ECG Interpretation: Normal ECG Flashcards
What are the key components of the ECG complex
P wave = atrial depolarisation QRS complex = Ventricular depolarisation T wave = ventricular depolarisation
PR interval =
spread of depolarization through atria to AVN
ST segment =
Time between ventricular depolarisation and repolarisation
Label the ECG complex
See diagram!
How can you easily distinguish between a segment and an interval
Segment- Between waveforms
Interval- Will include waveforms
Where should electrodes be placed
Not over muscle- as this can distort the readings
Over bony prominences only
What is each view of the heart described as
Each view of the heart is described as a lead- note that the term lead does not refer to the electrodes.
The rhythm of the heart can only be determined from one view i.e one lead (this requires two electrodes).
Summarise the six limb leads
One electrode is attached to each limb. These four electrodes provide six ‘limb leads’ or six different views of the heart in a vertical plane. These are called leads I,II,III VL, VF and VR.
Summarise the six chest leads
Six electrodes are attached to the chest, recording leads V1-V6. Accurate placement if these electrodes is essential for comparing later ECGs. These leads look at the heart from a. horizontal plane.
How do we record a 3-lead ECG using only the limb electrodes
‘Ride your Green Bike’
R or red and right arm
Y- yellow and left arm
G- green and left leg
B- black and right leg
Work clockwise from R- red and right arm.
What other things must you remember to do when recording the ECG
Make sure the patient is warm and relaxed
Check machine settings: standard paper speed of 25mm/sec; the voltage calibration should be set so that 1mV causes 1cm upwards deflection.
Make sure date and time are recorded and always ensure that the patient’s name is on the ECG
Write the patients symptoms and BP on the ECG.
Which segment represents the period in which the ventricles are fully activated
The ST segment.
Describe the bipolar limb leads
Bipolar limb leads
The potential difference shown by these leads is conventionally measured from:
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Lead I: right arm (aVR) to left arm (aVL); left arm positive.
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Lead II: right arm (aVR) to left leg (aVF); left leg positive.
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Lead III: left arm (aVL) to left leg (aVF); left leg positive.
These bipolar limb leads view the heart in the frontal plane. These three leads make up Einthoven’s triangle around the heart.
Describe the unipolar limb leads
Unipolar leads measure any positive potential difference directed towards their solitary positive electrode from an estimate of zero potential. They include aVL, aVR and aVF. They also view the heart in the frontal plane.
What does a positive deflection mean
Electrical activity is moving towards that lead
What regions of the heart does each lead look at
High lateral- I and VL Inferior- II,III and VF Septal- V1 and V2 Anterior- V2-V4 Lateral- V5-V6
Outline a systematic approach to ECG interpretation
Rate Rhythm Cardiac Axis P wave PR interval QRS complex ST segment T wave
Summarise the assessment of rate
Using standard paper speed of 25mm/sec:
1 small square = 0.04s
1 large square= 0.20s
So 5 big squares represents 1 second
When the rhythm is regular- you can work out rate using:
300/ number of large squares between consecutive R waves
When the rhythm is irregular, you can work out rate using:
Number of R waves multiplied by 6
How can you easily spot tachycardia and bradycardia
The closer together the QRS complexes are, the faster the heart is beating. As a rough guide. less than 3 large squares between each QRS complex indicates a rate over 100bpm (tachycardia) and more than 6 large squares indicates a rate of less than 50bpm (bradycardia)
Define what is meant by sinus rhythm and the criteria that it must fulfil
Sinus rhythm = depolarisation starts at the sino-atrial node (SAN)
Are there p waves present? – YES
Is every p wave is followed by a QRS complex? – YES Is every QRS complex is preceded by a p wave? – YES
How can you easily identify whether the rhythm is regular or irregular
A regular rhythm means that there is the same number of squares between each QRS complex.
Describe how we can classify arrhythmias based on their location
Atrial Rhythms
Abnormal p waves + narrow QRS complexes
Nodal / Junctional Rhythms
Absent p waves
+ narrow QRS complexes
Ventricular Rhythms
Absent p wave + wide QRS complexes + abnormal T waves
How else can we categorise arrhythmias
Bradycaria
Slow and sustained
Tachycardia
Fast and sustained
Extra-systoles
Early single beat
Fibrillation
Activation of the atria or ventricles is totally disorganised
Define what is meant by the term ‘Cardiac Axis’
Cardiac axis = the average direction of spread of depolarization through the ventricles (as seen in the frontal plane)
Describe the relationship between lead VR and II
They look at the heart from opposite directions. When seen from the front the wave of depolarisation normally spreads from 11 o’clock to 5 o’clock, so the deflections in lead VR are normally mainly downward (negative) and in lead II mainly upward (positive).
