ECG Interpretation Flashcards
What are the leads in a 12 lead ECG?
Recordings are made using 10 recording leads attached to electrodes:
- 4 limb leads with electrodes on the R Arm, L Arm, L Leg and R Leg
- 6 precordial leads across the chest V1, V2, V3, V4, V5 and V6
Using these, graphs of the electrical activity of the heart are recorded in 12
different views:
- The limb leads are used to create standard bipolar leads I, II, III
- The ‘augmented’ unipolar leads aVR, aVL and aVF
- The precordial
leads record the chest leads V1, V2, V3, V4, V5 and V6
Giving the full ‘12 lead’
ECG
What territories (and arteries) do each leads correspond to?
Lateral:
- I + aVL + V5 + V6
- L circumflex or diagonal of LAD
Inferior:
- II + III + aVF
- RCA and/or L circumflex
Anterior/septal:
- V1-V4 (V1-2 more septal, V3-4 more anterior)
- LAD
(aVR is a bit of a forgotten lead but does have come useful clinical applications… somewhere)
How is a standard ECG recorded?
Standard recording speed is 25 mm/sec. = one small square (1 mm) on standard ECG paper record equals 40 milliseconds (0.04 secs.)
- If you want visualise an tachyarrhythmia better, you can slow down the speed of the ECG to stretch out the morphology
The voltage
calibration is 10 mm/mV
What do positive and negative complexes indicate on ECG?
- +ve = a peak = depolarisation travels towards a positive electrode
- -ve = a trough = depolarisation travels away from a positive electrode
What are P waves?
Sequential depolarisation of right and left atria
- R before L but both summate to form P wave
- 1st 1/3rd = R, middle 2/3rd = both, 3/3rd = L
Vector is from the SAN to AVN
Should be <120ms or 3 small squares
Should be:
- Upright in leads I, and II and inverted in aVR
- Monophasic in most leads (e.g. II)
- Biphasic in V1 - as L and R waveforms move in opposite directions - +ve deflection then -ve deflection
Abnormalities most easily seen:
- Inferior leads (e.g. II) - As P waves are most prominent
- V1 - as biphasic and can split out looking at L and R atrial pathology
What are some common abnormalities of P waves and the underlying pathology?
P mitrale:
- Bifid/notched P waves in lead II
- Seen with L atrial hypertrophy
- Classically due to mitral stenosis
P pulmonale:
- Peaked/tall P waves in lead II
- Seen with R atrial hypertrophy
- Classically due to pulmonary HTN e.g. cor pulmonale from chronic resp. disease
P wave inversion:
- In inferior leads/II
- A non-sinus origin of P wave
- When PR interval <120ms = AV junction origin (e.g. accelerated junctional rhythm)
- When PR interval >120ms = origin within atria (e.g. ectopic atrial rhythm)
Variable P wave morphology:
- e.g. >3 different P wave morphology = multifocal atrial tachycardia (rare)
What is the PR segment?
Flat/isoelectric segment between end of P and beginning of QRS
Electrical conduction from AVN-Bundle of His-Bundle branches- Purkinje fibres and enter ventricles
PR segment changes in pathology are measured against the baseline formed by the T-P segment (the isoelectric period where no electrical activity is occurring)
What are some common abnormalities of the PR segment and the underlying pathology?
Pericarditis:
- Widespread PR segment depression + saddle shaped/concave ST elevation
- reciprocal PR elevation and ST depression in aVR and V1
Atrial ischaemia/infarction:
- PR segment depression or elevation (reciprocal) in patients with MI indicates concomitant atrial ischaemia or infarction
- Associated with complications + poor outcomes
What is the PR interval?
Reflects condition through the AV node
Normal time = 120-200ms or 3-5 small squares
What are some common abnormalities of prolonged PR interval (>5 small squares) and the underlying pathology?
First degree heart block/AV block
Second degree heart block - Mobitz type 1 (Wenckebach phenomenon)
What are the features of first degree heart block including its causes and management?
