integrates electrical information obtained by placing electrodes at
different locations on the body surface
The typical ECG has three recognizable waves:
1st Wave: P wave (depolarization of the atria)
2nd Wave: QRS complex (ventricular depolarization)
3rd Wave: T wave (ventricular repolarization)
Clinicians can use an ECG to access the performance of specific nodal,
conducting, and contractile components.
Abnormalities in the shape of the wave and changes in the timing could
indicate something is wrong.
Examples: An excessively large QRS complex often indicates that the
heart has become enlarged. When a portion of the heart has been
damaged by a heart attack the ECG reveals an abnormal pattern of
impulse conduction.
The Cardiac Cycle
• Each heartbeat is followed by a brief
resting phase, allowing time for the
chambers to relax and prepare for the next
heartbeat
• The period between the start of one
heartbeat and the beginning of the next is
a single cardiac cycle
• Includes alternating periods of contraction
and relaxation
The Cardiac Cycle Phases
Systole: the phase of contraction of a chamber
(the chamber contracts and pushes blood into
an adjacent chamber or into an arterial trunk)
Diastole: the phase of relaxation (the chamber fills
with blood and prepares for the next cardiac
cycle)
Blood Pressure
• Pressure that blood exerts against the
inner walls of the blood vessels
• Systolic Number – This is the TOP
number in your blood pressure…related to
the pressure of contraction in the heart
that is placed on the inside of the vessels
• Diastolic Number – This is the BOTTOM
number in your blood pressure…related to
the relaxed phase of your heart that is
then transferred to your vessels
• Normal blood pressure is 120/80
Cardiac Output
• For the body to function properly, the heart
needs to pump blood at a sufficient rate to
maintain an adequate and continuous
supply of oxygen and other nutrients to the
brain and other vital organs
• Cardiac output = describes the amount of
blood your heart pumps each minute
• A healthy heart pumps about 5-6 liters of
blood every minute when a person is
resting
Cardiac Output
• Cardiac Output: It is determined by:
1. The amount of blood pumped by the left
ventricle during each beat (stroke volume)
2. The number of heart beats per minute
• The amount of blood ejected by a ventricle
during each contraction (single beat) is called
stroke volume
• Cardiac output is an indication of the blood flow
through peripheral tissues; without adequate
blood flow, homeostasis cannot be maintained;
helps keep blood pressure at the levels needed
to supply oxygen-rich blood to your organs
Cardiac Output
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For an adult at rest…
Stroke volume = 80 mL (average)
Heart rate = 75 beats per minute (average)
Therefore, Cardiac Output (CO) = Stroke
Volume x Beats per Minute
CO = 80 mL/beat x 75bpm
CO = 6000mL/min (6L/min)
Arrhythmia
• Abnormal heart rhythm (irregular
heartbeats and heart rates)
• Bradyarrhythmia (Bradycardia) – heart
rates that are slow (less than 50bpm)
• Tachyarrhythmia (Tachycardia) – rapid
heart rates (faster than 100 bpm)
• About 5% of healthy individuals
experience a few abnormal heartbeats
each day
Causes of Arrhythmia
•Coronary artery disease (partial or
complete blockage of coronary
circulation)
•Electrolyte imbalances in your blood
•Injury from a heart attack
•Healing process after heart surgery
Example: Fibrillation (atrial or
ventricular) – rapid, uncoordinated
shuddering of the heart
High Blood Pressure/Hypertension
(Silent Killer)
• Normal bp = 90/60 (birth) and 120/80 (adults)
• Top number shows the pressure when the heart beats
• Lower number measures pressure at rest between heart
beats, when the heart refills with blood
• High blood pressure = 140/90 consistently
• High blood pressure can threaten healthy arteries and
lead to life-threatening conditions (leading cause of
stroke and major cause of heart attack)
• Significantly increases the workload on the heart, and
the left ventricle gradually enlarges
• More muscle mass means a greater oxygen demand
• Places physical stress on the walls of blood vessels
Hypertension
• Prehypertension – blood pressure is
consistently just above normal (120-139
& 80-89)
What causes hypertension?
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Smoking
Being overweight or obese
Lack of physical activity
Too much salt in the diet
Too much alcohol consumption
Stress
Genetics
Older age
Atherosclerosis
• Usual cause of heart attacks & strokes
• High blood pressure, smoking or high
cholesterol damage the endothelium (lines
the arteries) & cholesterol plaque forms
• The arteries harden and narrow and
block blood flow
• Plaque behaves in different ways: can stay
within the artery wall, cause blockages, or
rupture which allows blood to clot in the
artery
Heart Attack/Myocardial Infarction
• Part of the coronary circulation
becomes blocked, and cardiac muscle
cells die from lack of oxygen
• Coronary heart disease causes the
arteries to become narrow and blood
cannot flow as well
• Fatty matter, calcium, proteins, and
inflammatory cells build up within the
arteries to form plaque
Heart attack continued
• When the plaque that forms becomes
hard, the outer shell cracks, and blood
clots form around the plaque
• If a blood clot blocks the artery, the heart
muscle becomes “starved” for oxygen
• Within a short time, death of heart
muscle cells occurs, causing permanent
damage
Risk Factors of a Heart Attack
1. Smoking
2. High blood pressure
3. High blood cholesterol levels
4. Diabetes
5. Obesity
6. Sedentary lifestyle
7. Genetics
Pulmonary Edema
• An abnormal buildup of fluid in the air
sacs of the lungs, which leads to
shortness of breath
• In most cases heart problems cause
pulmonary edema but fluid can
accumulate for other reasons
• As pressure in the blood vessels increase,
fluid is pushed into the air spaces (alveoli)
in the lungs
• This fluid interrupts normal oxygen
movement in the lungs
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