ECGs and arrhythmias Flashcards
1
Q
How would you interpret an ECG?
The steps
A
- Determine rate
- 300/no. of large squares
- Determine rhythm
- Sinus rhythm = each P wave followed by QRS, constant PR interval
- Determine cardiac axis
- Normal = I & II postive
- Right axis deviation = I negative, III positive
- Left axis deviation = II & III negative
- Assess P waves
- Peak/tall in right atrial hypertrophy
- Notched/broad in left atrial hypertrophy
- Assess PR interval
- Time from beginning of the P wave to the beginning of QRS
- The normal range in 0.12-0.2 sec (3-5 small squares)
- If longer than 0.22s; first degree heart block
- Assess the QRS complex
- QRS width: wider than 120ms = ventricular origin/bundle branch block
- QRS height: tall R waves in V1 = Right ventricular hypertrophy, tall R waves in V6 = left ventricular hypertrophy
- Assess the ST segment
- Elevation in MI/pericarditis
- Depression in ischaemia/digoxin
- Assess for pathological T wave inversion
- T wave inversion in aVR, III and V1/2 can be normal
- Assess for Q waves
- Q waves indicate old infarction - normal in LV leads: I, VL, V5/6
- Assess the QT interval
- From beginning of QRS to end of the T wave
- Normal is <0.45s, approx. 2 large squares
3
Q
How should you correctly set up an ECG machine?
A
- Skin must be clean and dry
- V1/2 are positioned in the 4th intercostal space either side of the sternum
- Palpate the angle of Louis, and the 2nd intercostal space is adjacent
- V4 is positioned in the 5th intercostal space, mid-clavicular line
- V3 is placed between V2 and V4
- aVR and aVL go on right and left arms respectively
- aVF goes on the left ankle
- There is a neutral lead that is placed on the right ankle
4
Q
Describe the QRS complex
Abnormalities?
A
- If the first deflection is downward, then it is a ‘Q’ wave
- The first upward deflection is always the’R’ wave (regardless of if there has been a Q wave)
- Any deflection below baseline following the R wave is the S wave
- QRS is usually <120ms, and if this is prolonged it represents bundle branch block or depolarisation from a ventricular focus
5
Q
How should the T wave be seen on an ECG?
What are T wave abnormalities and causes?
A
- The T wave is normally inverted in aVR, III and V1/2
- Sometimes V3 in black people
- Otherwise, T wave inversion may represent several pathologies:
- Ischaemia
- Occurs in both STEMI and NSTEMI
- T waves become inverted over 24-48h, often permanent
- Ventricular hypertrophy
- In leads looking at the involved ventricles
- Bilateral bundle branch block
- Digoxin treatment
- Also causes classical sloped ST segments
- Ischaemia
6
Q
Why is the ECG trace pattern the way it is?
Why are the waves where they are?
How does it work?
A
- Myocardial contraction causes fibre depolarisation, which is detected by electrodes on the body
- The ECG trace should represent the normal electrical activity of the heart;
- SAN ⇒ atrial depolarisation ⇒ AVN delay ⇒ septal depolarisation ⇒ ventricular depolarisation via bundle of His
- If the electrical activity starts at the SAN, the heart is described as being ‘sinus rhythm’ i.e. a ‘P wave’ is present, representing atrial depolarisation
- Depolarisation of the larger mass of the ventricles causes the much larger ‘QRS complex
- The T wave then represents the repolarisation of the ventricular mass back to its resting state
8
Q
What happens in a Right bundle branch block?
How is it seen on an ECG trace?
A
- The septum is depolarized from the left side as normal, causing an R wave in V1 and a small Q wave in V6
- Excitation on the left ventricle causes an S wave in V1 and R wave in V6
- As it takes longer for excitation to reach the right ventricle, this depolarises after the left, causing a second R wave (R1) in V1, and a deep S wave in V6
- RBBB is best seen in V1 (RSR1)
- Ma_rr_oW: ‘M’ shape in V1, ‘W’ shape in V6
https://lifeinthefastlane.com/ecg-library/basics/right-bundle-branch-block/
9
Q
What happens in a left bundle branch block?
How is shown on an ECG trace?
A
- The septum will depolarize from right to left, causing Q wave in V1 and an R wave in V6
- The right ventricle is depolarised before the left ventricle, so there will be a small R wave in V1 and an S wave in V6 (although this often will appear only as a notch)
- Subsequent depolarization of the left ventricle causes an S wave in V1 and another R wave in V6
- LBBB is associated with T wave inversion in the lateral leads (I, VI and V5/6 altough not necessarily in all of these)
- LBBB is best seen in V6 (‘broad ‘M’ complex), and the ‘W’ pattern in V1 is often not fully developed
- ‘WilliaM’: ‘W’ shape in V1, ‘M’ shape in V6
https://lifeinthefastlane.com/ecg-library/basics/left-bundle-branch-block/
11
Q
What ST abnormalities in an ECG tracing can occur?
