Identifying types of MI

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Винницкий национальный медицинский университет им. Н.И.Пирогова

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ECG Interpretation Made Incredibly Easy (5th edition).pdf

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IDENTIFYING TYPES OF MI	271

In addition to rest, pain relief, and supplemental oxygen, such medications as nitroglycerin, morphine sulfate, beta-adrenergic blockers, calcium channel blockers, angiotensin-converting enzyme inhibitors, and antiarrhythmics are used. Aspirin or glycoprotein IIb/IIIa inhibitors may be used to reduce platelet aggregation. Thrombolytic therapy may also be prescribed to dissolve a thrombus occluding a coronary artery.

Identifying types of MI

The location of the MI is a critical factor in determining the most appropriate treatment and predicting probable complications. Characteristic ECG changes that occur with each type of MI are localized to the leads overlying the infarction site. (See Locating myocardial damage.) Here’s a look at characteristic ECG changes that occur with different types of MI.

Anterior wall MI

The left anterior descending artery supplies blood to the anterior portion of the left ventricle. The artery supplies blood to the

Improving blood flow

Increasing the blood supply to the heart of a patient who has had a myocardial infarction can help prevent further damage to his heart. In addition to medications, blood flow to the heart can be improved by:

•intra-aortic balloon pump

•percutaneous transluminal coronary angioplasty

•atherectomy

•laser treatment

•stent placement

•coronary artery bypass graft.

Locating myocardial damage

After you’ve noted characteristic lead changes of an acute myocardial infarction, use this chart to identify the areas of damage. Match the lead changes in the second column with the affected wall in the first column and the artery involved in the third column. Column four shows reciprocal lead changes.

Wall affected	Leads	Artery involved	Reciprocal changes

Anterior	V2 to V4	Left coronary artery, left anterior descending (LAD) artery	II, III, aVF
Anterolateral	I, aVL, V3 to V6	LAD artery, circumflex artery	II, III, aVF
Septal	V1 to V2	LAD artery	None
Inferior	II, III, aVF	Right coronary artery	I, aVL
Lateral	I, aVL, V5, V6	Circumflex artery, branch of left coronary artery	II, III, aVF
Posterior	V8, V9	Right coronary artery, circumflex artery	V1 to V4
Right ventricular	V4R, V5R, V6R	Right coronary artery	None

INTERPRETING A 12-LEAD ECG

272

ventricular septum and portions of the right and left bundlebranch systems.

When the anterior descending artery becomes occluded, an anterior wall MI occurs. (See Recognizing an anterior wall MI.)

Recognizing an anterior wall MI

This 12-lead ECG shows typical characteristics of an anterior wall myocardial infarction (MI). Note that the R waves don’t progress through the precordial leads. Also note the ST-segment elevation in leads V2 and V3. As expected, the reciprocal leads II, III, and aVF show slight ST-segment depression. Axis deviation is normal at +60 degrees.

Lead I

Lead aVR

Lead V1

Lead V4

Lead II

Lead aVL

Lead V2

Lead V5

Lead III

Lead aVF

Lead V3

Lead V6

IDENTIFYING TYPES OF MI	273

Complications of an anterior wall MI include varying second-degree atrioventricular blocks, bundle-branch blocks, ventricular irritability, and left-sided heart failure.

Changing the leads

An anterior wall MI causes characteristic ECG changes in leads V2 to V4. The precordial leads show poor R-wave progression because the left ventricle can’t depolarize normally. ST-segment elevation and T-wave inversion are also present.

The reciprocal leads for the anterior wall are the inferior leads II, III, and aVF. They show tall R waves and depressed ST segments.

Septal wall MI

The patient with a septal wall MI has an increased risk for developing a ventricular septal defect. ECG changes are present in leads V1 and V2. In those leads, the R wave disappears, the ST segment rises, and the T wave inverts.

Because the LAD artery also supplies blood to the ventricular septum, a septal wall MI commonly accompanies an anterior wall MI. (See Recognizing an anteroseptal wall MI, page 274.)

