Understanding Electrocardiography lesson-3

How to read a 12-lead ECG: examination sequence

Rhythm strip (lead II)
To determine heart rate and rhythm
Cardiac axis
Normal if QRS complexes +ve in leads I and II
P-wave shape
Tall P waves denote right atrial enlargement (P pulmonale) and notched P waves denote left atrial enlargement (P mitrale)

PR interval
Normal = 0.12-0.20 secs. Prolongation denotes impaired AV nodal conduction. A short PR interval occurs in Wolff-Parkinson-White syndrome (p. 565)
QRS duration
If > 0.12 secs then ventricular conduction is abnormal (left or right bundle branch block)
QRS amplitude
Large QRS complexes occur in slim young patients and in patients with left ventricular hypertrophy
Q waves
May signify previous myocardial infarction (MI)
ST segment
ST elevation may signify MI, pericarditis or left ventricular aneurysm; ST depression may signify myocardial ischaemia or infarction)
T waves
T-wave inversion has many causes, including myocardial ischaemia or infarction, and electrolyte disturbances
QT interval
Normal < 0.42 secs. QT prolongation may occur with congenital long QT syndrome, low K+, Mg2+ or Ca2+, and some drugs (see Box 18.35, p. 568)
ECG conventions
Depolarisation towards electrode: positive deflection
Depolarisation away from electrode: negative deflection
Sensitivity: 10 mm = 1 mV
Paper speed: 25 mm per second
Each large (5 mm) square = 0.2 s
Each small (1 mm) square = 0.04 s
Heart rate = 1500/RR interval (mm)
(i.e. 300 ÷ number of large squares between beats)

How to report an ECG(Shortly) : 1.Rhythm, 2.Conduction Intervals. 3.Cardiac axis.
4. A description of QRS complex, 5. A description of  the ST segments and T-wave.