A first look at the ECG Flashcards
What are the 3 main recordings made from any pair of electrodes?
- SLL I = left arm wrt right arm
- SLL II = left leg wrt right arm
- SLL III = left leg wrt left arm
Basic principles of ECG
- Fast events e.g. depolarisation and repolarisation of the AP, are transmitted well.
- Slow events, e.g. the plateau of the AP, are not.
What hypothetical triangle is created around the heart when electrodes are placed on both arms and the left leg?
Einthoven’s triangle
What does each side of the Einthoven’s triangle represent?
Numbered that correspond to 3 leads (or pairs of electrodes) used for recording
What are the 2 leads of SLL I (standard limb lead 1)?
Left arm and right arm
What are the 2 leads of SLL II?
Right arm and left leg
What are the 2 legs of SLL III?
Left leg and left arm
What is the difference between an AP and an ECG recording?
- An AP is one electrical event in a single cell recorded using an intracellular electrode.
- An ECG is an extracellular recording that represents the sum of multiple action potentials taking place in multiple heart muscle cells.
On SLL II, what occurs when the wave of depolarisation passes the electrode on the left leg?
It creates a positive potential relative to the electrode on the right arm.
In SLL II, how is the P wave caused?
P wave is caused by atrial depolarisation.
In SLL II, how is the QRS complex caused?
QRS complex is caused by ventricular depolarisation.
In SLL II, how is the T wave caused?
T wave is caused by ventricular repolarisation.
Why can’t you see atrial repolarisation on the recording from SLL II?
Because atrial repolarisation coincides (same time) with ventricular depolarisation.
- Ventricular depolarisation involves much more tissue depolarising much faster so it swamps any signal from atrial repolarisation.
Why is the QRS complex so complex?
-Because different parts of the ventricle depolarise so different times in different directions:
1st – the interventricular septum depolarises from left to right
2nd – the bulk of the ventricle depolarises from the endocardial to the epicardial surface
3rd – the upper part of the interventricular septum depolarises
Why is the T wave positive-going?
Because the action potential is longer in endocardial cells than in epicardial cells, so the wave of repolarisation runs in the opposite direction to the wave of depolarisation.
Why is the R-wave bigger in SLL II than in SLL I or SLL III?
Because the main vector of depolarisation is in line with the axis of recording from the left leg with respect to the right arm.
What would happen to the wave if the heart was rotated to the left, or developed hypertrophy on the left, or atrophy on the right?
Causes axis deviation
What extra information do the augmented limb leads give you?
- By recording from one limb lead with respect to the other two combines, it gives you 3 other perspectives on events in the heart.
- ie recordings from SLLs I, II, III and aVR, aVL, aVF give you 6 different views of events occurring in the frontal (or vertical) plane
What causes an “upward-going blip” in an ECG recording?
A wave of approaching depolarisation
How does the wave of depolarisation pass in SLL2 (standard limb lead)?
It passes from atria down to the ventricles and through the body fluids towards the electrode on the left leg
What potential does a wave of depolarisation cause?
POSITIVE potential
What potential does a wave of repolarisation cause?
NEGATIVE potential
What are the 2 intervals which exists in an ECG recording?
- PR interval (from P to Q)
- QT interval (from Q to T)
What does the PR interval tell us?
Time from atrial depolarisation to ventricular depolarisation mainly due to transmission through the AV node
How long does the PR normally last for?
Normally about 0.12-0.2 seconds
How long does the QRS complex last for?
Normally about 0.08 seconds
Function of which fibres are reflected by the QRS complex effectivity?
Bundle of His
Which interval will be longer in longer Plateau phase?
Longer QT interval
What are the 3 augmented limb leads?
- aVR (right arm)
- aVF (left leg)
- aVL (left arm)
What is the most positive limb lead?
aVF
What is the most negative limb lead?
aVR
What extra information do the precordial (chest) leads give you?
- These are arranged in front of the heart and therefore look at the same events, but in the horizontal (or transverse) plane
What are the 6 precordial (chest) leads?
V1,2,3,4,5,6
Where is the biggest negative blip in the precordial (chest) leads?
V1
Where is the biggest positive blip in the precordial (chest) leads?
V6
Where is the “flip over” part of the precordial (chest) leads?
V3 or V4
What do limb leads look at?
Spread of depolarisation and repolarisation from 6 angles in the frontal plane
What do precordial leads look at?
Spread of depolarisation and repolarisation from 6 angles in the transverse plane
What is the ECG rhythm in terms of time?
25mm/sec
What is the calibrating pulse?
0.2 sec =1 large square (5mm)
How many beats per minute is normal?
60-100 bpm
What is the term for heart rate< 60 beats?
Bradycardia
What is the term for heart rate> 60 beats?
Tachycardia
When is the PR interval too short?
<0.012 seconds
When is the PR interval too long?
> 0.02 seconds
When is the QRS complex too wide?
> 0.12 seconds
When is the QT interval too long?
> 0.42 seconds at 60bpm
What is STEMI?
- ST elevated myocardial infarction (on ECG)
- Developing complete occlusion of major coronary artery previously affected by atherosclerosis
- Full thickness damage of heart muscle
What is NSTEMI?
- Non-ST elevated myocardial infarction (on ECG)
- Developing complete occlusion of minor coronary artery or partial occlusion of major coronary artery previously affected by atherosclerosis
- Partial thickness damage of heart muscle
Is STEMI or NSTEMI worse?
STEMI is worse
- STEMI is more damage in tissue/severity of heart attack.
What are the 2 types of myocardial infarctions?
STEMI and NSTEMI
