- 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,
- Brugada syndrome,
- hypertrophic cardiomyopathy,
- benign early repolarization.
ST segment depression is caused by:
- 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.