Module 2 - ST Elevated Myocardial Infarction (STEMI)

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1Welcome CME Module 2
ST Elevated Myocardial Infarction (STEMI)
Dear Doctor,
Welcome to the online CME module.
In this module we will emphasize ST Elevated Myocardial Infarction (STEMI)
2Key Objectives:

  • Anterior Myocardial Infarction
  • Lateral STEMI
  • Patterns of Anterior Infarction
  • Evaluation of STEMI
At the end of this module you will know more about anterior and lateral STEMI, different patterns of anterior infarction and evaluation of STEMI.
Basics on ECG1
Before starting the module, first we will brush up our ECG basics.
Any ECG is based on the basic shape of an ECG trace:
The P Wave: Associated with contraction of the atria. The muscle mass of the atria is small when compared to the ventricles, so P waves are relatively small. A P wave should precede every QRS complex.
The PR Interval:
The QRS Complex: Associated with contraction of the ventricles. Large amplitudes in comparison with P waves.
The ST Segment: The non-deflected section of trace between the end of the S wave and the beginning of the T wave.
The T Wave: Associated with repolarisation of the ventricular muscle mass.
The electrical activity on an ECG2

The areas represented the precordial leads on the ECG


The areas represented the limb leads on the ECG
The areas represented on the ECG are summarized below:
V1, V2 = Right ventricle
V3, V4 = Septum
V5, V6 = Left side of the heart
Lead I = Left side of the heart
Lead II = Inferior territory
Lead III = Inferior territory
aVF = Inferior territory (remember ‘F’ for ‘feet’)
aVL = Left side of the heart
aVR = Right side of the heart
Comparsion of normal and abnormal ECG3
If you observe the comparison of normal and abnormal ECG, normally occurring Q waves are narrow and shallow, whereas abnormal Q’s are wider and deeper than normal Q waves. Abnormal Q waves indicate this person has experienced heart damage and as a result, there is a loss of electrical conduction due to tissue damage.
3
An example of an anterior ST segment elevation MI.
This ST elevation is concave downward and frequently overwhelms the T wave producing a "tombstoning" type appearance.
Coming to our module -
ST segment elevation indicates transmural (subepicardial) myocardial ischemia, and the leads in which it appears can indicate the area of the heart affected. The degree of ischemia leads to greater elevation of the ST segment. In this case, ST elevation is concave downward and overwhelms the T wave producing a "tombstoning" type appearance indicating severe ischemia.
Anterior STEMI - ECG4:

Anterior STEMI requires 2 mm of ST segment elevation in V2 and V3 in men, or 1.5 mm in women, aged older than 40 years
According to the American College of Cardiology/American Heart Association, anterior STEMI is defined as:
2 mm of ST segment elevation in V2 and V3 in men, or 1.5 mm in women, aged older than 40 years
4Clinical Relevance of Anterior Myocardial Infarction5
Anterior STEMI:

  • Affects the largest muscle mass of the heart and its ability to maintain cardiac output

  • Carries the worst prognosis of all infarct locations

  • Most commonly results from occlusion of the left anterior descending artery (LAD)

Clinical Relevance of Anterior Myocardial Infarction
Anterior STEMI affects the largest muscle mass of the heart and its ability to maintain cardiac output.
Therefore, anterior myocardial infarction carries the worst prognosis of all infarct locations.
Anterior STEMI most commonly results from occlusion of the left anterior descending artery (LAD).
Lateral STEMI – ECG:
Clinical Significance of lateral STEMI6

  • Lateral STEMI usually results from ischemia in branches of the left anterior descending (LAD), the obtuse marginal branch (OM) of the left circumflex (LCx) artery, or the ramus intermedius
  • Because branches of the large coronary arteries are involved, lateral STEMI usually occurs as part of a larger territory infarction, such as an anterolateral STEMI
  • Lateral extension of an anterior, inferior or posterior MI reflects significant myocardium at risk and a worse prognosis
  • Lateral STEMI is a stand-alone indication for emergent reperfusion
Clinical Significance of lateral STEMI
The lateral wall of the LV is supplied by branches of the left anterior descending (LAD) and left circumflex (LCx) arteries.
Infarction of the lateral wall usually occurs as part of a larger territory infarction, e.g. anterolateral STEMI.
Isolated lateral STEMIs are less common, but may be produced by occlusion of smaller branch arteries that supply the lateral wall, e.g. the first diagonal branch (D1) of the LAD, the obtuse marginal branch (OM) of the LCx, or the ramus intermedius.
Lateral extension of an anterior, inferior or posterior MI indicates a larger territory of myocardium at risk with consequent worse prognosis.
Lateral STEMI is a stand-alone indication for emergent reperfusion.
Anterolateral STEMI6

