A first look at the ECG Flashcards
What are your standard limb leads?
What does an approaching wave of depolarisation cause?
Upward going blip
Which events are better transmitted, fast or slow?
Fast
What is the PR interval and how long does it usually last?
Time from atrial depolarisation to ventricular depolarisation - mainly due to transmission through the AV node (normally about 0.1-0.2 sec)
Should really be called P - Q interval
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 can’t you see atrial repolarisation?
Because atrial repolarisation coincides with ventricular depolaristion. Ventricular depolarisation involves much more tissue depolarising much faster so it swamps any signal from atrial repolarisation.
How do you explain the three stages of the QRS complex?
Different parts of the ventricle depolarise at different times and also in different directions
- The interventricular septum depolarises from left to right
- The bulk of the ventricle depolarises from the endocardial to
the epicardial surface – travels towards the electrode on the left leg – hence the upwards spike
- The upper part of the interventricular septum depolarises
Why is the T - wave positive?
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
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.
What are the augmented limb leads?
What extra information do the augmented limb leads give you?
A. By recording from one limb lead with respect to the other two combined, it gives you 3 other perspective 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
Can you correctly label the vectors to the correct limb leads?
Should AVR be positive or negative?
Negative, travels away from the heart
Which wave is:
- aVL
- aVF
- aVR