1B ECG and rhythm disorders Flashcards
What are the kinds of abnormalities ECG can tell us about?
- Conduction
- Structural (e.g. ventricular hypertrophy)
- Perfusion (whether the muscle is ischaemic or infarct e.g. MI)
What are the advantages of an ECG?
- Relatively cheap and easy to undertake
- Reproducible between people and centres
- Quick turnaround on results/report
What do the deflections on ECG leads mean?
- Downwards deflections are towards the negative electrode
- Upward deflections are towards the positive electrode
- If the wave of depolarisation is moving perpendicular to the angle of the lead, the line will be isoelectric
What does steepness of the deflection mean?
The velocity of the action potential
What does the width of the deflection mean?
The duration of the event
What is a cardiac vector?
A vector is ‘a quantity that has both magnitude and direction’
Typically represented by an arrow in the net direction of movement, whose size reflects the magnitude
What does the P wave indicate?
The electrical signal that stimulates contraction of the atria (atrial systole)
What does the QRS complex indicate?
The electrical signal that stimulates contraction of the ventricles (ventricular systole)
What does the T wave indicate?
The electrical signal that signifies relaxation of the ventricles
Describe what this part indicates
Sinoatrial node (SAN)
- Autorhythmic myocytes
- Spontaneous depolarisation of atria
- Slow and not a lot of muscle so P wave is wide and small
What does this part indicate?
- AVN depolarisation
- Isoelectric ECG
- Slow signal transduction
- Protective
Cells are intentionally impeding conduction
What does this part indicate?
Bundle of His
- Rapid conduction
- Insulated
Insulation ends on left bundle branch
What does this part indicate?
Bundle branches
- Septal depolarisation
- This happens because bundle branches are insulated but bottom of left branch is more gappy and lets excitation escape so all of septum is innervated by these escaping signals
- This creates small deflection towards negative electrode
What does this part indicate?
Purkinje fibres
- Ventricular depolarisation (big up and down R bit). Thick muscle so bigger deflection.
- Late ventricular depolarisation (small negative S bit)- as current travels up the Purkinje fibres towards negative electrode
What does this part indicate?
Fully depolarised ventricles
- Isoelectric ECG
What does this part indicate?
Repolarisation
- Ventricular repolarisation
- Slow positive deflection- going in same direction but repolarising and not depolarising
What is important to keep in mind about the timing of the ECG wave and the actual heart contraction?
Electrical activity happens first and is quicker- heart muscle takes a little longer to react and pump (systole is green)
so e.g. P wave triggers atrial systole but it doesn’t reflect it
Rule of Ls for limb leads
- Lead I (1 L) → Right arm to Left arm
- Lead II (2 Ls) → Right arm to Left Leg
- Lead III (3 Ls) → Left arm to Left Leg
What does aVL read?
Compares the electrical activity between a positive electrode on the left arm with the average electrical activity between the right arm and left leg (lead II)
This same logic works for aVR and aVF
Label the electrode placement on the body
Explain this example of an ECG
Each of the leads gives a view from one angle of the heart (blue) and also of the muscle that a certain heart artery supplies (purple)
e.g. if we saw ST elevation in leads II, III and aVF we can guess the RCA has a blockage
Give a summary of a 12 lead ECG
Find the RR interval for this ECG
520ms
Find the P wave duration for this ECG
100ms
Find the PR interval for this ECG
160ms
Find the QRS duration of this ECG
80ms
Find the QT interval for this ECG
320ms
Find the T wave duration for this ECG
160ms
Find the heart rate of this ECG
115bpm
Rate = 300 / number of large squares between consecutive R waves
What’s the QRS axis of this ECG?
Can’t tell: need 2 more axes
Calculate the cardiac axis of the following ECG
1) Calculate the net deflections of both leads II and aVL
- Lead II → +6.5mm positive deflection - 2mm negative deflection = +4.5mm net deflection
- Lead aVL → +4.5mm positive deflection - 2.5mm negative deflection = +2mm net deflection
2) Draw out lines of the same length as the deflection and in the same directions as the leads
3) Align them and use trig to work out the angle between them
- tan(x) = 2/4.5
- x = tan-1 (0.444) = 24°
4) Use that angle to find the actual heart axis (by taking it away/adding it to the nearest known angle on the diagram)
- Angle → 60° - 24° = 36°
What is the ECG reporting procedure?
1) Is it the correct recording?
2) Review the signal quality and leads
3) Verify the voltage and paper speed
4) Review the patient background if available e.g. if the ECG has AF then reviewing notes will show if they know or not so you know whether or not to tell them
What do we review on the ECG?
1) Rate and rhythm- is RR interval normal
2) P wave and PR interval
3) QRS duration
4) QRS axis (normal range is -30° to 90°)
5) ST segment- is it of an appropriate duration and is it elevated or depressed?
You’re a junior doctor in an emergency where a patient flatlines. How appropriate are the following responses and why?
1) Defibrillate the patient
2) Administer adrenaline
1) Defibrillate: bad- this is not a shockable rhythm as there is no electrical activity to reset
2) Adrenaline: good- could help spontaneously restore shockable rhythm to this asystole (flatline) and get heart to work again
What is sinus rhythm and what are its features?
- Standard heart rhythm
- Each P wave is followed by a QRS wave in a 1:1 ratio
- Rate is regular (even RR intervals) and normal (60-100bpm)
- Otherwise unremarkable
What is sinus bradycardia and what are its features?
