Chapter 6: Myocardial Ischemia and Infarction

Broad term for urgent situations where blood supply to heart is acutely compromised

Acute coronary syndrome (ACS)

Acute coronary syndrome (ACS) can be classified into two categories:

1. ST segment elevation myocardial infarction (STEMI)

2. Non-ST segment elevation myocardial infarction (NSTEMI)

ACS is most often caused by

Rupture or erosion of atherosclerotic plaque → Formation of thrombus → Occlusion of coronary artery

What is angina?

Diffused chest pain or pressure

What causes angina?

Narrowing of the coronary arteries (atherosclerosis)

Angina is a classic symptom of

Cardiac ischemia

Angina symptoms

1. Pain → May radiate to neck, arms, back

2. SOB

3. N/V

4. Dizziness

5. Diaphoresis

What are the two types of angina?

Stable angina

→ Chest pain occurs only after a given level of exertion

→ Relieved with rest

Unstable angina

→ Symptoms occur with little or no exertion

→ Does not resolve with rest

What type of angina is predictable? What type isn't?

Unpredictable: Unstable angina

Predictable: Stable angina

What type of angina is more severe and lasts longer?

Unstable angina

In what type of myocardial infarction is blood flow through the coronary artery completely occluded?

STEMI

In what type of myocardial infarction is blood flow through the coronary artery reduced but not totally blocked?

NSTEMI

Characteristic feature of STEMI

Elevated ST segment

What happens to the ST segment in NSTEMI?

May have a normal or a depressed ST segment

Which type of myocardial infarction is a true emergency?

STEMI because the coronary artery is completely occluded

→ NSTEMI is still an urgent situation

Why does the ST segment change in STEMI?

→ Lack of oxygen changes how heart cells handle electricity

→ This creates a voltage difference between healthy and ischemic myocardium

→ Gradient creates an injury current that shifts the ST segment

What are the 3 main steps to diagnose a myocardial infarction?

1. Patient history and physical exam

2. Cardiac enzymes: CK and troponin

3. EKG

Risk factors for myocardial infarction

1. M > 45 and F > 55

2. Obesity

3. Smoking

4. Hypercholesterolemia

→ High LDL and low HDL

5. Sedentary lifestyle

6. Illicit drug use

7. Hypertension

8. Family history

9. Diabetes

What are the key cardiac enzymes used to diagnose a myocardial infarction?

1. Creatine kinase (CK)

2. Troponin

What cardiac enzyme has limited value in early diagnosis and why?

Creatine kinase (CK)

→ Limited value in early diagnosis because levels rise 6 hours after MI

How long does it take for creatine kinase (CK) levels to return to normal?

Within 48 hours

What cardiac enzyme is most commonly used today to diagnose a myocardial infarction?

Troponin

How soon do troponin levels rise after a myocardial infarction, and how long do they stay elevated?

Troponin levels rise within 2-3 hours and stay elevated for several days

Which cardiac enzyme has high sensitivity and high specificity for myocardial infarction?

Troponin

Can troponin be elevated in conditions other than myocardial infarction?

Yes, but not to the same extent

Conditions that can raise troponin levels:

→ Sepsis

→ CHF

→ Myocarditis/Pericarditis

In an emergency, should you wait for lab results before doing cardiac catheterization?

No, if history and EKG are consistent with myocardial infarction, proceed with cardiac catheterization without waiting for labs

True or false: EKG should be performed immediately on anyone suspected of having MI?

True

True or false: Initial EKG is almost always conclusive by itself

False

→ Initial EKG might not be conclusive by itself, needs serial EKGs

If the initial EKG is not conclusive for myocardial infarction, what should be done next?

Repeat EKG in 15-30 minutes, as serial EKGs can reveal evolving changes

What are the 3 main EKG changes seen in myocardial infarction, and what do they indicate?

T wave peaking/inversion → Ischemia (reversible)

ST elevation → Injury (potentially reversible)

Q waves → Cell death (irreversible)

First sign of Ischemia on EKG?

Peaked T waves

→ Amplitude approaches or even surpasses QRS amplitude

What EKG change often follows T wave peaking during myocardial ischemia?

T wave inversion

What are some other causes of T wave inversion besides ischemia?

1. Bundle branch block

2. Ventricular hypertrophy with repolarization abnormalities

How does T wave inversion differ between ischemia and ventricular hypertrophy?

Ventricular hypertrophy → Asymmetric inversion

Myocardial ischemia → Symmetric inversion

Ischemia causes previously inverted T waves to appear upright again, falsely suggesting a normal EKG

Pseudonormalization

True or false: T wave inversion is a nonspecific finding

True

In which leads are T waves normally inverted in children

V1, V2, and V3

Is an inverted T wave in lead aVR normal?

Yes

What is the ST elevation (STE) threshold for diagnosing myocardial injury in leads?

