ECG

Question 1

Describe normal conduction in the heart

Answer 1

• Start SA node
• Travel through both atria
• Pause at AV node about 120ms
• Bundle of His
• Left bundle (then ant and post fascicle) & right bundle
• Purkinje fibres

Question 2

What is meant by the cardiac axis?

What is the normal axis?

Answer 2

• General/overall direction of depolarisation through heart
• Normal axis is towards apex of heart

Question 3

What are the angles of the normal cardiac axis?

Answer 3

Between -30^o (aVL) and +90^o (aVF)

Question 4

Remind yourself of the different views of heart from limb leads- include the angle of each

Answer 4

Question 5

Which region of heart does each of the chest leads look at?

Answer 5

• V1 & V2= right ventricle
• V3 & V4= septum
• V5 & V6= left ventricle

Question 6

What does each of the following represent:

• P wave
• PR interval
• Q wave
• R wave
• S wave
• T wave

Answer 6

• P wave= atrial depolarisation
• PR interval= delay at AV node
• Q wave= depolarisation of septum (L to R)
• R wave= depolarisation of apex & free ventricular wall
• S wave= depolarisation spreads up to base of ventricles
• T wave= ventricular repolarisation (base to apex)

Question 7

State how long, in terms of time and squares, each of the following intervals should be:

• PR interval
• QRS interval
• QTc interval

Answer 7

• PR: 120-200ms (3-5 small squares)
• QRS: up to 120ms (up to 3 small squares)
• QT: 400-440ms- generally say (about 2 large squares)

Question 8

What is the QTc interval?

How do you calculate the QTc interval?

Answer 8

The corrected QT interval. The QT interval is inversely proportional to HR (e.g. slower HR, longer QT) hence you must adjust the QT interval for the heart rate.

Use Bazett’s formula as shown in image

Question 9

How long, in terms of time, is each of the rhythm strips in the 12 lead ECG?

Answer 9

Each rhythm strip is 10s

Question 10

Explain the shape of each of the QRS complexes in the chest leads (V1, V2, V3, V4, V5, V6)

Answer 10

Important points to note first:

• ​Septum depolarised first and septum depolarisation spreads left to right
• Then main mass of ventricle is depolarised. Since left ventricular wall has more muscle the left ventricle exerts more influence on ECG pattern than right ventricle (i.e. depolarisation more towards left)
• V1 & V2 look at right ventricle
• V3 & V4 look at septum
• V5 & V6 look at left ventricle

Shapes of QRS in Chest Leads

• Leads V1 & V2 (RV leads):
  
  • Deflection is firstly upwards as depolarisation of septum is L to R (upwards R wave)
  • Downward deflection as main muscle mass is depolarised (downwards S wave)
  • Depolarisation spreads from apex to bases- bases are situated more towards the right hence upwards deflection
• Leads V5 & V6 (LV leads):
  
  • First deflection is small and down because of depolarisation of septum L to R (downwards Q wave)
  • Large upward deflection as main muscle mass is depolarised and depolarisation is towards the lead (upwards R wave)
  • Downward deflection as depolarisation moves from apex to bases and bases are more to the right

IN SUMMARY:

• V1 & V2: will see R wave and S wave (mainly negative deflection)
• V3 & V4: transition leads
• V5 & V6: will see Q, R and S wave

Question 11

What do we mean we talk about a lead being + or -?

Answer 11

Comparing size of upward deflection of QRS to downward deflection of QRS to see which is bigger

Question 12

Which of the chest leads are negative and which are positive?

Answer 12

Remember we decide if negative or positive by comparing R and S wave. If R is larger than S= positive. If S is larger than R=negative

• Negative= V1 & V2
• Positive= V5 & V6

V3 & V4 are transition leads so will be - or +

Question 13

Which of the limb leads are negative and which are positive?

Answer 13

• Negative: aVR
• Positive: I, II, III, aVF, aVL

*****aVL varies but usually positive?

Question 14

T waves vary in different leads; discuss what leads you expect to see upright T waves in and what leads you expect to see downward T waves in

Answer 14

• Upright T waves: I, II, V2-V6
• Very commonly inverted: aVR
• Variable: III, aVF, aVL, V1

*Vijay said most commonly inverted in aVR, V1 and soemtimes III

Question 15

State the format in which you should report an ECG

Answer 15

• Patient details
  
  • ECG of (name) aged (age)
  • Taken on (date)
  • At (time)
• Rate
• Rhythm
• Axis
• Intervals
  
  • PR interval
  • QRS interval
  • QT interval
• Parts of ECG
  
  • P wave
  • QRS
  • ST segment
  • T wave

Question 16

How would you calculate HR on ECG if rhythm is regular?

How would you calculate HR on ECG if rhythm is irregular?

Answer 16

• Regular: 300/number of small squares between each R wave
• Irregular: count number of QRS in 10 seconds and multiply by 6

Question 17

What would you say when commenting on rhythm of ECG?

Answer 17

Whether it is:

• Regular
• Irregular
  
  • Regularly irregular
  • Irregularly irregular

Question 18

Describe how you work out the cardiac axis to determine if there is deviation

Answer 18

• Look at any 2 leads which are 90 degrees apart- typically use lead I and aVF
• In each of the leads, determine the size of the upward deflection and the downard defleciton of QRS in terms of small squares
• Determine whether it is + or -

Then, move on to the limb lead views diagram:

• Arrows show direction of depolarisation if number is positive; hence, opposite direction to arrows is the direction of depolarisation if number is negative
• Keeping your scale the same, plot the numbers on the axis (be this in + or - direction)
• Draw straight lines (vertical for point on lead I and horizontal for lead aVF)
• Point where they meet is the cardiac axis
• If cardiac axis is anticlockwise past aVL/between -30^o and -90^o = left axis deviation
• If cardiac axis is clockwise past aVF/between +90^o and -90^o =right axis deviation

Question 19

State the angles for:

• Left axis deviation
• Right axis deviation
• Extreme axis variation

Answer 19

Question 20

Explain why an inferior MI can cause left axis deviation

Answer 20

?

