3/ a first look at the ECG Flashcards
which pair of electrodes can recordings be made from?
any pair
which pair is referred to as SLL I?
left arm and right arm
which pair is referred to as SLL II?
left leg and right arm
which pair is referred to as SLL III?
left leg and left arm
what does a wave of approaching depolarisation cause on the recording?
an upward-going blip
which events are transmitted better between fast and slow ones?
fast
where is the typical ECG recorded from? which electrode is the reference?
standard limb lead II (SLL II), the recording is LL’s position relative to the RA
which direction does the main wave of depolarisation take?
down the ventricles, through the body fluids, towards the electrode on the left leg
a wave of depolarisation approaching the left leg will cause a positive/negative relative to the right arm?
positive
a wave of depolarisation going away from the left leg will cause a positive/negative relative to the right arm?
negative
a wave of repolarisation approaching the left leg will cause a positive/negative relative to the right arm?
negative
a wave of repolarisation going away from the left leg will cause a positive/negative relative to the right arm?
positive
what is the P wave caused by? (from SLL II)
atrial depolarisation
what is the QRS complex caused by? (from SLL II)
ventricular depolarisation
what is the T wave caused by? (from SLL II)
ventricular depolarisation
what does the PR interval correspond to? what does the impulse go through during this time? how long is the PR interval?
time from atrial depolarisation to ventricular depolarisation, mainly due to transmission through the AV node (0,12-0,2sec)
what does the QRS time correspond to? how long is it usually?
time for the whole of the ventricle to depolarise (normally about 0,08sec)
what does the QT interval correspond to? how long is the QT interval?
time spend while ventricles are depolarised (varies with HR but normally about 0,42sec at 60bpm)
is atrial repolarisation visible on the ECG?
no 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 does the Q part of the QRS complex correspond to?
the inter ventricular septum depolarises from left to right
what does the R part of the QRS complex correspond to?
the bulk of the ventricle depolarises from the endocardial to the epicardial surface
what does the S part of the QRS complex correspond to?
the upper part of the inter ventricular septum depolarises
why is the T-wave positive + going?
because the action potential is longer in endocardial cells than in epicardial cells, so th 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
