ECG Flashcards
What does ECG stand for?
- Elctrocardiogram
What does an ECG show?
- Representation of cardiac cycle
- Volume rise in systole
- Ejection as pressure exceeds systolic pressure
- Volumes drop at same time
- Muscle depolarisation and contraction, volume decreases as blood leaves chamber
What is ECG a measure of?
- Not a measure of contraction
- The measure of electrical activity
- Which then precedes the muscle contraction
What are the components of the ECG?
- SA node (pacemaker) depolarises first
- Atrial muscle and AV node depolarise at the same time, during P wave
- Purkinje fibres depolarise at QRS complex
What is the P wave?
- First event of cycle is SA node depolarisation
- Triggers adjacent cells in atria to depolarise
- Spreads through L and R atria
- Generates P wave- reflection of SAN
- P wave shows atrial function
What is the PR interval?
- Depolarisation spread though AVN towards ventricle
- Delay at AVN to allow ventricular filling
- Gap between top of P wave and R wave- usually flat
- Normally 0.12-0.2 secs, 3-5 small ECG squares
What is the QRS complex?
- Ventricular depolarisation
- Spread though AVN, down Bundle of His, though Purkinje fibres and up through epicardium
- 3D spread
- Produces different waveforms that make up QRS complex
Describe Q wave in QRS complex
- Negative wave preceding R, depolarisation of ventricular septum
- Only seen in left pointing leads (I, II, AvL, V5, V6)
- by def QRS does not need Q because only present in certain leads
Describe R wave in QRS complex
- Upwards deflection
- Wave always R, irrespective of whether Q is present
- Always R wave, unlike Q
Describe S wave in QRS complex
- Not a downward reflection of R
- Follows R but deflection below isoelectric line
- Once R wave back to baseline, S wave starts
- Occurs regardless of Q
What is the T wave?
- Repolarisation of ventricle- appears positive
- Same deflection of QRS
- I.e. if QRS negative, T is also
Why does repolarisation appear positive?
- Depolarisation flows from endo to epicardium
- Repolarisation flows opposite
- Epicardium to endocardium double negative- so appears positive
What are U waves?
- Small deflections in the same direction as T wave
- Unknown origin (possibly papillary muscles)
Where in the ECG does atrial repolarisation occur?
- Within QRS complex
- Tends not to show due to larger events occurring in ventricle at the same time
How many leads in the ECG?
12
What are the bipolar limb leads?
- Lead 1
- Lead 2
- Lead 3
- Make up Eindhoven’s triangle
Describe lead 1 of ECG
- Runs from negative electrode in right arm to positive electrode in left arm
- Records from right to left across the body
Describe lead 2 of ECG
- Right arm (negative) to left leg (positive), down and across the body
Describe lead 3 of ECG
- Negative electrode from the left arm to the positive electrode in left leg down across body
What is Eindhoven’s triangle?
- Equilateral triangle with equal angles of 60 degrees, recording vertically through the body
What are the unipolar limb leads?
- Augmented leads:
- Lead AvF
- Lead AvL
- Lead AvR
Describe lead AvF
- Lead 1 to left leg
- Joining the electrodes from left arm and right arm- collapse lead 1 into a midpoint
- Record to remaining electrode (left leg)
Describe lead AvL
- Lead 2 to left arm
- Collapse lead 2 into a midpoint and record to left arm
Describe lead AvR
- Lead 3 to right arm
- Collapse left arm and left leg lead 3 and record up to right arm
Describe the 2 axes of the ECG
- Limb and augmented leads look at heart in vertical plane (from sides and feet)
- Remaining 6 leads are in horizontal plane
- Chest leads look at horizontal
- Collapsing electrodes in RA, LA, LL and recording out to electrodes placed chest
Describe the chest leads
- Particular places- V1, 2, 3, 4, 5 & 6
- Specific orientation of chest leads
- They give a horizontal view of chest
Which leads give view of anterior surface?
- V1, 2, 3 and 4
Which leads give view of lateral surface?
- I, AvL, V5 and 6
Which leads give view of the inferior surface?
- Pointing downwards
- II, III and AvF
Which leads give view of right side of heart?
- Point from left to right
- AvR and V1
How do you report an ECG?
- Rhythm
- Conduction intervals
- Cardiac axis
- Description of QRS complexes
- Description of ST segment
- Are they normal?
How do you assess rhythm in an ECG?
- Rhythm, strip at bottom of ECG with numerous QRS complexes
- 60-100? tachy
- Is the pattern regular?
- Does every P wave produce a QRS complex?
- Are QRS complexes driven by P wave (SA node)?
How do you assess conduction intervals?
- ECG based on cardiac cycle, should reflect those timings
- PR interval 0.12-0.2s
- Shorter? Depolarisation near AVN
- Longer? heart block
- Is the timing of each event normal?
- QRS should be <3 squares
- Depolarisation of ventricles is rapid and spreads equally
How do you assess cardiac axis?
- Represents general depolarisation with heart
- Starts top right (SAN) and spreads to bottom left and apex
- Axis of 0 consistent with depolarisation that moved R to L across body
- Calculated by looking at heights of ECG leads
- If not in this axis, may be problematic
- Not migrating round to right- bigger ventricles and pulling more depolarisation (hypertrophy)
10 rules for normal ECG
1) PR interval 3-5 squares
2) QRS <3 squares
3) QRS mainly upward in I and II
4) QRS + T same direction
5) AvR waves downward (against depolarisation)
6) R wave should increase in size from V1-4
7) ST segment isoelectric (except V1 + V2- may be raised)
8) P wave upward in I, II, V2-6
9) No or only small wave in I, II, V2-6
10) T wave upward in I, II, V2-6
