12 lead ECG Essentials
 
An ECG records the heart’s electrical activity over time using electrodes placed on the skin. 

It provides information about: 
 
Heart rate 
Heart rhythm 
Conduction abnormalities 
Chamber enlargement 
Evidence of cardiac ischaemia or infarction 
 
Standard ECG setup 
 
12 leads: 6 limb leads (I, II, III, aVR, aVL, aVF) + 6 chest leads (V1–V6) 
Paper speed: 25 mm/second 
1 small box on standard ECG paper at this speed = 0.04 sec, 1 large box = 0.2 sec 
 
 
 
Normal Sinus Rhythm (example): 
 
 
 
Stepwise ECG Interpretation 
 
Rate – to assess the rate 
If Regular rhythm: 300 divided by the number of large boxes between R waves (in the example above this would be 300/4 = 75 bpm (heartrate). 
If Irregular rhythm: Count R waves in 10 seconds (30 large squares from the first R wave (this represents 10 seconds at a speed of 25mm/second) × 6 (note: a rhythm strip may be required to calculate this accurately) 
 
Rhythm – to assess the rhythm: 
Is it Regular or irregular? 
Is there a P wave before every QRS? 
Is the PR interval consistent? 
 
P waves (Atrial depolarisation) 
Normal: upright in II, inverted in aVR 
Absent → consider AFib or junctional rhythm 
Sawtooth pattern → Atrial flutter 
 
PR interval (atrial to ventricular delay) 
Measure the start of the P wave to the start of the QRS complex 
Normal: 0.12–0.20 sec (3 to 5 small squares) 
Prolonged → 1° AV block 
Variable → Mobitz I or II 
 
QRS complex (ventricular depolarisation)  
Normal: <0.12 sec (less than 3 small squares) 
Wide → bundle branch block, ventricular rhythm, or WPW (wolff parkinson white syndrome) 
 
ST segment (early ventricular repolarisation) and T wave (ventricular repolarisation) 
ST elevation/depression → ischaemia/infarction 
T wave inversion → ischemia or strain 
Peaked T → indicates hyperkalaemia (check bloods urgently) 
 
QT interval (beginning to the end of ventricular contraction and relaxation) 
Correct for rate: QTc (c- corrected interval) = QT (measure from start of Q(RS) to start of T wave / √RR (square root of R-R interval) 
QTc less than 440 ms (men) or less than 460 ms (women).   
Risk of torsades de pointes (polymorphic VT) if greater than 500 ms 
 
Common ECG Rhythms and Recognition 
 

Rhythm Strip Tips 
Use Lead II for rhythm recognition (best view of P waves) and gives the view across the heart 
Use V1 for differentiating atrial vs ventricular origin 
Always correlate the ECG with patient symptoms 
 
Practice and Tools 
To master ECG rhythm recognition: 
Practice….
Regularly review rhythm strips and real patient ECGs 
Mnemonics help: 
Rate, Rhythm, Axis, Intervals, ST-T, Morphology