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