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
