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
