This guide delves into the mechanisms of EKG alterations in ischemia and infarction, providing a detailed analysis to enhance understanding and clinical application.
EKG Changes in Ischemia and Infarction
ST-Elevation Myocardial Infarction (STEMI)
STEMI is characterized by specific EKG changes that typically appear in at least two contiguous leads. These changes reflect acute transmural myocardial injury.
Hyperacute T-Waves
The earliest electrocardiogram (ECG) indicator of STEMI is the presence of hyperacute T-waves, which are characterized by their symmetric and peaked appearance.
ST Elevation Criteria
ST elevation is a hallmark of acute MI. Abnormal ST elevation is measured at the J point—the junction where the QRS complex meets the ST segment. The criteria for significant ST elevation are:
- General Leads (excluding V2 and V3): Elevation of ≥0.1 mV (1 mm).
- Leads V2 and V3:
- Women: ≥0.15 mV.
- Men ≥40 years: ≥0.2 mV.
- Men <40 years: ≥0.25 mV.
Understanding these gender and age-specific thresholds is crucial for accurate diagnosis.
Differentiating ST Elevation Causes
Myocardial Infarction
In MI, the ST segment is typically convex upwards and may merge with the QRS complex, creating a “tombstone” appearance. This bowing upwards indicates significant myocardial injury.
J-Point Elevation (Early Repolarization)
J-point elevation is a benign variant often seen in healthy young adults. It features a slight elevation at the J point with a distinct ST segment and T-wave morphology. The ST segment is usually concave upwards, and there is no merging with the QRS complex.
Pericarditis
Pericarditis presents with diffuse ST elevation that is concave upwards and lacks reciprocal changes. A key distinguishing feature is PR segment depression in multiple leads except for lead aVR, where PR elevation may occur. The changes in pericarditis are more widespread compared to the localized patterns seen in MI.
Persistent ST Elevation and Ventricular Aneurysm
ST elevation usually resolves within a few hours in acute MI. If it persists beyond three weeks, consider the possibility of a ventricular aneurysm resulting from extensive myocardial damage.
Abnormal Q Waves
Q waves signify irreversible myocardial necrosis. Abnormal Q waves are defined as:
- Leads V2 to V3: Any Q wave ≥20 milliseconds or a QS complex.
- Other Leads (I, II, aVL, aVF, V4 to V6): Q wave ≥30 milliseconds and ≥0.1 mV deep in two contiguous leads or a QS complex.
The presence of Q waves without accompanying ST changes often indicates a previous infarct rather than an acute event.
Reciprocal Changes
Reciprocal changes enhance the diagnostic accuracy for MI. They occur in leads opposite the site of infarction and include:
- Tall R-Waves: Reciprocal of Q waves.
- ST Depression: Reciprocal of ST elevation.
- T-Wave Inversions: Reciprocal of hyper acute T waves.
Recognizing reciprocal changes helps confirm the diagnosis and localize the infarct.
Localization of Infarcts on EKG
Inferior Infarcts
Inferior MI involves the diaphragmatic surface of the heart and affects:
- Primary Changes: Leads II, III, and aVF.
- Reciprocal Changes: Leads I and aVL.
Note: Changes in V1 – V3 may be reciprocal; however, be alert for a concurrent posterior myocardial infarction if such changes are observed. An EKG with posterior leads can help confirm the diagnosis.
Right Ventricular Infarcts
Right ventricular infarction should always be considered in any patient with an inferior myocardial infarction, particularly under the following conditions:
- ST Elevation in Lead V1: V2 may show ST elevation or depression.
- ST Elevation in Lead III > Lead II: Because lead III is more rightward-facing.
Right ventricular involvement can significantly impact management and prognosis. An EKG with right sided leads can help confirm the diagnosis.
Anterior and Lateral Infarcts
Anterior MI affects the anterior wall of the left ventricle, while lateral MI involves the lateral wall.
Anteroseptal MI
- Leads Involved: V1 and V2.
- Significance: Indicates septal wall infarction.
Anteroapical MI
- Leads Involved: V3 and V4.
- Significance: Reflects damage to the apical region.
Anterolateral MI
- Leads Involved: I, aVL, V5, and V6.
- Significance: Involvement of the lateral wall.
High Lateral MI
- Leads Involved: I and aVL.
- Significance: Indicates infarction in the high lateral wall.
Poor R-Wave Progression
Poor R-wave progression across the precordial leads may signify anterior MI. However, it can also be caused by:
- Lung Disease: Alters electrical conduction.
- Right Ventricular Hypertrophy (RVH): Changes ventricular depolarization patterns.
Wellens’ Syndrome
Wellens’ syndrome is characterized by:
- EKG Findings: Symmetric T-wave inversions or biphasic T-waves in leads V2 and V3 without significant ST segment changes.
- Cardiac Enzymes: Typically normal.
- Clinical Significance: Indicates critical stenosis of the proximal left anterior descending (LAD) artery, posing a high risk for extensive anterior MI.
Posterior Infarcts
Posterior MI affects the posterior wall of the left ventricle. Needs to be suspected in all patients with inferior wall MI.
EKG Challenges
- Standard 12-Lead EKG: Lacks posterior leads.
- Reciprocal Changes in Leads V1 to V3:
- Tall R-Waves: Reciprocal of Q waves.
- ST Depression: Reciprocal of ST elevation.
- Upright T-Waves: Reciprocal of T-wave inversions.
Diagnostic Approach
- 15-Lead EKG: Incorporates posterior leads (V7 to V9) to detect posterior MI.
