ECG Flashcards
What is an ECG?
This is the recording of the electrical activity of the heart.
It is the vector sum of the depolarisation and repolarisation potentials of all myocardial cells.
Fluctuations of these potentials create the P-QRS-T pattern.
Depolarisation that spreads towards an electrode is an —– deflection?
Upwards/Positive.
Depolarisation moving AWAY from the heart is seen as a downwards/negative deflection.
How does an ECG work?
It records from 12 projections of the vector called leads.
6 are recorded from the limbs and the other 6 leads are recorded from the heart.
The 6 limbs are recorded using a combination of 4 leads so there are 10 actual electrodes connected to the patients.
Describe the electrical conduction pathway
SA node initiate depolarisation in the normal heart.
Depolarisation then spreads through atria and it is seen as the upward deflection (P wave).
Atria and ventricles are electrically isolated from each other so impulses have to pass through the AV node.
Conduction through the AV node is usually slow and it is seen as the PR interval.
Depolarisation continues through the rapidly-conducting Purkinje fibres- the bundle of His then down the left and right bundle branches to depolarise both ventricles.
This is seen as the narrow QRS complex.
Ventricular repolarisation is seen as the T wave.
Which aspect of the heart does each lead look at?
The 6 limb leads look at the heart from a coronal plane:
-aVR looks at the right atrium (all vectors will be negative in this lead)
-aVF, II, and III view the inferior/diaphragmatic surface of the heart
- I and aVL examine the left lateral aspect
The 6 chest leads look at the heart in a transverse plane:
- V1 and V2 look at the septum
- V3 and V4 look at the anterior aspect of the left ventricle
- V5 and V6 look at the lateral aspect of the left ventricle
How do you interpret an ECG trace?
First confirm the patient's name and age, and the ECG date. Check the paper speed and voltage. Rate Rhythm Axis P wave PR interval QRS complex QT interval ST-segment T wave J wave
How do you assess the rate on an ECG?
Rate- At usual speed (25 mm/s), each big square is 0.2s each. The normal rate is 60-100 bpm.
<60 bpm- bradycardia
>100 bpm- tachycardia
Normal way- 300 divided by the number of big squares between consecutive R waves
Cardiology way- count 30 squares, count the R waves then multiply the R waves by 10.
Students way- count the R waves then multiply by 6
How do you assess the rhythm on an ECG?
Rhythm: Check if the rhythm is regular or irregular.
Sinus rhythm is characterised by a P wave followed by a QRS complex.
Slight but regular lengthening and then shortening (with respiration)- sinus arrhythmia, common in the young
Regularly irregularly (types of heart block)
Irregularly irregular and no P waves (AF)
How do you assess the axis on an ECG?
Axis is the overall direction of depolarisation across the patient’s anterior chest.
To determine the axis, the limb leads needs to be examined.
There are four ways of interpreting axis on ECG:
Normal axis (-30 to +90)- When QRS complex is positive in lead I and aVF.
Left Axis Deviation (-90 to -30)- If the QRS is positive in lead 1 and negative in aVF N.B. This could still be normal. if QRS in lead II is also negative then its most likely LAD.
If the QRS complexes in 1 and II point away from each other, then LAD
Right Axis deviation (+90 to +180)- If the QRS is predominantly negative in lead I and positive in lead aVF, then the axis is rightward. The QRS complexes in I and II or III point towards each other, then RAD
Indeterminate Axis- If the QRS is downward (negative) in lead I and downward (negative) in lead aVF, then the axis is indeterminate and sometimes referred to as “northwestern axis.” This finding is uncommon and usually from ventricular rhythms; however, it can also be from paced rhythms, lead misplacement and certain congenital heart diseases.
What causes left axis deviation?
Normal variant
Left anterior hemiblock
Left ventricular hypertrophy (rarely with LVH; usually axis is normal)
Left bundle branch block (rarely with LBBB)
A mechanical shift of heart in the chest (lung disease, prior chest surgery, etc.)
Inferior myocardial infarction
Wolff-Parkinson-White syndrome with “pseudoinfarct” pattern
Ventricular rhythms (accelerated idioventricular or ventricular tachycardia)
Ostium primum atrial septal defect
What causes right axis deviation?
Normal variant Right bundle branch block Right ventricular hypertrophy Left posterior hemiblock Dextrocardia Ventricular rhythms (accelerated idioventricular or ventricular tachycardia) Anterolateral myocardial infarction Wolff-Parkinson-White syndrome Acute right heart strain/pressure overload — also known as McGinn-White Sign or S1Q3T3 that occurs in pulmonary embolism
What is the P wave?
P wave normally precedes the QRS complex.
P wave denotes atria depolarisation.
It is usually a positive deflection except in aVR.
What is the PR Interval?
Measure from the start of the P wave to the start of QRS.
Normal range: 0.12-0.2 seconds (3-5 small squares).
What is the QRS complex?
Represents ventricular depolarisation and comprises:
Q wave: if the first deflection from the isoelectric line is negative.
R wave: First positive upwards deflection- may or may not follow a Q wave.
S wave: a downwards deflection following the R wave.
Normal duration <0.12s.
<3 small squares.
What is the QT interval?
Measure from the start of QRS to end of T wave.
It varies with the rate.
Normal QT- 0.38-0.42s
What is the ST segment?
Usually in line with the plane.
