Common ECG Abnormalities Flashcards
Viewing from lead II, what does atrial depolarisation produce?
A small upwards deflection
Why is atrial depolarisation a small deflection?
Because there is only a small amount of muscle
What is the deflection from atrial depolarisation upwards (viewing from lead II)?
Because its moving towards the electrode
What is deflection caused by atrial depolarisation called?
The P wave
What does presence of the P wave show?
Atrial activity, at that the activity starts at the SAN
What is conduction from the atrium to ventricle via?
The AV node and bundle of His
How quick is conduction from atrium to ventricle?
Slow
Why is conduction from atrium to ventricle slow?
To allow time for atrial contraction to fill ventricle
What does conduction from the atrium to the ventricle produce on the ECG?
A isoelectric (flat) segment
How does electrical activity spread in the interventricular septum?
Down the left and right branches of the bundle of His, faster down the left bundle
What is the result of depolarisation being faster down the left bundle?
The muscle in the interventricular septum depolarises from right to left
What does depolarisation down the interventricular septum produce in the ECG wave?
A small downwards deflection
Why does spread down the interventricular septum produce a small downwards deflection (viewing from lead II)?
Downwards because moving away
Small because not moving directly away
What is the downwards deflection caused by the spread in the interventricular septum called?
Q wave
What does spread through the ventricle cause in the ECG (viewing from lead II)?
A large upwards deflection
Why does spread through the ventricle cause a large upwards deflection (viewing from lead II)?
Upwards because moving towards
Large because lots of muscle, and moving directly towards the lead
What is the large upwards deflection caused by spread through the ventricle called?
The R peak
What happens at the end of depolarisation?
Depolarisation spreads towards the base of the ventricles
What is produced in the ECG when depolarisation spreads upwards to the base of depolarisation (viewing from lead II)?
A small downwards deflection
Why is a small downwards deflection produced at the end of depolarisation (viewing from lead II)?
Downwards because moving way
Small because not moving directly away
What is the downwards deflection at the end of depolarisation called?
S wave
Where does ventricular repolarisation begin?
Epicardial surface
How does ventricular repolarisation?
In the opposite way to depolarisation
What does ventricular repolarisation produce in the ECG (viewing from lead II)?
Medium upwards deflection
Why does ventricular repolarisation cause an upwards deflection?
Because its moving away
What is the deflection caused by ventricular repolarisation called?
T wave
What is the first step in reading an ECG?
Assess rhythm
What do you need know to assess rhythm?
What it looks like in lead 2- don’t need to look at all 12 leads
What do you look at to assess rhythm?
The long rhythm strip
What do all ECG machines run at?
25m/sec
What do 5 large squares equal in time?
1 sec
How do you calculate rate from ECG if rhythm is regular?
Dividing 300 by the same of small squares in the R-R interval
How do you calculate rhythm if its irregular?
Count number of QRS complexes in 6 seconds, then times by 10
What is the normal PR interval?
0.12-2 seconds (3-5 small boxes)
What is meant by the QRS interval?
The width of the QRS complex
What is the normal QRS interval?
What is the normal QT interval?
It varies with heart rate, so must correct calculation for heart rate
Upper limit of corrected QT interval for males is 0.45, and for females 0.47s
What are the steps of assessing rhythm?
Assess P wave
Calculate PR interval
Check relationship between P waves and QRS complexes
Assess QRS complexes
What should be looked for when assessing P waves?
Is it absent or abnormal
Why do you need to calculate PR interval?
It estimates conduction in AV node and bundle of His
What should you be looking for when assessing relationship between P waves and QRS complexes?
Is every P wave followed by QRS, and vice versa
What do narrow QRS complexes indicate?
That rhythm starts in the atria/AVN
What do broad QRS complexes indicate?
Rhythm is originating in ventricles, or there is bundle branch block
Where is depolarisation initiated in sinus rhythm?
Depolarisation
What will be seen on the ECG in sinus rhythm?
Rate 60-100bpm P upright in leads I and II PR interval normal Every P wave followed by QRS Every QRS preceded by P wave Normal QRS width
What is sinus rhythm with rate
Sinus bradycardia
What is sinus rhythm with rate >100/min?
Sinus tachycardia
What happens in atrial fibrillation?
The base of the atrium begins to act as the pacemaker, so different bits of the atrium depolarise at different times
What will be seen in the ECG of atrial fibrillation?
No P wave
QRS complexes irregularly irregular, and
What does the PR interval show?
The time taken for conduction of impulse to ventricles
How is the PR interval calculated?
Time from start of P wave to start of QRS complex
What happens in first degree heart block?
Slow conduction in AV node and His bundle
What will be seen in the ECG of someone with first degree heart block?
P wave normal
PR interval prolonged to >5 small squares
QRS normal
What are the types of second degree heart block?
Type 1 second degree HB, aka. Mobitz type 1 HB, Wenkebach phenomenon
Type 2 second degree HB
What happens in type 1 second degree HB?
