ECG

Question 1

What is an ECG detecting

Answer 1

Changes in potential arising from currents flowing through myocardial tissue
Can be recorded by electrodes on the body surface
Produces a picture of the electrical activity of the heart

Question 2

Which parts of the heart does the ECG record the electrical activity of

Answer 2

It only shows up the electrical activity of large tissue masses like the atria & ventricles
Only these generate sufficient current to be detected at the body surface
Nodes are too small

Question 3

What information does the ECG provide

Answer 3

Information about cardiac rate and rhythm, chamber size, the electrical axis of the heart and is a main test to assess for myocardial ischaemia and infarction

Question 4

what is the electrical vector

Answer 4

A separation of positive and negative charge (dipole) that that has a particular direction and magnitude

Question 5

What is the clinical importance of the electrical vector

Answer 5

allows the electrical axis of the heart to be estimated

Question 6

What determines the direction and magnitude of the electrical vector

Answer 6

Magnitude - determined by the mass of cardiac muscle that is involved in the generation of the signal
Direction is determined by the overall activity of the heart (varies throughout cardiac cycle)

Question 7

What causes an upward deflection on the ECG

Answer 7

Depolarization moving towards the recording electrode

Question 8

What causes an downward deflection on the ECG

Answer 8

Depolarization moves away from the recording electrode

Question 9

What would cause a flat line on a ECG

Answer 9

there is no movement towards or away from the recording electrode
It is known as isopotential

Question 10

What are the 12 leads on a 12 lead ECG

Answer 10

3 standard limb leads
Three augmented voltage (aV) leads
Six chest leads

Question 11

Where are the electrodes for the 3 standard limb leads

Answer 11

Electrodes on the right and left arms and one on the left leg

Question 12

Where are the 3 standard limb leads

Answer 12

I - right arm to left arm (LA recording)
II - right arm to left leg (LL recording)
III - left arm to left leg (LL recording)

Question 13

What causes the P wave

Answer 13

Atrial depolarization spreading from the SA node inferiorly and to the left
towards recording so upward deflection
Reflects time for depolarization of atrial muscle to be complete

Question 14

What causes the Q wave

Answer 14

left to right depolarization of the interventricular septum

Moving slightly away from the recording electrode so downward deflection

Question 15

What causes the R wave

Answer 15

Depolarization of the main ventricular mass

Moves towards the recording electrode so upward deflection

Question 16

What causes the S wave

Answer 16

Depolarization of ventricles at the base of the heart

Moving away from the recording electrode so downward deflection

Question 17

What causes the T wave

Answer 17

Ventricular repolarization

Moving in a direction opposite to that of depolarization accounts for the usually observed upward deflection

Question 18

What is the PR interval

Answer 18

Start of the P wave to the start of the QRS complex
Reflects the time for the SA node impulse to reach the ventricles
normally 0.12 – 0.2 s (120 - 200 ms)
Influenced by delay through AV node

Question 19

What is the ST segment

Answer 19

End of the QRS complex to the start of the T wave.
Ventricles are contracting - systole
Normally isoelectric so changes are diagnostically important

Question 20

What is the QT interval

Answer 20

Start of the QRS complex to the end of the T wave
Reflects the time for ventricular depolarization and repolarization
Normally 0.44 s (440 ms) in males, 0.46 s (460 ms) in females.
Prolongation predisposes to disturbances of cardiac rhythm

Question 21

What are the augmented limb leads

Answer 21

The recording electrode ‘looks’ between the two other limb leads

Question 22

What is different about the aVR reading

Answer 22

The waves are negative so the ECG reading is ‘upside-down’ compared to the others
The depolarisation moves away from the recording electrode as its on the upper right

Question 23

Where are the 6 chest electrodes placed

Answer 23

V1 = 4th intercostal space immediately right of sternum 
V2 = 4th intercostal space immediately left of sternum 
V3 = midway between V2 & V4
V4 = 5th intercostal space, mid clavicular line 
V5 = same horizontal level as V4, anterior axillary line 
V6 = same horizontal level as V4, mid-axillary line

Question 24

How do you calculate heart rate from an ECG

Answer 24

300/ number of large squares between beats

or between R-R interval

Question 25

What is the rhythm strip of an ECG

Answer 25

Prolonged recording of one lead (usually lead 2) | allows detection of rhythm disturbance

Question 26

Why do you need 12 leads if one can show rhythm

Answer 26

the 12 creates a complete picture: You can determine the axis of the heart, look for ST segment or T wave changes in specific regions and look for voltage criteria changes

Question 27

What are the 6 steps to analysing an ECG

Answer 27

``` 1= Verify patient details: name and date of birth 2= Check date and time ECG was taken 3= Check the calibration of the ECG paper 4= Determine the axis, if possible 5= Workout the rhythm 6= Look at individual leads for voltage criteria changes OR any ST or T-wave changes ```

Question 28

What is the normal calibration of the ECG paper

Answer 28

25mm per second and correct voltage (10mm/mV)

Question 29

How do you work out the rhythm from an ECG (7 Q's)

Answer 29

Using rhythm strip: 1. Is electrical activity present? 2. Is the rhythm regular or irregular? 3. What is the heart rate? 4. P-waves present? 5. What is the PR interval? 6. Is each P-Wave followed by a QRS complex? 7. Is the QRS duration normal?

Question 30

What conditions may give an normal resting ECG

Answer 30

Myocardial Infarction - may or may not cause characteristic ECG changes Intermittent Rhythm Disturbance - monitor for 24hrs to properly check Stable Angina - ST changes usually absent at rest so instead do ECG during/ after exercise