Question 21

State 3 possible causes of left axis deviation

Answer 21

• Left ventricular hypertrophy
• Left hemiblock (left posterior fasicle not working)
• Inferior MI`

Question 22

State three possible causes for right axis deviation

Answer 22

• Right ventricular hypertrophy
• Wolff-Parkinson-White
• Lateral MI

Question 23

Explain why Wolff-Parkinson-White can cause right axis deviation

Answer 23

In WPW the accessory pathway is typically between right atria and right ventricle; it allows impulses to travel from atria to ventricle. The accessory pathway, unlike the AV node, isn’t able to slow down the impulses hence PR interval is shorter

Question 24

State some casues for abnormal P waves; for each, state what it would show as on an ECG

Answer 24

• Left atrial dilation (P mitrale on ECG)
• Right atrial dilation (P pulmonale on ECG)
• Atrial ectopics (inverted P waves on ECG)
• Atrial fibrillation (irregularly irregular rhythm)
• Atrial flutter (regularly irregular rhythm)

Question 25

What do we call a P wave that looks like this? What causes this? Explain how it causes this

Answer 25

* **P Mitrale** * **Left atrial dilation** * Since SA node is in right atrium, the P wave for the right atrium usually comes slightly before the P wave for the left atrium. Usually these just combine to give us our usual P wave. If there is left atrial dilation, then the conduction through/in the left atrium is 'stretched out' hence the P wave for the left atrium is delayed. This produces characteristic P Mitrale P wave *(small depression in middle of P wave).* P wave will also be longer \*NOTE: P Mitrale can show as a typical M shaped P wave in lead II and/or as enhanced negative in V1

Question 26

What do we call a P wave that looks like this? What causes this? Explain how it causes this

Answer 26

* P Pulmonale * Righh atrial dilation * Since SA node is in right atrium, the P wave for the right atrium usually comes slightly before the P wave for the left atrium. Usually these just combine to give us our usual P wave. If there is right atrial dilation, the impulses in right atrium are 'stretched out' so the P wave from the right atrium is delayed. Usually the P wave from right atrium would come slightly before P wave from left atrium but since the right atrial P wave is late when they two combine to give us our one P wave the P wave produced is taller

Question 27

Summarise the ECG findings in P Mitrale- include any timings or measurements if relevant

Answer 27

* P wave with notch/depression in it *(looks M shaped)* * P wave \>120ms

Question 28

Summarise the ECG findings in P Pulmonale- include any timings or measurements if relevant

Answer 28

* P wave \>2.5mm (\>2.5 small squares)

Question 29

What does this ECG show? What causes this?

Answer 29

* Inverted P waves * Atrial ectopics

Question 30

Describe the mechanism/pathophysiology behind atrial flutter

Answer 30

* Re-entrant circuit within atria * Typically located in right atrium involving a strip of tissue between IVC and tricuspid valve * Re-entrant ciruct can be anticlockwise (90%) or clockwise (10%) * Atria beat about 300bpm * AV node blocks some of the atrial depolarisations so that ventricles depolarise at a slower rate *(NOTE: this is the AV node doing its job, it is not heart block)*

Question 31

Describe the appearance of atrial flutter with anticlockwise re-entrant loop on an ECG

Answer 31

* 'Saw tooth' appearance * If anticlockwise, see * Negative/inverted flutter waves in leads I, II and aVF * Positive flutter waves in V1 * AV node block often in 2:1 or 3:1 ratio

Question 32

Describe the appearance of atrial flutter with clockwise re-entrant loop

Answer 32

* 'Saw tooth' appearance * If clockwise, see * Positive flutter waves in leads I, II and aVF * Negative/inverted flutter waves in V1 * AV node block often in 2:1 or 3:1 ratio *\*Flutter pattern, in terms of positive and negative, is opposite to pattern seen in anti-clockwise re-entratn loop*

Question 33

What does this ECG show?

Answer 33

Atrial flutter with anticlockwise re-entrant loop with variable 3:1 to 4:1 block *\*Always state the ratio for atrial flutter*

Question 34

Describe the mechanism/pathophysiology behind atrial fibrillation

Answer 34

_Ectopic Cause_ * Intracardiac pressure increases (which is most commonly increased by systemic hypertension but may be increased by other causes e.g. mitral stenosis) * Atrial pressure increases * Atrial dilation * Ectopic atrial tissue within pulmonary veins triggers repetitive atrial depolarisation * Atrial fibrosis occurs as a result of either ischaemia, aging or persistent dilation; varying fibrosis provided non-homogenity to refractory periods in atria _Re-entrant_ * Damaged area has shorter refractory period * SA node fires and whereas other tissue won't conduct as in refractory period this tissue will * Damaged area of tissue can excite other areas of atria

Question 35

Describe the appearance of atrial fibrillation on ECG

Answer 35

* Irregularly irregular * P waves absent/not clear * Absence of isoelectric baseline * Tachycardia * Narrow QRS

Question 36

What does this ECG show?

Answer 36

Atrial fibrillation

Question 37

What is coarse AF?

Answer 37

Atrial fibrillation that has a baseline that resembles atrial flutter; however, it can be distinguished from atrial flutter because AF is irregularly irregular whereas atrial flutter is regularly regular

Question 38

What does this ECG show?

Answer 38

Coarse AF * Irregularly irregular * V1 and V2 show 'sawtooth' pattern associated with atrial flutter