np2010-performing a 12 lead ecg and rhythm recognition · np2010-performing a 12 lead ecg and...
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![Page 1: NP2010-Performing a 12 Lead ECG and rhythm recognition · NP2010-Performing a 12 Lead ECG and rhythm recognition Author: Preston Acute Hospital NHS Trust Created Date: 1/29/2007 10:32:35](https://reader033.vdocument.in/reader033/viewer/2022042913/5f49f1a7a402bd441033b056/html5/thumbnails/1.jpg)
Basic Electrocardiography
Paula HignettCardio-Respiratory Department
Royal Preston Hospital
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Electrocardiography• Definition & Indications• The conduction system• The ECG in relation to the conduction
system• Normal ECG complex’s• Performing a 12 lead ECG
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What is an ECG?• Electrocardiography is a
method of obtaining electrical activity of the heart via surface electrodes placed on the body
• The machine used to record the waveforms is called an electrocardiograph and the tracing of the electrical activity of the heart is called an electrocardiogram(ECG)
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Indications For a 12 lead ECG• 1st line investigation when a more in-depth
knowledge about the hearts is required.• Can help identify or exclude suspected cardiac
pathologies including:• Enlarged chambers or thickened muscle within
the heart, • Electrolyte disturbances• Conduction abnormalities• Evidence of ischaemia or MI/MI screen• Pre and post operative assessment-guidelines
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Basic Principles
To be able to understand the waveforms seen on the ECG an understanding of the
electrical system of the heart is needed:The conduction system
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The Conduction System3.AV node 4.His bundle
1.SA Node
2.Internodal pathways
6.Right bundle branch
7.Purkinje fibres
5.Left Bundle
5a)posterior fascicle
5.b Anterior fascicle
1.
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Back-up Mechanism Of The Heart
• The normal spread of impulses starts at the SA node
• If the SA node fails to generate an impulse the next part the conduction system will try
• Any part of the conduction system is capable of generating an impulse
• Each component has its own ‘intrinsic ‘ rate
• The lower down the conduction system the lower the heart rate
60-100bpm
45-50bpm 40-45bpm
35-40
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The ECG in relation to the conduction system
• The P wave represents the spread of impulses across the atria resulting in contraction of the atria
• The P-R interval is the delay at the AV node• The QRS complex is the spread of impulses across the
ventricles which results in contraction of the ventricles• The T wave represents ventricular relaxation
P
Q
RS T
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Standard ECG settings
1. Standard paper speed is 25mm/sec. Each small square is equal to 0.04 secs and 1 large square is 0.2 secs
2. Standard setting for calibration is 10mm/mV. Allows measurement of amplitude. Ensures correct sensitivity of the machine
1 2
Horizontal linesShow amplitude
Vertical lines show Time intervals
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The P Wave
• The first positive deflection• Represents atrial contraction• Slow rounded wave• Duration between 0.08 and 0.11secs (2 ½ small squares)• Height less than 2.5mm (2 ½ small squares) • Upright in lead II (usual lead for a rhythm strip)• Inverted in aVR• Abnormal in atrial hypertrophy
P P P P
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The PR interval• The time it takes the
impulses to travel from the atria to the AV node (atrio-ventricular conduction time
• Measured from the onset of the P wave to the onset of the QRS complex
• No more than 5 small squares in duration (0.20secs)
• Prolonged PR interval >0.20secs is 1st degree heartblock
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1st Degree Heartblock
• Increased delay at the AV node• Prolonged PR interval > 1 large box or 5 small squares• PR interval is approximately the same for each complex• Normal P waves and QRS complexes• Regular rhythm • Usually benign
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The QRS Complex• Represents ventricular
contraction• Measured from the onset
of the Q to the end of the S wave
• Predominantly Upward in left sided leads and negative in negative in right sided leads
• Between 0.08 and 0.12 secs in duration (3 small squares)
Q
R
S
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Widened QRS complex
• A QRS > 0.12 seconds• Abnormal conduction of impulses across the ventricles• A block somewhere either in the his bundle or bundle
branches preventing normal sequence of events in the ventricles
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The Q wave
• Reflects impulses travelling down the septum
• The first negative component of the QRS complex
• Should be less than 25% of the associated R wave
• Abnormal Q waves can indicate previous MI
•
Abnormal Q waves
Normal
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The T wave
• Represents ventricular relaxation
• Rounded peak• Should be in the same
direction as the main QRS complex
• Abnormal if inverted, seen in LVH, Bundle branch blocks & ischaemia
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Abnormal T waves
T wave inversion e.gIschaemia/old MI
Deep symmetrical T wavee.g subendocardial MI
normal
Tall peaked T wave e.g Hyperkalaemia
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The ST segment
• Measured between the end of the QRS and the beginning of the T wave
• Should be no more than 1mm above or below the baseline
Baseline
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Abnormal ST segments
• ST segments which are below an imaginary baseline is called ST depression.Typically seen in angina.
