ECG intro

Question 1

how are standard limb leads arranged

Answer 1

Einthoven’s triangle surrounding the heart
always record with respect to the other
readings can be made from any pair of electrodes

Question 2

which planes do standard limb leads look at events in

Answer 2

vertical or frontal

Question 3

SLL I

Answer 3

L arm wrt R arm

Question 4

SLL II

Answer 4

L leg wrt R arm

biggest deflections so we tend to concentrate on this one

Question 5

SLL III

Answer 5

L leg wrt L arm

Question 6

basic principles of the ECG (3)

Answer 6

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

Question 7

recording from SLL II

Answer 7

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

Question 8

wave of depolarisation approaching the L leg will create a …

Answer 8

+ve potential relative to the R arm

upward going blip

Question 9

wave of depolarisation going away from the L leg will create a …

Answer 9

-ve potential relative to the R arm

downward going blip

Question 10

wave of repolarisation approaching the L leg will cause …

Answer 10

-ve potential relative to the R arm

downward going blip

Question 11

wave of repolarisation going away from the L leg will cause a …

Answer 11

+ve potential relative to the R arm

upward going blip

Question 12

P wave

Answer 12

atrial depolarisation

Question 13

QRS complex

Answer 13

ventricular depolarisation
time for the whole of the ventricle to depolarise
normally ~0.08s
depends on how good the rapid conduction system is

Question 14

T wave

Answer 14

ventricular repolarisation

Question 15

PR interval

Answer 15

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

Question 16

QT interval

Answer 16

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

Question 17

why isn’t atrial repolarisation visible

Answer 17

atrial repol coincides with ventricular depol

ventricular depol involves much more tissue depolarising much faster so it swamps any signal from atrial repol

Question 18

explain the 3 parts of the QRS complex

Answer 18

different parts of the ventricle are depolarising at different times in different directions

1. interventricular septum depolarises from L to R, small Q wave, moving away from the electrode
2. 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
3. upper part of the interventricular septum depolarises, S wave going away from the recording electrode

Question 19

why is the T wave +ve going

Answer 19

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)

Question 20

why is the R wave bigger in SLL II than SLL I or III

Answer 20

the main vector of depol is in line with the axis of recording from the L leg wrt R arm

Question 21

what would happen if the heart was rotated to the L/developed hypertrophy on L/atrophy on R /dextrocardia

Answer 21

axis deviation
SLL II would be smaller
SLL I would be larger

Question 22

aVR

Answer 22

R arm to aVL and aVF

-ve going bit on depol, spreading away from the electrode

Question 23

aVF

Answer 23

L foot to aVL and aVR

gives the biggest R wave

Question 24

aVL

Answer 24

L arm to aVF and aVR

Question 25

what extra information do augmented limb leads provide

Answer 25

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

Question 26

precordial chest leads

Answer 26

arranged in front of the heart and look at the same events in the horizontal/transverse plane

Question 27

what extra information do precordial leads give

Answer 27

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

Question 28

the rhythm strip

Answer 28

paper should run at 25mm/s

calibrating pulse is 0.2s = 1 large square (5mm)

Question 29

determining HR from the rhythm strip

Answer 29

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

Question 30

heart rate values

Answer 30

all these refer to at rest
60-100bpm = normal
<60bpm = bradycardia
>100bpm = tachycardia

Question 31

what else can the rhythm strip tell us

Answer 31

• 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)

Question 32

STEMI

Answer 32

ST elevated MI

Question 33

NSTEMI

Answer 33

non-ST elevated MI

Question 34

what is the difference between STEMI and NSTEMI

Answer 34

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