- Conditions Mimicking Posterior MI:
- RVH
- Right Bundle Branch Block (RBBB)
- Wolff-Parkinson-White (WPW) Syndrome
These conditions can cause early R-wave progression, complicating the diagnosis.
Non-ST-Elevation Myocardial Infarction (NSTEMI) and Unstable Angina
EKG Changes
Both NSTEMI and unstable angina may present with:
- ST Depression:
- Horizontal or down-sloping ST segment depressed ≥0.5 mm below baseline.
- Measured 0.08 seconds (2 mm) after the J point.
- Present in at least two contiguous leads.
- T-Wave Inversions:
- Especially significant if inversion exceeds 1 mm.
Differentiating NSTEMI from Unstable Angina
NSTEMI
- Persistent EKG Changes: ST depression and T-wave inversions persist for longer.
- Troponin Elevation: Indicates myocardial injury.
Unstable Angina
- Transient EKG Changes: Normalize as symptoms resolve.
- Normal Troponin Levels: No myocardial necrosis.
ST Elevation in Lead aVR
- Significance: ST elevation in lead aVR with widespread ST depression suggests:
- Left Main Coronary Artery Disease
- Severe Triple-Vessel Disease
Prinzmetal Angina
Pathophysiology
- Cause: Coronary vasospasm leading to transient myocardial ischemia.
- Occurrence: Often at rest and can be cyclical.
EKG Presentation
- Transient ST Elevation or Depression: Mimics acute MI.
- Resolution: EKG changes and symptoms resolve when the vasospasm subsides.
Occlusion Myocardial Infarction (OMI)
Electrocardiography (EKG) remains an indispensable tool in diagnosing and managing myocardial ischemia and infarction. Traditionally, the focus has been on identifying ST-Elevation Myocardial Infarction (STEMI) through specific EKG criteria to expedite reperfusion therapy. However, recent advancements advocate for a shift towards the Occlusion Myocardial Infarction (OMI) paradigm. This approach emphasizes the underlying pathophysiology—coronary artery occlusion—over strictly adhering to EKG criteria.
What Is OMI?
Occlusion Myocardial Infarction (OMI) refers to myocardial infarctions caused by a complete or near-complete blockage of a coronary artery, leading to significant myocardial ischemia and necrosis. Unlike the traditional STEMI classification, which relies heavily on EKG findings, OMI focuses on the presence of an occlusion, regardless of whether the classic EKG criteria for STEMI are met.
Why Shift from STEMI to OMI?
Limitations of STEMI Criteria: Up to one-third of acute coronary occlusions may not exhibit the traditional EKG characteristics of STEMI, potentially resulting in missed diagnoses and delayed treatment. Notable EKG changes to monitor include the new onset of bifascicular blocks, new onset LBBB, and pseudonormalization patterns in the appropriate clinical context. Pseudonormalization on an EKG refers to a phenomenon where previously abnormal waves, such as inverted T waves, seem to revert to their normal position, creating a misleading impression of recovery or improvement. However, this “normalization” is deceptive as it often indicates a deterioration of the underlying cardiac condition, such as progressing myocardial ischemia.
Pathophysiological Focus: OMI emphasizes identifying and treating the underlying coronary occlusion promptly to prevent extensive myocardial damage.
Improved Patient Outcomes: Early recognition and intervention can significantly reduce morbidity and mortality associated with myocardial infarctions.
Clinical Implications
Diagnostic Approach: Clinicians are encouraged to consider the possibility of OMI even when EKG findings are subtle or non-specific.
Treatment Strategies: Prompt reperfusion therapy should be considered based on clinical suspicion of occlusion and acuity, not solely on EKG criteria.
Decision to Perform Emergency Intervention: Both American and European guidelines advise that any patient experiencing ongoing ischemic symptoms despite initial treatment should be promptly considered for an immediate angiography even if they do not have a STEMI.
Atrial Ischemia and Infarct
When to suspect
Atrial infarction should be considered in patients with ventricular myocardial infarction who present with any type of atrial arrhythmia.
Criteria (Liu’s Criteria)
Before we delve into the criteria, let’s first explore the concept of the PTa segment. On an electrocardiogram (ECG), the “PR segment” refers to the flat section between the end of the P wave and the start of the QRS complex, representing the electrical delay between atrial and ventricular depolarization. In contrast, the “PTa segment” (sometimes referred to as PTA) is a less commonly used term that describes the entire section from the end of the P wave to the end of the atrial repolarization wave. Essentially, this encompasses the PR segment and a portion of the “T wave” of the P wave itself. Since atrial repolarization is often lost in the QRS complex, PTa segment changes are evaluated by looking at the PR segment itself.
- PR segment: A specific, short flat section between the P wave and QRS complex.
- PTa segment: A broader section including the PR segment and the tail end of the P wave repolarization.
Now let’s explore the Liu’s criteria for atrial ischemia/ infarct:
Major Criteria
- Elevation of the P-Ta segment of over 0.5 mm in V5 and V6, with reciprocal depression of the same segment in V1 and V2.
- Elevation of the P-Ta segment exceeding 0.5 mm in lead I, with reciprocal depression of the same segment in leads II or III
- Depression greater than 1.5 mm in the precordial leads and 1.2 mm in leads I, II, and III, particularly in the context of any atrial arrhythmia.
Note: Depression of the P-Ta segment of small amplitude without elevation of this segment in other leads cannot be regarded by itself as positive evidence of atrial infarction.
Minor Criteria
- Abnormal P waves: M-shaped, W-shaped, irregular or notched
Importance
Often linked to a poor prognosis, these criteria can aid in diagnosing an ischemic event even when the ventricular criteria for ischemia or infarct are not fully met.