Analysing an ECG Flashcards
Describe the electrical activity of the heart
The electrical activity of the heart begins at the sinoatrial node. The atria contract.
Slight delay at atrioventricular node, then depolarisation passes down bundle of His, down the right and left bundles, through the Purkinje fibres and through the ventricles.
What does a positive spike on an ECG show?
Depolarisation towards ECG electrode
What do P waves represent?
Atrial depolarisation
When does the PR interval start and end and what is a normal PR interval?
- The PR interval begins at the start of the P wave and ends at the beginning of the Q wave
- PR interval: 120-200ms
Why is there a flat line between P and Q?
With atrial depolarisation complete, the impulse is delayed at the AV node
What does the QRS complex represent?
Ventricular depolarisation, begining at the apex.
Normal QRS time
70-100ms
What does the ST segment represent?
The time between ventricular depolarisation and repolarisation (i.e. ventricular contraction).
What does the T wave represent?
The T wave represents ventricular repolarisation, begining at the apex.
When does the QT interval start and end?
What does the QT interval represent?
What is the normal QTc time?
The QT interval begins at the start of the QRS complex and finishes at the end of the T wave.
It represents the time taken for the ventricles to depolarise and then repolarise.
QTc: <440ms (men) and <460ms (women)
Q, R and S wave definitions
Q wave: first negative wave after a P wave
R wave: first positive wave after P wave
S wave: first negative wave after R wave
What are the steps of interpreting an ECG?
- Rate
- Rhythm
- Axis
- P waves
- PR interval
- QRS complex
- QT interval
- ST-segment
- T waves
How would you work out heart rate on an ECG?
If a patient has a regular heart rhythm:
Count the number of large squares present within one R-R interval
Divide 300 by this number to calculate heart rate
If a patient’s heart rhythm is irregular:
Count the number of complexes on the rhythm strip (each rhythm strip is typically 10 seconds long).
Multiply the number of complexes by 6 (giving you the average number of complexes in 1 minute).
Big box = 0.2 seconds
How would you conclude if heart rhythm is regular or irregular?
Mark out several consecutive R-R intervals on a piece of paper, then move them along the rhythm strip to check if the subsequent intervals are similar.
(Can be irregularly irregular or regularly irregular)
Causes of an irregularly irregular heart rhythm
- AF
- Ectopics
Causes of a regularly irregular heart rhythm
- Ectopics
- Second degree heart block
- Sinus arrhythmia
What does cardiac axis describe and what leads do you need to look at to determine cardiac axis?
Cardiac axis describes the overall direction of depolarisation within the heart.
To determine the cardiac axis you need to look at leads I, II and aVF
Draw the ECG vectors
What is the normal axis of the heart?
-30 to 90 degrees
(Bulk of the heart is left ventricle , so normal axis is towards the apex)
Axis when left axis deviation:
Axis when right axis deviation:
Left axis deviation: -30 to -90 degrees
RIght axis deviation: 90 to 180 degrees
Typical ECG findings for normal cardiac axis
Lead I and II are positive
Typical ECG findings for right axis deviation:
Right axis deviation: 90 to 180 degrees
Negative QRS in I and positive QRS in aVF
OR
Lead III has the most positive deflection and lead I should be negative.
Causes of right axis deviation
- Right ventricular hypertrophy
- Lung disease eg PE, COPD
- Lateral MI
- Left posterior fascicular block
- Wolff-Parkinson-White syndrome
Typical ECG findings for left axis deviation:
Left axis deviation: -30 to -90 degrees
Lead I has the most positive deflection. Leads II and III are negative.
Causes of left axis deviation
- Left ventricular hypertrophy
- Inferior MI
- Left bundle branch block
- Left anterior fascicular block
- Wolff-Parksinson-White syndrome
How would you analyse a P wave?
- Is P wave present?
- Is it associated with a QRS complex?
- PR interval? (3 to 5 small squares)
- P-mitrale or P-pulmonale?
P wave inversion is normal in which leads?
