CVS S7 - The ECG

Question 1

Describe the spread of excitation through the heart

You should be able to draw a diagram of this

Answer 1

Activity starts at the SAN and depolarisation spreads over the walls of the atria to the AVN

At the AVN there is a 120ms delay

Excitation then spreads down the septum through the bundle of His and the left and right bundle branches then out over the ventricular myocardium

Excitation through the myocardium progresses from the endocardial to epicardial surface, at this point, the entire ventricle is depolarised

There is a delay before ventricular repolarisation of 280ms

Repolarisation spreads in the opposite direction to depolarisation, from epicardial to endocardial surface

Question 2

What would an extracellular electrode placed near a myocardial cell see during each contraction?

Why does this differ from what an intracellular electrode sees?

Answer 2

Sees only two signals for each action potential/systole:

• One for depolarisation
• One for repolarisation

Electrodes outside the cell only record CHANGES in membrane potential

Question 3

How does what an electrode see depend on the direction of depolarisation and repolarisation?

Answer 3

If depolarisation is spreading:

• Toward electrode then upward signal
• Away from electrode then downward signal

If repolarisation is spreading:

• Toward electrode then downward signal
• Away from electrode then upward signal

Question 4

What determines amplitude of signal seen by an extracellular electrode?

Answer 4

Amplitude is larger when:

• There is more muscle depolarisaing/repolarising
• The spread is at a smaller angle to the electrode view (largest when directly towards/away)

Question 5

What are the different waves found on an ECG and what electrical events do they correspond to?

Answer 5

P wave - Atrial depolarisation

Q wave - Septal depolarisation spreading to ventricles

R wave - Main ventricular depolarisation

S wave - End ventricular depolarisation

T wave - Ventricular repolarisation

Question 6

Describe how the P wave’s form is produced

What happens following the P wave?

Answer 6

Atrial depolarisation produces a small upward deflection

Small and upward because it’s moving toward the electrode, but not directly

Following the P wave:

• 120ms delay before QRS complex

Question 7

Describe how the Q wave’s form is produced

Answer 7

Depolarisation spreads down the septum then out across the axis of the heart, producing a small downward deflection

Small and downward because it’s away from the electrode, but not directly

Question 8

Describe how the R wave’s form is produced

Answer 8

Depolarisation spreads thorough the ventricular muscle along an axis slightly left of the septum producing a large upward deflection

Large and upward as there’s lots of muscle and depolarisation is moving directly toward the electrode

Question 9

Describe how the S wave’s form is produced

What happens directly after the S wave?

Answer 9

Depolarisation spreads up the ventricles towards the atria producing a small downward deflection

Small and downwards as it’s moving away from the electrode, but not directly

Afterwards, there is a delay of 280ms before the T wave

Question 10

Describe how the T wave’s form is produced

Answer 10

Repolarisation spreads from epicardial to endocardial surfaces producing a medium upward deflection

Upward because repolarisation is moving away

Medium sized because repolarisation timing in different cells is dispersed

Question 11

Label each of the black boxes

Answer 11

From left to right

P wave

Q wave

R wave

S wave

T wave

At the top:

QRS complex

Question 12

Where is atrial repolarisation on an ECG trace?

Answer 12

It can’t be seen as it gets lost in the QRS complex

Question 13

Describe how the R wave would change if an electrode is moved around the heart

Answer 13

Viewing directly in line with the cardiac axis results in a large upward deflection

Viewed at a 90 degree angle to the cardiac axis there is no signal

Viewed at a 180 degree angle sees a large downward deflection

Varies predictably between these points

Question 14

How many electrode comprise are involved in a normal ECG reading?

How many are recording?

Where are these electrodes? (In the broadest of terms)

Answer 14

10 electrodes in a 12 lead ECG

9 recording, right lower limb is neutral

4 Limbs

6 Chest

Question 15

Where are each of the limb electrodes placed?

Answer 15

Red - Right upper limb

Yellow - Left upper limb

Green - Left lower limb

Blue - Right lower limb

Question 16

What are all the limb leads called?

State the equivalent single electrode views of all the leads

Answer 16

All veiwing the heart in the coronal plane, all angles given relative to Lead I, which views the heart directly from the left

Lead I - 0 degrees

Lead II at +60 degrees

Lead III at +120 degrees

aVL at -30 degrees

aVR at -150 degrees

aVF at +90 degrees

Question 17

What are the limits of the normal cardiac axis?

Answer 17

Between aVL and aVF ( -30 to +90 degrees)

Question 18

Describe the placement of each of the chest electrodes

Answer 18

V1 - Right 4th intercostal space next to the sternum

V2 - Left 4th intercostal space next to the sternum

V3 - Directly between V2 and V4

V4 - 5th intercostal space along the left midclavicular line

V5 - Level with V4 at the left anterior axillary line

V6 Level with V5 along the left midaxillary line (directly under the midpoint of the armpit)

Question 19

What leads correspond the the chest electrodes?

