Cardiac output =
Stroke Volume x Heart rate
Electrocardiograms (ECG) show voltage against time. A normal ECG will show five distinct peaks
P Wave –
excitation of the atria
QRS complex –
excitation of the ventricles
T wave – repolarisation of the ventricles
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The sinoatrial
node is responsible for the coordination
of the heart beat and is found in the wall of the right atrium
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The sinoatrial
node sends out a wave of depolarisation
that spreads over the left and right atria. This stimulates the myocardium (heart muscle) of both atria to contract.
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The wave of excitation then reaches the atrioventricular node (found in the
right atrium) which then sends a wave of depolarisation to the bundle of His and purkyne fibres (located in the septum)
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The wave of excitation continues along the purkyne fibres to the apex of the heart and then up through each ventricle. This causes the
ventricle to contract, forcing blood
out of the heart.
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Note there is a slight delay after the atria
contract to ensure the ventricles are full of blood and to insure that the atrioventricular
valves are closed.
Using ECGs to
Diagnose Heart Disease
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If the heart behaves abnormally, the normal
electrical activity of the heart is disrupted, causing the rhythm to change. Different
rhythms are known as arrhythmias and can be detected using ECGs.
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If the heart is starved of oxygen (ishaemic)
then it cannot empty or fill
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Atrial fibrillation : atria beat too fast,
increasing chance of a clot
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Tachycardia: heart beats too quickly, can be
caused by stress
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Ventrical fibrillation: ventricles contract
weakly and erratically, hence little blood is pumped out of the heart. It is
often a rapid drop in blood pressure, so the brain, body and heart are rapidly
starved of oxygen and glucose. This can lead to a heart attack.
In some cases, an abnormal heart rate can be corrected by
giving the heart a large electrical shock with a defibrillator.
Medicine is becoming closer and closer to technology with the time
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