7. ECG analysis Flashcards
How is the heart rate shown on an ECG and how can you calculate this?
Heart rate is given by the RR intervals - the QRS complex is ventricular depolarisation
Count the number of R waves in 15 large squares and multiply by 20 (30 squares correlates to 6 seconds)
NB. the heart rate will usually be provided below on the rhythm strip
What is meant by the rhythm on an ECG?
The rhythm is essentially the regularity
Are the RR intervals the same throughout the whole strip? Or are they irregular?
What is indicated by the P wave and how long should this be?
P wave indicates atrial depolarisation
Should not be more than 2 squares long
What is indicated by the PR interval?
PR interval is the time taken for the electrical impulse to cross the AV node and propagate via bundle of His to the ventricles
i.e. from the atria to the ventricles
What is the normal range for the PR interval and what might an abnormal PR interval indicate?
PR interval should be 12-200ms i.e. 3 to 5 small squares
PR interval over 200ms usually indicates some form of heart block
Prolongation of the PR interval can also be associated with hypokalemia, acute rheumatic fever or carditis
Where is the QRS complex measured from?
From the start of the Q wave to the end of the S wave
What is indicated by the QRS complex?
Indicates the synchronisation of contraction of ventricular muscle
What is a long QRS complex indicative of?
Indicates that part of the ventricular muscle is not contracting normally
Give the normal values on an ECG for: rate, rhythm, P wave, PR interval, QRS duration
Rate: 60-100bpm Rhythm: Regular P wave: less than 80ms PR interval: less than 200ms QRS duration: less than 120ms
What is meant by sinus rhythm?
Sinus rhythm is a normal ECG with nothing abnormal
i.e. rate, rhythm, QRS duration, P wave, R wave and PR intervals are all normal
What is meant by sinus bradycardia?
This is where there is a low heart rate of less than 60 beats per minute
More spaced RR intervals but everything else is normal
Briefly describe sinus bradycardia
The rate is less than 60 beats per minute but everything else is normal i.e. normal rhythm, QRS duration, P waves and RR intervals
Why might sinus bradycardia occur?
May occur naturally in athletes where they have an enlarged heart
The increased size of the heart means that the diastolic filling time is longer - hence longer RR intervals
Can also be seen in patients on beta blockers
What is meant by sinus tachycardia?
Where there is a very high heart rate - greater than 100 beats per minute - originating at the SAN
The RR intervals are reduced
Everything else will be normal (including the PR intervals)
When might sinus tachycardia occur?
May be due to stress, fright, illness, exercise
Can be triggered by shock
NB. if no apparent trigger - medications may be required to suppress the rhythm
What is meant by ventricular tachycardia (pathophysiology)?
This is where improper electrical activity arises in the ventricles of the heart
The ventricles contract synchronously and produce a cardiac output but this is not controlled via the atrial contraction (SAN)
Can lead to rapid and irregular heart rhythm which can result in cardiac arrest
How is ventricular tachycardia shown on an ECG?
This ECG is very abnormal No P wave is seen QRS duration is prolonged Very very high rate of around 180-190 beats per minute The rhythm is generally regular
How does ventricular tachycardia differ to ventricular fibrillation?
VT is much more regular than VF
The rhythm and amplitude of the QRS complex is regular - it is just much faster than normal
What is meant by ventricular fibrillation?
Disorganised electrical signals - no sign of organisation or control of the electrical impulses
The ventricles quiver rather than contract in a rhythmic fashion
Blood is not pumped to the brain - the patient is generally unconscious
When might ventricular fibrillation occur and what how must the patient be treated?
May occur during or after a myocardial infarct
Patient should be immediately treated by a defibrillator (either internal or external)
How does ventricular fibrillation appear on an ECG?
Rhythm - irregular
Rate - very high, greater than 300
QRS duration - not recognisable
P wave - not seen
Very unfamiliar ECG pattern
What is meant by ‘heart block’ and where can it occur?
How can general heart block show on an ECG?
This is a disease/inherited condition
Causes a fault within the SAN due to an obstruction or block of the electrical conduction of the heart
A PR interval greater than 5 small squares indicates heart block
Can occur at the SAN, the AVN or just below the Bundle of His
What are the different types of heart block?
SAN blocks
AVN blocks - first second or third degree
Infra-Hisian blocks
Describe SAN blocks
This type of block will rarely give any severe symptoms because a total block at the SAN would simply result in the natural pacemaker of the heart being shifted to the AVN
Describe first degree AVN block and how it shows on an ECG
Regularly irregular PR intervals
PR interval lengthened beyond 0.20 at a regular rate - PR interval greater than 5 small squares
This condition rarely causes any problems by itself (minor)
Many athletes have this
Give the different names of second degree heart block?
Type 1 - Wenkenbach
Type 2
Describe the pathopysiology of type 1 (Wenkenbach) second degree heart block and how it appears on an ECG
Each successive impulse from the atria finds it more difficult to pass the AVN - problem with the electrical conduction from the atria to the ventricles
How does type 1 (Wenkenbach) second degree heart block appear on an ECG?
