1. review normal electrical flow through the heart. 2. discuss normal coronary artery anatomy and...
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What Does your “I” See: Ischemia, Injury or Infarction?
Carol Fahje MS, RN, BCNursing Education Specialist
Emergency DepartmentMayo Clinic, Rochester, MN
Objectives
1. 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.
Conduction System
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 60-100
› 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
Coronary ArteriesBlood supply to myocardium itself achieved by three major coronary arteries
Location› Originate in aortic arch just underneath
flap of aortic valve
Openings are very small Fill only during diastole
Left main coronary artery
Left Circumflex (Left Circ)
Left anterior descending
(LAD)
Right coronary artery(RCA)
Coronary Blood Supply
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
Three Main Coronary Arteries
Lead Placement
Limb Lead Placement: Standard 5 Lead
Lead I High Lateral
Lead II Inferior
Lead IIIInferior
aVFInferior
aVLHigh Lateral
aVRRight Atrium
12 Lead Reference:Leads Reflecting Heart Walls
aVRV1
V2
V3
V4
V5
V6
aVL
aVFIII
II
I
ISCHEMIA• ST Depression• T Wave Inversion• Flattened T waves• Hyperacute T waves Injury
• ST Elevation• Hyperacute T
waves
Infarction• Pathological Q
waves
Posterior MI• Tall R waves in V1, V2
and/or V3 along with ST Depression
ST Segment and T Wave Changes
After ventricular depolarization, normal myocardial cells are at nearly the same action potential. This is reflected during the ST Segment
Ischemia: Myocardial demand exceeds supply
Two characteristic changes seen: 1. ST Depression2. T Wave Inversion
T-waves should be upright in all leads EXCEPT: aVR V1 (50% of the population are inverted….)
You may also see Flattened T waves in ischemia
Hyperacute T-Waves 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)
Injury: ST Elevation
Occurs in the setting of abrupt loss of blood flow to the myocardium
ST Elevation
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
Reciprocal Changes
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)
aVR: The Forgotten Lead
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
Infarction 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
Pathological Q-Wave in Lead II
Normal Pathological Q Wave
Pathological Q waves Inferior Leads affected Note presence of ST elevation as well
indicating this is recent
Pathological Q Waves in Anterior Septal Wall
References http://lifeinthefastlane.com/ecg-library/lmca/ http://lifeinthefastlane.com/ecg-library/myocardial-ischaemia/ http://
www.uptodate.com/contents/electrocardiogram-in-the-diagnosis-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.
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