• ST segments above the baseline are evident during the acute stage of an MI and indicates muscle injury. Can also be seen in pericarditis and printzmental angina
ST Depression ST Elevation
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Sinus Rhythms
• Beats are ‘sinus’if the P wave have originated from the SA node and the impulses have been conducted in the normal way. Meaning they have a normal PQRST complex.
• Normal sinus rhythm 60-100beats/min• Sinus Bradycardia Below 60bpm• Sinus Tachycardia Above 100bpm
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Performing a 12 lead ECG
• Lie patient in a relaxed supine position with arms resting by their side
• Prepare skin in accordance to the manufacturers recommendations. ‘Biotabs’ require skin abrasion to reduce skin impedance. Shaving may be necessary.
• Apply the electrodes • Attach the limb and chest leads to the electrodes
via crocodile clips…….
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(Extremity) limb leads• Colour coded electrodes
are attached to the wrists and ankles
• Right leg electrode functions as an electrical ground only. It prevents alternating current interference and can be ignored
• Can be placed anywhere on the arms or legs providing they are placed at least 10cm from the heart
aVF
N
N
aVR aVL
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Limb leads-aVR, aVL & aVF (Unipolar)
• The abbreviation ‘a’ refers to augmented. A definition of augmented is to increase in size which simply means that the ECG machine amplifies the signal to make it more readable.(the machine augments the reading by 50%)
• The ‘V’ refers to voltage• R, L and F refer to right
arm, left arm and left foot
aVF
aVR aVL
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Viewpoints of aVR, aVL, aVF
• The limb leads look at the heart from the side at a particular angle
• aVR looks down at the heart from the patients right shoulder
• aVL looks at the heart from the left shoulder
• aVF looks up at the heart from the left leg
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Leads I, II, II• Difference in voltage
between 2 extremities• Lead I-difference in
voltage between the left arm and right arm
• Lead II-difference in voltage between the left leg and right arm
• Lead III-difference in voltage between the left leg and left arm
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The 6 Chest Leads (V1-V6)• Leads are placed in a
horizontal plane• They record voltages
moving anteriorly • Each of the leads has its
own sight and a region of the heart it views best
• V1-V2 Right side heart• V3-V4 Interventricular
septum towards apex• V5-V6 Left side heart
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Chest Lead placement• V1-4th intercostal space
on the right sternal border• V2-4th intercostal space
left sternal border• V3-horizontally between
V2 + V4• V4-5th intercostal space
in the mid-clavicular line• V5-same horizontal plane
as V4 at the anterior axillary line
• V6-same horizontal plane as V4 in the mid axillary line
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Lead Systems Summary• The 12 leads provide a 3-D view of the electrical activity
of the heart• 6 extremity leads record voltages from the frontal plane of
the heart. These leads consist of the bipolar (I,II,III) and unipolar leads(AVR,AVL,AVF). These are recorded from the limb leads
• The chest leads record voltages from the heart in the horizontal plane by placing electrodes in specific anatomical positions
• Although we only use 10 leads we are getting 12 bits of information, which represent different areas of the heart.
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The 12 lead ECG
I
V3
II
III
II
aVR
aVF
aVL
V4
V5
V6
V1
V2
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Useful Tips
• Ensure paper speed and calibration signal is correct
• Chest electrodes are identified using external surface markers-don’t just guess!
• Ensure don’t transpose leads• Long rhythm strip(lead II) most useful identifying
rhythms.May need 2 pages to look for patterns.• Lead II often has waveforms with a good
amplitude for assessing complexes in more detail.
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Thank-you Good luck with the rest of your study
Any Questions?