ECG

Question 1

What is an ECG

Answer 1

Graph of voltage against time

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

Question 2

What occurs for a myocyte (cardiac cell) to contract

Answer 2

Potential difference across the cell membrane must change from negative to positive compared to a cell

Changes occur through flow of ions

Question 3

V1

Answer 3

4th intercostal space. Right of sternum

Question 4

V2

Answer 4

4th intercostal space. Left of sternum

Question 5

V3

Answer 5

Equidistant between V2-V4

Question 6

V4

Answer 6

5th intercostal space. Midclavicular line

Question 7

V5

Answer 7

Left anterior axillary line in horizontal line with V4

Question 8

V6

Answer 8

Mid axillary line, horizontal with V4 and V5

Question 9

P wave/PR interval

Answer 9

1) Hump before QRS
2) Depolarisation of both atria
3) Time taken from SA node to ventricle
4) Measurement of AV node conduction time
5) Typically 120-200ms

Question 10

QRS complex / interval

Answer 10

1) Depolarisation of ventricles
2) Following traversing AV node/His/Purkinje system
3) Specialised conduction cells = fast conduction
4) Beginning of QRS to its end
5) Typical range: 80-110ms

Question 11

Nomenclature of QRS

Answer 11

First negative deflection - Q
First positive deflection - R
The 2nd negative deflection or if a negative follows a positive - S

Question 12

QT interval

Answer 12

1) Onset of QRS to end of T wave
2) Ventricular depolarisation and repolarisation
3) Long QT syndrome = sudden death
4) Typically 350 - 420 ms

Question 13

ST segment / T wave

Answer 13

1) Isoelectric (flat) segment between end of QRS and start of T wave
2) Ischaemia manifests here

Depression = ischaemia
Elevation = myocardial infarct

3) Some elevation normal
4) Repolarisation of ventricles

Question 14

U wave

Answer 14

1) A broad deflection of low amplitude appear after T wave

2) Most easily visible in V3 or V4

Question 15

Tachycardia

Answer 15

fast heart rate

sinus tachycardia > 100bpm

Question 16

Bradycardia

Answer 16

slow heart rate

sinus bradycardia <50bpm

Question 17

Normal sinus rhythm

Answer 17

Rate = 50 - 100bpm
R-R interval - regular
PR interval - normal and constant

Question 18

The 2 categories of arrythmias

Answer 18

1) Conduction abnormalities e.g. blocks

2) Abnormal impulse initiation e.g. ectopic, VT

Question 19

Describe conduction abnormalities

Answer 19

1) SA node fails to initiate impulse - results in no P wave/ QRS
2) R-R irregular
3) PR normal
4) Characterised by a delay or interruption in conduction, can be ischaemic heart disease or valve fibrosis

Question 20

Describe abnormal impulse initiation

Answer 20

Every other beat is ventricular ectopic, alternating with an intrinsic beat

Question 21

List the mechanisms of tachycardia

Answer 21

1) Altered automatically; (either normal or abnormal)
2) Triggered activity where normal action potential suddenly becomes positive again allowing another depolarisation to occur abnormally
3) Re-entry

Question 22

Describe the changes in ECG during myocardial ischaemia

Answer 22

Ischaemic myocytes have reduced membrane potentials compared with healthy myocytes
Difference in potential between the ischaemic region and healthy region displaced the ST segment - INJURY CURRENT EFFECT