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Writer's pictureMedicine Revision Crash Course

ECG Interpretation

Updated: Jan 7, 2020

Introduction:

  • Confirm the name and date of birth of the patient matches the details that on the ECG strip

  • Confirm the date and time the ECG was performed, and note if any others have been performed recently for this patient (to act as a comparison)


 

1. Heart rate


Bradycardia = <60bpm

Normal = 60-100bpm

Tachycardia = >100bpm


There are a couple of ways to calculate the heart rate:




 

2. Rhythm


Regular - recurrent pattern of P, QRS and T waves at regular intervals


Irregular rhythms can be:

  • Regularly irregular (A recurrent pattern of irregularity - Ectopic beats)

  • Irregularly irregular (Completely disorganised - Atrial fibrillation)


How to check if the rhythm is regular?

  • 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 the same.


 

3. Axis


People often find it difficult to determine if there is any deviation in the axis!


Below is a table that summarises normal axis vs right axis deviation vs left axis deviation.


  • LAD= Leaving

Physiological – Age related, short/stocky

Indicates: LVH, LBBB, LAFB, Inf MI, paced, VEB


  • RAD= Reaching

Physiological – Thin patients or children

Indicates: RVH, RBBB, LPFB, latMI, COPD, PulHTN, PE


 

4. P waves


Duration = Less than 120 ms wide and less than 2.5 mm high


  • Are P-waves present?

  • Is each P-wave followed by a QRS complex?

  • Do the P-waves look normal? (check duration, direction and shape)

 

5. P-R interval


Duration = 120 to 200 ms wide (aka 3-5 small squares)


A prolonged PR interval suggests there is atrioventricular delay (AV block)


  • Long PR

Heart block - First (>0.20ms)

Mobitz-type I heart block= Prolonged >20ms then drop QRS


  • Short PR

Pre-excitation = WPW (wide QRS, delta waves)

AV nodal rhythm


 

6. QRS complex


Duration = 0.12 seconds


Pathological Q wave = Previous MI


A narrow QRS complex occurs when the impulse is conducted down the bundle of His and the Purkinje fibre to the ventricles. This results in well organised synchronised ventricular depolarisation.


A broad QRS complex occurs if there is an abnormal depolarisation sequence - eg: A bundle branch block results in a broad QRS because the impulse gets to one ventricle rapidly down the intrinsic conduction system then has to spread slowly across the myocardium to the other ventricle.



WilliaM - Left bundle branch block. W in V1 and M in V6. May be due to AS, IHD, HTN, MI, HTN

MarroW - Right bundle branch block. M in V1 and W in V6. May be due to RVH, IHD, PE.

 

7. ST segment


  • ST elevation – Acute MI >1mm 2 consistent, or >2mm chest

  • ST depression - MI

  • Saddle shaped - Acute pericarditits



Anterior STEMI - V1-V4

Lateral STEMI – I avL and V5-6

Inferior STEMI - II, III and aVF


 

8. T wave


Should all be positive except for aVR and V1


  • T wave inversion - ischaemia, hypokalaemia

  • Peaked T waves - hyperkalaemia ('Tall tented')

  • Flattened - ischaemia or electrolyte imbalance


T waves are tall if they are:

  • >5mm in the limb leads AND >10mm in the chest leads


 

9. QT interval


  • Men 440ms

  • Women 460ms

  • Increased HR = decreased QT

  • Prolonged- Risk of torsade de pointe

  • Acquired- amiodarone, antipsychotics/ Ads, electrolytes

  • Congenital – Romano Ward


 

10. U wave


  • After T, same direction

  • Seen more in a patient with bradycardia

Prominent U waves: >1-2mm, same as T

Bradycardia

Hypokalaemia


Inverted U waves:

Cardiac disease + chest pain= MI


 

Time to practice:



1.

1. Atrial Fibrillation

  • Irregularly irregular rhythm

  • No P waves

  • Absence of an isoelectric baseline

  • Variable ventricular rate



2.

2. Atrial flutter

  • Narrow complex tachycardia

  • Regular atrial activity at ~300 bpm

  • Flutter waves (“saw-tooth” pattern) best seen in leads II, III, aVF

  • Loss of the isoelectric baseline



3.

3. Hyperkalaemia

  • Prolonged PR interval

  • Broad, bizarre QRS complexes — these merge with both the preceding P wave and subsequent T wave

  • Peaked T waves ('Tall tented')



4.

4. Anterior STEMI

  • ST elevation is maximal in the anteroseptal leads (V1-4)

  • Q waves are present in the septal leads (V1-2)

  • There are hyperacute (peaked) T waves in V2-4

  • These features indicate a hyperacute anteroseptal STEMI



5.

5. 1st degree heart block

  • Sinus bradycardia with 1st degree AV block

  • PR interval > 300 ms



6.

6. Inferior STEMI

  • Marked ST elevation in II, III and aVF with early Q-wave formation

  • Reciprocal changes in aVL

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