Lateral wall MI

A lateral wall MI is usually caused by a blockage in the left circumflex artery and shows characteristic changes in the left lateral leads I, aVL, V5, and V6. The reciprocal leads for a lateral wall infarction are leads II, III, and aVF. (See Recognizing a lateral wall MI, page 275.)

This type of infarction typically causes premature ventricular contractions and varying degrees of heart block. A lateral wall MI usually accompanies an anterior or inferior wall MI.

Inferior wall MI

An inferior wall MI is usually caused by occlusion of the right coronary artery and produces characteristic ECG changes in the inferior leads II, III, and aVF and reciprocal changes in the lateral leads I and aVL. (See Recognizing an inferior wall MI, page 276.) It’s also called a diaphragmatic MI because the heart’s inferior wall lies over the diaphragm.

Patients with inferior wall MI are at risk for developing sinus bradycardia, sinus arrest, heart block, and premature ventricular contractions. This type of MI occurs alone, with a lateral wall MI, or with a right ventricular MI.

Memory jogger

To remember which

leads are critical in diagnosing a lateral wall MI, think of the L’s in “lateral MI” and “left lateral leads.”

INTERPRETING A 12-LEAD ECG

274

Recognizing an anteroseptal wall MI

This 12-lead ECG shows typical characteristics of an anteroseptal wall myocardial infarction (MI). Note the poor R wave progression, the elevated ST segments, and the inverted T waves in leads V1, V2, and V3. Reciprocal changes are seen in leads II, III, and aVF with depressed ST segments and tall, peaked T waves.

Lead I

Lead aVR

Lead V1

Lead V4

Lead II

Lead aVL

Lead V2

Lead V5

Lead III

Lead aVF

Lead V3

Lead V6

IDENTIFYING TYPES OF MI	275

Recognizing a lateral wall MI

This 12-lead ECG shows changes characteristic of a lateral wall myocardial infarction (MI). In leads I, aVL, V5, and V6, note the pathologic Q waves, the ST-segment elevation, and the T-wave inversion.

Lead I

Lead aVR

Lead V1

Lead V4

Lead II

Lead aVL

Lead V2

Lead V5

Lead III

Lead aVF

Lead V3

Lead V6

Right ventricular MI

A right ventricular MI usually follows occlusion of the right coronary artery. This type of MI rarely occurs alone. In fact, 40% of all patients with an inferior wall MI also suffer a right ventricular MI.

INTERPRETING A 12-LEAD ECG

276

Recognizing an inferior wall MI

This 12-lead ECG shows the characteristic changes of an inferior wall myocardial infarction (MI). In leads II, III, and aVF, note the T-wave inversion, ST-segment elevation, and pathologic Q waves. In leads I and aVL, note the slight ST-segment depression, a reciprocal change. This ECG shows left axis deviation at –60 degrees.

Lead I

Lead aVR

Lead V1

Lead V4

Lead II

Lead aVL

Lead V2

Lead V5

Lead III

Lead aVF

Lead V3

Lead V6

A right ventricular MI can lead to right-sided heart failure or right-sided block. The classic changes are ST-segment elevation, pathologic Q waves, and inverted T waves in the right precordial leads V4R to V6R. (See Recognizing a right ventricular MI.)

IDENTIFYING TYPES OF MI	277

Recognizing a right ventricular MI

This 12-lead ECG shows typical characteristics of a right ventricular myocardial infarction (MI). Note the T-wave inversion in leads V3R, V4R, V5R, and V6R. Pathologic Q waves and ST-segment elevation are also present.

Lead I

Lead aVR

Lead V1

Lead V4R

Lead II

Lead aVL

Lead V2

Lead V5R

Lead III

Lead aVF

Lead V3R

Lead V6R

Take the right lead

Identifying a right ventricular MI is difficult without information from the right precordial leads. If these leads aren’t available, you can observe leads II, III, and aVF or watch leads V1, V2, and V3 for ST-segment elevation. If a right ventricular MI has occurred, use lead II to monitor for further damage.

