ELECTROCARDIOGRAM
Indications for Ordering an Electrocardiogram:
1. To determine cardiac rate
2. To accurately define cardiac rhythm
3. To diagnose old or new myocardial infarction
4. To identify intracardiac conduction disturbances
5. To aid in the diagnosis of ischemic heart disease, pericardittis, myocarditis,
electrolyte abnormalities, and pacemaker malfunction
Position of Chest Leads
V1 4th ICS at the right sternal border
V2 4th ICS at the left sternal border
V3 Halfway between V2 and V4
V4 5th ICS at the left midclavicular line
V5 5th ICS at the left anterior axillary line
V6 5th ICS at the left midclavicular line
V3R Halfway between V1 and V4R
V4R 5th ICS at the right midclavicular line
Six components of ECG Interpretation
Mnemonics: RRAHIM
1. Rate
2. Rhythm
3. Axis
4. Hypertrophy
5. Ischemia and Infarction
6. Miscellaneous findings
RATE
Rate Interpretation
Three possibilities only:
1. Bradycardia (<60 beats per minute)
2. Normal rate (60-100 beats per minute)
3. tachycardia (>100 beats per minute)
Rate Analysis
Mnemonics: memorize “300, 150, 100,75,60,50)
Heart rate Number of big squares bet RR interval Interpretation
300 1 tachycardia
150 2 tachycardia
100 3 normal rate (maximum)
75 4 normal rate
60 5 normal rate (minimum)
50 6 bradycardia
Shortcut
If R to R interval >5 big squares: Bradycardia
If R to R interval between 3-5 big squares: Normal rate
If R to R interval <3 big squares: Tachycardia
Formula:
Heart rate= 1500 or 300
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# of small boxes # of big boxes
RHYTHM
A. Common Rhythm interpretations:
1. Sinus Rhythm
2. Common Supraventricular Arrhythmias:
a.Atrial Fibrillation
b. Atrial Flutter
c.Supraventricular Tachycardia
3. Heart Blocks
a.First degree AV block
b. Second Degree AV block Mobitz type I (Wenkeback)
c.Second degree AV block Mobitz type II
d. Third degree AV block
e. Left and Right Bundle Branch Block (complete and incomplete)
4. Ventricular Arrhythmias
a.Premature Ventricular Contractions
b. Ventricular Tachycardia (sustained and unsustained)
c.Ventricular Fibrillation
RHYTHM ANALYSIS:
1. Identify the P wave
Determine from the configuration if this is a sinus P
2. Check the relation of P wave to QRS
a. P wave is before QRS (normal)
b. P wave is buried or after QRS (e.g SVT, complete heart block)
3. Check PR interval (Normal PR interval: 0.12 – 0.20 sec)
a. Short PR (WPW syndrome)
b. Normal PR
c. Prolonged PR (1st and 2nd degree AV block)
4. Check QRS duration (Normal QRS duration <0.10 sec)
a. Normal QRS
b. Wide Bundle (Bundle branch blocks)
5. Check the relation of R-R and P-P interval
a. Equal R-R and P-P interval
b. P-P interval shorter than R-R interval (Complete heart block)
c. P-P interval longer that R-R interval (Av disssociation)
AXIS
A. Axis Interpretation:
Has four possibilities only:
1. Normal Axis
2. Left Axis Deviation (LAD)
3. Right Axis Deviatio (RAD)
4. Indeterminate Axis
Getting the Axis Deviation:
(+) QRS deflection: Average QRS vector above the baseline in Leads I or AVF
(- ) QRS deflection: Average QRS vector below the baseline in Leads I or AVF
Lead I Lead AVF
Normal Axis + +
Left Axis Deviation + -
Right Axis Deviation - +
Indeterminate Axis - -
DIFFERENTIAL DIAGNOSIS FOR LAD AND RAD
QRS Left Axis Deviation QRS Right Axis Deviation
Normal Variant (short, Fat Normal Variant (thin, tall
individuals) individuals)
Left Ventricular Hypertrophy
(e.g. due to hypertension) Right Ventricular Hypertrophy
Inferior Wall Infarction (e.g. due to COPD, Cor
Left Bundle Branch Block pulmonale)
Left Anterior Fascicular Block Lateral Wall Infarction
WPW Syndrome Pulmonary Embolism
Left Posterior Fascicular Block
WPW Syndrome
HYPERTROPHY
A. Hypertrophy Intepretation:
1. No Hypertension
2. Left Ventricular Hypertrophy (LVH)
3. Right Ventricular Hypertrophy (RVH)
4. Left Atrial Enlargement (LAE)
5. Righy Atrial Enlargement (RAE)
6. Combination of the above
B. Hypertrrophy Analysis:
Three Left Ventricular hypertrophy (LVH) ECG criteria:
These criteria are distorted by the presence of complete LBBB but not by complete
RBBB.
