ECG Flashcards
Describe what SLL 1,2 and 3 is
SLL I = left arm wrt right arm
SLL II = left leg wrt right arm
SLL III = left leg wrt left arm
Describe the basic principle behind fast events
eg depolarisation and repolarisation of the AP, are transmitted well
Describe the basic principles behind slow events
Slow events, eg the plateau of the AP, are not
What is SLL 2 in terms of potential?
Standard limb lead II records from the left leg with respect to the right arm - ie it is the difference in potential between the left leg and right arm that counts
What charge of potential with from RA to LL cause?
Positive
What is the PR interval?
time from atrial depol. to ventricular depol. - is mainly due to transmission through the AV node (normally about 0.12-0.2 sec)
What is the QRS interval?
time for the whole of the ventricle to depolarise
(normally about 0.08 sec)
What is the QT interval?
time spent while ventricles are depolarised (varies with heart rate, but normally about 0.42 sec at 60 bpm)
Why cant 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
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. ie a wave of repolarisation moving away from the recording electrode produces another positive-going blip.
Q. Why is the R-wave bigger in SLL II than in SLL I or SLL II?
Because the main vector of depolarisation is in line with the axis of recording from the left leg with respect to the right arm.
Q. So what would happen if the heart was rotated to the left, or developed hypertrophy on the left, or atrophy on the right?
A = 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 combined, it gives you 3 other perspective on events in the heart
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
Because the main vector of depolarisation is as shown by the arrow, it will produce a negative going blip when recorded from V1, a positive going blip from V6, and flip over somewhere around V3 or V4. “Progression”