Prolonged PR >200ms (5+ small squares)
- Delayed condition through the AV node
Sinus rhythm or sinus bradycardia = common
Aetiologies:
- Increased vagal tone i.e. from athletic training
- Inferior MI
- Mitral valve surgery
- Myocarditis
- Electrolyte abnormalities e.g. hyperkalaemia
- AV nodal blocking drugs e.g. betablockers, CaBs, digoxin, amiodarone
Management:
- Does not cause haemodynamic instability
- No specific treatment is required
What are the features of second degree heart block/Mobitz type 1 (Wenckebach phenomenon) including its causes and management?
PR interval is prolonged and elongates with each successive beat until a QRS is dropped/there is a non-conducted P wave and the cycle repeats
- With a shorter PR interval that gradually increases again
- May present in P:QRS ratios of 3:2, 4:3 or 5:4
- R-R interval remains constant amongst the clusters despite prolongation of PR
Aetiologies:
- Usually due to reversible conduction block at AV node
- Beta-blockers, CaBs, Digoxin, amiodarone
- Inferior MI, myocarditis, cardiac surgery
Management:
- Minimal haemodynamic instability
- Low risk of progression to 3rd degree heart block
- If asymptomatic - does not require treatment
- If symptomatic - usually respond to atropine
- Permanent pacing rarely needed
What are some common abnormalities of shortened PR interval and the underlying pathology?
Preexcitation syndromes
- Wolff-Parkinson-White (WPW) = most notable
AV nodal (junctional) rhythm
- Narrow complex, regular rhythms arising from AVN instead of SAN
- Absent or inverted P waves + short PR = retrograde P waves
What are the features of Wolff-Parkinson-White syndrome including its causes?
Due to the presence of a congenital accessory pathway (Bundle of Kent) connecting atria and ventricles which can (mostly) conduct in both directions
- This pathway conducts faster = shortened PR <120ms/3 small squares
- Also serves as an anatomical re-entry circuit = paroxysmal supraventricular tachycardia (SVT, specifically atrioventricular re-entry tachycardia = AVRT) (2x subtypes)
- Can also occur alongside AF and atrial flutter - which because of the pathway can lead to VT or VF as conduction bypasses AV node
ECG:
- Short PR <120ms
- Broad QRS >110ms
- Delta wave = slurred upstroke to the QRS (will also be reciprocal in relevant leads)
- Features may become more obvious with increased vagal tone e.g. during Valsalva manoeuvres or with AV blockade
How do you manage the complications of WPW?
AVRT:
- Depending on subtype and stability of patient, may respond to vagal manoeuvres +/- adenosine or CaBs OR amiodarone
- If haemodyamically unstable, urgent synchronised DC cardioversion is required
Atrial fibrillation and flutter:
- AV nodal blocking drugs - adenosine, CaBs, beta-blockers - may INCREASE CONDUCTION VIA ACCESSORY PATHWAY AND RESULT IN INCREASE IN VENTRICULAR RATE AND DEGENERATION TO VT OR VF SO AVOID
- Synchronised DC cardioversion is the therapy of choice
What is a Q wave?
Any negative deflection that precedes an R wave
- Represent normal L->R depolarisation of intraventricular septum
Leads:
- Small = normal in most leads
- Deeper (>2mm) may be a normal variant in III and aVR
- Normally not seen in right sided leads V1-3
What are the features of pathological Q waves? What do they indicate?
Features: - > 40 ms (1 mm) wide - > 2 mm deep - > 25% of depth of QRS complex - Seen in leads V1-3 Usually indicate a current or prior MI
May also indicate:
- Hypertrophic CM
- Rotation of the heart
- Lead placement error e.g. upper limbs placed on lower
Absence of Q waves in V5-6 is also abnormal:
- Most commonly due to LBBB
What are QRS complexes?
