Interpreting ECGS: Lecture 1

Question 1

Define depolarisation

Answer 1

Change within cell of electric charge distribution leading to a less negative charge inside the cell. This happens through movement of ions eg Na+ , K+ , Ca2+

Question 2

Define repolarisation

Answer 2

Change within cell of electric charge distribution leading to more negative charge inside cells.

This happens through movement of ions - Na+ , K+ , Ca2+

Question 3

What does the resting membrane potential measure ?

Answer 3

How much more negative the inside of the cell cell compared with the outside.

Question 4

How does propagation of cell depolarisation spreads from cell to cell ?

Answer 4

X

Question 5

How does the electrical activity spread throughout the heart at tissue level ?

Answer 5

1. Pacemaker cells located within the SA node depolarise , generating an electrical impulse which spreads across the atria and causes the atria to contract.
2. There is a fibrous band of tissue ( Annulus fibrosis) that separates the atria from the ventricles. This prevents the impulse from spreading directly to the ventricles.

3, instead the impulse reaches the AV node which is located between the atria ( INTER- ATRIAL SEPTUM)

4, the Av node delays the pulse for a short period of time before transmitting the impulse down the Bundle of His.

5. The bundle of his divides into the right bundle branch which travels along the right side of the inter ventricular septum. And the left bundle branch which travels along side the left side of the inter ventricular septum.
6. The right and left bundle branch terminate into the extensive network of Purkyne fibres which would depolarise the ventricles.

Question 6

Which has a faster rate of depolarisation- the SAN or the AVN ?

Answer 6

The SAN has the fastest rate of depolarisation in the heart.

Question 7

What is the intrinsic firing rate of the SAN ?

Answer 7

60-100 times / minute

Question 8

What is the intrinsic firing rate of the AVN ?

Answer 8

40-60 times / minute

Question 9

What is the intrinsic firing rate of the left bundle and right bundle branch ?

Answer 9

20-40 times / minute

Question 10

What does an ECG measure ?

Answer 10

Changes in electrical potential produced in successive areas of myocardium during the cardiac cycle - measures and records these changes over time.

Question 11

What do we use to measure an ECG ?

Answer 11

1. An electrode which is a conductive pad that attaches to the skin and enables recording of electrical currents.
2. Leads : which is a cable used to connect the electrode to the ECG recorder. BUT a lead ALSO means the electrical view of the heart obtained from any one combination of electrodes.

Question 12

When recording an ECG , how many electrodes and cables do we use ? And how many views does this produce ?

Answer 12

10 electrodes ( 6 on the chest and 4 on the limbs )

Connected by 10 cables to ECG machine

This gives us 12 VIEWS / LEADS

Question 13

What does limb lead 1 measure the voltage difference between ?

Answer 13

Voltage difference between the electrode on the right arm and left arm. The positive electrode is on the left arm.

Question 14

What dies limb lead 11 measure the voltage difference between ?

Answer 14

Voltage difference between right arm and left leg. The positive electrode is on the left leg.

Question 15

What does limb lead 111 measure the voltage difference between ?

Answer 15

The voltage difference between electrode on the left arm and left leg. The positive electrode is placed on the left leg.

Question 16

What is important about the right leg electrode ?

Answer 16

It is a grounding electrode - it is it actually used for any views/leads.

Question 17

Are limb leads 1,11,111 unipolar or bipolar ?

Answer 17

They are bipolar because they have both positive and negative electrodes.

Question 18

What is different between the augmented limb leads and limb leads 1,11,111 ?

Answer 18

Augmented limb leads are unipolar - they only have a positive electrode

Question 19

What are augumented limb leads ?

Answer 19

They are termed unipolar leads because they have a single positive electrode that is. Referenced against combination of the other limb electrodes.

The positive electrodes for these augmented leads are located on the : aVl ( Left arm ) , aVR ( right arm ) and the aVf ( left leg / foot)

Question 20

Depolarisation spreading towards a positive electrode would yield what type of deflection ?

Answer 20

Upward deflection from the isoelectric point.

Question 21

Depolarisation spreading away from the positive electrode would yield what type of deflection ?

Answer 21

Downwards deflection

Question 22

Repolarisation spreading towards a positive electrode would yield what type of deflection ?

Answer 22

Downwards deflectiom

Question 23

Repolarisation spreading away from a positive positive electrode would yield what type of deflection?

Answer 23

Upwards

Question 24

Using a view from ECG lead 11 , what does the P wave represent ? And outline why there is an upwards deflection ?

Answer 24

P wave represents depolarisation of the atria.

