1. The
Electrocardiogram
ECG
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ECG
 Shows electrical pattern generated by
heart as it’s activated from atrium to
ventricle
 Muscle undergoes depolarization and
repolarization during each cardiac cycle
 Leadsprovide us with 3D vectors that
indicate direction of depolarization
 Looking at different projections in different
directions of body

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 Cardiac
depolarization
spreads from upper
right to lower left (in
reference to the
body as a whole),
considering the tilt of
the heart.
 This is important in
understanding the
various ECG lead
tracings

4. Conduction System of the Heart
Sinoatrial node
AV node
Bundle of His
Bundle Branches
Purkinje fibers
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Pacemakers of the Heart
 SA Node - Dominant pacemaker with an
intrinsic rate of 60 - 100 beats/minute.
 AV Node - Back-up pacemaker with an
intrinsic rate of 40 - 60 beats/minute.
 Ventricular cells - Back-up pacemaker
with an intrinsic rate of 20 - 45 bpm.

6. Impulse Conduction & the ECG
Sinoatrial node
AV node
Bundle of His
Bundle Branches
Purkinje fibers
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The normal electrocardiogram

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Electrophysiology of the heart
Different waveforms for each of the specialized cells

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The ECG Paper
 Horizontally
 One small box - 0.04 s
 One large box - 0.20 s
 Vertically
 One large box - 0.5 mV

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The ECG Paper (cont)
3 3
sec sec
 Every 3 seconds (15 large boxes) is
marked by a vertical line.
 This helps when calculating the heart rate.
NOTE: the following strips are not marked
but all are 6 seconds long.

11. The “PQRST”
P wave - Atrial
depolarization
 QRS – Ventricular
depolarization
T wave – Ventricular
repolarization
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12. The PR Interval
Atrial depolarization
+
delay in AV junction
(AV node/Bundle of
His)
(delay allows time for the
atria to contract before
the ventricles contract)
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The ECG Tracing: Waves
 P- wave
 Marks the beginning of the cardiac cycle and
measures the electrical impulse that causes atrial
depolarization and mechanical contraction
 QRS- Complex
 Measures the impulse that causes ventricular
depolarization
 Q-wave- may or may not be evident on the ECG
 R-wave- first upward deflection following P wave
 S-wave- the first downward deflection following the R-
wave
 T- wave
 Marks ventricular repolarization that ends the
cardiac cycle

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Intervals and Segments
 P-R interval-
 Time interval for impulse to go from the SA to the AV
node
 normal 0.12-0.20 secs
 QRS Interval
 Time interval for impulse to go from AV node to stimulate
Purkinjie fibers
 Less than 0.12 secs
 QT Interval
 Time interval from beginning of depolarization to the end
of repolarization
 Should not exceed ½ the length of the R-R
 ST segment
 end of the S to the beginning of the T

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CARDIAC CYCLES
 Recorded on graph paper.
 Time is measured across (x-axis)
 Voltage is measured up and down. (y-axis)
 Small squares 1mm high 0.04 secs wide
 5 small squares = 1 large square
 1 large square is 5mm high and 0.20 sec wide

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Principles of
Electrocardiograph
 Electrocardiograph – is the instrument that
records the electrical activity of the heart
 Electrocardiogram (ECG) is the record of
that activity

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ELECTRODE PLACEMENT
 LIMB LEADS
 Bipolar leads: I II III
 Augment leads: Avr Avl
Avf
 CHEST LEAD
 6 UNIPOLAR LEADS: V1,
V2, V3, V4, V5 & V6

18. Limb Leads
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Chest Leads

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I and AVL
V3 & v4
V1 & v2
II, III and AVF
V5 & v6
Where the positive electrode is positioned,
determines what part of the heart is seen!

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Normal ECG

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ECG Changes: Infarct
 Significant Q-wave where none previously existed
 Why?
 Impulse traveling away from the positive lead
 Necrotic tissue is electrically dead
 No Q-wave in Subendocardial infarcts
 Why?
 Not full thickness dead tissue
 But will see a ST depression
 Often a precursor to full thickness MI
 Criteria
 Depth of Q wave should be 25% the height of the R wave
 Width of Q wave is 0.04 secs
 Diminished height of the R wave

25. Evolving MI and Hallmarks of AMI
Q wave
ST Elevation
1 year T wave inversion
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A Normal 12 Lead ECG

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A Normal 12 Lead ECG

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Color Coding ECG’s Anterior
 Yellow indicates V1,
V2, V3, V4
 Anterior infarct with ST
elevation
 Left Anterior Descending
Artery (LAD)
 V1 and V2 may also
indicate septal involvement
which extends from front to
the back of the heart along
the septum
 Left bundle branch block
 Right bundle branch block
 2nd Degree Type2
 Complete Heart Block

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Anterior MI

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Color Coding ECG- Inferior
 Blue indicates leads
II, III, AVF
 Inferior Infarct with ST
elevations
 Right Coronary Artery
(RCA)
 1st degree Heart Block
 2nd degree Type 1, 2
 3rd degree Block
 N/V common, Brady

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Inferior MI

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Color Coding ECG- Lateral
 Redindicates
leads I, AVL, V5,
V6
 Lateral Infarct with
ST elevations
 Left Circumflex
Artery
 Rarely by itself
 Usually in combo

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Lateral MI

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Color Coding ECG- Posterior
 Green indicates
leads V1, V2
 Posterior Infarct with ST
Depressions and/ tall R
wave
 RCA and/or LCX Artery
 Understand Reciprocal
changes
 The posterior aspect of
the heart is viewed as a
mirror image and
therefore depressions
versus elevations
indicate MI
 Rarely by itself usually in
combo

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Posterior MI

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Color Coding ECG- SubEndo
 No color for
SubEndocardial
infarcts since they
are not transmural
 Look for diffuse or
localized changes
and non – Q wave
abnormalities
 T-wave inversions
 ST segment
depression

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SubEndo MI

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More than one color shows
abnormality
A combination of infarcts such as:
 Anterolateral yellow and red
 Inferoposterior blue and green
 Anteroseptal yellow and green

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Putting it ALL together

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