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.