S6 Interpreting ECGs Flashcards
What are the 3 types of atrioventricular conduction blocks?
- First degree heart block
- Second degree heart block - Mobitz type 1 and Mobitz type 2
- Third degree heart block
What is an atrioventricular conduction block?
Delay/failure of conduction of impulses from the atria to the ventricles via the AV node (more than physiological/normal) and Bundle of His
What are the 4 causes of heart blocks?
- degeneration of the electrical conducting system with age (sclerosis and fibrosis)
- acute myocardial ischaemia
- medications
- valvular heart disease
What happens in first degree AV/heart block (describe the rhythm, PR interval and QRS complex)?
Conduction is slowed without skipped beats - all normal P waves are followed By QRS complexes but with a longer PR interval (a partial block) * Rhythm - regular * PR interval - more than 0.2 seconds * QRS complex - usually normal Usually asymptomatic
What happens in Mobitz type 1/Wenkebach second degree AV/heart block?
There are successively longer PR intervals until one QRS is dropped/lost (electrical signal is not conducted through ventricles), then this cycle starts again (an increasing block)
What happens in Mobitz type 2 second degree AV/heart block?
The PR intervals don’t lengthen, there is just a sudden drop in QRS complex without any PR changes (the p waves are regular). Ventricular rhythm is irregular
This is symptomatic and has a high risk of progression to a complete heart block
Where can Mobitz type 2 second degree heart/AV block, block?
- level of Bundle of His
- at bilateral bundle branches
- at trifascicular bundle branches
What happens in third degree AV/heart block? What does the QRS complex look like? What is required in this type urgently?
The atria and ventricles are depolarising independently due to complete failure of AV conduction. So the ventricular pacemaker takes over (at 20-40bpm which is too slow to maintain blood pressure)
Wide
A pacemaker
What is a bundle branch block? What are the P waves, PR intervals and QRS complexes like?
Delayed conduction in the bundle branches (left or right bundle branch block)
- p wave - normal
- PR interval - normal
- QRS complex - wide - because ventricle depolarisation takes longer
(V1 - W in S wave, V6 - M in S wave)
Where may abnormal arrhythmias arise from?
- Atria (supraventricular arrhythmias)
* sinus node
* atrium
* AV node - Ventricles (ventricular arrhythmias)
What are the QRS complexes like in a supreventricular and ventricular arrhythmias?
Supraventricular - normal
Ventricular - wide and bizzare
What is atrial fibrillation? What are the impulses like? Describe the P waves, R-R intervals and QRS complex.
Supraventricular arrythmia - arises from multiple atrial foci, rapid chaotic impulses
- P waves - non-existent - just a wavy baseline
- R-R intervals - irregular
- QRS complex - normal as ventricles are depolarised normally as not all impulses at AV node are conducted
What are the ECG variations of atrial fibrillation (Afib)?
- slow - ventricular response is less than 60bpm
- fast - ventricular response is over 100bpm
- normal rate - 61-99bpm
- coarse fibrillation - amplitude above 0.5mm
- fine fibrillation - amplitude below 0.5mm
What happens to atrial and ventricle contractions in atrial fibrillation?
The atrial contraction is lost (atria just ‘quiver’), the ventricles contract normally
What happens to the heart rate and pulse in atrial fibrillation?
Both are ‘irregularly irregular’
What does the loss of atrial contraction in atrial fibrillation mean for the haemodynamics (of blood in atria)?
Increases blood stasis (this is more evident in L atrium) so small clots form in L atrium - this is a risk factor for ischaemic stroke
What are premature ventricular ectopic beats/contractions (PVCs)?
- ectopic foci in ventricle muscles provides an extra contraction that doesn’t spread via the His-Purkinje system
- adds an additional QRS complex in every now and then
- ventricles have a lower intrinsic heart rate so these extra complexes have a wider QRS
What is ventricular tachycardia (VTACH)?
A broad complex tachycardia - when you have 3 or more consecutively PVCs (premature ventricular contractions) - this is dangerous and requires urgent treatment and has high risk progression to ventricular fibrillation
What is ventricular fibrillation?
Abnormal, chaotic and fast ventricular depolarisations due to impulses from numerous ectopic sites in the ventricle - there is no cardiac output as there is no coordinated contraction, the ventricles just ‘quiver’ and if it is sustained, cardiac arrest occurs
How can coronary ischamia or infarction occur?
Due to narrowing/occlusion of a coronary artery
How does ST Segment Elevation Myocardial Infarction (STEMI) occur?
Due to complete occlusion of coronary artery (full thickness)
What change in ST in STEMI?
ST elevation
What are the evolving ECG changes in a STEMI?
- acute - ST elevation
- hours - ST elevation, decreased R wave, Q wave begins
- day 1 to 2 - inversion of T wave, Q wave deepens
- days later - ST normalises, the T wave is inverted
- weeks later - ST and T are normal, Q wave persists
Are all Q waves a sign of an old infarct or depolarisation of septum?
No - pulmonary embolism may also lead to a Q wave in lead III
How can you tell a pathologic Q wave?
- more than 1 small square wide
- more than 2 small squares deep (except in leads III and aVR where a bigger Q wave could be normal)
- if depth of Q wave is 1/4 heigh of subsequent R wave
What are the ECG changes in non-STEMI and ischaemia?
- ST segment depression
* T wave inversion
What are the signs and symptoms of hypokalaemia?
- potassium levels: low - below 3.5mmol/L, moderate - below 3.0mmol/L, severe - below 2.5mmol/L
- muscle weakness, palpitations, arrythmia, cardiac arrest
What does an ECG look like for someone who has hypokalaemia?
- peaked P waves
- T wave flattened/inverted
- U waves form
What are the signs and symptoms of hyperkalaemia?
- potassium levels: above 5mmol/L
* muscle weakness, palpitations, arrhythmia, cardiac arrest
What does an ECG look like for someone with hyperkalaemia?
- tall tented T waves (5.5-6.5)
- loss of P wave (6.5-7.5)
- widening QRS (7.5-8.5)
- widening QRS approaching sine wave (over 8.5)