ECG Flashcards
What plane do limb leads record in?
Frontal/ vertical
What do the limb leads (1,2,3) record from?
SLL I (Standard limb lead 1)= from left arm to right arm SLL II = from left foot in respect of the right arm SLL III = from the left foot with respect of left arm SLIII = from the left foot in respect of the right arm
Approaching depolarisation results in an upward or downward blip?
upward, apraching the limb first mentioned (as from) eg SLII a depolarisation towards the left foot would result in a upward blip as the left foot becomes more positive (depolarised) in comparison to the right arm.
If it was to travel the other way, eg Q ( as the wave travels left to right across bundle of his) will result in a downward as right arm will become more positive than left foot.
In repolarisation will be more negative but as it travels upwards, the hand will be more negative than the foot and so the foot is more positive than the hand resulting in an upward blip.
Which events are transmitted well and not so well by the limb leads?
Fast events are transmitted well (wg depolerisation and repolarisation on ventricles)
Slow events eg plateau of action potential are not transmitted well.
Which direcction does the ventricle wave of depolarisation go?
generally down, should be in the plane of the SLL2 (left foot in relation to right arm)
Which lead are we comparing to?
You compare the first limb mentioned in relation to the other kim mentioned, eg SLL I is the left arm in relation to the right arm
What is the PR interval?
From the initiation of the P wave until the initiation of the QRS complex, so fom the start of depolarisation of the atrium until the depolarisation of the ventricles. Should be between 0.12-0.2 seconds
What is the QRS time and reliant on what?
The time taken from the start of the QRS complex to the end (start to end of the depolarisation of the ventricles). Time should be approx 0.08s = 2 mini squares). Time taken is mainly detwemined by the fast conducting system ( Purkinje fibers and bundle of his) is able to pass on signal to the rest of the heart.
What is the QT interval and what should it usually be?
Time from the beginning of the depolarisation of the ventricle to the end of the repolarisation of the ventricle (Q start to the end of T). around 0.42 seconds at 60bpm (just over 2 big squares), but decreases as HR increases, and represents the time the ventricle spends depolarised - the plateau phase.
Why isn’t atrial repolarisation visible on an ecg?
Because it coincides with the ventricle depolarisation. which includes many more cells and depolarises at a much faster rate.
Why is the QRS down UP down in SLL II?
Because of the directions of depolarisations.
In relation to the SLL II:
The initial down Q is because the the first part to depolarise is the interventricular septum, which depolarises from left to right and so the signal will be travelling towards the right hand, making the right hand more positive than the left foot and so a downward blip.
The big UP (R) is the ventricle muscle wall contracting, which has a net depolarisation down towards the foot. (the inner myocardial cells depolarise first, so the wave travels towards the foot).
The final little downward S blip is the depolarisation of the upper part of the interventricular septum. ( so same reason as the initial down)
Why is the T wave upwards?
Because so when the heart depolarises it goes from the endocardial cells to the epicardial cells, so depolarisation moves outwards. However the epicardial cells have a shorter plateau and so repolarise before the endocardial cells, thus sending the repolarisation away from the leg lead and making the right arm more negative than the left leg.
What happens to the relative heights of SLI,II and III in hypertrophy od the left ventricle and what is this change called?
So in hypertrophy of the left ventricle, the extra muscle shifts the mean depolarisation axis to the patients left (more towards the outside of the body, more across).
This means that:
SLL1: the QRS complex will be bigger because the vector of depolarisation is more in the direction of the limb leads (across the body, right to left arm)
SLL II: The QRS complex would be smaller as it will have deviated away from the direction of the limb lead.
SLL III: will be smaller again as it has shifted away from the direction of the limb leads
It is called Axis deviation
What is an augmented limb lead and what do they tell you?
It is comparing one limb lead in comparison to the other 2 limb leads. They give you another 3 views of events in frontal/vertical plane.
What are the precordial leads?
The chest leads