INTERPRETING A 12-LEAD ECG

278

Posterior wall MI

A posterior wall MI is caused by occlusion of either the right coronary artery or the left circumflex arteries. This MI produces reciprocal changes on leads V1 and V2.

Recognizing a posterior wall MI

This 12-lead ECG shows typical characteristics of a posterior wall myocardial infarction (MI). Note the tall R waves, the depressed ST segments, and the upright T waves in leads V1, V2, and V3. These are reciprocal changes, because the leads that best monitor a posterior wall MI (V7, V8, and V9) aren’t on a standard 12-lead ECG.

Lead I

Lead aVR

Lead V1

Lead V4

Lead II

Lead aVL

Lead V2

Lead V5

Lead III

Lead aVF

Lead V3

Lead V6

IDENTIFYING TYPES OF MI	279

Classic ECG changes in a posterior wall MI include tall R waves, ST-segment depression, and upright T waves. (See

Recognizing a posterior wall MI.) Posterior infarctions usually accompany inferior infarctions. Information about the posterior wall and pathologic Q waves that might occur can be obtained from leads V7 to V9 using a posterior ECG.

That’s a wrap!

12-lead ECG interpretation review

Looking at the waves

• P waves: Upright; possibly inverted in lead aVR or biphasic or inverted in leads III, aVL, and V1

•PR intervals: Always constant, like QRScomplex durations

•QRS-complex deflections: Variable in different leads

•Q waves: Possibly pathologic; duration of less than 0.4 second when normal

•T wave: Slightly rounded and upright; normal deflection upward in leads I, II,

and V3 through V6; inverted in leads

III, aVF, aVR, aVL, and V1; shouldn’t be tall, peaked, or notched

•ST segments: should be isoelectric or have minimal deviation

Electrical axis

• Average direction of the heart’s electrical activity during ventricular depolarization

Methods of determining axis

• Quadrant method: Determines main deflection of QRS complex in leads I and

aVF

• Degree method: Identifies axis by degrees on the hexaxial system

•Examination of the waveforms from the six frontal leads: I, II, III, aVR, aVL, and aVF

•Normal axis: Between 0 and +90 degrees

•Left axis deviation: Between 0 and –90 degrees

•Right axis deviation: Between +90 and +180 degrees

•Extreme axis deviation: Between –180 and –90 degrees

Disorders affecting 12-lead ECGs

Angina

• Possible ECG changes: Peaked or flattened T wave, T-wave inversion, and ST-segment depression with or without T-wave inversion

Bundle-branch block

•QRS complex: Width increases to greater than 0.12 second with bundlebranch block

•Lead V1 (to right of heart) and V6 (to left of heart): Used to determine whether

block is in right or left bundle

•RBBB: QRS complex has rsR’ pattern

and T-wave inversion in lead V1 and widened S wave and upright T wave in

lead V6

Identifying a right ventricular MI is difficult without the right information from the right percordial leads.

(continued)

INTERPRETING A 12-LEAD ECG

280

12-lead ECG interpretation review (continued)

• LBBB: Wide S wave (which may be preceded by a Q wave or small R wave) and small positive T wave in lead V1; tall, notched R wave or slurred R wave and T-wave inversion in lead V6

Myocardial infarction

• Three pathologic changes on

ECG — ischemia, injury, and infarction

Pathologic ECG changes

•Zone of ischemia: T-wave inversion

•Zone of injury: ST-segment elevation

•Zone of infarction: Pathologic Q wave in transmural MI

Locating the MI

•Anterior wall: Leads V2 to V4; involves LAD artery

•Septal wall: V1 to V2; involves LAD artery

•Inferior wall: Leads II, III, aVF; involves right coronary artery

•Lateral wall: Leads I, aVL, V5, V6; involves left circumflex artery

•Posterior wall: Leads V7, V8, V9; involves right coronary or left circumflex arteries

•Right ventricular wall: V4R, V5R, V6R; involves right coronary artery

Quick quiz

1. Your patient’s ECG shows positively deflected QRS complexes in leads I and aVF. Using the four-quadrant method for determining the electrical axis, you determine that he has a:

A.normal axis.

B.left axis deviation.

