Introduction To ECGs

Question 1

What is on the axis of an ECG?

Answer 1

Voltage on the Y and time on the X

Question 2

What does an ECG stand for?

Answer 2

Electro cardiogram

Question 3

What is the difference between an action potential graph and an ECG.

Answer 3

An action potential is for one cell, where as an ECG is for the whole heart.

Question 4

How does the impulse travel from the SAN to cause ventricular contraction?

Answer 4

From the SA node — atrial muscle — AV node (delay) — common bundle — left and right bundle — Purkinje fibres — ventricular muscle

Question 5

In an ECG when the line comes up above the baseline what does its mean and likewise when it goes down what does it mean?

Answer 5

Up = positive deflection
There is current towards the lead

Down = negative deflection there is current away from the lead

(In a bipolar lead positive deflection is when when the current flows from the negative to positive lead)

Question 6

What is the typical speed and voltage of an ECG?

Answer 6

Speed = 25mm/sec therefore 25mm on the graph would show a period of 10 seconds. Each small square is 1mm and hence shows 0.04 seconds.
5 big squares is 1 second (this is 25 squares).

voltage = 10mm/mV hence 10 small squares is a mV

Question 7

There a few ways of calculating heart rate using an ECG, a particularly useful one =

Answer 7

(QRS complexes in 10 seconds) x 6

10 seconds is 25mm of the graph (10seconds). So count this portion and then x 6.

Question 8

What is the order of waves on an ECG?

Answer 8

P,QRS,T

Question 9

What does the P wave show?

How does it appear on the graph?

Answer 9

Atrial depolarisation

1st wave - 1st thing that happens

Small uniform first wave

Question 10

What does the QRS complex show?

How does it appear on the graph?

Answer 10

Ventricular depolarisation

Small dip down to Q then up to a peak at R and down to another dip at S.

Question 11

What does the T wave represent?

How does it appear on the graph?

Answer 11

T wave = ventricular repolarisation

Longest, middle height peak after the QRS complex

Question 12

What is the usual time for the PR interval?

QRS complex?

QT interval?

Answer 12

PR = 120-200ms (3-5 small squares)

QRS interval = less than 120ms (less than 3 small squares)

QT = shows time taken for repolarisation
Normal men = 250-440ms
Women = 350-460ms

Question 13

The two types of lead ECGS?

Answer 13

ECGs can be 12 lead or 3 lead.

12 leads give you and overview of the heart using 10 electrodes, 3 bipolar limb leads I,II,III, 3 unipolar limb leads aVL, aVF, aVR and 6 unipolar chest leads V1-V6

All together they provide information on rate, rhythm, conduction and show what aspect of the heart may be effected.

3 lead is used to keep an eye on rhythm using the three bipolar leads I,II,III leads

Question 14

What is the difference between an electrode and a lead?

Answer 14

An electrode is a physical connection to the patient, there are 10 electrodes for 12 leads.

A Lead is a graphical representation of electrical activity in that particular area.

Question 15

What is the difference between an bipolar and unipolar lead?

Answer 15

Bipolar:
Measure the voltage between two electrodes. One positive and the other negative.

Unipolar:
Measure the voltage between an electrode and a combined reference electrode which is negative.

Question 16

What is the job of the right leg electrode?

Answer 16

It is to reduce artefact and is not actually directly involved in an ECG.

Question 17

What are the colours of the Right arm, left arm, right leg and left leg electrodes?

Answer 17

RA = red
RL= black
LA= yellow
LL= green

Question 18

What does the I lead measure?

II?

III?

Answer 18

I = from the right arm to the left arm. Right arm is negative and left arm is positive.
This looks at a lateral aspect of the heart at 0 degrees to the left.

II= from the right arm to the left leg. Right arm again negative, left leg is positive.
60 degrees to the 0 degrees.

III = Left arm to the left leg, here the left arm is negative and the left leg is positive.
120 degrees to the 0 degrees lead

The three bipolar leads form a triangle between the three limbs and form a equilateral triangle

Question 19

What effects the size of the deflection of the bipolar leads?

Answer 19

The direction of the impulse the normal axis is between -30 and 90 degrees to the 0 degrees lead/ lead 1.

The closer the angle of impulse is to the lead the larger the deflection, likewise the more opposite the angle is the more negative the deflection.

Question 20

What plane does the aVR, aVF and aVL leads look at?

Chest leads?

Answer 20

Coronal plane

Axial plane

Question 21

Where are the aVF, aVR and aVL leads?

Answer 21

augmented Vector Foot - left leg

augmented Vector Left -left arm

augmented Vector Right- right arm

Question 22

How do the aVR, aVF and aVL leads differ to that of I, II, III?

Answer 22

As they are unipolar they measure the impulse towards the lead, where as the bipolar leads measure the activity in the direction of one lead to another.

Hence the aVR is always negative as the normal axis is away from this lead.

Question 23

How are the chest leads placed?

Answer 23

There are 6 leads V1-6 and they are placed from the right side of the septum to the most lateral aspect of the heart.

Question 24

What leads cover the lateral aspect of the heart?

Answer 24

Lead 1, aVL, V5, V6

Question 25

What leads cover the inferior aspect of the heart?

Answer 25

Lead II, III and aVF

Question 26

What leads cover the anterior aspect of the heart?

Answer 26

V2, V3, V4

Question 27

What leads cover the septal aspect of the heart?

Answer 27

V1 and possibly V2

Question 28

See lecture for how the leads are arranged on the graph:

Answer 28

3 lines

I,AVR,V1,V4

II,aVL,V2,V5

III,aVF,V3,V6

See pattern

At the bottom there is a rhythm strip