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The Cardiac Cycle and Electrocardiogram

•An overview of cardiac pacemaker cells and how the electrical events of the ECG (electrocardiogram) are generated;
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the cardiac cycle and ventricular systole
the events of the electrocardiogram (ECG)
ECG intervals, sinus dysfunction and AV block
pacemaker and cardiac muscle action potentials

The Cardiac Cycle

• The heart beats about once every 0
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8 seconds at rest:
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75 – 80 beats per minute for up to 100 years
2
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5 x 1012) heart beats in an average life time

• Each beat involves diastole (dilation) and atrial systole (contraction) followed by ventricular diastole and

systole:
1) atria relax and fill with blood
2) atria contact and fill left and right ventricles
3) right ventricle contacts to pump blood to the lungs whilst the left ventricle contracts to pump blood around
the body

• The stroke volume (SV) depends on how much the chambers fill by the end of diastole
~ End Diastolic Volume (EDV)
 venous return

 EDV

 SV  Cardiac Output

Electrocardiograms (ECG)

• ECGs are used to measure:

~ the rate and regularity of the heart beat
~ the size / position of the heart chambers
~ damage to the heart
~ the effects of drugs or devices such as pacemakers
• ECGs monitor electrical activity of the heart:
~ they record electrical events generated by depolarisation and repolarisation of heart muscle (action
potentials)
~ these ‘action potentials’ promote muscle contraction
• ECGs record the electrical events that reach the skin:
~ they don’t measure muscle contraction
~ they don’t measure membrane permeability
ECG Lead Placement

• Simple 3 lead or multi-channel 12 lead ECGs systems are used to assess the heart and problems with its
conduction systems

3 lead recordings
12 lead recordings

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Provides simple recordings
Provides multi-channel recordings

ECG Events
• P-Wave:
~ atria depolarisation
• QRS-complex:
~ ventricular depolarisation
~ largest spike due to larger muscle mass of ventricles
• Atrial repolarisation:
~ hidden by the QRS complex
~ it is relatively small
• T-Wave:
~ ventricular repolarisation
ECG Events During the heart cycle
SA node & Atria depolarisation
AV-node depolarisation
Bundle of His depolarisation
Ventricle depolarisation
Other ECG Intervals

• P-R interval (012 – 0
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07 – 0
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05):
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an isoelectic period after ventricle depolarisation

• Q-T segment (0
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4 sec):
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ventricle depolarisation plus repolarisation time

ECG following a Heart Attack
• Myocardial infarctions (heart attacks) are caused by blockages of coronary arteries
~ blood is prevented from reaching cardiac muscle (ischemia) causing necrosis
~ ischemia depresses the ST segment
• The ECG due to an injury (infarct) usually has an elevated ST segment
• Damage is permanent so any altered ECG characteristic will remain with the patient:
~ doctors can easily tell if a patient has had a heart attack in the past
Sinus Node Dysfunction and AV block

• Problems with generating AV node action potentials (APs) or failure to transmit APs to the AV node
• Bradycardia:
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slow heart rate
• Sinus node arrest:
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no p-wave

• Exit block:
• Tachy / Brady Syndrome:

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impulse not transmitted
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bursts of activity
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sick sinus syndrome
• AV block is classified according to the extent of the problem:
~ ranges from delayed conduction of APs from atria to ventricles to no synchronisation between contractions
• Severe block disorders require artificial pacemakers
Atrial and Ventricular Pacemaker cells

• The sinoatrial node is located in the posterior wall of the right atrium near the entrance of the vena cava
• The sinoatrial node has a fast pacemaker rhythm of about 100 beats / minute and is the primary cardiac pacemaker:
~ parasympathetic vagal tone slow the resting heart rate
~ sympathetic tone increases the heart rate (activity / stress)
• The AV node is larger but has a slower intrinsic rhythm:
~ therefore its rate is controlled by the sinoatrial node
~ maximum rate is 230 APs / min which decreases with age
~ it is located in the floor of the right atrium
• Pacemaker cells and myocardial cells (muscle cells) have long action potentials due to calcium ion channels
Pacemaker and Cardiac Action Potentials
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SA node pacemaker depolarisation is due to inward Na+ and Ca++ currents
Repolarisation involves outward K+ currents
Atria depolarisation occurs before ventricle depolarisation
Ventricular muscle APs have a long plateau due to inward Ca++ currents



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