Approach to ECG

Confirm details

• Name
• Age
• ECG date

Rate

• 25mm/s standard (0.04s per small square, 0.2s per big square)
• 300 divide by number of big squares between each R-R interval

Rhythm

• Intervals should be equal (card method)
• Different rates either 100% irregular (AF) or multiples (varying block)
• Sinus rhythm  is P wave followed by QRS 

Axis

• Normal -30 to +90 (Complexes in leads I and II are usually both normal)
• Left axis deviation -30 to -90
• Left anterior hemiblock
• Inferior MI
• VT from LV focus
• Some WPW syndromes
• Right axis deviation +90 to +180
• RVH
• PE
• Anterolateral MI
• Left posterior hemiblock
• Some WPW syndromes

P wave

• Normally one before QRS complex
• Absent
• AF
• Sinoatrial block
• Junctional (AV nodal) rhythm
• Dissociated from QRS
• Complete heart block
• Bifid
• P mitrale (left atrial hypertrophy)
• Peaked
• P pulmonale (right atrial hypertrophy)
• Pseudo P pulmonale (K+ lowered) 

PR interval 

• Start of P to start of QRS
• Normal 0.12s to 0.2s (3-5 small squares)
• Prolonged
• Delayed AV conduction (1st degree heart block)
• Shortened
• Unusually fast AV conduction down an accessory pathway (e.g. WPW)

QRS complex

• Normal less than 0.12s
• > or = 0.12 suggests ventricular conduction defects (e.g. bundle branch block) 
• Large complexes (vertical) suggest ventricular hypertrophy
• Q wave
• <0.04s wide and <2mm deep (although V5, V6, AVL and I would be normal)
• Pathological seen a few hours after acute MI

QT interval

• Start of QRS to end of T
• Calculate QTc because it varies with rate
• QTc = QT / square root of RR = 0.38s to 0.42s
• Prolonged
• Acute MI
• Myocarditis
• Bradycardia (e.g. AV block)
• Head injury
• Hypothermia
• U&E imbalance (decreased K+, Ca2+ and/or Mg2+)
• Congenital (Romano-Ward and Jervell-Lange-Nielson syndromes)
• Drugs
• Sotalol
• Quinidine
• Antihistamines
• Macrolides (e.g. erythromycin)
• Amiodarone
• Phenothiazines
• Tricyclics

ST segment

• Usually isoelectric
• >1mm elevation = Infarction
• >0.5mm depression = Ischaemia

T wave

• Normally inverted in AVR, V1 and occasionally V2
• Abnormal if in I,II,V4,V5,V6
• Peaked in hyperkalaemia
• Flattened in hypokalaemia

J wave

• Abnormal peak right after S wave
• Hypothermia, subarachnoid haemorrhage, hypercalcaemia