2. • The 12 conventional ECG leads record the difference in potential
between electrodes placed on the surface of the body.
• These leads are divided into two groups:
• Six limb (extremity) leads and six chest (precordial) leads.
• The limb leads record potentials transmitted onto the frontal plane,
and the chest leads record potentials transmitted onto the horizontal
plane.
3. • The six chest leads are unipolar recordings obtained by electrodes in the
following positions;
• lead V1, fourth intercostal space, just to the right of the sternum;
• lead V2, fourth intercostal space, just to the left of the sternum;
• lead V3, midway between V2 and V4:
• Lead V4, midclavicular line, fifth intercostal space;
• and lead V5, anterior axillary line, same level as V4;
• and lead V6, midaxillary line, same level as V4 and V5.
4.
5.
6. • smallest (1 mm) horizontal divisons correspond to 0.04 (40 ms), with
heavier lines at intervals of 0.20 s (200 ms).
7.
8. READING 12-LEAD ECGS
• The best way to read 12-lead ECGs is to develop a step-by-step approach
(just as we did for analyzing a rhythm strip). In these modules, we present
a seven-step approach:
• Calculate RATE
• Determine RHYTHM
• Determine QRS AXIS
• Check individual WAVES
• Calculate INTERVALS
• Assess for HYPERTROPHY
• Look for evidence of infarction/dyselectrolytemia.
9. Step 1: Determining the Heart Rate
• Rule of 300
Count the number of “big boxes” between two QRS complexes, and
divide this into 300 for regular rhythms.
• Second Rule ECGs record 6 seconds [30 boxes] of rhythm per page
Count the number of beats present on the ECG in 6 seconds Multiply
by 10 This is useful for irregular rhythms
12. Step 2: Determine Regularity
• Look at the R-R distances (using a caliper or markings on a pen or
paper). Regular (are they equidistant apart)? Occasionally irregular?
Regularly irregular? Irregularly irregular?
13. Step 3: Determining the Axis
• Normal QRS axis from −30° to +110°.
• −30° to −90° is referred to as a left axis deviation (LAD).
• +110° to +180° is referred to as a right axis deviation (RAD)
• −180° to −90° is referred as north-west axis/extreme axis/axis in no man’s land
14.
15.
16. Step 4: Check Individual Waves
P WAVE
• Always positive in lead I and II
• Always negative in lead aVR
• <2.5 small squares in duration
• <2.5 small squares in amplitude
• Commonly biphasic in lead V1
17. • Best seen in leads II
• Tall (>2.5 mm), pointed P waves (P pulmonale)—suggests right atrial
enlargement
• Seen in chronic obstructive pulmonary disease (COPD), atrial septal
defect (ASD), TS, Ebstein anomaly (Himalayan P waves)
• Notched/bifid (“M” shaped) P wave (P “mitrale”) in limb leads—
suggests left atrial enlargement Seen in MS, MR, and systemic
hypertension
• Absent P waves—atrial fibrillation/flutter
20. Decreased—low voltage QRS (<5 mV in limb
leads/<10 mV in chest leads)
• Obese patient
• Restrictive cardiomyopathy
• Pericardial effusion
• Hypothyroidism
• Hypothermia
• Myocarditis
21. Q Waves
• The normal Q wave in lead I is due to septal depolarization
• It is small in amplitude—less than 25% of the succeeding R wave, or
less than 3 mm
• Its duration is <0.04 sec or one small box
22. • THE PATHOLOGICAL Q WAVE
• It is deep in amplitude—more than 25% of the succeeding R wave, or
more than 4 mm. Its
• duration is >0.04 sec or >1 small box
• Pathological Q waves may be seen in Infarction, cardiomyopathies—
hypertrophic obstructive cardiomyopathy (HOCM), infiltrative
myocardial disease
24. U Waves
• The U wave is not always seen. It is typically small, and, by definition,
follows the T wave. U waves are thought to represent repolarization
of the papillary muscles or Purkinje fibers
Prominent U waves are most often seen in
• Hypokalemia
• Hypercalcemia
• thyrotoxicosis, or exposure to digitalis, epinephrine
• in intracranial hemorrhage.
25.
26. Assess for Hypertrophy
Right Ventricular Hypertrophy (RVH)
• Criteria of RVH
• Tall R in V1 with R >S, or R/S ratio >1
• Deep S waves in V4, V5, and V6
• Associated right axis deviation, right atrial enlargement (RAE)
27. Cause of RVH
• Long-standing mitral stenosis
• Pulmonary hypertension of any cause
• Ventricular septal defect (VSD) or atrial septal defect (ASD) with initial
L to R shunt
• Congenital heart with RV over load
• tricuspid regurgitation, pulmonary stenosis.
28. Left Ventricular Hypertrophy (LVH)
• Criteria of LVH
• High QRS voltages in limb leads:
• Sokolow and Lyon criteria: S (V1) + R (V5 or V6) >35 mm
• Cornell criteria: S (V3) + R (aVL) >28 mm (men) or >20 mm (women)
• Others: R (aVL) >13 mm.
29. Causes of LVH
• Pressure overload—systemic hypertension and aortic stenosis
• Volume overload—AR or MR-dilated cardiomyopathy
• Ventricular septal defect—cause both right and left ventricular
volume overload
• Hypertrophic cardiomyopathy.