End of S wave to the start of the T wave.
What is the T wave?
Ventricular repolarisation.
What is the J wave?
The J point is where the S wave finishes and ST-segment starts.
Abnormalities of rate in ECG
Sinus means all impulses are initiated in the SA node.
Sinus Tachycardia- rate >100 bpm
Causes:
Drugs (adrenaline), Pain, exertion, anxiety, anaemia, PE, pregnancy, cardiac failure
Sinus Bradycardia- rate <60 bpm:
Causes:
Physical fitness, vasovagal attacks, sick sinus syndrome,
drugs (beta-blockers, digoxin, amiodarone), hypothyroidism, hypothermia, Increased intracranial pressure, cholestasis, inferior MI.
Abnormalities of rhythm in ECG
Regularly irregularly- Heart block
Irregularly irregular - AF
Abnormalities of P wave in ECG
Supraventricular tachycardias- arise in the atria or the AV node where QRS complexes appear normal.
There are 4 main causes: AF, atrial flutter, junctional tachycardia and reentry tachycardia.
What is AF?
Disorganised contraction of the atria in the form of rapid, irregular twitching.
There will be no P waves on the ECG.
Irregularly irregular QRS complexes.
Normal appearance of QRS and ventricular rate may be increased.
Causes of AF: Ischaemic heart disease, thyroid disease, HTN, PE, rheumatic mitral or tricuspid valve disease.
What is atrial flutter?
This is the abnormal rapid contraction of the atria.
Contractions are not disorganised or random, unlike AF, but are fast and inadequate for the normal movement of blood.
Instead of P waves, the baseline will have a typical ‘saw-tooth’ appearance.
Causes are similar to AF
What is junctional tachycardia?
The area in or around the AV node depolarizes spontaneously, the impulse will be immediately conducted to the ventricles.
The QRS complexes will be normal, but no P waves will be seen.
Causes: digoxin toxicity, sick sinus syndrome, ischaemia of the AV node, especially with acute inferior MI, acutely after cardiac surgery, diphtheria and other drugs (most anti-arrhythmic agents)
What is Wolff-Parkinson-White syndrome?
There’s an accessory pathway known as the bundle of Kent. The bundle of Kent doesn’t slow down impulses as the AV node does. This means there’s earlier ventricular depolarisation which gives a shorter PR interval and a slurred upstroke of the QRS complex (delta wave)
What is re-entry tachycardia?
An accessory pathway which may allow electrical activity to be conducted from the ventricles back up to the atria, which causes them to contract again. Narrow complex tachycardia
A P wave that occurs after the QRS complex (a short PR interval)
Abnormalities of PR Interval
WPW syndrome
Re-entry tachycardia
Causes of heart block
What is heart block?
Disturbance of normal conduction at the AV node leads to heart block.
Causes of 1st and 2nd-degree heart block include:
A normal variant, athletes, sick sinus syndrome. IHD (esp. inferior MI), acute myocarditis, drugs (digoxin, beta-blockers).
Causes of 3rd-degree heart block include: IHD (esp. inferior MI), idiopathic (fibrosis) of the conduction system, congenital, aortic valve calcification, cardiac surgery/trauma, digoxin toxicity, infiltration (abscesses, granulomas, tumours, parasites)
How many types of heart block are there?
3 with 2 subtypes of second-degree heart block
What is a first-degree heart block?
PR interval fixed but prolonged; no missed beats.
What is a second-degree heart block?
Not every P wave is followed by a QRS complex.
Mobitz type 1: PR interval becomes longer and longer until a QRS is missed, the pattern then resets. This is a Wenckebach phenomenon.
Mobitz type 2: PR interval is fixed but not every P wave is followed by a QRS complex. The relationship between P waves and QRS complex maybe 2.1 or random.
What is a third-degree heart block?
Also called complete heart block.
There is no conduction of the impulse through the AV junction.
Atrial and ventricular depolarisation occur independently of one another.
The QRS complex is an abnormal shape as the electrical impulse doesn’t travel through the ventricles via the normal routes.
P waves may be seen ‘merging’ with QRS complexes if they coincide.
As tissue distal to the AVN paces slowly, the patient becomes very bradycardic and may develop a haemodynamic compromise. Urgent treatment is required.
Abnormalities of QRS complexes
This occurs when depolarisation occurs through non-specialised ventricular myocardium, this is slower than normal conduction.
QRS complex will become prolonged and is described as ‘broad’
These abnormalities include RBBB and LBBB. Also bifascicular block (RBBB and left bundle hemiblock- LAD)
Trifascicular block- bifascicular block plus 1st degree heart block.
What is RBBB?
Delayed right ventricular depolarisation, giving a 2nd R wave.
RSR pattern seen in V1.
T wave flattening or inversion (V1 or V3).
Causes include hyperkalaemia, congenital heart disease, PE, cor pulmonale, fibrosis of conduction system.
MaRRoW- RBBB
What is LBBB?
Delayed left ventricular depolarisation.
M pattern seen in V6.
T wave flattening or inversion (V5-V6).
Causes include HTN, IHD, acute MI, aortic stenosis, cardiomyopathies, fibrosis of conduction system.
Abnormalities of ST segment
ST elevation: cause include- normal variant, acute MI (STEMI), Prinzmetal’s angina, acute pericarditis (saddle-shaped), left ventricular aneurysm.
