first look at ECG

Question 1

where does the standard limb leads look at on the body?

Answer 1

events in the vertical, or frontal, plain

Question 2

what pair of electrodes calculate left arm wrt right arm

Answer 2

SLL I

Question 3

what pair of electrodes calculate left leg wrt right arm

Answer 3

SLL II

Question 4

what pair of electrodes calculate left leg wrt left arm

Answer 4

SLL III

Question 5

what are the electtrodes actually recording?

Answer 5

difference in potential between the left leg and right arm that counts

Question 6

where does the wave of electrodes go?

Answer 6

The main wave of depolarisation passes down the ventricles and through the body fluids towards the electrode on the left leg

Question 7

A wave of depolarisation approaching the left leg will cause a … potential relative to the right arm.

Answer 7

positive

Question 8

It follows that a wave of depolarisation going away from the left leg will cause a … potential relative to the right arm.

Answer 8

negative

Question 9

A wave of repolarisation approaching the left leg will cause a … potential relative to the right arm.

Answer 9

negative

Question 10

It follows that a wave of repolarisation going away from the left leg will cause a … potential relative to the right arm.

Answer 10

positive

Question 11

Why can’t I see atrial repolarisation?

Answer 11

Because atrial repolarisation coincides with ventricular depolarisation. Ventricular depolarisation involves much more tissue depolarising much faster so it swamps any signal from atrial repolarisation.

Question 12

what is the P wave caused by?

Answer 12

atrial depolarisation

Question 13

what is the QRS complex caused by?

Answer 13

ventricular depolarisation

Question 14

what is the T wave caused by?

Answer 14

ventricular repolarisation

Question 15

what does the timing tell you ?

Answer 15

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)

Question 16

what is the PR interval

Answer 16

timefrom atrial depol, to ventricular depol

Question 17

how many seconds is the PR interval?

Answer 17

0.12-0.2

Question 18

what is the QRS interval?

Answer 18

time for the whole of the ventricle to depolarise

Question 19

how long does it take for the whole ventrile to depolarise?

Answer 19

0.08

Question 20

what is the QT interval?

Answer 20

time spent while ventricles are depolarised

Question 21

how long is the qt interval?

Answer 21

0.42 sec at 60bpm

Question 22

Why is the QRS complex so complex?

Answer 22

Because different parts of the ventricle depolarise at different times in different directions:

Question 23

what direction does the interventricular septum depolarise?

Answer 23

left to right

Question 24

the bulk of the ventricle depolarises from the … to the … surface

Answer 24

endocardial to epicardial

Question 25

does the upper part of the inter ventricular septum depolarise or hyperpolarise

Answer 25

depolarise

Question 26

Why is the T-wave positive-going?

Answer 26

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.

Question 27

Why is the R-wave bigger in SLL II than in SLL I or SLL II?

Answer 27

Because the main vector of depolarisation is in line with the axis of recording from the left leg with respect to the right arm.

Question 28

So what would happen if the heart was rotated to the left, or developed hypertrophy on the left, or atrophy on the right?

Answer 28

Causes axis deviation.

Question 29

What extra information do the augmented limb leads give you?

Answer 29

By recording from one limb lead with respect to the other two combined, it gives you 3 other perspective on events in the heart

Question 30

What extra information do the precordial (chest) leads give you?

Answer 30

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

Question 31

how many angles do the limb leads look at?

Answer 31

6

Question 32

What can the rhythm strip tell you?

Answer 32

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

Question 33

what range of heart rate is normal?

Answer 33

60-100

Question 34

what heart rate would suggest bradycardia

Answer 34

below 60

Question 35

What else can the rhythm strip tell you?

Answer 35

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)?

Question 36

what is STEMI

Answer 36

ST elevated myocardial infarction

Question 37

what is NSTEMI?

Answer 37

non-ST elevated myocardial infarction

Question 38

what is worse stemi or non stemi?

Answer 38

stemi