Progressive lengthening of PR interval, until one 1 is not conducted, allowing time for the AVN to recover and the cycle begins again
What happens in type 2 second degree HB?
PR interval normal, but theres a sudden lack of conduction of beat, and so a dropped QRS (P wave, but no QRS)
What is the problem with type 2 second degree HB?
High risk of progression to complete heart block
What does type 2 second degree HB require?
Insertion of pacemaker
What happens in third degree heart block?
P waves at normal rate, but not conducted to the ventricle, and so the ventricular pacemaker takes over, producing a ventricular escape rhythm
What is seen in the ECG of someone with third degree heart block?
Rate very slow, 30-40bpm
Usually wide QRS complexes
What is the problem with third degree HB?
HR slow to maintain BP and perfusion
What does third degree HB require?
Urgent pacemaker insertion
What are ectopic beats?
Premature contractions
What are ectopic foci?
Abnormal pacemaker sites within the heart muscle
What do ectopic foci display?
Automaticity
What normally happens to ectopic foci?
Thet are suppressed by the higher rate of the SA node in overdrive suppression
Where can ectopic foci occur?
Within atria (atrial ectopics) or ventricles (ventricular ectopics)
What happens in ventricular ectopic beats?
Ectopic focus is in ventricle
Depolarisation spreads form muscle, but not via Purkinje system
What is the result of depolarisation not spreading through the Purkinje system with ectopic beats?
There is a much slower depolarisation
What is seen in an ECG of ectopic beats?
Wide QRS complex
Different shape to usual QRS
What is ventricular tachycardia?
A run of 3 or more consecutive ventricular ectopics
What is ventricular fibrillation?
Abnormal, chaotic, fast ventricular depolarisation producing no coordinated contraction, and therefore a very rapid, irregular heart rhythm
What causes ventricular fibrillation?
Impulses arising in numerous ectopic sites in ventricular muscle
How do atrial and ventricular fibrillation differ?
In atrial fibrillation, atrial depolarisation is chaotic, whereas in ventricular fibrillation, ventricular depolarisation is chaotic
What happens to the ventricles in atrial fibrillation?
Impulses are conducted irregularly to the ventricles, so there is ventricular depolarisation and contraction
What happens to the ventricles in ventricular fibrillation?
There is no coordinated ventricular contraction
What happens to the pulse and heart rate in atrial fibrillation?
The pulse and heart rate are irregularly irregular
What happens to the pulse and heart rate in ventricular fibrillation?
There is none
How do the limb leads view the heart?
In vertical plane
What leads view the inferior surface of the RV and LV?
II, III and aVF
What leads view the free wall of the LV?
I and aLV
What does the aVR lead view?
Atrial and ventricular cavities
How do the chest leads view the heart?
In horizontal plane
What leads face the right ventricle?
V1 and V2
What leads face the interventricular septum?
V3 and V4
What leads face the left ventricle?
V5 and V6
What causes ischaemic heart disease?
Reduced myocardial perfusion
What can cause reduced myocardial perfusion?
Partial occlusion of coronary arteries
Acute occlusion by thrombus
What does partial occlusion of coronary arteries lead to?
Poor myocardial perfusion, particularly on exercise, leading to pain on exercise
What is shown in the ECG of someone with partial occlusion of coronary artery?
ECG often normal at rest, but changes seen in exercise
What can sub-endocardial ischaemia cause?
ST depression facing the affected area
What does acute occlusion by thrombus cause?
Death of part of the myocardium- a myocardial infarction
What are the ECG features of a fully evolved myocardial infarction?
Abnormal Q waves
ST segment elevation
T wave inversion
What causes abnormal Q waves in myocardial infarctions?
Myocardial necrosis
What causes ST segment elevation in myocardial infarctions?
Subepicardial injury
Where do ECG changes occur when there is a myocardial infarction?
ECG leads facing the infarcted area
What are pathological Q waves?
Those that are more that 0.04s (1 small square) wide, and more than 2mm deep
When are pathological Q waves present?
In full thickness myocardial infarction, and remain after other changes resolve
What is the cardiac axis?
The average (overall) direction of the spread of ventricular depolarisation
In what direction is the cardiac axis normally?
Downwards and to the left- between -30º and +90º
What is considered to be a right axis?
More than +90º
What is right axis deviation associated with?
Right ventricular hypertrophy
What happens to the QRS complex in right axis deviation?
Lead I is inverted
Lead III is upright
What is considered to be left axis?
Less than -30º
What is left axis deviation associated with?
Left ventricular hypertrophy
Conduction blocks in anterior part of left bundle
What happens to the QRS complex in left axis deviation?
Lead I is upright
Lead III is inverted
What should be reported on when reporting an ECG?
Rate Rhythm Conduction intervals- PR, QRS, QT Axis P wave- LA or RA enlargement Description of QRS complex ST segment T wave
