EKG Basics
Jason Ryan, MD, MPH
SA
AV
HIS
Bundle
Bundle
Branches
Purkinje
Fibers
AV Node
HIS Bundle
Bundle Branches
Purkinje Fibers
R
SA
AV
T
P
Q
Atrial
Depolarization
HIS
Bundle
Bundle
Branches
S
Ventricular
Depolarization
Purkinje
Fibers
EKG
EKG Electrical Activity
SA
AV
LBB
His
RBB
Purkinje
Fibers
EKG Electrical Activity
EKG Electrical Activity
AVR
I
V1 V2 V3 V4 V5 V6
AVL
II III AVF
EKG
EKGs
Key Principles
• #1: Waves represent repolarization/depolarization
• #2: EKGs have 12 leads
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Each lead watches the same thing
Each lead watches from different vantage point
Electrical activity toward lead = upward deflection
Electrical activity away from lead = negative deflection
Pacemakers
• SA node is dominant pacemaker of the heart
• Other pacemakers exist but are slower
• If SA node fails, others takeover
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SA node (60-100 bpm)
AV node (40-60 bpm)
HIS (25-40 bpm)
Bundle branches (25-40 bpm)
Purkinje fibers (25–40 bpm)
Conduction Velocities
• SLOWEST conduction is through AV node
• Very important so ventricle has time to fill
• Purkinje fibers  fastest conduction
• Purkinje > Atria > Vent > AV node
Determining Heart Rate
• 3 – 5 big boxes
between QRS
complex
300 150 100 75 60 50
QRS Axis
SA
AV
LBB
His
RBB
Purkinje
Fibers
QRS Axis
-90o
+180o
0o
Normal QRS Axis
-30 and +90 degrees
+90o
QRS Axis
Left Axis Deviation
LBBB
Ventricular Rhythm
-90o
+180o
0o
Normal QRS Axis
-30 and +90 degrees
+90o
QRS Axis
Left Axis Deviation
LBBB
Ventricular Rhythm
-90o
Right Axis Deviation
RBBB
RVH
+180o
0o
Normal QRS Axis
-30 and +90 degrees
+90o
(-)
Determining Axis
-90o
Lead F (+)
LAD
Lead I (-)
(+)
+180o
0o
Normal Axis
-30 to +90
RAD
+90o
Axis Quick Method
• First, glance at aVr.
• It should be negative
• If upright, suspect limb lead reversal
Normal
Axis Quick Method
• If leads I and II are both positive, axis is normal
Lead I
Lead II
Axis 0 to 90°
Axis Quick Method
• For left axis deviation:
• All you need is lead II
Lead I
Axis -30 to -90°
Lead II
Lead II
Axis 0 to -30°
Physiologic
Left Axis
Axis Quick Method
• For right axis deviation:
• All you need is lead I
• Negative = RAD
Lead I
Lead II
Axis 90 to 180°
Axis Quick Method
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Look at aVr: Make sure its negative
Look at I, II: If both positive, axis is normal
If II is negative: LAD
If I is negative: RAD
Normal
Lead I
Lead II
Phys Left
Left
Right
PR Interval
Normal PR
120-200ms
Prolonged PR
1° AV block
Short PR - WPW
QRS Interval
Normal QRS
<120ms
Right Bundle
Branch Block
Left Bundle
Branch Block
Qt Interval
Normal Qt
Short Qt: Hypercalcemia
Prolonged Qt
Hypocalcemia
Drugs
LQTS
Calcium
Myocyte Action Potential
Phase 1
IK+ (out)
Phase 2
ICa+ (in) & IK+ (out)
0mv
Phase 0
INa+ (in)
-85mv
Phase 3
IK+ (out)
Phase 4
Torsade de Pointes
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Feared outcome of Qt prolongation
Results in cardiac arrest
Antiarrhythmic drugs
Hypokalemia, hypomagnesemia
Rarely from hypocalcemia
Congenital Long Qt Syndrome
• Rare genetic disorder
• Abnormal K/Na channels
K
Na
Prolonged QT
Congenital Long Qt Syndrome
• Family history of sudden death (torsades)
• Classic scenario: Young patient recurrent “seizures”
• EKG shows long Qt interval
• Jervell and Lange-Nielsen Syndrome
• Norway and Sweden
• Congenital deafness
Acquired Long Qt Syndrome
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Antiarrhythmic drugs
Levofloxacin (antibiotic)
Haldol (antipsychotic)
Many other drugs
Congenital LQTS: need to avoid these drugs
T waves
Peaked T waves
↑K
Early ischemia
(hyperacute)
U waves
Origin unclear
T
U
May represent
repolarization of
Purkinje fibers
Can be normal but
also seen in
hypokalemia
T
U