- Similar to sinus- each P wave followed by QRS wave in 1:1 ratio
- Rate is regular (even RR intervals) but slow (<60bpm)
What is sinus bradycardia caused by?
- Can be healthy
- Caused by medication
- Caused by vagal stimulation
What is sinus tachycardia and what are its features?
- Each P wave followed by a QRS wave in a 1:1 ratio
- Rate is regular (even RR intervals) and fast (>100bpm)
- Often a physiological response (i.e. secondary)
What is a sinus arrhythmia and what are its features?
- Each P wave is followed by a QRS wave
- Rate is irregular (variable RR intervals) and normalish (65-100bpm)
What does RR interval vary with in sinus arrhythmia and what are its features?
- Breathing cycle
- The heart wants to natively beat at 110bpm but vagus nerve (through parasympathetic stimulation) slows it down to 70bpm
- As we breathe in, in many young healthy adults the vagal stimulation is turned down a bit- leads to small and big gaps in ECG between heart beats- leads to varying heart rate
- Very common and not pathological a lot of the time
What is atrial fibrillation and what are its features?
- Oscillating baseline (atria contracting asynchronously)
- Rhythm can be irregular and rate may be slow
- Atria not essential for cardiac cycle
What does turbulent flow pattern increase risk of?
Clots
What is atrial flutter and what are its features?
- Regular saw tooth pattern in baseline (II, III and aVF)
- Atrial to ventricular contractions are at a 2:1 ratio, 3:1 ratio or higher
- Saw-tooth not always visible in all leads
What is first degree heart block and what are its features?
- Prolonged PR interval caused by slower AV conduction
- Regular rhythm → 1:1 ratio of P waves to QRS waves
What are first degree heart blocks caused by?
- Most are benign heart block
- Can be a progressive disease of ageing
What is second degree heart block (Mobitz I) and what are its features?
- aka Wenckebach phenomenon
- Gradual prolongation of PR interval until a QRS is skipped
- Most P waves followed by QRS but some aren’t
Why is the rate regularly irregular in Mobitz I second degree heart block?
Diseased AV node
What is second degree heart block (Mobitz II) and what are its features?
- P waves are regular but only some are followed by QRS
- No PR prolongation
- Rate is regularly irregular- successes to failures (e.g. 2:1) or random
What is the issue with second degree heart block (Mobitz II)?
Can rapidly deteriorate into 3rd degree heart block if left untreated and not monitored
What is third degree (complete) heart block and what are its features?
- P waves are regular (PP interval is standard), QRS are regular (RR interval is standard), but no relationship
- P waves don’t lead to QRS complexes (work independently of one another)
- P waves can be hidden within bigger vectors
- A truly non-sinus rhythm → backup pacemaker needs to be in action
What is ventricular tachycardia and what are its features?
- P waves hidden- dissociated atrial rhythm
- Rate is regular and fast (100-200bpm)
- It is a shockable rhythm so defibrillators can be used
What is ventricular tachycardia at high risk of?
Deteriorating into fibrillation (cardiac arrest)
What is ventricular fibrillation and what are its features?
- Heart rate is irregular and 250bpm and above
- Heart unable to generate an output because heart can’t fill before it contracts
- Shockable rhythm so use defibrillator
What is ST elevation and what are its features?
- P waves visible and always followed by QRS (1:1)
- Regular rhythm and normal rate (60-100bpm)
- ST segment is elevated >2mm above isoelectric line
What is ST elevation caused by?
Infarction (tissue death caused by hypoperfusion)
What is ST depression and what are its features?
- P waves visible and followed by QRS always
- Regular rhythm and normal rate (60-100bpm)
- ST segment is depressed >2mm below isoelectric line
What is ST depression caused by?
Myocardial ischaemia (coronary insufficiency)
What can cause a right axis deviation?
Where the functional myocardium is more heavily concentrated on right hand side of heart
- Right ventricular hypertrophy- increases mass of muscle in right ventricle which causes bigger deflections and will move axis around to right
What can cause right ventricular hypertrophy?
As a result of another condition e.g. in COPD you can’t ventilate lungs so pulmonary system detects reduced pO2 and vasoconstricts so pulmonary circuit becomes higher pressure so right ventricle hypertrophies to pump harder to deal with this
- Infarction of muscle in left ventricle which disables it and pushes axis to right
Describe the ECG lines at different angles for leads I and aVF
They are perpendicular
How can perpendicular ECG lines be used to find the axis of the heart from an ECG?
- Pick any 2 pairs of perpendicular leads (I and aVF, II and aVL, III and aVR)
- Compare the size of the QRS complexes and use it and a diagram like below to figure it out depending on how big the complexes are and whether they’re positive/negative
Identify 3 pathological features from this ECG
1) ST elevation in leads II, III and aVF
- Point to a STEMI in the right coronary artery (because that’s the artery those leads are associated with)
2) Third degree heart block
- Regular P waves and regular QRS complexes but they’re independent
3) Bradycardia
- 37.5bpm
What diagnostic imaging would you need for this person and what would you see?
- Angiogram- release dye during diastole because that’s when heart muscle is perfused
- Would see RCA being tapered at one point and thinner- there is a blockage here
- Most likely atherosclerotic plaque
How do we treat this person?
- Stent- put in a balloon to distend artery and put in a metal mesh stent to support and keep artery patent
- Bypass- obstruction is still there but you’re getting blood supply from a different vessel
Diagnose this patient based on their ECG
Second degree Mobitz II heart block