C.right axis deviation.

D.extreme axis deviation.

Answer: A. If the QRS-complex deflection is positive or upright in both leads, the electrical axis is normal.

2. Your patient’s ECG shows a negatively deflected QRS complex in lead I and a positively deflected one in lead aVF. Using the four-quadrant method for determining electrical axis, you determine that he has a:

A.normal axis.

B.left axis deviation.

C.right axis deviation.

D.extreme axis deviation.

Answer: C. When lead I points downward and lead aVF is upright, right axis deviation exists. Both waves pointing down signals indeterminate axis deviation.

QUICK QUIZ	281

3. If your patient has a T-wave inversion, ST-segment elevation, and pathologic Q waves in leads II, III, and aVF, suspect an acute MI in the:

A.anterior wall.

B.inferior wall.

C.lateral wall.

D.septal wall.

Answer: B. Leads II, III, and aVF face the inferior wall of the left ventricle, so the ECG changes there are indicative of an acute inferior wall MI.

4.If a patient’s QRS complex has an R wave in V1, suspect:

A.right ventricular hypertrophy.

B.left ventricular hypertrophy.

C.LBBB.

D.RBBB.

Answer: D. In RBBB, depolarization of the right ventricle takes longer than normal, thereby creating an R wave in lead V1.

5. Myocardial injury is represented on an ECG by the presence of a:

A.T-wave inversion.

B.ST-segment elevation.

C.pathologic Q wave.

D.ST-segment depression.

Answer: B. ST-segment elevation is the ECG change that corresponds with myocardial injury. It’s caused by a prolonged lack of blood supply.

6.On a 12-lead ECG, a posterior wall MI produces:

A.deep, broad Q waves in leads V1 through V4.

B.inverse or mirror image changes in V1 and V2.

C.raised ST segments in all leads.

D.poor R-wave progression in the precordial leads.

Answer: B. Leads V1 and V2 show reciprocal changes when a posterior wall MI occurs. Look at the mirror images of these leads to determine the presence of a posterior wall MI.

7. An ST segment located 1.5 mm above the baseline is considered:

A.normal.

B.slightly depressed.

C.abnormally elevated.

D.isoelectric.

So far, so good. Now check out the strips on the

next page!

Answer: C. ST segments should be isoelectric or have minimal deviation. ST-segment elevation greater than 1 mm above the baseline is considered abnormal.

INTERPRETING A 12-LEAD ECG

282

Test strips

Okay, it’s time to try out a couple of ECG test strips.

8. In the 12-lead ECG below, you’ll see an rsR pattern in lead V1, as well as T-wave inversion. Looking at lead V6, you’ll note a widened S wave and an upright T wave. These changes indicate:

aVR

aVL

III

aVF

A.right ventricular MI.

B.LBBB.

C.RBBB.

D.angina.

Answer: C. The rsR configuration in lead V1 and the wide

S wave in V6 indicate RBBB.

QUICK QUIZ	283

9. Your patient says he thinks he might have had a heart attack about 3 years ago but, because he never went to the doctor, he doesn’t know for sure. Looking at his admission ECG, you think he definitely had an MI at some point. You base your assessment on the presence of:

aVR

aVL

III

aVF

A.pathologic Q waves in leads II, III, and aVF.

B.inverted T waves in leads I, aVL, and V6.

C.depressed ST segments in leads II, aVL, and V5.

D.axis between 0 and +90 degrees.

Answer: A. Old damage to the myocardium is indicated by the presence of pathologic Q waves, in this case in leads II, III, and aVF. Those findings lead you to suspect the patient suffered an inferior wall MI at some point.

INTERPRETING A 12-LEAD ECG

284

Scoring

If you answered all nine questions correctly, wow! You’ve graduated with honors from the School of 12-Lead ECG Interpretation.

If you answered seven or eight questions correctly, super! You’re ready to take a graduate course on 12-lead ECG interpretation.

If you answered fewer than seven questions correctly, you can still get on the right track. Just review your notes from 12-Lead ECG Interpretation 101.

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IDENTIFYING TYPES OF MI