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Electrocardiogram (ECG) Interpretation
This guide can be used to read an ECG in a systematic manner
We begin by confirming the patient’s details and introducing the ECG for presentation.
- “This is a XX second 12-lead ECG trace of…”
- Check Name & Age & Date of exam
- Are there presenting complaints written at the top?
Eg. “This is a 10 second 12-lead ECG trace of John Smith, a 60-year-old gentleman, taken 2 hours ago in the Emergency Department. It is noted that he experienced shortness of breath and chest pain at the time of examination”
Rate & Rhythm
Looking at the rate strip (usually lead 2, and located along the bottom of the trace):
- Is the patient in sinus rhythm? – Are there P waves preceding every QRS complex? Are there QRS complexes after each P wave?
- What is the rate? Calculate by 300/number of large squares between each R wave, or the number of complexes over the strip x 6 (if it is a 10s strip).
- Is the rate regular, or irregular (regularly irregular or irregularly irregular)?
- Narrow complex tachycardia – Normal QRS complex duration with an increased heart rate.
- Broad complex – extended QRS complex duration with a reduced heart rate
- Are they in sinus? If so, it is Ventricular based. If not, Atrial based.
- Is it a regular rhythm (so Tachycardia) or irregular / not uniform (so Fibrillation)?
- Generally – if not in sinus, fast rate, and irregular = Atrial Fibrillation.
The ECG axis demonstrates the direction of overall electrical activity within the heart. Normally, overall conduction would travel in downward right motion (from R atrium to L ventricle).
Calculating Axis Deviation using the Quadrant Method and Leads I and aVF
- Look at Leads 1 and aVF. Are the QRS complexes Positive, Equiphasic or Negative?
- Using the quadrant method to determine overall axis deviation.
- Lead 1 – right half is for positive.
- Lead aVF – the bottom half is positive.
- If Lead 1 is positive, then it will point towards the right. If aVF is positive, then it will point downwards.
- The average of these 2 (where they overlap) determine axis. In the above diagram, light blue is Left deviation, Pink is right deviation. Dark blue is normal.
- If Left Axis Deviation – most likely CAD.
- If Right Axis Deviation – most likely chronic lung disease or PE
P Wave & PR Interval
- We have already observed the P wave during the rhythm check,
- P waves correlate to Atrial activity. Absence means loss of atrial activity.
- PR Interval should be between 120-200ms (3-5 small squares)
- A prolonged interval:
- Suggests the presence of atrioventricular delay (AV block)
- A fixed PR interval consistency over 200ms (5 squares) suggests First-Degree Heart Block.
- An increasing PR interval until QRS is dropped suggests Type 1 Second-Degree Heart Block
- A fixed PR interval with an intermittent drop of QRS suggests Type 2 Second-Degree Heart Block.
- Completely random PR intervals (P waves and QRS complexes with no association) is suggestive of Third-Degree Heart Block (complete AV block)
- Shortened PR interval:
- Can be normal (due to SAN closer to the AV node)
- Can indicate impulses are taking a shortcut to the ventricles (will have Delta wave)
- Is the complex Narrow (<0.12s) or Broad (>0.12s)
- Narrow indicates well-organised conduction and synchronised ventricular depolarisation
- Broad indicates abnormal depolarisation.
- Can be a Bundle Branch Block, where depolarisation spreads slowly through ventricular myocardium from one ventricle to the other.
- Are they short or tall?
- Q waves
- Should be <25% of following R wave or <2mm in height and <40ms in duration.
- R and S waves
- Progression. In leas V1, the S wave should be dominant compared to the R wave. This should gradually shift so that at around V3 and V4, the R wave is larger than the S wave.
- Is the R waved “notched”?
- If it’s an M shape in V5/6 – Left Bundle Branch Block
- If it’s a W shape in V1 – Right Bundle Branch Block
- Q waves
- A normal ST segment is an isoelectric line between the end of the S wave and the start of the T wave.
- Is there ST Elevation?
- Greater than 1mm in 2 or more limb leads, or >2mm in 2 or more chest leads
- Commonly caused by full-thickness myocardial infarction.
- Is there ST Depression?
- >0.5mm depression in 2 leads.
- Indicates myocardial ischaemia.
T waves represent repolarisation of the ventricles
- Are the waves tall?
- >5mm in limb leads and >10mm in chest leads
- Can be a sign of Hyperkalaemia or Hyperacute STEMI
- Are the waves Inverted?
- They are normally inverted in V1 and Lead III
- In other leads, is pathogenic.
- Can be a sign of Bundle Branch Block, previous ischaemia, PE, general illness.
- Not normally seen. If so, often >0.5mm deflection after the T wave in V2 or V3.
- Classically U waves are seen in various electrolyte imbalances, hypothermia and secondary to antiarrhythmic therapy
Recap and Summarise
- Don’t go back through the whole ECG, only mention significant changes that you have observed.
- You should also present a diagnosis or differential diagnosis during the recap, if you have formed one.
- Don’t worry if you can’t determine a diagnosis. It is likely that having a diagnosis is only a couple of marks. If you have analysed each wave and picked up on abnormalities, you will have a good standing to pass the OSCE station.