What do they mean?
A
- The ST segment should be ‘isoelectric’, i.e. at the same level as the part between the T wave and the next P wave
- ST elevation indicates acute myocardial injury; either a recent infection or due to pericarditis
- Pericarditis is not localised and thus it causes ST elevation in most leads
- ST depression is usually a sign of ischaemia rather than MI
12
Q
What is a branch block?
How is it seen on an ECG trace?
A
- The depolarization waves reaches the septum normally, so PR interval is normal, yet there is abnormal conduction through left/right bundle branches (of His)
- Delayed depolarization of the ventricles thus leads to a wide QRS (>120ms)
13
Q
The ECG is made up of 12 characteristic ‘leads’ that view the heart from different directions. What are the directions of each lead?
A
- The six limb leads (I, II, III, VR, VL, VF) look at the heart in a vertical plane
- aVR: right ventricle
- aVL, I: left heart surface
- II, III & aVF: inferior surface
- The six V (chest leads - V1-6) look in a horizontal plane
- V1/2: right ventricle
- V3/4: septal area
- V5/6: left ventricle
16
Q
What does the PR interval show?
What if it is longer in length?
A
- Time from the start of P to the start of the QRS complex
- Represents the time taken for excitation to spread from the SAN to the ventricular muscle
- Time is mostly made up of AVN delay
- If >0.2s - 5 squares, there may be heart block
17
Q
What is first degree heart block?
How is it seen on an ECG?
What can it indicate?
A
- First-degree atrioventricular block is a disease of the electrical conduction system of the heart in which the PR interval is lengthened beyond 0.22 seconds.
- PR interval >0.22 seconds
- First degree heart block is not patholigical in itself but can indicate:
- Coronary artery disease
- Acute rheumatic fever
- Electrolyte disturbances
- Digoxin toxicity
18
Q
Where is the ST segment?
What may changes show?
How big must these changes be?
A
- Interval between the end of the S wave and the beginning of the T wave
- Changes may represent myocardial ischaemia
- ST elevation must be >2mm in a chest lead, >1mm in a limb lead, on two leads to be significant
19
Q
What is complete/third degree heart block?
How is it seen on an ECG trace?
What happens to ventricle contraction?
What can cause 3rd degree heart block?
A
- Atrial contraction is normal but no beats are conducted to the ventricles
- P waves will be dissociated from the QRS complexes
- The ventricles are excited by a ‘slow escape mechanism’ from a depolarising focus within the ventricles, giving a wide QRS
- Can occur acutely following MI or be a chronic state (the chronic state indicates conducting tissue disease)
20
Q
Where is the QT interval?
What can cause a prolonged QT interval?
What may this cause?
A
- Time from the start of Q to the end of T
- Varies with heart rate but can be prolonged by drugs/electrolyte abnormalities
- A prolonged QT interval may lead to ventricular tachycardia
24
Q
Describe atrial fibrilation?
What is seen on an ECG?
Symptoms of AF?
A
- Irregular baseline with no P waves
- The AVN is bombarded and thus will depolarize irregularly, leading to ventricular contraction at an irregular rate;
- Usually 450-600 atrial contractions per minute
- Normal QRS as conduction from the AVN is not abnormal
- Can be asymptomatic, or present with dyspnoea, palpitations, syncope, chest pain or stroke/TIA
25
Q
Describe atrial flutter?
What is seen on an ECG?
Similarities and differences with atrial fibrilation?
A
- If the atrial rate is above 250/min and no flat rate baseline between P waves exists, atrial flutter is present
- Normally 300-450 contractions per minute
- Classic ‘saw-toothed’ baseline
- It can be thought of similar to atrial fib. in that the normal co-ordination of the atria is lost, however some element of synchronicity still exists
26
Q
Compare atrial fibrilation and atrial flutter on an ECG
A
27
Q
What is Wolff-Parkinson-White (WPW) syndrome?
What is seen on an ECG?
A
- Wolff–Parkinson–White syndrome (WPW) is a disorder due to a specific type of problem with the electrical system of the heart which has resulted in symptoms.
- Some people have an accessory conducting bundle alongside the bundle of His, usually in the left hand side of the heart, and unconnected to the AVN
- As there is no AVN delay, ‘pre-excitation’ occurs with a short PR interval and the QRS shows an early, alurred upstroke called a ‘delta wave’
- The second part of QRS is normal as the bundle of His conduction catches up
- The ECG rhythm is sinus, but there is right axis deviation, short PR interval and widened QRS with ‘delta wave’ of pre-excitation
- The obnly clinical significance of this is that it can cause paroxysmal supraventricular tachycardia
