ECG 1: The Normal Electrocardiogram

Question 1

What is an electrocardiogram?

Answer 1

A recording from the body surface of the electrical activity spreading through the heart BEFORE each heartbeat.
This is a non-invasive way of indirectly visualing the hearts electrical activity.

VOLTAGE/TIME recording

Question 2

How does an ECG work?

Answer 2

Recorded by electrodes at different SKIN sites on the body. Via these skin sites they are able to measure the potential difference between body sites. It measure the electric field as it changes through the cardiac cycle

Question 3

How can the electrical activity be recorded from a site distant from the heart?

Answer 3

Because body tissues acts as conductors

Question 4

Draw and label a generic ECG description “sinus rhythm”

Answer 4

…

Question 5

Three main deflections

Answer 5

P: atrial depolarization
QRS: Ventricular depolarisation
T: Ventricular repolarization

Question 6

P wave

Answer 6

Atrial Depolarisation

• relatively small mass therefore small height deflection
• quite a slow event, width reflects time taken

Question 7

PR segment

Answer 7

Atria have depolarised- isoelectric

• Refects time taken for wave to pass through AV node, AV bundle, BB’s
• As their depolarisation is of such a small mass it’s not seen on the body surface

Question 8

QRS complex

Answer 8

Ventricular Depolarisation

• greater magnitude then P wave (greater mass)
• Relatively shorter then P wave, due to rapide spread via purkinje fibres

**ALSO atrial repolarisation (not visible/lost)

Question 9

PR interval

Answer 9

Reflects total time for a wave to pass from atria to ventricles

Question 10

ST segnment

Answer 10

Isoelectric (@baseline)

• all depolarised so no moving wavefront
• plateau of ventricular AP

Question 11

T wave

Answer 11

Asynchronous ventricular repolarisation

• slower than depolarisation
• Very complicated event

Question 12

U wave

Answer 12

Uncertain origin

Question 13

QT interval

Answer 13

Reflection of ventricular AP duration

Question 14

Normally cells are polarised (- inwards, + outwards), due to lots of Na+ trying to get in. When depolarised…

Answer 14

change to + inwards and - outwards

Question 15

Orientation, length and direction of a dipole

Answer 15

Orientation: orientation of dipole
Length: Magnitude or strength
+ dipole: towards + charge
- dipole: away from + charge

Question 16

A dipole is?

Answer 16

A pair of equal but opposite charges separated by a small distance

** this varies continuously in size and orientation throughout the cardiac cycle

Question 17

Q:
R:
S:

Answer 17

Q: ventricular septum depolarising
R: Vantricular apex depolarising
S: Ventricular Base depolarizing

Question 18

Ventricular Depolarization is from….

Ventricular repolarisation is from …..

Answer 18

Depol: Endocardium (inwards) to epicardium (outwards) (+ QRS)
repol: Epicardium (outwards) to endocardium (Inwards) (+ T wave)

This is due to AP of inside surfaces being much longer.

Question 19

Bipolar Leads

Answer 19

Potentials recorded between LA, RA and LL
Einthoven’s Triangle: 3 electrodes to 3 corners of a triangle, heart at the centre

Lead I= LA-RA
Lead II= LL-RA
Lead III = LL-LA

Einthoven’s Law: I + III = II

Question 20

Unipolar Leads

Answer 20

Potential of a SINGLE electrode relative to some constant reference. To use we need a ‘reference lead’.
THis is done by conneting all three limb electrodes together through resistors, resulting in a lead that’s always at a constant value, which we assume to be 0.

if dipole is facing exploring electrode, a positive deflection is recorded.

VR (right), VL (left), VF (foot)

but usually augmented limb leads are used instead

Question 21

Augmented unipolar limb leads.

Answer 21

aVR, aVL, aVF
eg; aVR= 1.5 * VR

Done because the real signal is usually too small

Question 22

Unipolar chest leads

Answer 22

Give us electrical activity in the horizontal plane, from front and side
V1-V6

Question 23

A 12-lead ECG is made up of

Answer 23

6 frontal/coronal plane leads: I, II, III, aVR, aVL, aVF

6 Horizontal plane leads: V1-V6

Question 24

We usually look at a rhythm strip from lead___?

Answer 24

II

Question 25

What types of features of an ECG do we look at?

Answer 25

Rate
Rhythm - sinus shythm is normal, originates at SA node
P waves - atrial depol
PR interval- slowed conduction between A and V
QRS
ST segment- ischaemia
T waves