11. Interpreting ECGs Flashcards
Define depolarisation
This is a change within a cell that causes the charge inside of the cell to become less negative. This occurs through the movement of ions e.g the movemnt of Na+ ions out of the cell.
Define repolarisation
This is a change within a cell that causes the charge inside of the cell to become more negative. This occurs through the movement of ions e.g the movement of K+ ions out of the cell.
Define resting membrane potential
This refers to the charge across the membrane of the cell when the cell is at rest and not undergoing any significant changes in ion concentration.
Define action potential
This is a brief reversal of the cell membrane polarity which is then propagated from cell to cell.
Define deflection
This refers to a deviation from the baseline.
In an ECG this is either an upward or downward wave from the baseline.
What is the function of an ECG?
An ECG is an electrocardiogram and it’s used to record the electrical activity of the heart from different angles to help to identify and locate pathology.
What causes contraction and relaxation of cardiac muscle?
Contraction and relaxation of cardiac muscle results from the depolarisation and repolarisation of myocardial cells
How does the electrical activity spread through the heart at the cell level?
- Electrical activity spreads through the cardiac myocytes of the heart.
- The action potential is generated in the primary pacemaker cells of the sinoatrial node in the right atrium.
- The action potential will be propagated from cardiac cell to cardiac cell in a depolarisation wave.
- All the cardiac cells are electrically connected through gap junctions which allows the organised transmission of electrical activity.
- The depolarisation wave causes coordinated contraction of the atria and ventricles through excitation-contraction coupling.
How does the electrical activity spread through the heart at the tissue level?
- The action potential is initiated at the SAN
- The depolarisation will pass through the right and left atrium.
- The electrical activity will then reach the AVN. The function of the AVN is to slightly slow down the action potential to allow full contraction of the atria.
- From the AVN the action potential travels down the bundle of HiS, these are fibres that travel through the Annulus Fibrosis.
- The bundle of His will enter into the interventricular septum where it’ll divide into the right and left bundle branch
- Each bundle branch travels along their respective interventricular septum side
- The right and left bundle branches will terminate at the Purkinje fibres at the apex and these will continue to conduct the depolarisation wave up to the ventricles
Where is the SAN?
at the junction of the right atrium and the superior vena cava.
Where is the AVN?
located in the inter-atrial septum near the tricuspid valve
What is the bundle of His?
Wide, fast, conducting muscle fibres that travel through the annulus fibrosis
Where does the right bundle branch travel and what does it excite?
travels along right side of interVENTRICULAR septum
– excites right ventricle
Where does the left bundle branch travel and what does it excite?
travels along left side of interVENTRICULAR septum
– excites left ventricle
Describe the Annulus fibrosis, including its function
- The annulus fibrosis is the fibrous skeleton of the heart.
- It’s used to anchor the myocardium and cardiac valves.
- It also acts as an electrical insulator between the atria and the ventricles.
- It consists of 4 fibrous rings.
List and describe the 3 parts of the heart that can initiate an action potential within myocardial cells
The SAN, AVN and LBB& RBB all possess an electrical conducting system and can all respectively initiate action potentials.
However the speed of depolarisation varies between them and the fastest conducting system(SAN) will over power the rest.
SAN> AVN> LBB & RBB
What is the function of the SA node?
- Sets heart rate and rhythm– SINUS rhythm
* Fastest rate of depolarisation in the heart
What are the 2 meanings of ECG lead
- It’s the cable that’s used to connect the electrode to the ECG recorder
- It’s the electrical view of the heart that’s obtained from any one combination of electrodes
What is an electrode?
An electrode is a conductive pad that’s attached to the skin (limbs and chest) and is used to record electrical activity.
What does an ECG measure?
Electrical activity of the heart as it is transmitted to chest wall and limbs
Measures it over a time
Respectively how many leads and electrodes are there in an ECG?
In a 12 lead ECG you have 12 leads(12 views of heart) but only 10 physical electrodes - 4 on limbs (A) and 6 on chest (B).
What is the function of the Right leg electrode?
grounding electrode – Not used for any leads/views
What is the order of waves and intervals in an ECG?
P wave
P-R interval
QRS complex
S-T segment
What does the P wave represent?
The P wave represents atrial depolarisation.
If the patient is in sinus rhythm there will ALWAYS be a p wave before the QRS complex.