What will a normal cardiac axis look like
The depolarising wave will be spreading towards leads I,II and III, and is therefore associated with a. predominately upward deflection in all these leads.
The deflection will be greater in lead II than in lead I or III.
How do you know when the axis is at right angles to the lead
When the R and S waves of the QRS complex are equal
Describe what happens in right axis deviation
E.G right ventricular hypertrophy due to cor pulmonate
The deflection in lead 1 will become negative because depolarisation is spreading away from it, and the deflection I lead III becomes more positive because deflection is spreading towards it.
Right = Reaching (leads I and III will reach towards each other).
Describe what happens in left axis deviation
The QRS complex will become predominately negative in lead III.
Left axis deviation is not significant until the QRS complex deflection is also predominately negative in lead II.
Although left axis deviation can be due to excess influence of an enlarged left ventricle, in fact this axis change is usually due to a conduction defect rather than to increased bulk of the left ventricle.
What is the cardiac axis normally measured in
Normally measured in degrees, although this is not particularly useful clinically.
Lead I is taken to look at the heart from O degrees, lead II from +60; lead VF from +90 and lead III from +120.
Leads VL and VR look from -30 and -150 respectively.
What is the range of the normal cardiac axis.
-30- 90 degrees.
If the S wave in lead II is greater than the R wave, the axis will be more than 90 degrees away from lead II. In other words, it must be at a greater angle than -30 and closer to the vertical, and left axis deviation is present
Similarly, if the size of the R wave equals that of the S wave in lead I, the axis is at right angles to lead I. This is the limit of normality towards the right.
If the S wave is greater than the R wave in lead I, the axis is at an angle of greater than 90 degrees, thus right axis deviation is present.
Should we worry about the cardiac axis?
In themselves, deviations are seldom significant- minor degrees occur in tall, thin individuals and short, obese individuals. However, the presence of a deviation should alert you to look for other signs of right and left ventricular hypertrophy.
Right- could be P.E
Left- could be a conduction defect.
Summarise the cardiac axis
QRS deflection
Axis
Lead I Lead II Lead III
Positive Positive Positive Normal -30° to +90°
Positive Negative Negative LAD -30° to -90°
Negative Positive Positive RAD +90° to -90
Describe the normal atrial waveform
The Atrial Waveform – Relationship to the P wave
Atrial depolarisation proceeds sequentially from right to left, with the right atrium activated before the left atrium.
The right and left atrial waveforms summate to form the P wave.
The first 1/3 of the P wave corresponds to right atrial activation, the final 1/3 corresponds to left atrial activation; the middle 1/3 is a combination of the two.
In most leads (e.g. lead II), the right and left atrial waveforms move in the same direction, forming a monophasic P wave.
However, in lead V1 the right and left atrial waveforms move in opposite directions. This produces a biphasic P wave with the initial positive deflection corresponding to right atrial activation and the subsequent negative deflection denoting left atrial activation.
This separation of right and left atrial electrical forces in lead V1 means that abnormalities affecting each individual atrial waveform can be discerned in this lead. Elsewhere, the overall shape of the P wave is used to infer the atrial abnormality.
Describe normal P-wave morphology in lead II
Normal P-wave Morphology – Lead II
The right atrial depolarisation wave (brown) precedes that of the left atrium (blue).
The combined depolarisation wave, the P wave, is less than 120 ms wide and less than 2.5 mm high.
Describe right atrial enlargement in lead II
Right Atrial Enlargement – Lead II
In right atrial enlargement, right atrial depolarisation lasts longer than normal and its waveform extends to the end of left atrial depolarisation.
Although the amplitude of the right atrial depolarisation current remains unchanged, its peak now falls on top of that of the left atrial depolarisation wave.
The combination of these two waveforms produces a P waves that is taller than normal (> 2.5 mm), although the width remains unchanged (< 120 ms).
This is P pulmonale- due to pulmonary HTN
Describe left atrial enlargement in lead II
Left Atrial Enlargement – Lead II
In left atrial enlargement, left atrial depolarisation lasts longer than normal but its amplitude remains unchanged. Therefore, the height of the resultant P wave remains within normal limits but its duration is longer than 120 ms. A notch (broken line) near its peak may or may not be present (“P mitrale”).
