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All things ECG
Basics - ECG Leads
The standard EKG has 12 leads:
3 Standard Limb Leads
3 Augmented Limb Leads
6 Precordial Leads
The axis of a particular lead represents the viewpoint from
which it looks at the heart.
Basics – precordial leads
Precordial leads
Standard Limb Leads
Lead I:
• Left arm to right arm
Lead II:
• Left leg to right arm
Lead III:
• Left leg to left arm
Waveforms
What do they mean?
 P Wave.
 Caused by depolarization of the atria.
 With normal sinus rhythm, the P wave is upright in leads I, II, aVF,
V4, V5, and V6 and inverted in aVR.
 QRS Complex.
 Represents ventricular depolarization
 Q Wave.
 The first negative deflection of the QRS complex (not always
present and, if present, may be pathologic).
 To be significant, the Q wave should be > 25% of the QRS
complex.
 T Wave.
 Caused by repolarization of the ventricles and follows the QRS
complex
 Normally upright in leads I, II, V3, V4, V5, and V6 and inverted in
aVR
ECG paper
How to read an ECG systematically
1.
2.
3.
4.
Rate
Rhythm
Axis
Interval
1. Rate- Rule of 300
Take the number of “big boxes” between neighboring
QRS complexes, and divide this into 300. The result will
be approximately equal to the rate
Although fast, this method only works for regular
rhythms.
Abnormalities




Bradycardia:
Heart rate < 50 beats/min
Tachycardia:
Heart rate > 100 beats/min
What is the heart rate?
www.uptodate.com
(300 / 6) = 50 bpm
What is the heart rate?
(300 / 1.5) = 200 bpm
The Rule of 300
It may be easiest to memorize the following table:
# of big boxes
Rate
1
300
2
150
3
100
4
75
5
60
6
50
2. Rhythm
 Sinus Rhythms
 Normal:
 Each QRS preceded by a P wave (which is positive in II
and negative in aVR) with a regular PR and RR interval
and a rate between 60 and 100 beats/min
http://www.bem.fi/book/19/19.htm
Lead II : sinus arrhythmia in a healthy 26 yearold woman. Note the marked variation in the
P-P intervals induced by respiration.
Courtesy of Morton Arnsdorf, MD.
http://www.uptodate.com/contents/image?imageKey=CARD/2987
Normal rhythm strip in lead II. The PR interval
is 0.15 sec and the QRS duration is 0.08 sec.
Both the P and T waves are upright.
Courtesy of Morton Arnsdorf, MD.
http://www.uptodate.com/contents/image?imageKey=CARD/2987
3. The QRS Axis
The QRS axis represents the net overall direction of the
heart’s electrical activity.
Abnormalities of axis can hint at:
Ventricular enlargement
Conduction blocks (i.e. hemiblocks)
The QRS Axis
By near-consensus, the normal
QRS axis is defined as ranging
from -30° to +90°.
-30° to -90° is referred to as a
left axis deviation (LAD)
+90° to +180° is referred to as a
right axis deviation (RAD)
Determining the Axis
Predominantly
Positive
Predominantly
Negative
Equiphasic
The Quadrant Approach
1. Examine the QRS complex in leads I and aVF to determine if
they are predominantly positive or predominantly negative.
The combination should place the axis into one of the 4
quadrants below.
The Quadrant Approach
2. In the event that LAD is present, examine lead II to determine if
this deviation is pathologic. If the QRS in II is predominantly
positive, the LAD is non-pathologic (in other words, the axis is
normal). If it is predominantly negative, it is pathologic.
Quadrant Approach: Example 1
The Alan E. Lindsay ECG
Learning Center
http://medstat.med.utah.
edu/kw/ecg/
Negative in I, positive in aVF  RAD
Quadrant Approach: Example 2
The Alan E. Lindsay ECG
Learning Center
http://medstat.med.utah.
edu/kw/ecg/
Positive in I, negative in aVF 
Predominantly positive in II 
Normal Axis (non-pathologic LAD)
4. Intervals
 PR <0.2 sec, QRS <0.12 sec, QT <0.4 sec
 P wave




Always positive in leads I and II
Always negative in lead aVR
<3 small squares in duration
<2.5 small squares in amplitude
 QRS
 A normal QRS width should be less than 0.12 s.
 ST segment
 Normally isoelectric
What are these?
ST
elevation
T waves
 Normal T wave is asymmetrical
 First half having more gradual slope than the second
half
 Amplitude rarely exceeds 10 mm
 Abnormal T waves are symmetrical, tall, peaked,
biphasic or inverted
 As a rule T waves follows direction of main QRS
deflection
 Normal T wave is always negative in lead aVR and
positive in lead II
QT interval
http://www.slideshare.net/cksheng74/tutorial-in-basic-ecg-for-medical-students
 QT interval decreases when HR increases
 Should not be more than half of the interval between
adjacent R waves (R-R interval)
Acute anterior wall ST elevation MI (STEMI)
Note the marked ST elevations and
hyperacute T waves in the anterior/lateral
leads, including V2-V5, I and aVL.
Acute transmural anterior wall myocardial
infarction
Chronic anterior wall myocardial infarction
A chronic anterior wall infarction is
diagnosed by the presence of initial deep and
broad Q waves in any of the precordial leads;
in this case they are present in leads V1 to V4.
 http://www.uptodate.com/contents/ecg-tutorialbasic-principles-of-ecganalysis?source=related_link#H18
 ECG made easy page 138 (7th edn.) has list of all
problems and what to expect on an ECG