Isolated lateral infarction:
ST elevation is present in the anterior (V2-4) and lateral leads (I, aVL, V5-6).
Q waves are present in both the anterior and lateral leads, most prominently in V2-4.
There is reciprocal ST depression in the inferior leads (III and aVF).
This pattern indicates an extensive infarction involving the anterior and lateral walls of the left ventricle.
There is early ST elevation with hyperacute T waves in the anteroseptal leads (V1-4).
There is also subtle ST elevation in the high lateral leads (I and aVL); this may be easily missed.
Anterior-inferior STEMI5
ST elevation is present throughout the precordial and inferior leads.
There are hyperacute T waves, most prominent in V1-3.
Q waves are forming in V1-3, as well as leads III and aVF.
This pattern is suggestive of coronary occlusion occurring with an anatomic “type III” or “wraparound” LAD
Left main coronary artery4

ST Elevation
ECG tracing is from a patient with an acutely occluded left main coronary artery. This ECG shows dramatic ST depression that is downsloping consistent with ischemia.
Inferior STEMI with a posterior MI5
Since the blood supply to the posterior heart and the inferior portions of the heart (which may also receive some blood supply from the distal branches of the LAD and LCx) are supplied by the right coronary artery (RCA), occlusion of this vessel may result in a posterior-inferior MI. When this occurs, ST segment depression (not elevation) is seen in the septal and anterior precordial leads (V1-V4). This is because these ECG leads will see the MI “backwards”; the leads are placed anteriorly, but the myocardial injury is occurring posteriorly, causing what are referred to as reciprocal changes. A R/S wave ratio greater than 1 in leads V1 or V2 are suggestive of a posterior STEMI. This may be confirmed by placing posterior ECG leads V7-V9 which will show ST segment elevation.
Evaluation of ST Segment Elevation Myocardial Infarction4
Killip Classification4

  • Class I: No evidence of HF (mortality 6%)
  • Class II: Findings of mild to moderate HF (S3 gallop, rales < halfway up lung fields or elevated jugular venous pressure (mortality 17%)
  • Class III: Pulmonary edema (mortality 38%)
  • Class IV: Cardiogenic shock defined as systolic blood pressure < 90 mm Hg and signs of hypoperfusion such as oliguria, cyanosis and sweating (mortality 67%)
Now let us discuss about last topic of this module – evaluation of STEMI
The Killip Classification is a method used to predict mortality during STEMI based on clinical findings indicating the degree of myocardial injury.
Class I: No evidence of HF (mortality 6%)
Class II: Findings of mild to moderate HF (S3 gallop, rales < halfway up lung fields or elevated jugular venous pressure (mortality 17%)
Class III: Pulmonary edema (mortality 38%)
Class IV: Cardiogenic shock defined as systolic blood pressure < 90 mm Hg and signs of hypoperfusion such as oliguria, cyanosis and sweating (mortality 67%)
Heart sounds

An S4 heart sound may be present during myocardial ischemia due to ischemia-related metabolic changes that impair left ventricular relaxation.
Heart sounds may also be helpful in evaluating patients with STEMI. During myocardial ischemia, an S4 HEART SOUND may be present due to ischemia-related metabolic changes that impair left ventricular relaxation.
Cardiac biomarkers

No Q WaveLeft Bundle Branch Block

  • QRS duration greater than 120 milliseconds
  • Absence of Q wave in leads I, V5 and V6
  • Monomorphic R wave in I, V5 and V6
  • ST and T wave displacement opposite to the major deflection of the QRS complex

Depolarization
Ventricular hypertrophy

The amplitude of the QRS complex, representing ventricular depolarization, is increased.
Non-atherosclerotic (vasospasm or Prinzmetal’s angina)

Occurs with coronary artery vasospasm resulting in myocardial ischemia

Cardiac biomarkers — Cardiac enzymes are released into the circulation when myocardial necrosis occurs, as seen in MI.
Biomarkers include myoglobin, troponin and creatine kinase, with troponins being the most sensitive and specific for myocardial injury. Historically, lactate dehydrogenase, or LDH, was also used but is nonspecific.
Left bundle branch block (LBBB)
LBBB resulting from an acute transmural infarction is relatively uncommon, and is more often a pre-existing marker of underlying structural heart disease and increased risk for cardiovascular mortality and morbidity. Diagnosing MI in the setting of LBBB is challenging because the electrical changes that occur in LBBB may obscure the typical ECG findings in myocardial ischemia and infarction.
The Sgarbossa criteria are an ECG interpretation tool to assist in the diagnosis of MI in the presence of LBBB, although their absence is not able to exclude MI. The three criteria suggesting MI in a patient with LBBB are:

