8. ECG Interpretation: Myocardial Ischaemia & Infarction (STEMI/NSTEMI)
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When the ECG shows a condition in one lead, it is typically present in all appropriate leads; however, bundle branch blocks require you to examine multiple leads to interpret the pattern correctly.
In exams or clinical cases, focus on conditions that require looking at all leads rather than a single lead alone.
Myocardial Ischaemia & Infarction: STEMI, NSTEMI, and Ischaemia
STEMI (ST-Elevation Myocardial Infarction) is a transmural infarct (through the full thickness of the heart wall) and is characterized by:
ST segment elevation
T-wave inversion
Significant Q-wave (defined as a Q-wave ≥ 1 mm wide or ≥ 1/4 the amplitude of the R wave)
NSTEMI (Non-Q Wave MI) is subendocardial and presents with:
ST segment depression
T-wave inversion
May be seen at rest and can be persistent; nitrates may not relieve it
Ischaemia (without infarction) presents with:
ST segment depression
T-wave inversion
Usually brought on by exertion and often reversible with rest or administration of nitrates
Key question: What is the difference between Ischaemia and Infarction?
Ischaemia shows ST depression and/or T-wave inversion due to reversible conditions; infarction shows tissue death with Q waves and evolving ST/T changes.
Coronary Anatomy and Blood Supply (Region-to-Leads mapping)
Regions and corresponding leads/arteries:
Lateral
Leads: I, aVL, V5, V6
Main artery: Circumflex + LAD (territory overlap)
Anterior/Septal
Leads: V1, V2, V3, V4
Main artery: LAD
Inferior
Leads: II, III, aVF
Main artery: Right Coronary Artery (RCA)
Inferior mapping may overlap with other territories depending on dominance
Practical note: Knowledge of territory helps in localizing the infarct and guiding management.
Regions, Leads, and Arteries (Table summarized)
Anterior wall
Leads: V2 to V4
Artery: Left Anterior Descending (LAD)
Reciprocal changes: II, III, aVF
Anterolateral wall
Leads: I, aVL, V3 to V6
Arteries: LAD and Circumflex
Reciprocal changes: II, III, aVF
Septal wall
Leads: V1 to V2
Artery: LAD
Reciprocal changes: None
Inferior wall
Leads: II, III, aVF
Artery: Right Coronary Artery (RCA)
Reciprocal changes: I, aVL
Lateral wall
Leads: I, aVL, V5, V6
Artery: Circumflex (branch of the left coronary artery)
Reciprocal changes: II, III, aVF
Evolution of STEMI: Timeline and EKG Changes
General idea: The age of MI progresses through phases with characteristic ECG changes.
Timeline (as presented in the material):
Immediately before MI starts
T wave inversion indicates early ischemia.
Within hours after MI starts (Acute stage)
Marked ST elevation with upright T waves (acute injury).
Hours later (Acute stage progression)
Significant Q waves develop with ST elevation and upright T waves (injury progressing to infarction).
Infarction edges progressing
Significant Q waves with T wave inversion begin to appear.
Infarction is complete
Infarction is complete; some injured tissue may remain, but ischemia has ceased.
Weeks, months, years later (old infarct)
Persistent Q waves indicate permanent tissue death.
Implications: This evolution helps in dating the infarct and understanding tissue viability at different times after onset.
EKG change overview:
Early: T wave inversion may appear as initial sign of ischemia.
Acute injury: ST segment elevation with upright T waves (classic STEMI pattern).
Infarction progression: Q waves develop; ST elevations may persist or evolve; T waves may invert.
Chronic stage: Persistent pathological Q waves indicate established infarct; T waves may normalize or remain inverted depending on the case.
Diagnosis of STEMI: Criteria and Features
Diagnostic criteria for STEMI:
ST elevation > 1\ \text{mm} in two contiguous limb leads or > 2\ \text{mm} in two contiguous chest leads.
R-wave morphology should be convex (often described as a tombstone appearance).
Look for the presence of significant Q-waves.
Look for reciprocal changes in opposite leads.
Practical cues:
A convex or tombstone-shaped ST elevation strongly supports an acute STEMI.
Q waves indicate that infarction has occurred and may be evolving into a chronic pattern.
Reciprocal changes (opposite leads) support the diagnosis and help localize the infarct.
Reciprocal Changes and Locating Myocardial Damage
Concept: Changes in one region of the heart (damaged/dysfunctional area) are often accompanied by reciprocal changes in the opposite leads.
Reciprocal changes are most commonly seen in leads opposite the injured area.
