3. A First Look At The ECG Flashcards
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
How many leads are used at a time when an ECG is recorded?
1 lead at a time
How many electrodes are there per lead?
2 electrodes; positive electrode and negative electrode (third one is inactive)
Leads look at events in the heart activity in what planes?(2)
vertical or frontal planes
What 3 body parts make up the Einthoven triangle?
- left arm
- right arm
- left leg
What are the 2 leads of SLL I (standard limb lead 1)?
- left arm
2. right arm
What are the 2 leads of SLL II (standard limb lead 2)?
- right arm
2. left leg
What are the 2 leads of SLL III (standard limb lead 3)?
- left leg
2. 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
What does an ECG tracing show?
It shows a summed electrical potential generated by all cells of the heart
What causes an “upward- going blip” in an ECG recording?
A wave of approaching depolarisation
Are fast or slow events transmitted better across the heart?
fast events are transmitted better
When recording from SLL II for example, what is actually recorded?
the DIFFERENCE in potential between the left leg and right arm which counts
How does the wave of depolarisation pass in SLL2 (standard limb lead)?
passes from atria down to the ventricles and through the body fluids towards the electrode on the left leg
What is meant by ECG electrodes?
- They are wires that are attached to patients to record the ECG.
- These electrodes allows leads to be recorded
What potential does a wave of depolarisation cause?
POSITIVE potential
What potential does a wave of repolarisation cause?
NEGATIVE potential
What does the standard limb lead tell you?
- lead refers to imaginary line between two ECG electrodes
- electrical activity of this lead is measured and recorded as part of the ECG
What allows leads to be calculated?
electrodes
What is P wave caused by?
atrial depolarisation
What is QRS complex caused by?
ventricular depolarisation
What is T wave caused by?
ventricular repolarisation
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
What does the QRS complex show us?
the time for the whole of the ventricle to depolarise
How long does the QRS complex last for?
normally about 0.08 seconds
What does the QT interval tell us?
time spent while ventricles are depolarised (interval between ventricles depolarising and repolarising)
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
Why can’t we see atrial repolarisation?
Because atrial repolarisation coincides with ventricular depolarisation. Ventricular depolarisation involves much more tissue depolarising much faster so it swamps any signal from atrial repolarisation (atrial repolarisation hides behind the larger QRS complex showing ventricular contraction)
In which ECG section is the Plateau phase taking place in?
In the QRS complex
Why is the QRS complex so complex?
Because different parts of the ventricles depolarise at different times in different directions.
What are 3 ways ventricles depolarise?
- the interventricular septum depolarises from left to right
- the bulk of the ventricle depolarises from the endocardial to the epicardial surface
- the upper part of the interventricular septum depolarises
Why is T wave positive if it’s repolarising?
Because the AP is longer in the endocardial cells than in epicardial cells, so wave of repolarisation runs in the OPPOSITE direction to the wave of depolarisation. Ie a wave of repolarisation moving away from the recording electrode produces another positive going blip
Which cells depolarise first, the cells on the endocardial or epicardial surface?
endocardial
Why is the R-wave in QRS complex bigger in SLL2 than in SLL1 and SLL3?
because the MAIN VECTOR of depolarisation is in the line of the axis of recording from the left leg with respect to right arm
What would happen if heart was rotated to the left or developed hypertrophy (enlargement) on the left or atrophy (wasting away) on the right?
causes axis deviation
What extra info do the augmented limbs leads give you?
by recording from one limbs lead with respect to the other two combined, it gives you 3 other perspective on events in the heart
What are the 3 augmented limb leads?
- aVR (right arm)
- aVF (left leg)
- aVL (left arm)
How many different views of events can be seen in frontal (or vertical) plane including all leads?
6:
- SLL1, 2 and 3
- aVR, aVL and aVF
What is the most positive limb lead?
aVF
What is the most negative limb lead?
aVR
What extra info 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 will affect the ECG the most?
changes in the SPREAD of depolarisation
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 R waves are there in 6 seconds?
7 R waves (then x 10 in one minute which means 70 beats/min)
How many beats per minute is normal?
60-100
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?
<0.12 seconds
When is the PR interval too long?
> 0.2 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
What is an elevation of the ST segment an indicator of?
- something has gone seriously wrong
- identifies severity of the heart attack (how bad it is)
What is the correlation between ST elevation and heart attack severity?
the bigger the ST elevation, the worse the heart attack is (myocardial infarctions can be graded from mild to severe)
Is STEMI or NSTEMI worse?
STEMI is worse
What are 2 types of myocardial infarctions?
- STEMI
2. NSTEMI
