A normal adult 12-lead ECG

Normal adult 12-lead ECG
The diagnosis of the normal electrocardiogram is made by excluding any recognised abnormality. It's description is therefore quite lengthy.

normal sinus rhythm
each P wave is followed by a QRS
P waves normal for the subject
P wave rate 60 - 100 bpm with <10% variation
rate <60 = sinus bradycardia
rate >100 = sinus tachycardia
variation >10% = sinus arrhythmia

normal QRS axis

normal P waves
height < 2.5 mm in lead II
width < 0.11 s in lead II
for abnormal P waves see right atrial hypertrophy, left atrial hypertrophy, atrial premature beat, hyperkalaemia

normal PR interval
0.12 to 0.20 s (3 - 5 small squares)
for short PR segment consider Wolff-Parkinson-White syndrome or Lown-Ganong-Levine syndrome (other causes - Duchenne muscular dystrophy, type II glycogen storage disease (Pompe's), HOCM)
for long PR interval see first degree heart block and 'trifasicular' block

normal QRS complex
< 0.12 s duration (3 small squares)
for abnormally wide QRS consider right or left bundle branch block, ventricular rhythm, hyperkalaemia, etc.
no pathological Q waves
no evidence of left or right ventricular hypertrophy

normal QT interval
Calculate the corrected QT interval (QTc) by dividing the QT interval by the square root of the preceeding R - R interval. Normal = 0.42 s.
Causes of long QT interval
myocardial infarction, myocarditis, diffuse myocardial disease
hypocalcaemia, hypothyrodism
subarachnoid haemorrhage, intracerebral haemorrhage
drugs (e.g. sotalol, amiodarone)
Romano Ward syndrome (autosomal dominant)
Jervill + Lange Nielson syndrome (autosomal recessive) associated with sensorineural deafness

normal ST segment
no elevation or depression
causes of elevation include acute MI (e.g. anterior, inferior), left bundle branch block, normal variants (e.g. athletic heart, Edeiken pattern, high-take off), acute pericarditis
causes of depression include myocardial ischaemia, digoxin effect, ventricular hypertrophy, acute posterior MI, pulmonary embolus, left bundle branch block

normal T wave
causes of tall T waves include hyperkalaemia, hyperacute myocardial infarction and left bundle branch block
causes of small, flattened or inverted T waves are numerous and include ischaemia, age, race, hyperventilation, anxiety, drinking iced water, LVH, drugs (e.g. digoxin), pericarditis, PE, intraventricular conduction delay (e.g. RBBB)and electrolyte disturbance.

normal U wave

Reference ECG Library