Example mappings (reciprocal relationships):
Anterior wall infarct (V2–V4): Reciprocal changes in II, III, aVF.
Anterolateral wall infarct (I, aVL, V3–V6): Reciprocal changes in II, III, aVF.
Septal infarct (V1–V2): Typically none documented as reciprocal in this chart.
Inferior infarct (II, III, aVF): Reciprocal changes in I, aVL.
Lateral infarct (I, aVL, V5–V6): Reciprocal changes in II, III, aVF.
Practical use: Identifying reciprocal changes helps confirm infarct location and can aid in determining the involved coronary artery.
Myocardial Ischaemia: ST Depression and T-Wave Inversion
Ischaemia signs on ECG:
ST segment depression and/or new or worsening T-wave inversion are indicators of ischaemia.
Inverted T-waves in leads with upright QRS complexes are abnormal and concerning.
Morphology of ST depression:
Upsloping ST depression: Not always pathological; may be seen in some normal variants or benign conditions.
Downsloping ST depression: More often considered pathological.
Horizontal ST depression: Pathological and strongly suggests ischaemia.
Diagnostic threshold for ischaemia (ECG):
Horizontal or downsloping ST depression ≥ 1\,\text{mm} at the J-point in ≥ 2 contiguous leads indicates myocardial ischaemia.
Ischaemia symptoms:
Chest pain or discomfort, often described as pressure, squeezing, or heaviness.
Arm pain, which may radiate to the neck, jaw, back, or stomach.
Shortness of breath, which may occur with or without chest discomfort.
Nausea, light-headedness, or cold sweats.
Ischaemia vs Normal vs ST Depression Illustrations
Normal ECG vs ST depression patterns can be visualized in typical diagrams (not reproduced here).
ST depression patterns (A = upslope, B = downslopes, C = horizontal) illustrate how different morphologies relate to pathology.
Practical takeaway: Horizontal or downsloping patterns, especially across multiple contiguous leads, are more strongly associated with ischaemia.
Exercise Stress Testing: Pathological ST Depression During Exercise
Exercise ECG situations show dynamic ischaemia:
At rest, there may be no abnormalities, but with increased workload, ST segment depressions can appear.
Example: In a coronary artery disease patient, depressions appear at the J-60 and J-80 points and become more pronounced with higher workloads.
Interpretation: Exercise-induced ST depressions support the presence of flow-limiting coronary disease and help assess functional significance.
Practical Example: Rhythm Strip and Lead Readouts (Illustrative Notes)
Rhythm strips (e.g., Lead II at 25 mm/sec; 1 cm/mV) are used to confirm rhythm and evaluate ST-T changes in context.
While specific rhythm patterns are not exhaustively listed here, the general approach includes:
Correlating rhythm with regional infarct patterns (based on lead changes).
Assessing ST segments, T waves, and Q waves across multiple leads to localize infarct.
Quick Reference: Key Definitions and Thresholds
STEMI: transmural infarction with ST elevation and/or Q waves; diagnostic criteria include:
ST\ elevation > 1\ \text{mm} in two contiguous limb leads or > 2\ \text{mm} in two contiguous chest leads.
Convex R-wave evolution and potential tombstone pattern.
Significant Q-waves indicating infarct maturation.
NSTEMI: subendocardial infarction with ST depression and/or T-wave inversion; may not present with ST elevation.
Ischaemia: ST depression and/or T-wave inversion without established infarction; often exertional and reversible with rest/nitrates.
ST depression morphologies: upslope, downslope, horizontal (the latter two are more strongly associated with ischaemia).
Threshold for ischaemia on ECG: horizontal/downsloping ST depression ≥ 1\,\text{mm} at the J-point in ≥ 2 contiguous leads.
Reciprocal changes: opposite changes in leads opposite the damaged area; useful for confirming infarct localization.
Summary of Exam-Relevant Concepts
Distinguish STEMI, NSTEMI, and ischaemia by ECG patterns, Q waves, and ST-T changes.
Know lead-to-territory mappings to localize infarct and predict the involved artery.
Recognize the evolution of STEMI on ECG over time and how this informs clinical dating and management.
Use reciprocal changes as a corroborating sign of infarct location.
In exercise testing, stress-induced ST depressions indicate flow-limiting coronary disease and guide risk stratification.
Note: The content above consolidates the key ideas and specifics from the provided transcript, including ST elevations, Q waves, ischaemia criteria, lead mappings, and the evolution of STEMI. The LaTeX-formatted thresholds and equations are embedded where appropriate to aid precise study and exam preparation.