2. 2
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
3. 3
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
4
5. 5
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
6
9. 9
The ECG Paper
Horizontally
One small box - 0.04 s
One large box - 0.20 s
Vertically
One large box - 0.5 mV
10. 10
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.
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)
12
13. 13
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
14. 14
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
15. 15
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
24. 24
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
25
28. 28
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
32. 32
Color Coding ECG- Lateral
Redindicates
leads I, AVL, V5,
V6
Lateral Infarct with
ST elevations
Left Circumflex
Artery
Rarely by itself
Usually in combo
34. 34
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
36. 36
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
38. 38
More than one color shows
abnormality
A combination of infarcts such as:
Anterolateral yellow and red
Inferoposterior blue and green
Anteroseptal yellow and green