1. S wave in V1 + R in V5 or V6 > 35 mm (commonly used)
(Sensitivity = 43%, Specificity = 97%)
2. R in AVL >11 mm
(Sensitivity = 11%, Specificity = 100%)
3. Romhit and Estes Critera (Best Criteria)
(Sensitivity = 50%, Specificity =95%)
Total score: Possible LVH = 3 points
Probable LVH = 4 points
Definite LVH > 5 points
a. Amptitude: (any of the following) 3 points
largest R or S wave inn the limb leads > 20 mm
S wave in V1 or V2 > 30 mm
R wave in V5 or V6 > 30 mm
b. ST-T segment changes typical of LV strain pattern
without digitalis 3
points
with digitalis 1 point
c. LAE: terminal nnegativity of the P wave in V1 is 3
points
1 mm or more in depth with a duration of 0.04 seconds more
d. LAD: 30 degrees or more 2 points
e. QRS duration > 0.09 seconds (but <0.12 sec) 1 point
f. Intrinsicoid deflection in V5 and V6 > 0.05 sec 1 point
Right Ventricular Hypertrophy (RVH) ECG Criteria:
1. Right axis deviation of + 110 degrees or more, with any of the following:
2. Lead V1: R wave > S wave
3. Deep S wave in leads V5 and V6
4. ST depression and T-wave inversion in V1-V3
*Differential Diagnosis of ”RVH” ECG Pattern*
1. Tall R wave in lead V1 (R wave > S wave)
Normal variant (counter-clockwise rotation)
Normal in young adults and children
Right ventricular hypertrophy including displacement of the heart due to
pulmonary disease, COPD
Right bundle branch block
True posterior infarction
WPW syndrome
2. Deep S wave in leads V5 and V6
(R/S ratio less than 1 in leads V5 and V6)
Right ventricular hypertrophy esp. due to COPD
Left Atrial Enlargement (LAE) ECG Criteria: (p mitrale)
Any of the following:
1. In lead V1: Wide terminal component of P wave which is > 1 mm wide (0.04 sec)
and > 1 mm deep.
2. In any lead: P wave wider than 0.12 sec (> 3 small squares) or with a > 1 mm
notched in the middle.
Right Atrial Enlargement (RAE) ECG Criteria: (p pulmonale)
Any of the following:
1. In lead V1: Tall initial component of P wave which is > 2 mm wide (0.08 sec) and
> 2 mm tall.
2. In any lead: P wave > 2.5 mm tall
Biventricular Hypertrophy Diagnostic ECG Criteria:
Any of the following:
1. The ECG meets one or more of the diagnostic criteria for isolated left and right
ventricular hypertrophy.
2. The precordial leads show signs of left ventricular hypertrophy, but the QRS axis
in the frontal plane is greater than +90 degrees (RAD).
Biatrial Enlargement Diagnostic ECG Criteria:
Any of the following:
1. In lead V1, the presence of a large diphasic P wave with the initial positive
component > 2 mm tall (RAE) and the terminal negative component > 1 mm
deep and > 0.04 second in duration (LAE).
2. In any lead, an increase in both the amplitude which is 2.5 mm or greater (RAE)
and duration of 0.12 second or more of the P wave (LAE).
ISCHEMIA AND INFARCTION
A. Ischemia and Infarction Interpretation
Have four possibilities only:
1. Within Normal Limits (WNL): no ischemia or infarction present by ECG
Note 50% of patients with CAD and chronic stable angina have a normal ECG
2. None specific ST-T wave changes (NSSTTWC)
3. Myocardial ischemia changes:
a. identify which myocardial area is ischemic
4. Myocardial Infarction changes:
a. Identify which myocardial area is infarcted
b. Determine the timing of infarction
B. Ischemia and Infarction Analysis
Correspondence of Specific ECG leads and Left Ventricular Myocardial Area
Leads Involved Corresponding LV Areas or “Names”
II, III & AVF Inferior wall
I & AVL High lateral wall
V1, V2 Septal wall
V3, V4 Anterior wall
V5, V6 Lateral wall
V1-V3 Anteroseptal wall
V3-V6, I, AVL Anterolateral wall
V5, V6, II, III, & AVF Inferolateral wall
Almost all leads Diffuse/ global/ massive
Mirror image of V1, V2* Posterior LV wall
V3R & V4R RV wall
* A sudden increase in R forces in leads V1 and V2 suggests posterior wall MI.
