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Review normal electrical flow through
the heart.
2. Discuss normal coronary artery
anatomy and associated leads
reflecting ischemic changes.
3. Identify ECG indications of ischemia,
injury and infarction.
4. Analyze case studies.
1.
 Sinoatrial
Node (SA node, sinus node)
› Normal pacemaker of heart, because it
possesses the fastest inherent rate of
automaticity
› Initiates a rhythmic impulse at a rate of 60100
› Located in right atrium near superior vena
cava
 Intra-atrial
pathways
› Conducts impulse from SA node through
atrial musculature to atrioventricular (AV)
node
› Consists of:
 Anterior tract (Bachmann’s): through left
atrium
 Middle tract (Wenckebach’s): through right
atrium
 Posterior tract (Thorel’s): through right atrium
› Located in atrial tissue between SA and
AV nodes

AV Node
› Delays impulse from atria before it moves to
ventricles
› Allows for ventricular filling
› Serves as a protective mechanism against rapid
supraventricular impulses
› Located in the floor of right atrium, close to the
tricuspid valve

Junctional Tissue
› Serves as back-up pacemaker
› Intrinsic rate 40-60
› General term to describe the tissue in the lower AV
node but above the bifurcation of bundle of HIS
 Bundle
of HIS/Right and Left Bundle
Branches
› Arises from AV node and conducts impulses to
the ventricles via the bundle branches
› Intrinsic rate less than 40
 Purkinje
System
› Conducts impulses from the distal portion of
bundle branches to the sub-endocardial layers
of the ventricles
› Located distal to the bundle branches
› The terminal conduction system
 Location
› Originate in aortic arch just underneath flap
of aortic valve
 Openings are very small
 Fill only during diastole
Left main coronary
artery
Right coronary
artery
(RCA)
Left Circumflex
(Left Circ)
Left anterior descending
(LAD)

Left Main has two
major branches
› Left Anterior
Descending (LAD)
 Supplies all of bundle
branches
 Anterior wall of LV, part of
RV
 Anterior 2/3 of
interventricular septum
› Left Circumflex
 Supplies lateral wall of
LV
 AV node in 10% of
population
 SA node in 45% of
population
 Right
Coronary
Artery
› Supplies AV node
and inferior wall
of myocardium in
90% of population
› Supplies SA node
in 55% of
population
12 Lead Reference:
Leads Reflecting Heart Walls
aVR
I
aVL
II
aVF
III
Infarction
•
ISCHEMIA
•
•
•
•
ST Depression
T Wave Inversion
Flattened T waves
Hyperacute T waves
V4
V1
Pathological Q
waves
Injury
•
•
ST Elevation
Hyperacute T
waves
V2
V5
V3
V6
Posterior MI
•
Tall R waves in V1, V2 and/or
V3 along with ST Depression

After ventricular depolarization, normal
myocardial cells are at nearly the same
action potential. This is reflected during
the ST Segment

Two characteristic changes seen:
1. ST Depression
2. T Wave Inversion
 T-waves should be upright in all leads EXCEPT:
 aVR
 V1 (50% of the population are inverted….)
Sign of significant ischemia and a
precursor to acute injury
 Must be at least 7 mm high
 HOWEVER, may indicate other conditions
(e.g. hyperkalemia if widespread across
the 12 lead)


Occurs in the setting of abrupt loss of
blood flow to the myocardium
Usually stays elevated for 1-2 days but
should return to baseline within two
weeks
 Must be elevated greater than 1 mm in
at least 2 contiguous leads
 Again…seen in leads immediately
looking at the are of injury

ST depression found in electrically
opposite leads showing ST Elevation (e.g.
inferior wall ST elevation (II, III, avF)
reciprocates with ST depression in lateral
wall leads (I and avL)
 Speculation that STEMIs presenting with
reciprocal changes have a larger
myocardial area at risk
(Journal of Cardiovascular Magnetic
Resonance, 2013)


ST elevation >1 mm indicates:
› LAD/Left main coronary artery (LMCA)
occlusion or severe 3 vessel disease
› Predicts the need for bypass surgery
Differentiates LMCA from proximal LAD
occlusion if ST elevation in aVR is > than
ST elevation in V1
 Absence of ST elevation in aVR almost
entirely excludes significant LMCA

http://www.apiindia.org/medicine_update_2013/chap22.pdf
Pathological Q waves are the classic
indication of myocardial necrosis
 Reflect the fact that electricity must travel
great distances around the necrosed
tissue
 Appear several hours or days after the MI
 Criteria:

› Must be > 0.04 seconds wide
› Should be greater than 25% the height of any
accompanying R wave
Normal
Pathological Q Wave
Inferior Leads affected
 Note presence of ST elevation as well
indicating this is recent







http://lifeinthefastlane.com/ecg-library/lmca/
http://lifeinthefastlane.com/ecg-library/myocardial-ischaemia/
http://www.uptodate.com/contents/electrocardiogram-in-thediagnosis-of-myocardial-ischemia-and-infarction
http://www.apiindia.org/medicine_update_2013/chap22.pdf
Journal of Cardiovascular Magnetic Resonance 2013, 15(Suppl
1):P172 doi:10.1186/1532-429X-15-S1-P172
Mayo Clinic Advanced ECG Workshop class content.