The PR intervals are lengthening throughout the ECG until the P wave is then not followed by a QRS complex - the system resets and repeats
Describe type 2 second degree heart block
The AVN randomly fails to respond to atrial impulses
The rate is irregularly irregular
Describe the pathophysiology of third degree heart block and what problems this can lead to
This is much more serious
Where conduction from the atria to the ventricles has failed completely
The ventricles are generating their own signal from a focus somewhere else in the ventricle
The ventricle often does not contract synchronously in this case and can lead to ventricular fibrillation
Bradycardia is often present as the abnormal ventricular contraction is usually slow
Patients with this will require a pacemaker
What is meant by atrial fibrillation?
Give the pathophysiology
Many separate sites in the atria are generating electrical impulses rather than just the SAN
Results in an irregular conduction of impulses to the ventricles
There is a general tachycardia
Can be felt when palpating a pulse
Very common in the eldery
How does atrial fibrillation appear on an ECG?
Cannot distinguish any P waves
Hence, cannot measure the PR intervals
Very high heart rate - of about 100-160 beats per minute (slower if on medication)
Normal QRS complex
What symptoms may be associated with atrial fibrillation?
Many cause no symptoms Palpitations Fainting Chest pain Congestive heart failure
What is meant by atrial flutter?
Give the pathophysiology
The SAN is firing too fast - rather than going into a proper refractory period, it fires immediately
SO there is synchronous contraction but more than one - the atria contract an extra time whilst the ventricles are still contracting (v similar to AF but only one abnormal excitatory focus)
Very high heart rate - around 110 beats per minute
How does atrial flutter appear on an ECG?
Very high heart rate so very frequent P waves - the P waves are replaced with multiple F (flutter) waves
Cannot tell the PR interval
Regular rhythm
Normal QRS complex
Compare the pathophysiology and the causes of atrial fibrillation to that of atrial flutter
Atrial fibrillation:
Multiple ectopic atrial pacemaker sites that compete with the SAN to stimulate atrial contraction
Can be caused by atrial dilation or enlargement associated with heart failure
Atrial flutter
There is only one atrial pacemaker re-activating itself and competing with the SAN
Can be due to an electrolyte disturbance or e.g. cardiomyopathy
What is meant by ‘junctional rhythm’?
This is where there is damage to the SAN/conduction pathway in the atria
THE AVN TAKES OVER AS THE PACEMAKER
Bradycardia is present
How will junctional rhythm appear on an ECG?
Normal QRS complex
Absent P wave or if P wave is visible then it may be inverted in lead II
Hence, variable PR interval
What is meant by ‘supraventricular tachycardia’?
General term for tachycardia in the atria
Can be due to atrial fib, atrial flutter or Wolff-Parkinson White syndrome
Can occur in all age groups
Very very high heart rate of around 140-220 bpm
How does supraventricular tachycardia appear on an ECG?
Regular rhythm
Normal QRS complex
P wave may be absent or there may be multiple
What is the patholphysiology of Wolff-Parkinson White syndrome?
Where there is an abnormal extra conduction pathway between the atria and the ventricles
This abnormal pathway is known as the bundle of Kent
Can stimulate the ventricles to contract prematurely
What is meant by AV nodal reentrant tachycardia (AVNRT)?
Give the pathophysiology
Most common type of sypraventricular tachycardia
Main symptom of palpitations
The AVN is reacitvated shortly after the first activation
Circuit usually involves two pathways in the right atrium - a fast one and a slow one
What are the different conditions that can cause variations in the P wave on an ECG?
Atrial fibrillation Atrial flutter Junctional rhythm Supraventricular tachycardia AVNRT
What condition can cause variations in the QRS complex on an ECG?
Bundle branch block
Give the pathophysiology of bundle branch block
Abnormal delay of the depolarisation of the ventricular muscle
Right bundle branch block - indicates a problem on the right hand side of the heart
Left bundle branch block - indication of heart disease
How does bundle branch block appear on an ECG?
There will be a prolonged QRS complex - delay in ventricular depolarisation
There may be a double R wave/a notch on the R wave - this is inducative of bundle branch block (good images in MIllar lecture)
Everything else will appear normal
What is represented by the ST segment?
The time when both ventricles are fully depolarised
How should the ST segment normally appear on an ECG and why?
The ST segment should be ‘isoelectric’ i.e. at the same voltage as the line immediately preceding the P wave
Hence should not be elevated or depressed
What is ST depression indicative of?
Can indicate coronary ischaemia or hypokalemia
ST depression can be present only on an exercise ECG and indicate angina or coronary heart disease
What is ST elevation indicative of?
ST elevation indicates STEMI - ST elevation Myocardial Infarct
For STEMI to be a diagnosis, the ST elevation must show in two or more adjacent ECG leads
How does STEMI appear on an ECG?
ST elevation
May be pathological Q waves (region of myocardium has undergone irreversible death)
R wave is prolonged and doesn’t come all the way down to the isoelectric point
What is NSTEMI?
Myocardial infarction without ST elevation
Usually a less serious form - the cardiac ischaemia is present but not necessarily cardiac death
What is meant by the ‘electrical axis’ of the heart?
This is the mean direction of the action potentials travelling through the ventricles during depolarisation
How can you determine the electrical axis of the heart?
Use the QRS complex
Determine if the QRS complex is grossly positive or grossly negative in lead I and lead aVF
If overall positive, then the electrical axis is in the normal quadrant (between 0 and 90 degrees)
If it is positive in lead I but negative in aVF then the electrical axis for the heart is left axis deviated
If QRS complexes are both negative then the electrical axis is right axis deviated
