ECG

Question 1

What does an ECG plot?

Answer 1

Voltage against time

Question 2

What does an ECG show?

Answer 2

A visual representation of how the electrical potentials of heart muscle cells vary from moment to moment as the signals that stimulate and coordinate contraction of the muscle travel through the heart.

Question 3

What must the potential difference be in order for cardiac myocytes to contact?

Answer 3

For the myocyte (cardiac cell) to contract, the potential difference across the cellular (semi-permeable) membrane and must change from negative to positive in relation to the inside of the cell.

Question 4

How are changes in potential difference created?

Answer 4

Changes in potential difference occur through the flow of ions through specialised ion channels in the cellular membrane and also relatively freely through gap junctions.

Question 5

What does a potential difference between one part of the cell and another (a dipole (lead)) allow?

Answer 5

• that allows the electrical activity of the heart to be ‘viewed’ from a defined position or angle.
• The electric dipole (lead), as shown, consists of two equal and opposite charges,
• +q and –q, separated by a distance d.
• The dipole is a vector
• It has direction as well as magnitude (arrow).

Question 6

What do the six frontal leads/ limb leads consist of?

What are they designed to do?

Answer 6

Three bipolar leads

The bipolar leads are so designated because each records the difference in electrical potential between two limbs.

Question 7

Where can these three bipolar leads be placed?

Answer 7

• L – left wrist
• R – right wrist
• F – left ankle
• N – Typically right ankle

Question 8

How is a 12 leads ECG different?

Answer 8

Leads are placed on arms, legs and across the chest

Question 9

In a six lead ECG where are the leads placed?

Answer 9

• V1 – 4th IC, right sternal edge
• V2 – 4th IC, left sternal edge
• V3 – equidistant V2-V4
• V4 – 5th IC, midclavicular line
• V5 – left anterior axillary line In horizontal line with V4,
• V6 – mid axillary line, horizontal with V4 and V5,

Question 10

Why do we prefer to use a 12 lead ECG?

Answer 10

We measure as 12 lead so we can get a whole view round the heart. Each part of the ECG looks at a particular part of the heart.
2,3 and AVF look at the inferior wall

Question 11

Describe the electrical conduction of the heart

Answer 11

In the heart you have the SA node, AV node, internodal pathways and backmans bundle (takes the wave of depolarisation from the right side to the left side.

The AV node allows the ventricles to fill (up to 200ms).

The his bundle transmits the depolarisation to the apex to facilitate contraction.

Question 12

What does the P wave represent (hump before QRS complex)?

Answer 12

Atrial depolarisation (of both atria)

Question 13

What does the PR segment represent on the ECG?

Answer 13

AV node delay

Question 14

How long is the PR interval?

Answer 14

Typically between 120-200ms

Question 15

What does the QRS complex wave represent?

Answer 15

Depolarisation of ventricles

Question 16

When does the QRS complex occur?

Answer 16

Following transversing AV node/ His/ Purkinje system

Question 17

How long is the QRS complex?

Answer 17

Typically 80-110ms

Question 18

In the QRS complex, what is the negative deflection and what is the positive deflection?

Answer 18

Q - 1st negative deflection
R - 1st positive deflection
S - 2nd negative deflection

Question 19

What does the QT represent?

Answer 19

Ventricular depolarisation and repolarisation

Question 20

How long is the QT interval?

Answer 20

Typically 350-420ms

Question 21

What does an elevation and depression of the ST segment indicate?

Answer 21

Depression = ischaemia

Elevation = myocardial infarct

Question 22

What does ST-segment represent

Answer 22

Repolarisation of the ventricles

Question 23

What is a U wave?

What part of the ECG is the U wave visible?

Answer 23

A broad deflection of low amplitude appearing after the T wave

V3 or V4

Question 24

What is tachycardia?

Answer 24

Fast heart rate

Question 25

What is sinus tachycardia?

Answer 25

> 100bpm

Question 26

What is bradycardia?

Answer 26

Slow heart rate

Question 27

What is Sinus bradycardia?

Answer 27

<50bpm

Question 28

What is an arrhythmia?

Answer 28

An arrhythmia is an abnormal cardiac rhythm

Question 29

What are the two types of arrhythmias?

Answer 29

• Conduction abnormalities, e.g. blocks

* Abnormal impulse initiation e.g. ectopics, VT

Question 30

What happens in terms of the ECG if the SA fails to initiate impulse?

Answer 30

Results in no P wave/ QRS complex

Question 31

What can ischaemic heart disease or valve fibrosis cause on an ECG?

Answer 31

3rd degree AV Block / complete AV block

Characterised by a delay or interruption in conduction

Question 32

Where does ventricular tachycardia originate from?

Answer 32

The ventricles

Question 33

What can VT cause?

Answer 33

Ventricular tachycardia may impair cardiac output, consequently hypotension, collapse, and acute cardiac failure. This is due to extreme heart rates and lack of coordinated atrial contraction

Question 34

What are the three basic Arrhythmiagenic mechanisms responsible for initiating tachyarrhythmia?

Answer 34

Altered automaticity; (either normal or abnormal)

Triggered activity’ where normal action potential suddenly swings positive again allowing another depolarisation to occur abnormally. due to – early after depolarisation or Delayed after depolarisations e.g. ectopics

Re-entry

Question 35

what is SVT?

Answer 35

Supraventricular tachycardia

Question 36

Which pateints is SVT common in?

Answer 36

May occur in young and healthy patients as well as those suffering chronic heart disease.

Question 37

Describe AVNRT?

Answer 37

AVNRT is typically paroxysmal and may occur spontaneously or upon provocation i.e. exertion, caffeine, alcohol, beta-agonists i.e. (salbutamol) or amphetamines.

Question 38

Describe the ECG changes in angina and infarcts
Myocardial ischaemia (inadequate O2 supply)

Answer 38

ST depression caused by injury potential difference

Ischaemic myocytes have reduced membrane potentials compared with healthy myocytes.

The difference in potential between the ischaemic region and healthy region displaces the ST segment.

This is called the ‘injury current’ effect.

Question 39

Describe the changes shown in an ECG when during an MI, Ischemia and injury

Answer 39

Myocardial infarction –MI. (sudden chest pain due to acute arterial obstruction)
A. Ischaemia – manifests ECG with ST depression +/- T wave inversion; result from altered repolarisation. (1)
B. Injury – ST segment elevation +/- loss of R wave; ‘current of injury’ (1)
C. Infarction – deep Q waves (>0.06 sec / > 25% R wave) (1) resulting from absence of depolarisation current from dead tissue (transmural). Manifests receding currents from opposite side of heart as an ‘electrical window’ is formed. (1)
D. Surface ECG on the opposite wall can show reciprocal voltage changes to the infarcted site. (1)