first look at ECG Flashcards
where does the standard limb leads look at on the body?
events in the vertical, or frontal, plain
what pair of electrodes calculate left arm wrt right arm
SLL I
what pair of electrodes calculate left leg wrt right arm
SLL II
what pair of electrodes calculate left leg wrt left arm
SLL III
what are the electtrodes actually recording?
difference in potential between the left leg and right arm that counts
where does the wave of electrodes go?
The main wave of depolarisation passes down the ventricles and through the body fluids towards the electrode on the left leg
A wave of depolarisation approaching the left leg will cause a … potential relative to the right arm.
positive
It follows that a wave of depolarisation going away from the left leg will cause a … potential relative to the right arm.
negative
A wave of repolarisation approaching the left leg will cause a … potential relative to the right arm.
negative
It follows that a wave of repolarisation going away from the left leg will cause a … potential relative to the right arm.
positive
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.
what is the P wave caused by?
atrial depolarisation
what is the QRS complex caused by?
ventricular depolarisation
what is the T wave caused by?
ventricular repolarisation
what does the timing tell you ?
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)
QRS = time for the whole of the ventricle to depolarise
(normally about 0.08 sec)
QT interval = time spent while ventricles are depolarised (varies with heart rate, but normally about 0.42 sec at 60 bpm)
what is the PR interval
timefrom atrial depol, to ventricular depol
how many seconds is the PR interval?
0.12-0.2
what is the QRS interval?
time for the whole of the ventricle to depolarise
how long does it take for the whole ventrile to depolarise?
0.08
what is the QT interval?
time spent while ventricles are depolarised
how long is the qt interval?
0.42 sec at 60bpm
Why is the QRS complex so complex?
Because different parts of the ventricle depolarise at different times in different directions:
what direction does the interventricular septum depolarise?
left to right
the bulk of the ventricle depolarises from the … to the … surface
endocardial to epicardial
does the upper part of the inter ventricular septum depolarise or hyperpolarise
depolarise
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.
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.
So what would happen if the heart was rotated to the left, or developed hypertrophy on the left, or atrophy on the right?
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
how many angles do the limb leads look at?
6
What can the rhythm strip tell you?
Timing is all important – paper should run at 25mm/sec
Calibrating pulse is 0.2 sec = 1 large square (5mm)
Heart rate:
Measure the R-R interval and work out how many occur in 60 sec, or better ..
Count the R waves in 30 large squares (= 6 sec) and multiply by 10
60-100 beats per min = normal
Below 60 beats per minute = bradycardia
Above 100 beats per minute = tachycardia
what range of heart rate is normal?
60-100
what heart rate would suggest bradycardia
below 60
What else can the rhythm strip tell you?
Is each QRS complex preceded by a P-wave?
Is the PR interval too short (<0.12 sec) or too long (>0.2 sec)?
Is the QRS complex too wide (>0.12 sec)?
Is the QT interval too long (>0.42 sec at 60 bpm)?
what is STEMI
ST elevated myocardial infarction
what is NSTEMI?
non-ST elevated myocardial infarction
what is worse stemi or non stemi?
stemi
