ECG intro Flashcards
how are standard limb leads arranged
Einthoven’s triangle surrounding the heart
always record with respect to the other
readings can be made from any pair of electrodes
which planes do standard limb leads look at events in
vertical or frontal
SLL I
L arm wrt R arm
SLL II
L leg wrt R arm
biggest deflections so we tend to concentrate on this one
SLL III
L leg wrt L arm
basic principles of the ECG (3)
fast events (e.g. depolarisation and repolarisation) are transmitted well to the periphery slow events (e.g. AP plateau) are not transmitted well to the periphery a wave of approaching depolarisation causes an upward going blip
recording from SLL II
L leg wrt R arm
the difference in potential between the two is what counts
the main wave of depolarisation passed down the ventricles and through the body fluids towards the electrode on the L leg
as the wave of depolarisation passes the electrode on the L leg it creates a +ve potential relative to the electrode on the R arm
wave of depolarisation approaching the L leg will create a …
+ve potential relative to the R arm
upward going blip
wave of depolarisation going away from the L leg will create a …
-ve potential relative to the R arm
downward going blip
wave of repolarisation approaching the L leg will cause …
-ve potential relative to the R arm
downward going blip
wave of repolarisation going away from the L leg will cause a …
+ve potential relative to the R arm
upward going blip
P wave
atrial depolarisation
QRS complex
ventricular depolarisation
time for the whole of the ventricle to depolarise
normally ~0.08s
depends on how good the rapid conduction system is
T wave
ventricular repolarisation
PR interval
start of P wave to start of QRS
Time from atrial depol to ventricular depol
mainly due to transmission through the AVN
normally ~0.12-0.2s
QT interval
time from the start of the QRS to the end of the T wave
time spent while ventricles are depolarised
varies with HR, normally ~0.42s at 60bpm
longer AP = longer QT interval and vice versa
why isn’t atrial repolarisation visible
atrial repol coincides with ventricular depol
ventricular depol involves much more tissue depolarising much faster so it swamps any signal from atrial repol
explain the 3 parts of the QRS complex
different parts of the ventricle are depolarising at different times in different directions
- interventricular septum depolarises from L to R, small Q wave, moving away from the electrode
- bulk of the ventricle depolarises from the endocardial to the epicardial surface, large R wave, main part of ventricular depol going towards the L leg
- upper part of the interventricular septum depolarises, S wave going away from the recording electrode
why is the T wave +ve going
because the AP is longer in endocardial cells than epicardial cells (epicardial cells depol later but are ready to repol sooner)
the wave of repol runs in the opposite direction to the wave of depol (wave of repol away from the L leg so +ve going)
why is the R wave bigger in SLL II than SLL I or III
the main vector of depol is in line with the axis of recording from the L leg wrt R arm
what would happen if the heart was rotated to the L/developed hypertrophy on L/atrophy on R /dextrocardia
axis deviation
SLL II would be smaller
SLL I would be larger
aVR
R arm to aVL and aVF
-ve going bit on depol, spreading away from the electrode
aVF
L foot to aVL and aVR
gives the biggest R wave
aVL
L arm to aVF and aVR
what extra information do augmented limb leads provide
by recording from one limb lead wrt the other 2 combined, it gives you 3 other perspectives on events in the heart
recordings from SLL I, II and III AND aVR, aVL and aVF give you 6 different views of events occurring in the frontal/vertical plane
precordial chest leads
arranged in front of the heart and look at the same events in the horizontal/transverse plane
what extra information do precordial leads give
the main vector of depolarisation will produce a -ve going blip when recorded from V1, +ve going from V6 and flips around V3/4
this is known as progression
the rhythm strip
paper should run at 25mm/s
calibrating pulse is 0.2s = 1 large square (5mm)
determining HR from the rhythm strip
measure R-R interval and work out how many occur in 60s OR
count R waves in 30 large squares (6s) and multiply by 10
heart rate values
all these refer to at rest
60-100bpm = normal
<60bpm = bradycardia
>100bpm = tachycardia
what else can the rhythm strip tell us
- is each QRS preceded by a P wave
- is the PR interval too short (<0.12s) or too long (>0.2s)
- is the QRS complex too wide (>0.12s)
- is the QT interval too long (>0.42s at 60bpm)
STEMI
ST elevated MI
NSTEMI
non-ST elevated MI
what is the difference between STEMI and NSTEMI
elevation of ST segment is an indication something has gone very wrong
it is used to classify the severity of a heart attack
STEMI is worse than NSTEMI
