Section 3, Chapter 15
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Transcript of Section 3, Chapter 15
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Electrocardiogram
An electrocardiogram, or ECG (or EKG) is a recording of the electrical changes in the myocardium during the cardiac cycle.
Section 3, Chapter 15
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P Wave
• Represents atrial depolarization
• Leads to atrial contraction
• Conduction of electrical impulse
from right to left and downward
QRS Complex
• Represents depolarization of
ventricles
• Leads to ventricular contraction
• This massive wave hides the atria
repolarization
Electrocardiogram
from right to left and downwardrepolarization
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T Wave
• Represents repolarization of
ventricles
Electrocardiogram
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Figure 15.22d. An ECG pattern with the corresponding systole and diastole shown above.
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ECG of a regular heart rhythm at 75 beats per minute
Heart Arrhythmias:
normal
ECG of a regular heart rhythm at 75 beats per minute
Atrial Flutter. Atria fire 250-350 times per minute. For every QRS
complex there may be 4 or more P waves.
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Bradycardia – cardiac rhythm less than 60 beats per minute.
Examples of Heart Arrhythmias. Arrows indicate p Wave.
Bradycardia – cardiac rhythm less than 60 beats per minute.
Tachycardia– cardiac rhythm greater than 100 beats per minute.
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Atrial fibrillation. Instead of contracting, the atria become quivering
chambers. The ventricles respond only to impulses that make it to the
AV node.
Examples of Heart Arrhythmias, fibrillation
Ventricular fibrillation = Life threatening arrhythmia. Ventricles quiver, and are
unable to pump blood properly. Requires immediate defibrillation.
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The heart rate is controlled intrinsically by the SA
node, but sympathetic and parasympathetic fibers
alter the rate at which the pacemaker fires.
Regulation of cardiac cycle
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Cardiac Control Center
• Within Medulla Oblongata
• Receives sensory impulses from throughout the cardiovascular system and relays motor impulses to heart in response.
• Cardioinhibitor & cardioaccelerator reflex centers
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• Cardioinhibitor reflex center
– Parasympathetic fibers from vagus nerves innervate SA & AV nodes.
– Acetylcholine (ACh) released from fibers decreases the firing rates of SA
& AV nodes.
– Heart rate decreases
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• Cardioaccelerator reflex center
– Sympathetic fibers from accelerator nerves innervate SA & AV nodes.
– Norepinephrine released from fibers increases the firing rates of SA & AV
nodes.
– Heart rate and force of contraction increases
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Cardioinhibitor & cardioaccelerator reflex centers alter the heart
rate in response to sensory impulses from receptors
Baroreceptors – monitor blood pressure
• Within aortic arch and carotid sinuses
• Rising blood pressure stimulates cardioinhibitor center
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Figure 15.24b Illustration of the baroreflex arc
End of Section 3, Chapter 15