ECG Findings in Myocardial Ischemia:
Diagnostic Criteria:
1. At least 1 mm ST-segment depression
2. Symmetrical or deeply inverted T waves
3. Abnormally tall T waves
4. Normalization of abnormal T waves
5. Prolongation of the QT interval in addition to the above
6. Others: Arrhythmias, bundle branch blocks or AV blocks, electrical patterns
Note: Criteria 1 and 2 are more specific
Differential Diagnosis of ST Depression
(Aside From Myocardial Ischemia)
Digitalis Effect
Hypokalemia
Left ventricular hypertrophy (V5-V6)
Right ventricular hypertrophy (V1-V2)
Left bundle branch block
Right bundle branch block
Subendocardial (non-Q) myocardial infarction
Differential Diagnosis of Peaked T waves
Myocardial Ischemia
Hyperacute myocardial infarction
Hyperkalemia
Normal variant in young athletes
ECG Criteria for Myocardial Infarction:
Diagnostic Criteria: (any of the following)
1. ST elevation > 2 mm in 2 or more chest leads or > 1 mm in 2 or more limb leads
2. Q waves > 0.04 sec (1 small square)
Seven Useful Rules Concerning Q waves:
1. Q waves in lead AVR are never significant.
2. Q waves in lead V1 are not significant unless with abnormalities in other
precordial leads.
3. Q waves in lead III are not significant unless with abnormalities in leads II and
AVF.
4. Q waves associated with ST changes are more reliable than without ST changes.
5. Q waves in the presence of LBBB* are not significant if located in leads V1 to V3
6. Q waves in RBBB are significant.
7. The most significant criteria for pathologic Q waves are:
a. > 0.04 seconds duration
b. > 25% of the R wave amplitude
MI in the Presence of Bundle Branch Block:
1. RBBB- usual MI criteria
2. LBBB- diminishing R wave forces in the precordial leads (reverse R wave
progression) or Q waves at V5 and V6.
Differential Diagnosis of ST Elevation
(Aside From Acute Myocardial Infarction).
Acute Pericarditis
Ventricular Aneurysm
Severe LV wall hypokinesia
Early repolarization changes
Variant (prinzmental) angina
Patterns that may Mimic Myocardial Infarction.
Complete left bundle branch block
Early repolarization pattern
Left ventricular aneurysm
Hyperkalemia
Pericarditis
Intracranial hemorrhage
Idiopathic hyertrophic subaortic stenosis
Wolff-Parkinson White syndrome
Ventricular tachycardia
Electronic pacing of right ventricle
Pulmonary disorders (emphysema, pneumothorax, cor
pulmonale)
