Electrocardiogram Interpretation - The Basics

Electrocardiogram interpretation

The Basics

• The electrocardiogram (ECG) is normally recorded so that a deflection of 10mm = 1mV. 
• The recording rate is 25mm/sec, 1mm = 0.04sec, 
• 1 large square = 0.2sec. 

Rate Calculate the rate by dividing 300 by the number of large squares in one R–R interval.

Frontal plane axis Normally lies between –30 * and + 90 *. With a normal axis, QRS complexes in I and II are both + ve. An axis more –ve than –30 * (I + ve, aVF and II –ve) is left axis deviation (causes: left anterior hemiblock, inferior myocardial infarction (MI), ventricular tachycardia (VT), Wolf Parkinson White (WPW) syndrome). An axis more + ve than + 90 * (I –ve, aVF + ve) is right axis deviation (causes: pulmonary embolism (PE), cor pulmonale, lateral MI, left posterior hemiblock).

P wave Normally <0.12sec wide and <2.5mm tall. They are best seen in leads II and V 1 which are chosen for rhythm strips or monitoring.

• A tall peaked P wave in II may reflect right atrial hypertrophy; 
• a widened bifid P wave left atrial hypertrophy. 
• P waves are absent in atrial fibrillation (AF).

PR interval Normally 0.12–0.2sec (<5 small squares).

• A short PR interval (abnormally fast conduction between atria and ventricles) implies an accessory pathway (eg Wolf Parkinson White syndrome).
• A prolonged PR interval occurs in heart block (first, second or third degree.

QRS width Normally 0.05–0.11sec (<3 small squares).
Prolonged QRS complexes may be due to: right bundle branch block (RBBB) (RsR' or M shape in V 1 ), left bundle branch block (LBBB) (QS or W shape in V 1 with RsR' or M shape in V 6 ), tricyclic antidepressant poisoning, hypothermia, ventricular rhythms, and ectopics.

QRS amplitude The QRS amplitude can indicate left ventricular hypertrophy (LVH). Signs of LVH are: (S in V 2 + R in V 5 ) > 35mm; R in I > 15mm; R in aV L > 11mm.

Q waves May be normal in III, aV R , and V 1 , but are abnormal in other leads if > 0.04sec or > ½ of the height of the subsequent R wave.

ST segment elevation is caused by:

• acute MI, pericarditis (concave up),
• ventricular aneurysm, 
• Prinzmetal’s angina, 
• LVH, 
• Brugada syndrome,
• hypertrophic cardiomyopathy, 
• benign early repolarization.

ST segment depression is caused by:

• ischaemia, 
• digoxin, 
• LVH with strain.

QT interval = start of Q wave to end of T wave.
QT c = QT⁄√R–R (Bazett’s formula). Normal QT c is <440msec.
At rates of 60–100/min, QT should be <1/2 R–R interval.
A prolonged QT c predisposes to ‘torsades de pointes’ ( Broad complex tachyarrhythmias, ) and is caused by acute MI, hypothermia, hypocalcaemia, drugs (quinidine, tricyclic antidepressants), certain
congenital diseases (eg Romano–Ward syndrome).

T waves Abnormal if inverted in V 4–6 .

• Peaked T waves are seen in early acute MI and hyperkalaemia . 
• Flattened T waves (sometimes with prominent U waves) occur in hypokalaemia.