The Electrocardiogram (ECG)

Question 1

What do standard limb leads measure?

Answer 1

They record the spread of depolarisation in one limb with respect to another

Question 2

What are the standard limb leads?

Answer 2

From:
Right Arm -> Left Arm (SSL1)
Right arm -> Left Leg (SSL2)
Left Arm -> Left Leg (SSL3)

Question 3

Which standard limb lead gives us the classes P wave, QRS, T wave pattern?

Answer 3

SSL2

The right arm -> Left Leg as its in line with the main vector of depolarisation down the interventricular septum.

Question 4

How does the spread of depolaristion appear on an ECG?

Answer 4

Approaching depolarisation causes an upward blip

Question 5

In SSL2 which cardiac events correspond to the P Wave, QRS Complex and T Wave?

Answer 5

P Wave - Atrial Depolarisation
QRS - Ventricles depolarising
T Wave - Ventricles Repolarising.

Question 6

Why are there 3 parts to the QRS complex?

Answer 6

Different parts of the ventricles depolarise at slightly different times.
First, Interventricular Septum depolarises from left to right
Second, Bulk of ventricle depolarises from endocardial to epicardial surface
Thirds, Upper part of interventricular septum depolarises.

Question 7

Why is the R wave bigger than the Q & S?

Answer 7

Because when the bulk of the ventricle depolarises from the endocardial to epicardial surface the mean vector of depolarisation is in line with the SLL2 recording.

Question 8

What is the PR interval?

Answer 8

Start of P wave to start of QRS complex
Its the time from the start of atrial depolarisation to the start of ventricular depolarisation, mainly transmission in the AV node.

Question 9

How long does the PR interval last?

Answer 9

0.1-0.2 seconds.

Question 10

How long does the QRS complex last?

Answer 10

Around 0.08 seconds

Question 11

What is the QT interval?

Answer 11

Time from the start of the QRS complex to the end of the T wave.
Essentially the time the ventricle spends depolarised.

Question 12

How long is the QT interval?

Answer 12

It varies with HR.

The standard is 0.42 seconds at 60 BPM.

Question 13

Since the T wave is a repolarisation why isn’t it negative on the ECG?

Answer 13

Because it actually runs in the opposite direction to the depolarisation (away from the recording electrode), causing it to flip.
This is because the AP is longer in the endocardial than epicardial cells.

Question 14

What do augmented limb leads record?

Answer 14

The wave of depolarisation from one SSL to the third limb.

Question 15

What are the 3 augmented limb leads?

Answer 15

aVR - Midpoint of Left Arm/Left Leg -> Right Arm
aVL - Midpoint of Right Arm/Left Leg -> Left Arm
aVF - Straight down from arms line to left leg.

Question 16

What do precordial (Chest) leads do?

Answer 16

Measure the same events as the limb leads but in the transverse plane as opposed to the vertical.

Question 17

What are the precordial leads?

Answer 17

Theres 6.

V1 is -ve and V6 is +ve, the flip occurs around V3/V4

Question 18

How many perspectives do we have from which to observe cardiac events?

Answer 18

12:
3 - SSL
3 - Augmented limb leads
6 - Precordial Leads

Question 19

What is the rhythm strip?

Answer 19

A 10 second recording from SSL2

Question 20

What do we use the rhythm strip for?

Answer 20

Calculating HR
Checking for presence of P waves
Checking regularity of QRS complexes
Checking for appropriate length of the PR interval, QRS complex and QT interval.

Question 21

What does STEMI/NSTEMI stand for?

Answer 21

(Non) ST elevated myocardial infarction.

ST as in end of QRS complex to T wave