A first look at the ECG Flashcards
What does SLL stand for?
standard limb lead (SLL I , SLL II, SLL III)
What are the augmented leads?
aVR , aVF, aVL
What are the six chest leads or precordial leads?
V1-V6
What do standard limb leads look at?
these leads look at electrical events in the heart across the vertical or frontal plane
What is the rhythm strip?
from standard limb lead II and a this is a longer continuous recording of that
What do we record in SLL 1 ?
right arm to left arm (negative electrode at right arm)
What do we record in SLL II?
right arm to left leg
What do we record in SLLIII?
left arm to left leg (negative electrode at left leg)
What are basic principles of ECG?
fast events such as depolarisation and repolarisation of the action potentials are transmitted really well through the skin to to these recording electrodes
whereas slow events like the plateau phase of the action potential is not transmitted at all
a wave of depolarisation that’s approaching the recording electrode will be represented by an upward going blip.
Describe recording of SLLII?
As the wave of depolarisation passes the electrode on the left leg it creates a positive potential relative to the electrode on the right arm.
(negative electrode to positive electrode)
and therefore a wave of depolarisation going away from the left leg will cause a negative potential relative to right arm
A wave of repolarisation approaching the left leg will cause a negative potential relative to the right arm
Therefore, a wave of repolarisation going away from the left leg will cause a positive potential relative to the right arm.
What event in heart does P wave represent?
atrial depolarisation
What event does QRS complex represent?
ventricular depolarisation
What does T wave represent?
ventricular repolarisation
What is the PR interval?
from start of P wave to start of QRS complex
time from atrial depolarisation to ventricular depolarisation, mainly due to transmission through the AV node (normally about 0.12-0.2 s)
What is the time taken for QRS?
time for the whole of the ventricle to depolarise (normally about 0.08 s)
What is QT interval?
time for ventricles to depolarise and repolarise (varies with heart rate, but normally about 0.42 s at 60 bpm)
Increase of PR interval?
suggest block
Broad QRS complex?
longer for conduction
Why can’t I 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.
Why is the QRS complex so complex?
Because different parts of the ventricle depolarise at different times in different directions:
1st – the interventricular septum depolarises from left to right (negative Q wave)
2nd – the bulk of the ventricle depolarises from the endocardial to the epicardial surface (positive R wave as going to left leg)
3rd – the upper part of the interventricular septum depolarises (negative S wave as moving away from left leg electrode)
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 i.e. a wave of repolarisation moving away from the recording electrode produces another positive-going blip.
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 extra information do the augmented limb leads give you?
By recording from one limb lead with respect to the other two combined, it gives you 3 other perspective on events in the heart
i.e. recordings from SLLs I, II, III and aVR, aVL, aVF give you 6 different views of events occurring in the frontal (or vertical) plane.
Where is aVR recorded?
recorded from the right arm with respect to left arm and the left leg
Where is aVL recorded?
recorded from the left arm relative to the right arm and the left leg
Where is aVF recorded?
recorded from the left leg relative to the right arm and left arm
R wave in aVR?
R wave negative as depolarisation is moving away from its axis
aVL?
unsure R wave as perpendicular to the depolarisation
avF?
depolarisation generally moving in same direction as its axis , positive R wave
What extra information do the precordial (chest) leads give you?
arranged round the front of the heart and they’re going to be looking at the electrical events in the heart but in horizontal or transverse plane
What polarity would you expect the R waves at V5 and V6 to be?
positive as main way of depolarisation in the ventricles that gives us R waves moves from endocardial surface to epicardial surface
R waves of V1?
Negative
R waves going from V2 to v4?
negative to position
‘progression’
What speed is the paper of a rhythm strip running at?
25mm/s
What is the calibrating pulse?
0.2s= 1 large square (5mm)
How to work out heart rate?
count the R waves in 30 large squares (=6s) and multiply by 10
What is normal heart rate?
60-100 bpm
What is heart rate of brachycardia?
below 60bpm
What is heart rate of above 100bpm?
tachycardia
What else does rhythm strip tell us?
Is each QRS complex preceded by a P wave? - if not then possible heart block or AV block (problem with conduction spreading to atrialventricular node)
Is the PR interval too short (<0.12 s) or too long (>0.2 s)?- if too long could be AV conduction problem
Is the QRS complex too wide (>0.12 s)?- could indicate that there is a problem with the fast conducting system (bundle of His and purkinje fibres)- can get in L and R bundle branch block
Is the QT interval too long (>0.42 s at 60 bpm)?
What is a STEMI AND NSTEMI?
STEMI = ST segment elevation myocardial infarction (heart attack)
NSTEMI = non-ST segment elevation myocardial infarction
What does elevation of ST segment indicate?
something gone seriously wrong