The P wave is upwards because the depolarisation wave is towards the positive electrode in lead II. Direction of atrial depolarisation: Downwards & to the left (Towards AV node)
What is the PR interval?
The PR interval is from the start of the P-wave to the start of the Q wave.
It represents the time taken for electrical activity to move between the atria and ventricles. It’s caused by the AVN. It’s a straight line because there’s not enough electrical activity to initiate a wave.
What does the QRS complex represent?
The QRS complex represents depolarisation of the ventricles.
• depolarisation of the septum happens from left to right, seen as a downward deflection as the Q wave(away from electrode except in AVR)
• upwards deflection caused by the depolarisation spreading down the bundle of his is the R wave
• the negative deflection is the S wave and is the wave of depolarisation spreading up the walls of the ventricles via the purkinje fibres
What is the S-T segment?
The ST-segment starts at the end of the S-wave and finishes at the start of the T-wave.
The ST segment is an isoelectric line that represents the time between depolarization and repolarization of the ventricles. This is the period where contraction of the ventricles is taking place. If it is raised or depressed this indicates myocardial infarction or ischaemia.
What does the T wave represent?
The T-wave represents ventricular repolarisation.
It is seen as a small wave after the QRS complex.
Give the positioning of electrodes on the body of the patient when taking an ECG
There are 10 electrodes, 4 electrodes on the limbs and 6 on the chest.
Chest:
• V1 - 4th intercostal space - right sternal edge
• V2 - 4th intercostal space - left sternal edge
• V3 - midway between V2 and V4
• V4 - 5th intercostal space - midclavicular line
• V5 - left anterior axillary line - same horizontal level as V4
• V6 - left mid-axillary line - same horizontal level as V4 &
Limbs: • LA - left arm • RA - right arm • LL - left leg • RL - right leg However the RL is neutral and not included in calculations
Which limb leads are bipolar?
Limb Leads I, II and III - negative and positive electrodes
Describe limb lead I
voltage difference between electrode RA and LA; LA (+)electrode
Describe limb lead II
voltage difference between electrode RA and LL; LL (+)electrode
Describe limb lead III
voltage difference between electrode LA and LL; LL (+)electrode
What are the 3 augmented limb leads?
aVR, aVL, aVF
Describe the charges of the augmented limb leads
– Augmented Limb Leads aVR, aVL and aVF unipolar- only have a positive electrode
– Other “electrode” actually represents the average of the remaining 2 electrodes and is designated “neutral” or reference
– The positive electrodes for these augmented leads are located on
• Right arm for aVR
• Left arm for aVL
• Left leg aVF (F for foot)
do augmented limb leads and standard limb leads use the same electrodes
Augmented unipolar leads use same electrodes used for standard limb leads - only thing that changes is how these electrodes are connected
Why is it so important to understand the
concept of a positive electrode in the context
of ECG?
1) Cardiac view provided by a lead is from the perspective of the positive electrode
2) By convention – if electrical current (i.e. depolarisation current) of heart part being looked at is travelling to the positive electrode of lead – ECG wave will have positive deflection
Explain the cause of the different delefections in an ECG
• Depolarisation spreading towards a positive recording electrode yields an upward deflection
• Depolarisation spreading away from a positive recording electrode yields a downward deflection
• Repolarisation spreading towards a positive recording electrode yields a downward deflection
• Repolarisation spreading away from a positive recording
electrode yields an upward deflection
What does the height of deflection depend on?
The amplitude (height) of the deflection depends on the size of muscle changing potential and how fast. It also depends on how directly the wave of activity is travelling towards the electrode • directly towards /away yields a large signal • obliquely towards/away yields a smaller signal • spread at right angles yields no signal
How do you find the 4th intercostal space
Find the sternal angle (the slight dip that be felt between the manubrium and body of sternum) this is next to the 2nd ICS
Count down 2 more spaces
What is the view of limb lead I?
Lateral view
What is the view of limb lead II?
Inferior view
What is the view of limb lead III?
Inferior view
What is the view of aVR?
Lateral view
What is the view of aVL?
Lateral view
What is the view of aVF?
Inferior view
What is the view of V1, V2, V3, V4, V5, V6?