> 1.0 mm

How many leads must show ST elevation to support the diagnosis of myocardial injury?

At least two leads overlying the same region of the heart

What is the best reference baseline for measuring ST segment elevation or depression?

TP segment

How long does it take for the ST segment to return to baseline after a myocardial infarction?

Few hours

Where end of QRS meets ST segment

J point

J point elevation is often seen in

Young/healthy patient

J point elevation is particularly visible in lead

V1,V2, and V3

True or false: J point elevation has no pathologic significance/risk?

True

How to differentiate between STE and JPE?

ST elevation → ST segment bows upwards (sad face)

JPE → ST segment bows downward (smile)

Serial EKG changes for STE and JPE

→ J Point Elevation will not change from one EKG to the next

→ ST Elevation will continue to evolve if ischemia is untreated

Q waves are diagnostic of __________

True myocardial infarction

How long does the Q wave usually persist after MI?

Usually persists for the rest of the patient's life

Myocardial cells become __________ when they die

Electrically silent

Electrical current will be directed ______ (to/away) from infarcted areas (negative deflection). Redirection of the current can also result in _________ in leads on the opposite side of the heart

1. Away

2. Reciprocal changes

Reciprocal changes:

→ May see _______ opposite of new Q waves

→ May see __________ opposite of STE

1. Tall R waves

2. ST Depression

Normal Q waves usually appear in leads _____________

Lateral leads (aVL, I, V5, V6)

Pathological Q wave duration and magnitude

Duration: >0.04 sec

Depth: ≥ 25% height of R wave

True or false: Pathological Q waves will almost always show in multiple leads that look at the affected region of the heart

True

What part of the heart does the right coronary artery (RCA) supply?

The entire right side of the heart; it runs between the RA and RV and wraps around to the posterior side

In most people, which artery supplies the AV nodal artery?

The right coronary artery (RCA)

→ 90% of the people

What are the two main branches of the left coronary artery (LCA)?

1. Left anterior descending (LAD)

2. Left circumflex (LCx)

What does the LAD artery supply?

1. Anterior wall of the heart

2. Interventricular septum

What does the left circumflex (LCx) artery supply?

The lateral wall of the left ventricle

In 10% of people, which artery supplies the AV nodal artery instead of the RCA?

Left circumflex artery (LCx)

Which artery is involved in an anterior myocardial infarction?

Left Anterior Descending (LAD) artery

Which artery is involved in a lateral myocardial infarction?

Left Circumflex (LCx) artery

Which artery is usually involved in an anterolateral infarction?

Left Main Coronary artery

Which artery is involved in an inferior myocardial infarction?

Right Main Coronary Artery (RCA)

→ Or its descending branch

Which artery is involved in a posterior myocardial infarction?

Right Main Coronary Artery (RCA)

True of false: Posterior infarct rarely occurs in isolation

True

→ Usually accompanied by inferior infarct (Inferoposterior)

Which lead is not used for diagnosing ischemia?

aVR

Which EKG leads show changes in an anterior myocardial infarction?

V1-V4

In anterior MI, where might you see reciprocal changes?

In the inferior leads

What is a common R wave finding in anterior myocardial infarction?

Poor R wave progression

Which EKG leads show signs of a lateral myocardial infarction?

Leads I, aVL, V5, and V6

Where might reciprocal changes be seen in a lateral infarction?

In the inferior leads

Which EKG leads show signs of an inferior myocardial infarction?

Leads II, III, and aVF

Where are reciprocal changes seen in an inferior infarction?

In the lateral and/or anterior leads

What happens to Q waves in about 50% of patients after an inferior myocardial infarction?

They lose the criteria for significant Q waves within 6 months

How can a posterior myocardial infarction be identified on an EKG?

Look for reciprocal changes

→ Large R wave and upright T wave in lead V1

What EKG changes suggest a right ventricular (RV) infarction?

T wave changes and ST segment elevation in lead V1

What type of infarction almost always accompanies an RV infarction?

Inferior infarction

In RV infarction, which leads show greater ST elevation: lead II or lead III?

Lead III

What are the two main goals in treating a myocardial infarction (MI)?

1. Reduce myocardial oxygen demand

2. Increase myocardial oxygen supply

What EKG changes are seen in NSTEMIs?

ST depression or T wave inversion

What causes an NSTEMI?

Partial occlusion of a coronary artery or a small offshoot

Can NSTEMIs be localized to a specific region of the heart on EKG?

No

Are cardiac enzymes elevated in NSTEMIs?

Yes

Do NSTEMIs lead to the development of Q waves on EKG?

No

Takotsubo Cardiomyopathy

Broken heart syndrome

Prinzmetal's Angina

Coronary artery vasospasm

What are the key treatments for myocardial infarction (MI)?

M: Morphine

O: Oxygen

N: Nitroglycerin

A: Aspirin

B: Beta blockers