This is P mitrale
Describe P wave morphology in V1
Normal P-wave Morphology – Lead V1
The P wave is typically biphasic in V1, with similar sizes of the positive and negative deflections.
Right Atrial Enlargement – Lead V1
Right atrial enlargement causes increased height (> 1.5mm) in V1 of the initial positive deflection of the P wave.
Left Atrial Enlargement – Lead V1
Left atrial enlargement causes widening (> 40ms wide) and deepening (> 1mm deep) in V1 of the terminal negative portion of the P wave.
Describe some common P wave abnormalities
Common P wave abnormalities include:
P mitrale (bifid P waves), seen with left atrial enlargement.
P pulmonale (peaked P waves), seen with right atrial enlargement.
P wave inversion, seen with ectopic atrial and junctional rhythms.
Variable P wave morphology, seen in multifocal atrial rhythms.
P mitrale
The presence of broad, notched (bifid) P waves in lead II is a sign of left atrial enlargement, classically due to mitral stenosis.
What feature should you assess in the PR interval
Its duration
Summarise the assessment of heart block
First degree
Constant PR interval >200ms
Second degree
Presence of non-conducted P waves:
Mobitz 1:
Second-degree block can be divided into Mobitz type I and Mobitz type II. In type I (Wenckebach), the degree of block increases over a few beats (P–R interval increases over three or four beats, followed by an isolated P wave). This is analogous to a ‘lazy’ AV node which can still function. Type II is characterized by an unexpected non-conducted atrial impulse. Thus, the P–R and R–R intervals between conducted beats are constant. This is analogous to a fracture in the His–Purkinje system which is about to become completely severed. This frequently progresses to complete heart block and is associated with sudden cardiac death.
Third degree
No relationship between P waves and QRS complexes
In third-degree heart block, the atria and ventricles beat independently of each other. The ventricular rate is usually about 20–40 bpm (P waves and QRS complexes have no fixed relationship).
What features of the QRS complex should you look at
Features to look at: complex voltage (i.e. height)
complex width
What can be determined from QRS voltage height
Right or left ventricular hypertrophy
General Rule: hypertrophy if S + R ≥ 7 squares
Describe some problems associated with a narrow or wide QRS
Narrow (<100ms):
Sinus ryhtym
Atrial flutter
Ventricular tachycardia
Wide (>100ms):
Ventricular tachycardia
Ventricular fibrillation
Electrical activation through ventricles must be slow- either because conduction through the ventricles is abnormal- or because the electrical impulse erroneously began in the ventricular tissue rather than coming through the bundle of his.
What are deep Q waves indicative of
If the QRS complex starts with a deep downward deflection then this may be a Q wave due to an old M.I
Summarise bundle branch block
When conduction is blocked in one of the bundle branches of the interventricular septum, the affected areas of myocardium will be stimulated later by conduction from unaffected areas of myocardium. This leads to widening and disruption of the QRS complexes (>0.12 s). Looking at leads V1 and V6 in right bundle branch block there is:
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A second R wave (R′) in V1 and a deeper, wider S wave in V6.
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The last part of the QRS in lead V1 is positive. This is because of the delayed right ventricular depolarization.
In left bundle branch block:
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There is a Q wave with an S wave in V1.
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There is a notched R wave in V6.
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The last part of the QRS in lead V1 is negative. This reflects the delayed depolarization of the left ventricle.
How can you easily distinguish between right and left BBB
To determine the type of bundle branch block, look at leads V1 and V6 and think of WiLLiaM MaRRoW. In LBBB, there is a W pattern in lead V1, and an M pattern in V6 (WiLLiaM). In RBBB, there is an M pattern in lead V1, and a W pattern in V6 (MaRRoW).
Which part of the myocardium do impulsed pass through first
The left
This is why we see two R waves in RBB- the second R wave, R’, is the impulse coming from the left ventricle
What should you look at in the ST segment
It's morphology ST elevation (STEMI)
ST depression (ischeamia)-whether downssloping, upscoping or horizontal.
What should you look for in the T waves
Normally inverted in VR and V1.
Inverted in other leads- could be ischaemia or ventricular hypertrophy.
May have peaked T waves.
Describe some ECG red flags in an unwed patient
Ventricular rate above 120 bpm or below 45 bpm – iscahemia, hypotension or sepsis
Atrial fibrillation — valve disease, alcoholism, ischaemia or infarction
Complete heart block – any heart disease
ST elevation or depression- infarction or ischaemia
Abnormal T wave inversion – infarction, ischaemia or pulmonary embolism
Wide QRS — any heart disease
