S6 Understanding ECGs Flashcards
What is the definition of deflection?
A deviation away from the straight line e.g. in an ECG either an upwards to downward wave/peak from the baseline
What is the definition of depolarisation?
Change in electrical charge distribution within a cell leading to a less negative charge inside the cell (due to movement of ions)
What is the definition of repolarisation?
Change in the electrical charge distribution within a cell leading to a more negative charge inside the cell (due to movement of ions)
What is the definition of the resting membrane potential?
It’s created by a separation of charges across the cell membrane - measure of the electrical imbalance between inside and outside cell (measured in mV)
What is the definition of an action potential?
Brief reversal of the cell membrane electrical polarity (depolarisation) that is then propagated cell to cell
How many views are there involved in an ECG?
12
How does electrical activity spread through the hard at the tissue level?
- Initiated at SA node
- Depolarisation of right and left atria
- Impulse at AV node, delayed slightly
- Impulse goes to the Bundle of His
- The Bundle of His separates into the right and left bundle branches
- These branches terminate in the Purkinje fibres which continue to conduct the depolarisation wave through the ventricles
What is the Bundle of His?
Wide, fast, conducting muscle fibres that travel through the annulus fibrosis
What is the annulus fibrosis?
Connective tissue that separates the atria from the ventricles
Where is the SA node found?
Where is the AV node found?
- at the junction of the right atrium and superior vena cava
- in the inter-atrial septum near the the tricuspid valve
What sets the heart rate and rhythm? What is this rhythm called? What is the firing rate of this part of the heart?
SA node
Sinus rhythm
60-100bpm
What is the intrinsic firing rate of the AV node?
40-60bpm
What is the intrinsic firing rate of the left and right bundle branches?
20-40bpm
What are the two meanings for ECG leads?
- the cable used to connect the electrode to the ECG recorder
- the electrical view of the heart obtained from any one combination of electrodes
What is an electrode in an ECG?
A conductive pad that is attached to the skin and enables recording of electrical currents
How many electrodes are there? Where are they? How many views/leads do the 10 cables give?
10 electrodes
4 on the limbs
6 on the chest
12
What is the role of the right leg electrode?
The grounding electrode - not used for any leads/views
Where are the limb electrodes positioned?
Right arm (right shoulder) Left arm (left shoulder) Left leg (left side of waist below the umbilical cord) Right leg (right side of the waist below the umbilical cord)
Where are the 6 chest leads located?
V1 - 4th intercostal (right)
V2 - 4th intercostal (left)
V3 - between V2 and V4
V4 - midclavicular (in line with mid-collarbone)
V5 - 5th intercostal space (anterior axillary line)
V6 - 5th intercostal (midaxillary line)
Which electrodes is the limb lead I the voltage difference between? Which is the positive electrode?
The right arm and left arm
The left arm
Which electrodes is the limb lead II the voltage difference between? Which is the positive electrode?
Right arm and left leg
Left leg
Which electrodes is the limb lead III the voltage difference between? Which is the positive electrode?
Left arm and left leg
Left leg
Are the augmented limb leads unipolar or bipolar? Where are they located?
Unipolar (only have a positive electrode - the other electrode is represented by the average of the 2 other electrodes)
aVF - right arm
aVL - left arm
aVF - left leg
How are the augmented unipolar leads linked to the standard limb leads?
They use the same electrodes, but vary in how the electrodes are connected (ECG recorder does the switching/rearranging of whether electrode is negative/positive/averaged)
How is the cardiac view related to the positive electrode?
The cardiac view provided by a lead is from the perspective of the positive electrode (charge is towards the positive electrode)
If depolarisation of heart is travelling to the positive electrode of the lead, what will the deflection on the ECG wave be like?
Positive deflection
In what plane do the limb leads look at the heart?
Vertical plane (top to bottom)
How are the limb lead views related to the electrical current direction?
Opposite
E.g. Lead II is RA to LL, Lead II view is LL to RA
Are the precordial/chest leads unipolar or bipolar?
Unipolar, the other electrode is the average of the limb electrodes and positions in the middle of the chest (the ground/reference lead)
In what plane for the precordial/chest leas look at the heart?
The horizontal plane (front to back and right to left)
What two things does the height/depth of a deflection depend on?
- how directly the depolarisation wave is coming towards/away from a positive electrode
- number of cells generating the signal (muscle mass)
Describe the QRS complex if the depolarisation wave is
i. Directly towards the +ve electrode
ii. Obliquely towards +ve electrode
iii. 90 degrees to electrode
iv. Going directly away from the +ve electrode
i. Tall, upright QRS complex
ii. Smaller, upright QRS complex
iii. Biphasic or no complex
iv. Deep -ve QRS complex
Is the SA node depolarisation seen on the ECG?
No, the signal is insufficient, due to few cells able to generate signal, to register on the surface ECG
How is atrial depolarisation seen on a lead II ECG?
Small upward deflection - p wave
- lasts 80-100 ms
- towards +ve electrode (direction is downwards and to the left - towards LL)
How is the delay at the AV node seen on a lead II ECG?
Isoelectric (flat line) segment (flat line on ECG after p wave)
- signal is very small
How is spread of depolarisation from atrium to ventricle (BUndle of His) seen on a lead II ECG?