V1 - Septal view of heart V2 - Septal + anterior view of heart V3 - Anterior view of heart V4 - Anterior + lateral view of heart V5 - Lateral view of heart V6 - Lateral view of heart
Which ECG leads face which parts of the
ventricles?
• Inferior surface of ventricles = II, III and aVF
• Septum and anterior surface of ventricles = V1,V2, V3,V4
• Right ventricle and septum = V1 and V2 & aVR
• Apex and anterior surface of ventricles = V3 and V4
• Lateral surface of ventricle = Lead 1, aVL, V5,
V6
explain the spread of electrical activity through the heart corresponding to the ECG
1) Atrial depolarisation
- Spreads along atrial muscles fibres and internodal (SA-AV nodes) pathways
- Throughout both right and left atria
- Will produce a small upward deflection - the p wave
explain the spread of electrical activity through the heart corresponding to the ECG
2) Delay at AV node
- Conduction is slowed down at AV node
- Allows time for atrial contraction to fill ventricle
- Signal is very small
- Isoelectric (flat line) segment – flat line on ECG after p wave
explain the spread of electrical activity through the heart corresponding to the ECG
3) Bundle of His -Spread of depolarisation from atrium to ventricle
• Fibrous ring only crossed by Bundle of His
∴ Depolarisation can only reach ventricle via
conduction through Bundle of His - also gives an Isoelectric (flat) segment
• Thereafter rapidly conducted thru ventricle
• via left bundle and right bundle branches (LBB & RBB) and the Purkinje system
• 120 – 200 ms from start of atrial depolarisation
to start of ventricular muscle depolarisation
explain the spread of electrical activity through the heart corresponding to the ECG
4) Depolarisation of the of the interventricular septum
• First part to depolarise is muscle in interventricular septum
• Depolarisation spreads from left to right
• May produce a small downward deflection because moving obliquely away - to the sides (so not straight
towards the +electrode) (or no deflection may be seen)
• Termed a Q wave = first downward deflection after p
explain the spread of electrical activity through the heart corresponding to the ECG
5) Depolarisation of apex and free ventricular walls
• Produces a large upward deflection
• Termed the R wave
– Upward because depolarisation moving directly towards electrode
explain the spread of electrical activity through the heart corresponding to the ECG
6) Last part of depolarisation
- Depolarisation finally spreads upwards to the base of the ventricles
- Produces a small downward deflection
- ‘S’ of the QRS
- downward because moving away
- small because not moving directly away
- Complete ventricular muscle depolarisation (QRS complex) takes 80 -120 ms
explain the spread of electrical activity through the heart corresponding to the ECG
7) Ventricular repolarisation
- Begins on the epicardial surface of the heart
- Spreads in the opposite direction to depolarisation
- Produces a medium upward deflection - The T wave
- Upward because it is a wave of repolarisation moving away from electrode
Summarise the order of ECG
P wave = depolarisation of atria not contraction of atria
QRS : depolarisation of ventricles not contraction of ventricles
T wave: repolarization of ventricles not relaxation of ventricles
BUT
• Contraction of the atria immediately follows the p wave
• Contraction of the ventricles immediately follows the QRS complex
• Relaxation of the ventricles immediately follows the T wave
Why is the R wave large?
Large because large muscle mass – more electrical activity
If left ventricle is hypertrophied, how is the R wave affected?
R wave will be correspondingly taller because more muscle so more electrical activity
describe the charges of the pericordial leads
- Unipolar – the other “electrode” is the average of the limb electrodes and positions in the middle of the chest – called ground/reference lead
- Positive electrodes placed across chest
Where is the ground lead in the pericordial leads?
Ground lead being in centre of chest, precordial leads measure electrical activity that is moving in a front-back direction and rightleft direction – horizontal plane
Give the limb leads that can be used to see the lateral side of the heart and the pathologies that can be identified as a result
The lateral wall of the LV of the heart can be studied using leads I and aVL.
It can identify muscle necrosis due to occlusion of the branch of the left coronary artery.
This indicates a lateral wall myocardial infarction.
Give the limb leads that can be used to see the inferior side of the heart and the pathologies that can be identified as a result
The inferior surface of the heart can be studied using the II, III and the AVF.