Represent the depolarisation of both ventricles
- Normally 70-100ms
Width: determines origin -
- Narrow - <100ms (2.5 small squares) = supraventricular in origin
- Broad - >100ms = either ventricular in origin, or due to aberrant conduction of supraventricular complexes
What are some examples of conditions with narrow QRS complexes and their features?
Arise from 3x main places:
1) SAN = normal conduction, P wave present
2) Atria = abnormal P wave/flutter wave/fibrillatory wave
- Condition = atrial flutter (saw tooth pattern)
- ECG = always rate of 150, regular, narrow complex tachycardia
3) AV node/junction = either no P wave, or abnormal P wave with a PR interval <120ms
- Condition = junctional tachycardia
What are some examples of conditions with broad QRS complexes?
QRS >100ms = abnormal
QRS >120ms = required for the Dx of BBB or ventricular rhythm
Other causes of aberrant condition leading to broad complexes:
- BBB
- Hyperkalaemia
- Na-channel blockade e.g. tricyclic poisoning
- Pre-excitation syndromes e.g. WPW
- Ventricular pacing
- Hypothermia
- Intermittent aberrancy
What are bundle branch blocks? (BBBs)
Arise from a defect in the bundle branches which themselves are divisions of the Bundle of His
- Right bundle contains one fascicle
- Left bundle subdivides into two fascicles (left anterior and left posterior; the left then bifurcates itself)
- Ultimately the fibres terminate in Purkinje fibres which innervate individual myocytes allowing for rapid/coordinated contraction
The defects may arise from:
- Ischemic heart disease
- HTN
- MI
- Hyperkalaemia
- Digoxin toxicity
- Rheumatic heart disease
- Cardiomyopathy
- Degenerative disease of the conduction system
- Congenital heart disease e.g. ASD
etc.
This changes the pathways involved in depolarisation:
- Impulse may move through muscle fibres in a way that slows electrical movement and changes the direction of the impulses
- Ventricles do not synchronise, their depolarisation is prolonged and CO may drop
What are the features of RBBB?
Activation in RV is delayed as depolarisation has to spread across the septum from the left ventricle
- Early part of QRS unchanged as LV is intact
Delayed RV depolarisation produces:
- A second R wave (R’) in V1-3 (= ‘M’ shaped QRS)
- A wide, slurred S wave in the lateral leads 1, aVL, V5+6
- A broad QRS >120ms
These 3 features are needed for diagnosis
- If QRS <120ms = incomplete RBBB
Associated features:
- ST depression + T wave inversion in V1-3
What are the features of LBBB?
Normally the septum is activated from L->R, producing small Q waves in lateral leads
- LBBB reverses the spread of depolarisation as impulse travels first to the RV from the RBB then to the LV via the septum
This sequence of activation produces:
- R waves in lateral leads (I, aVL, V5+6) that are either ‘M’ shaped, notched (little bump on downslope), monophasic (single peak) or an RS complex (absence of Q wave + R peak + S trough)
- Deep S waves in V1-3, especially in V1; possibly as part of an rS complex (small R wave, deep S wave)
- Prolonged R wave peak time >60ms in V5+6
- Absence of Q waves in I, V5+6
- A broad QRS >120ms
These features are needed for diagnosis
Associated features:
- Appropriate discordance = ST segments and T waves always go in the opposite direction to the main vector of the QRS complex
- Poor R wave progression in chest leads
- Left axis deviation
What are some mnemonics/memory aids for distinguishing between LBBB and RBBB?
‘WiLLiaM MaRRoW’
WiLLiaM = in LBBB there is a ‘W’ in lead V1 and an ‘M’ in V6
MaRRoW = in RBBB there is an ‘M’ in lead V1 and a ‘W’ in V6
Though this is pretty basic and if you take these as the only indications of BBB then you will miss them
Alternatively, a quick hack is:
- If you see a broad QRS, hold the ECG sheet in front of you and turn it 90 degrees clockwise - if the QRS complex in V1 is pointing to the right = RBBB or to the left = LBBB