What views do they provide?

Answer 19

Leads correspond to each electrode (V1, 2, 3, 4, 5, 6)

They provide horizontal views of the heart

Question 20

Describe how leads are amplified

Answer 20

One electrode is used as positive

One is negative (can be directly opposing positive or offset)

Negative signal is inverted and the signals are combined

The signal is then amplified further

If the negative as opposing positive, we get an amplified view directly from the positive (E.g. Lead I)

If the negative was offset we get and amplified view from between the positive and equivalent positive of the negative (directly opposite the negative) (E.g. Lead II and III)

Question 21

Label each of the black boxes with the corresponding limb lead, include degrees offset from Lead I

Answer 21

Starting from the lead directly to the left and going clockwise

Lead I at 0 degrees

Lead II at +60 degrees

aVF at +90 degrees

Lead III at +120 degrees

aVR at -150 degrees

aVL at -30 degrees

Question 22

How do you calculate a regular heart rate from an ECG?

Where do you read this rate from?

Answer 22

All ECG machines run at a standard rate of 300 small squares per minute

To find the rate, divide 300 by the number of squares in the R -R interval.

The rate is commonly read from the ‘rhthym strip’ but can be read from any lead

Question 23

How would you calculate an irregular rhythym from an ECG?

Answer 23

A larger interval is used than a single R- R interval

For example, multiply the number of beats in 10 seconds by 6

Question 24

What are ventricular ectopic beats and how do they present in an ECG?

Answer 24

Ventricular cells that gain pacemaker capability can cause ventricular contraction before underlying rhythym depolarises the ventricles, the ectopic beat

The resultant ECG often appears wider and taller than the underlying rhthym

May occur every other beat, every third beat, every fourth beat etc. or in couplets or triplets.

Question 25

How does atrial fibrillation appear on an ECG?

Answer 25

P waves replaces by fibrillatory waves

Variable ventricular rate

Question 26

What is ventricular fibrillation?

How does ventricular fibrillation appear on an ECG?

Answer 26

Uncoordinated contraction of the ventricular myocardium

Rapid irregular waves of varying amplitude

Question 27

Describe frist degree heart block

Answer 27

P-R interval is legthened from its normal 120 - 200ms

There is a conduction delay through the AVN but all electrical signals reach the ventricles

Question 28

Describe type 1 second degree heart block

Answer 28

The P-R interval is erratic

P-R interval follows a pattern of elongating until a QRS complex is dropped then resetting to normal and progressively elongating again

Some but not all atrial beats reach the ventricles

Question 29

Describe type 2 second degree heart block

Answer 29

Electricla conduction sometimes fails to pass through the AVN or bundle of His

P-R interval is constant but not all atrial beats get through to the ventricles

Question 30

Describe third degree heart block

Answer 30

No association between the atrial pace and the ventricular pace

Normal atrial excitation doesn’t reach the ventricles which are then controlled by ectopic pacemaker cells resulting in two separate rhythms

Question 31

Describe how a bundle branch block might change the shape of an ECG trace

Answer 31

Damage to the conduction pathways alters the route of spread of depolarisation

Lengthens and changes the shape of the QRS complex

Many variations depending on location

Question 32

What is the electrical axis of the heart?

Answer 32

Relates to the main spread of depolarisation through the wall of the ventricle (R wave)

Combination of depolarising right and left ventricles generates a single vector normally pointing slightly to the left (as there is more depolarising muscle in the left ventricle)

Question 33

What will cause changes in the electrical axis of the heart and why?

Answer 33

Will change in response to changes in the relative amount of depolarising muscle in each ventricle

Increase in muscle in the left of the heart will create a left axis shift

Increase in muscle in the right of the heart will create a right axis shift

Question 34

How is electrical axis of the heart determined?

Describe both normal and abnormal axis determination

Answer 34

Look at the limb leads

Normal axis = positive Lead I and III deflection

Left axis deviation = Positive Lead I deflection, negative lead III deflection

Right axis deflection = Negative lead I deflection, positive lead III deflection

Question 35

How does damage or stress to the myocardium alter electrical conductivity?

Answer in broad terms

Answer 35

Stressed, dying or dead myocardium generates current flows during systole

These produce extra signals in the S-T segment

Question 36

What is the effect on the heart of temporary ischaemia?

Answer 36

Reduced blood flow leads to angina and ST depression in most cases

Question 37

Explain the most common changes in the ECG response during a myocardial infarction?

Answer 37

Dying tissue generates injury currents often leading to ST elevation

Also:

Pathological Q waves (Wider than 1 small square and more than 2mm deep) present in full thickness MI and remain after resolution

Inverted T waves

Question 38

How might you determine the location of a myocardial infrction from an ECG?

Answer 38

The view with the most prominent abnormality helps you identify:

• Which part of the heart is affected and how badly
• Which coronary artery is blocked
• Whether the whole thickness of the ventricular wall is affected