Isoelectric (flat) segment
How long is it between the start of atrial depolarisation and the start of ventricular muscle depolarisation?
120-200 ms
How is depolarisation of the interventricular septum seen on a lead II ECG?
Downward deflection (1st after p wave) - Q wave
- downward as moving obliquely away from +ve electrode (depolarisation spreads left to right (+ve on left))
How is depolarisation of the apex and free ventricular walls seen on a lead II ECG?
Large upward deflection - R wave
- upward as depolarisation moves towards electrode
- large due to a large muscle mass so more electrical activity
In left ventricular hypertrophy what will happen to the R wave?
Will be taller
How is the last part of ventricular depolarisation seen on a lead II ECG?
Small downward deflection - S wave
- downward as it’s moving away from the +ve electrode
- small because it’s not moving directly away (its obliquely moving away)
How long does complete ventricular muscle depolarisation (QRS complex) take?
80-120 ms
What is the last part of ventricular depolarisation?
When the depolarisation spreads upwards towards the base of the ventricle (towards atria)
How does ventricular repolarisation occur? What type of ECG lead II wave does it produce?
- begins on the epicardial surface of the heart
- spreads in the opposite direction to depolarisation
Medium upward deflection - T wave
Why is the repolarisation wave a medium upwards deflection?
Upwards as the wave of repolarisation is moving away from the electrode (when moves away from +ve electrode, produces upward deflection - opposite of depolarisation)
In summary, what does the P wave, QRS complex and T wave represent?
P wave - atrial excitation/depolarisation (not contraction)
QRS complex - ventricular excitation/depolarisation (not contraction)
T wave - ventricular recovery/repolarisation (not relaxation)
Contraction/relaxation immediately follows each though
What leads show abnormal complexes on the inferior aspect? Which coronary artery is most often responsible?
II, III and aVF
Right coronary artery
What leads show abnormal complexes on the right ventricle and septum? Which coronary artery is most often responsible?
V1 and V2
Left anterior descending artery
What leads show abnormal complexes on the anteroapical aspect? Which coronary artery is most often responsible?
V3 and V4
Left anterior descending artery (distal)
What leads show abnormal complexes on the anterospetal aspect? Which coronary artery is most often responsible?
V1 and V4
Left anterior descending artery
What leads show abnormal complexes on the lateral aspect? Which coronary artery is most often responsible?
V5, V6, I, aVL
Circumflex artery
What leads show abnormal complexes on the extensive anterior aspect? Which coronary artery is most often responsible?
V1, V2, V3, V4, V5, V6, I, aVL
Proximal left coronary artery
What leads show abnormal complexes on the posterior aspect? Which coronary artery is most often responsible?
V1, V2
Right coronary artery
Which of the chest leads are septal leads?
V1 and V2
Which of the chest leads are anterior leads?
V2, V3 and V4
Which of the chest leads are lateral leads?
V4, V5 and V6
What leads show abnormal complexes on the left side of heart (lateral wall of L ventricle)? Which coronary artery is most often responsible?
I and aVL
Left coronary artery
Which of the chest leads are anterior-septal leads?
V1, V2, V3, V4
How many seconds are there for a small square and large square on ECG paper?
Small - 0.04s
Large - 0.2s
At what speed does ECG paper move?
25mm/sec
How many squares are there in a minute of ECG paper?
300 large squares
How do you calculate heart rate when rhythm is regular?
- Each cardiac cycle represents one heart beat (e.g. one P QRS T complex) - count R-R interval as one heart beat
- How many big boxes are there on the ECG?
- No. big boxes x 0.2s
- 60/(No. big boxes x 0.2)
- Answer to 4 x no. R-R intervals
- This is the bpm
How do you calculate heart rate when rhythm is irregular?
- Calculate the heart rate by counting the number of QRS complexes in 6 seconds
- Multiply this by 10 to get total heart beat in 1 min (60 secs)
- Answer is the bpm
What is the PR interval? What does this indicate? What is a normal range of PR interval?
Start of P wave to the start of the Q wave
The longer the PR interval, the slower the conduction from the atria to ventricle (1st degree heart block) - prolonged if more than 1 large box
0.12s to 0.20s/3-5 small boxes
What is the QRS interval? What does this indicate? What is a normal QRS interval?
Start of Q wave to end of S wave
If have a wider QRS interval, indicates ventricular depolarisations aren’t initiated by the normal conductance mechanism (His-Purkinje system) - so take longer e.g. bundle branch block
Less than 0.12s/less than 3 small boxes
What is the QT interval? What does this indicate?
Start of Q wave to end of T wave
A prolonged QT interval suggests prolonged repolarisation of ventricles - can lead to arrhythmias
What does the QT interval vary with? What do you do to account for this? What are the upper limits for QTc?
Varies dependent on heart rate
Calculate corrected QT (QTc) intervals so can compare to a normal heart rate, etc.
Less than or equal to 0.44-0.45s/11 small boxes
What is the RR interval? What does this indicate?
From peak to peak of R waves
A shorter interval means faster heart rate
What is the ST segment? What does this indicate?
End of S wave to start of T wave
ST should be isoelectric, if it is raised/depressed indicates myocardial infarction/ischaemia
