A structured ECG examination reviews rhythm, axis, atrial activity, conduction, ventricular complexes and repolarisation, using standard recording conventions.
Rhythm, axis and P waves
•
Use the lead II rhythm strip to determine heart rate and rhythm.
•
The cardiac axis is normal when QRS complexes are positive in leads I and II.
•
Tall P waves denote right atrial enlargement (P pulmonale), while notched P waves denote left atrial enlargement (P mitrale).
PR interval and QRS complex
•
The normal PR interval is 0.12–0.20 seconds. Prolongation denotes impaired atrioventricular nodal conduction; a short PR interval occurs in Wolff–Parkinson–White syndrome.
•
A QRS duration greater than 0.12 seconds indicates abnormal ventricular conduction, including left or right bundle branch block.
•
Large QRS complexes occur in slim young patients and in patients with left ventricular hypertrophy.
•
Q waves may signify previous myocardial infarction.
ST segment, T waves and QT interval
•
ST elevation may signify myocardial infarction, pericarditis or left ventricular aneurysm. ST depression may signify ischaemia or infarction.
•
T-wave inversion has several causes, including myocardial ischaemia or infarction and electrolyte disturbances.
•
The heart-rate-corrected QT interval is normally less than 0.44 seconds in males and 0.46 seconds in females.
•
QT prolongation may occur with congenital long QT syndrome, low potassium, magnesium or calcium, and some drugs.
Recording conventions and rate calculation
•
Depolarisation towards an electrode produces a positive deflection; depolarisation away produces a negative deflection.
•
Standard sensitivity is 10 mm per 1 mV, and paper speed is 25 mm per second.
•
A large 5 mm square represents 0.2 seconds; a small 1 mm square represents 0.04 seconds.
•
Heart rate equals 1500 divided by the RR interval in millimetres, or 300 divided by the number of large squares between beats.