  • ST-elevation of ≥1 mm and concordant with the QRS complex
  • ST-segment depression ≥1 mm in lead V1, V2, or V3 (3 points)
  • ST elevation ≥5 mm and discordant with the QRS complex

The ECG criteria for a left bundle branch block include:

  • QRS duration greater than 120 milliseconds
  • Absence of Q wave in leads I, V5 and V6
  • Monomorphic R wave in I, V5 and V6
  • ST and T wave displacement opposite to the major deflection of the QRS complex

Left ventricular hypertrophy

It can be diagnosed on ECG with good specificity. When the myocardium is hypertrophied, there is a larger mass of myocardium for electrical activation to pass through; thus the amplitude of the QRS complex, representing ventricular depolarization, is increased.
Non-atherosclerotic vasospasm
It is also known as variant angina or angina inversa — occurs with coronary artery vasospasm resulting in myocardial ischemia. The smooth muscle in the coronary wall contracts without explanation, resulting in decreased blood flow to the myocardium which causes symptoms of angina.

To summarize:

  • ST-Elevation Myocardial Infarction (STEMI) is a very serious type of heart attack during which one of the heart’s major arteries is blocked.

  • STEMI is an important diagnosis to make in a timely fashion due to its associated morbidity and mortality.

  • Despite relatively codified definitions for the diagnosis, there remains some clinical subjectivity in the diagnostic components, including both the specific ECG findings and the timing of ECG testing.

References:

MCQS:

1. What is the Q wave present during ST segment elevation myocardial infarction?

A.Narrow and shallow

B.Wider and deeper than normal Q waves

C.Both

D.None of the above

If you observe the comparison of normal and abnormal ECG, normally occurring Q waves are narrow and shallow, whereas abnormal Q’s are wider and deeper than normal Q waves.

2. Which of the following alterations of heart sounds is present during active myocardial ischemia?

A.Fixed split S2 heart sound

B.The presence of an S3 heart sound

C.The presence of an S4 heart sound

D.Decreased intensity of the S1 heart sound

An S4 heart sound may be present during myocardial ischemia due to the lack of ATP production impairing left ventricular relaxation.

3. Which of the following is considered diagnostic ECG changes for anterior STEMI?

A. Total of 1 mm ST elevation in leads V1-V3 in a male > 40-year-old.

B.Total of 0.5 mm ST elevation in aVF and lead III in female > 40 years old

C.Total of 2 mm ST elevation in leads V2-V3 in a male < 40-year-old

D.Total of 1.5 mm ST elevation in leads V2-V3 in a female > 40-year-old

A total of 1.5 mm of ST elevation in leads V2-V3 is required to diagnose anterior STEMI in women older than 40 years of age.

4. Which of the following best describes the ECG findings of a true posterior myocardial infarction?

A.ST elevation in V5-V6 with R: S ratio < 1 in lead V1

B.ST depression in the inferior leads with R: S ratio > 1 in lead V1

C.ST depression in V1-V2 with R: S ratio > 1 in lead V1

D.ST depression in V1-V2 with R: S ratio < 1 in lead V1

The ratio of the R wave to the S wave in leads V1 or V2 is > 1

5. Which of the following statements is true?

A.Most myocardial infarctions originate from rupture of nonocclusive coronary plaques

B.Coronary vasospasm is a common cause of myocardial infarction

C.Plaque characterization to distinguish vulnerable from stable coronary plaques is a commonly utilized risk-stratification tool

D.A critical hemodynamic coronary stenosis is a prerequisite of an acute coronary syndrome

Plaque rupture and thrombosis frequently occur at the site of modest coronary stenosis (less than 50% luminal narrowing); thus even if stress testing is normal, the risk of an acute coronary syndrome is still present.

6. Which of the following describes a patient with a myocardial infarction that is Killip Class II?

A.Findings of mild to moderate heart failure (S3 gallop, rales < half-way up lung fields or elevated jugular venous pressure)

B.Pulmonary edema

C.Cardiogenic shock defined as systolic blood pressure < 90 mmHg and signs of hypoperfusion such as oliguria, cyanosis and sweating

D.No evidence of heart failure

Class II: Findings of mild to moderate heart failure (S3 gallop, rales < half-way up lung fields or elevated jugular venous pressure

7. What are the different bio markers involved/released in MI?

A.Myoglobin

B.Troponin

C.Creatine kinase

D.All of the above

Biomarkers include myoglobin, troponin and creatine kinase, with troponins being the most sensitive and specific for myocardial injury.

8. Which of the following ECGs is consistent with a localized anterior ST elevation myocardial infarction in a 60-year-old male?

A.  B.  C. 
D.   E.  

Choice E shows a typical, “tombstoning” appearance of ST elevation during an anterior myocardial infarction.