ECG Flashcards
Describe normal conduction in the heart
- 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
What is meant by the cardiac axis?
What is the normal axis?
- General/overall direction of depolarisation through heart
- Normal axis is towards apex of heart
What are the angles of the normal cardiac axis?
Between -30o (aVL) and +90o (aVF)
Remind yourself of the different views of heart from limb leads- include the angle of each
Which region of heart does each of the chest leads look at?
- V1 & V2= right ventricle
- V3 & V4= septum
- V5 & V6= left ventricle
What does each of the following represent:
- P wave
- PR interval
- Q wave
- R wave
- S wave
- T wave
- 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)
State how long, in terms of time and squares, each of the following intervals should be:
- PR interval
- QRS interval
- QTc interval
- 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)
What is the QTc interval?
How do you calculate the QTc interval?
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
How long, in terms of time, is each of the rhythm strips in the 12 lead ECG?
Each rhythm strip is 10s
Explain the shape of each of the QRS complexes in the chest leads (V1, V2, V3, V4, V5, V6)
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
What do we mean we talk about a lead being + or -?
Comparing size of upward deflection of QRS to downward deflection of QRS to see which is bigger
Which of the chest leads are negative and which are positive?
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 +
Which of the limb leads are negative and which are positive?
- Negative: aVR
- Positive: I, II, III, aVF, aVL
*****aVL varies but usually positive?
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
- 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
State the format in which you should report an ECG
-
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
Describe how you work out the cardiac axis to determine if there is deviation
- 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 -30o and -90o = left axis deviation
- If cardiac axis is clockwise past aVF/between +90o and -90o =right axis deviation
State the angles for:
- Left axis deviation
- Right axis deviation
- Extreme axis variation
What do we call a P wave that looks like this?
What causes this?
Explain how it causes this
- 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
What do we call a P wave that looks like this?
What causes this?
Explain how it causes this
- 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
What does this ECG show?
What causes this?
- Inverted P waves
- Atrial ectopics
Describe the mechanism/pathophysiology behind atrial flutter
- 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)
Describe the appearance of atrial flutter with anticlockwise re-entrant loop on an ECG
- ‘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
What does this ECG show?
Atrial flutter with anticlockwise re-entrant loop with variable 3:1 to 4:1 block
*Always state the ratio for atrial flutter
Describe the appearance of atrial fibrillation on ECG
- Irregularly irregular
- P waves absent/not clear
- Absence of isoelectric baseline
- Tachycardia
- Narrow QRS
What does this ECG show?
Atrial fibrillation
What is coarse AF?
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
What does this ECG show?
Coarse AF
- Irregularly irregular
- V1 and V2 show ‘sawtooth’ pattern associated with atrial flutter