V1 and aVR
(also acceptable in V2, III and aVF)
P waves:
What is P-mitrale? What does it signify?
What is P-pulmonale? What does it signify?
Bifid P wave (left atrial enlargement)
Peaked P wave: >2.5mm (right atrial enlargement)
What would an absent P wave indicate?
What would a sawtooth P wave indicate?
Atrial fibrillation
Atrial flutter
How would you differentiate between atrial fibrillation and artial flutter?
Atrial flutter: regular rhythm, sawtooth P waves
Atrial fibrillation: irregularly irregular rhythm, no P waves
PR interval: causes of abnormalities
Long?
Short?
Depressed?
- Long: heart block (AVN)
- Short: accessory pathways (bypassing AVN)
- Depressed: pericarditis
QRS complex: causes of abnormalities
Widened?
Tachycardia?
Large amplitude?
- Widened: bundle branch block
- Tachycardia:
- Narrow QRS complex: supraventricular
- Broad QRS complex: ventricular
- Large amplitude: ventricular hypertrophy
Causes of long QTc interval
<440ms (men) and <460ms (women)
- Drugs: macrolides, tricyclics, antihistamines, haloperidol, ondansetron
- Inherited: long QT syndrome
- Infarction
- Electrolytes:↓K, ↓Ca, ↓Mg
Causes of ST segment abnormalities
Elevated
Depressed
- Elevated: STEMI, pericarditis
- Depressed: NSTEMI, unstable angina
Causes of T waves abnormalities
Inverted
Peaked
Flat
- Inverted: ischaemia/infarction
- Peaked: hyperkalaemia
- Flat: hypokalaemia
Causes of:
U waves
J waves
Delta waves
- U waves: hypokalaemia
- J waves: hypothermia
- Delta waves: Wolff Parkinson White
Describe first degree heart block
First-degree heart block involves a fixed prolonged PR interval (>200 ms).
Describe second degree AV block (type 1), also known as Mobitz type 1 AV block.
Typical ECG findings in Mobitz type 1 AV block include progressive prolongation of the PR interval until eventually the atrial impulse is not conducted and the QRS complex is dropped.
AV nodal conduction resumes with the next beat and the sequence of progressive PR interval prolongation and the eventual dropping of a QRS complex repeats itself.
Describe second degree AV block (type 2) is also known as Mobitz type 2 AV block.
Typical ECG findings in Mobitz type 2 AV block include a consistent PR interval duration with intermittently dropped QRS complexes due to a failure of conduction.
The intermittent dropping of the QRS complexes typically follows a repeating cycle of every 3rd (3:1 block) or 4th (4:1 block) P wave.
Describe 3rd degree heart block
Third-degree (complete) AV block occurs when there is no electrical communication between the atria and ventricles due to a complete failure of conduction.
Typical ECG findings include the presence of P waves and QRS complexes that have no association with each other, due to the atria and ventricles functioning independently.
In 3rd degree heart block how would you diferentiate between rhythm that origionate above and below the bundle of His?
Narrow-complex escape rhythms (QRS complexes of <0.12 seconds duration) originate above the bifurcation of the bundle of His.
Broad-complex escape rhythms (QRS complexes >0.12 seconds duration) originate from below the bifurcation of the bundle of His.
ST elevation in lead II, III and aVF
What type of infarction would cause these changes?
Which cornonary artery would be affected?
Inferior MI
Right coronary
ST elevation in V1-4
What type of infarction would cause these changes?
Which cornonary artery would be affected?
Anteroseptal
Left anterior descending
ST elevation in V4-6, I, aVL
What type of infarction would cause these changes?
Which cornonary artery would be affected?
Anteriolateral
Left anterior descending or left circumflex
ST elevation in I, aVL +/- V5-V6
What type of infarction would cause these changes?
Which cornonary artery would be affected?
Lateral
Left circumflex
Tall R waves V1-V2
What type of infarction would cause these changes?
Which cornonary artery would be affected?
Posterior
Usually left circumflex, also right coronary artery