• this is where the electrical impulses spread along the atrial muscle fibres and intermodal ( between SA -AV nodes) pathways.
• the direction of impulses is downwards and to the left - towards the AV node.
• this produces a small upwards deflection because the electrical impulses are travelling towards the positive electrode ( Left Leg).
• this lasts 80-100 ms.

Question 25

Using a view from ECG lead 11 , what does the isoelectric segment after the P wave indicate ?

Answer 25

The delay at the AV node. This allows time for atrial contraction to fill the ventricles.

Question 26

What produces the downwards deflection ( Q wave) using an ECG view of lead 11 ?

Answer 26

Depolarisation of the interventricular septum There is a downwards deflection because electrical impulses are not moving straight towards the positive electrode - some depolarisation spreads from the left to the right septum.

Question 27

Using a view from ECG lead 11, what produces the large upwards deflection - termed the R wave ?

Answer 27

Upwards because depolarisation moving Directly towards the positive electrode ( LL ) as the apex and free ventricular walls become depolarised. R wave larger than other waves because there is large muscle mass therefore more electrical activity.

Question 28

Using a view from ECG lead 11, why does depolarisation of the ventricles produce a small downwards defection - known as the S wave

Answer 28

Downwards deflection because the electrical impulses are moving away from the positive electrode as they move upwards to the top of the ventricles.

Question 29

Using a view from ECG lead 11 , why is. There an upwards deflection for the repolarisation of the ventricles ?

Answer 29

Because it is a wave of repolarisation moving away from the positive electrode. When repolarisation moves away from lead produces upwards deflection. This is termed the T wave

Question 30

What does the T wave indicate ?

Answer 30

Ventricular repolarisation But relaxation of the ventricles immediately follows the T wave.

Question 31

Leads 11, 111 and AVF are best at looking at what view of the heart ?

Answer 31

INFERIOR surface of the heart AND Muscle necrosis due to occlusion of the right coronary artery.

Question 32

Leads 1 ,aVL , V5 and V6 are best at looking at what view of the heart ?

Answer 32

Lateral wall of the left ventricle.. Muscle necrosis due to occlusion branch left coronary artery - lateral wall MI.

Question 33

Where do we place the chest electrodes ?

Answer 33

V1 = fourth intercostal space ( right) V2 = 4th intercostal space ( left ) V3 = in between the V2 and V4 V4 = 5th Left intercostal space ( mid clavicular ) V5 = between V4 and V6 V6= 5th intercostal left space : mid axillary line.

Question 34

What is the mnemonic to remember the colour of the limb leads ?

Answer 34

RIDE ( red limb lead for right upper limb ) YOUR ( yellow limb lead for left upper limb ) GREEN ( green limb lead for lower left limb) BIKE ( black limb lead for right lower limb)

Question 35

Chest leads V5 and V6 are good at giving us what view of the heart ?

Answer 35

Lateral left view of the heart

Question 36

Which ECG leads give the view of the apex and anterior surface of the ventricles ?

Answer 36

V3 and V4

Question 37

Which ECG leads give a view of the septum and right ventricle ?

Answer 37

V1 , V 2 and AVR

Question 38

How many large boxes represent 60s?

Answer 38

300 large boxes = 60s

Question 39

How to calculate heart rate if the rhythm is regular ? ( ie. the R-R intervals are consistently the same )

Answer 39

If 300 boxes = 60s You count the number of boxes between R-R peaks. You then do 300 divided by the number of boxes between R-R. This should give you heart rate. If there was 1 box and 4 mini boxes - you would do 300/1.8. This is because If 1 Box = 5 mini boxes then 4 mini boxes would mean 0.8 Boxes. So 1.8.

Question 40

How to calculate the heart rate is the rhythm is irregular ?

Answer 40

REMEMBER: 39 large boxes in 6s. You have to count the number of QRS complexes in 6s. You then multiply the number of QRS complexes by 10. This will give you the total heart beats in 60s.

Question 41

What is the P-R interval ?

Answer 41

Start of the P Wave to the start of the Q wave. It is usually 3-5 small boxes. It is prolonged if it is greater than one large box.

Question 42

What is the QRS. Interval ?

Answer 42

Start of the Q wave to the end of the S wave. Wider QRS complexes are assosiated with ventricular depolarisations that are not initiated by the normal conductance mechanisms (His-purkyne system) Normal = less than 3 small boxes.

Question 43

What is the QT interval ?

Answer 43

Start of the Q wave to the end of the T wave. Prolonged QT interval indicates prolonged ventricular repolarisation Longer than 11 small boxes