Timing of Myocardial Infarction
Interpretation Mnemonic Q ST T wave Approx. Timing of
wave Elevation MI
Hyperacute MI (-) (-/+) Peaked 0-6 hours
Acute MI (-/+) (++) (-/+) 6-24 hours
Recent MI (++) (++) Inverted 24-72 hours
Undetermined age of MI (++) (-) Inverted 72 hours-6 weeks
Old MI (++) (-) Upright >6 weeks
Miscellaneous ECG Findings
A. List of 17 miscellaneous ECG Findings:
1. Hypokalemia
2. Hyperkalemia
3. Hypocalcemia
4. Hypercalcemia
5. Digitalis Effect
6. Digitalis Toxicity
7. Electrical alternans of QRS complexes
8. Poor R wave progression
9. Persistent S wave at V5-V6
10. Early repolarization changes
11. Juvenile T wave inversion
12. Low voltage QRS complexes
13. High voltage QRS complexes
14. Cerebral T waves of intracranial hemorrhage
15. Wrong lead placement
16. Wrong speed
17. Artifacts or Noise
B. Short Diagnostic Criteria to the Miscellaneous ECG Findings:
1. Hypokalemia: U wave as tall or taller than the T wave that leads V2 and V3.
Normal serum potassium: 3.6-5.5 mEq/L
General Correlation of ECG Hypokalemic
Changes and Serum Potassium Levels
Serum potassium, 3.0-3.5 mEq/L
ECG may be normal
Prominent U wave at V2 and V3 (may be as tall as the T
waves)
Serum potassium, 2.7-3.0 mEq/L
U waves become taller than the T waves in Leads V2 and V3
Serum potassium, < 2.6 mEq/L
Almost always accompanied by ECG changes
ST segment depression associated with tall U waves
Fusion of T and U waves
2. Hyperkalemia: In the chest leads, height of T wave > 10 mm in most leads.
In limb leads, height of T wave > 5 mm in most leads
General Correlation of ECG Hyperkalemic Changes and
Serum Potassium Levels
Serum potassium, 5.5 to 6.6 mEq/L
In Chest leads: Height of T wave > 10 mm in most leads
In Limb leads: Height of T wave > 5 mm in most leads
Serum potassium, 6.7 to 8.0 mEq/L
QRS widening
Slurring of both initial and terminal portions of the QRS
ST segment elevation
Low, wide P waves
Various arrhythmias: First and second degree atrioventricular block,
atrial arrest, bradycardia
Serum potassium, > 8.0 mEq/L
Marked widening of QRS complex
Distinct ST-T wave may not be noted
High risk for Ventricular fibrillation or asystole
3. Hypercalcemia: Prolonged QT interval, i.e. longer than half of the RR interval by
eyeballing
Measurement of the Normal QT interval
Short Cut: (by eyeballing)
Normal QT is less than half the RR interval.
Basset’s Formula
QTc = QTa _
√RR interval in sec
QTc: Corrected QT interval
QTa: Actual QT interval
Normal value for QTc = 0.35-0.44 sec
4. Hypercalcemia: Shortened QT interval.
5. Digitalis effect: (1) Prolonged PR interval (2) scooping of the ST segment, and (3)
short QT interval.
6. Digitalis toxicity: All types of arrhythmias, usually PVC’s or paroxysmal atrial
tachycardia.
7. QRS electrical alternans: Height of QRS varies from beat to beat
Differential Diagnosis of QRS Electrical Alternans
Cardiac tamponade
Large pericardial effusion
Low cardiac output
Chronic obstructive pulmonary disease
Tension pneumothorax
8. Poor R wave progression: Height of the R wave in V3 is less than 3 mm.
Differential Diagnosis of Poor R Wave Progression
Old antero-septal wall MI
Left ventricular hypertrophy
Normal variant: Heart rotated clockwise
Left bundle branch block
9. Persistent S at V5-V6: Prominent S waves in V5 and V6.
Differential diagnosis: Right ventricular hypertrophy, heart rotated clockwise.
10. Early repolarization changes: Normal variant in young males; St segment
elevation of 2-3 mm in leads V2-V4, usually found in males < 40 years old
Differential diagnosis: Acute anteroseptal wall Mi, acute pericarditis.
11. Juvenile T wave inversion: Normal variant in young females; T wave inversion in
V1-V3 in females < 30 years old
Differential diagnosis: Anteroseptal wall ischemia.
12. Low voltage QRS complexes: The amplitude of the entire QRS complex in all the
limb leads is < 5 mm.
Differential Diagnosis of Low
Voltage QRS complexes.
Normal elderly patients
Obese or edematous patient
Cardiac tamponade
Large pericardial effusion
Pneumothorax
Hypothyroidism
Dilated cardiomyopathy
13. High voltage QRS complexes: Large QRS complexes.
Differential diagnosis : Young patient age > 35, left ventricular hypertrophy, right
ventricular hypertrophy, left bundle branch block, right bundle branch block.
14. Cerebral T waves (of intracranial hemorrhage): Wide, prominent and deeply
inverted T waves with a long QT interval.
Differential diagnosis: Subendocardial myocardial infarction, myocardial
ischemia.
15. Wrong lead placement: (1) An upright P wave in lead AVR accompanied by (2) a
normal R wave progression in the precordial leads (to differentiate this from
dextrocardia)
16. Wrong speed: (1) ECG shows widened PR and QRS intervals, and (2) patient’s
heart does not tally with ECG heart rate. ECG speed used is 50 mm/sec instead of
25 mm/sec.
17. Artifacts or noise: Irregular spikes or undulations on the ECG baseline are not
found in other segments. This is usually due to patient’s movement, shivering or
poor contact between ECG electrodes and the patient’s skin.