They can be used to identify muscle necrosis as a result of occlusion of the right coronary artery. This indicates an inferior myocardial wall infarction
Give the limb leads that can be used to see the anterior wall side of the heart and the pathologies that can be identified as a result
The anterior wall of the heart can be be studied using precordial leads V3 and V4. They can be used to identify necrosis of the anterior cardiac wall. This indicates an occlusion in the left anterior descending artery/ anterior interventricular branch of the left coronary artery.
How many views does the pericordial leads give and how?
Precordial/Chest leads – 6 views of the heart in the horizontal plane
• 6 chest electrodes, V1 – V6
• Leads V1 to V4 “ antero-septal” leads
• V1 & V2 face the RV & septum (‘septal leads’)
• V3 & V4 faces the apex and anterior wall of RV & LV
• Leads V5 and V6 face the LV (‘lateral leads’)
Which ECG leads face which parts of the ventricles?
- Inferior surface of ventricles = II, III and aVF
- Septum and anterior surface of ventricles = V1,V2, V3,V4
- Right ventricle and septum = V1 and V2 & aVR
- Apex and anterior surface of ventricles = V3 and V4
- Lateral surface of ventricle = Lead 1, aVL, V5, V6
Explain how to calculate heart rate when rhythm is regular on an ECG
Each small square represents 0.04 seconds
- Each large square on the paper represents 0.2 seconds
- 5 large squares therefore = 1 second
- 300 large squares = 1 minute
- count how many large squares between two consecutive R waves.
- divide 300 by the number of large squares
Explain how to calculate heart rate when rhythm is irregular on an ECG
Calculate heart rate by counting the number of QRS complexes in 6
seconds, then multiply by 10 to get total heart beats in 60 seconds = 1 minute
What’s the normal PR interval
The range is between 0.12-0.20 seconds (3-5 small boxes).
What does an elongated PR interval indicate?
A prolonged PR interval suggests that there’s delay of conduction through the AVN and the bundle of His , causing an AV block. The PR interval is classed as elongated if it’s greater than 1 large box.(often seen in heart block, ischaemia heart diease, hypokalaemia). Longer P-R intervals indicate slow conduction
from the atria to the ventricle (first-degree heart block).
What does a shortened PR interval indicate?
- The P wave is originating from somewhere Closer to the the AV node so the conduction takes time (SAN position isn’t the same in everyone).
- The atrial impulse is getting to the ventricle by a faster shortcut instead of conducting across the atrial wall(seen in Wolff-Parkinson-white syndrome)
What does the QRS interval represent?
QRS interval (width of QRS complex) = Time taken for ventricular depolarisation
Explain what can cause a broad QRS complex
depolarisation is arising ectopically in the ventricles, the wave of depolarisation isn’t spreading via the his-purkinje system so its taking longer for the wave to spread (seen in bundle branchblock, hyperkalamia, ventricular ectopic)
Give the normal range for the width of the QRS complex
Should normally be 2-3 small boxes as ventricular depolarisation happens
What is the QT interval
time taken for depolarisation and repolarisation of ventricle - from beginning of Q to end of T
Give the normal range for the QT interval
normally 10-12 small boxes, but varies with heart rate and so it is often standardised.
Upper limit of corrected QT (QTc) interval: ≤ 0.44 – 0.45 seconds (11 small boxes)
why would QTinterval be prolonged
- Prolonged QTc: indicates prolonged ventricular repolarisation
- Prolonged QTc associated risk for dangerous arrythmias
What is a sinus rhythm
normal heartbeat triggered by the SA node
60-100bpm
P waves must be present (upright in I and II) and followed by QRS
What are the 6 steps to analysing an ECG
1) calculate the heart rate
2) regular rhythm? Mark the distance between the QRS complex
3) are there p waves? Showing atrial activities
4) QRS present? Showing ventricular activities
5) P wave followed by a QRS complex (checking for blocks)
6) measure the intervals!!
Does ventricular repolarisation follow the same route as depolarisation?
All the ventricular myocardial cells depolarise before any start to repolarise. Repolarisation does not follow the same sequence across the heart, as the cells at the outside of the ventricle depolarise first, so the direction of spread of repolarisation is opposite to that of depolarisation.
What is The ‘overall’ direction of the wave of depolarisation defined as?
defined as the ‘electrical axis’ of the heart, which is directed towards the apex
What is the R-R interval
From peak to peak of R-waves
- Shorter interval → faster heart rate
