Cardiac Rhythm Simulator

Lateral Myocardial Infarction

MISW Condition

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Clinical Overview

Lateral myocardial infarction is necrosis of the lateral wall of the left ventricle — the territory viewed by leads I, aVL, V5, and V6 — following interruption of its blood supply. That territory is most often supplied by the left circumflex (LCx) coronary artery or one of its branches (commonly the obtuse marginal), though when the infarction is confined to the “high” lateral segment (leads I and aVL, with only subtle V5-V6 change) the culprit is frequently the first diagonal branch of the left anterior descending (LAD) artery instead; LCx occlusion tends to produce the broader pattern extending into V5-V6 (Burns & Buttner, LITFL, 2024; StatPearls, “Lateral Wall Myocardial Infarction,” 2024). This dataset’s own label for this acronym, “Myocardial Infarction In The Side Wall,” is its own annotation vocabulary for the same lateral-wall concept. A SNOMED-mapping review of this dataset found that MISW’s underlying SNOMED CT code resolves, on independent lookup against the HL7 FHIR SNOMED CT terminology server (SNOMED CT International edition, version 20250201), only to the generic finding “ECG: myocardial infarction” — none of that code’s preferred term or synonyms carry a lateral- or side-wall-specific qualifier. The dataset’s own raw annotation table shares this identical code across five separate wall-specific labels (MI, MIBW/back wall, MIFW/front wall, MILW/lower wall, and MISW/side wall), so the code itself cannot confirm or refute that any given MISW-labeled record is genuinely lateral rather than a different territory. This page treats MISW as the dataset’s intended lateral (side)-wall finding, grounded in the label plus the independent clinical literature on lateral-wall MI cited throughout, but the territory-specific claim rests on the dataset’s own labeling convention rather than a verified SNOMED mapping [CLINICAL REVIEW NEEDED: whether MISW-labeled records are reliably lateral-wall-localized cannot be confirmed from the SNOMED code alone].

Mechanistically, lateral MI follows the same sequence as any acute coronary occlusion: an atherosclerotic plaque in the culprit vessel ruptures or erodes, triggering platelet aggregation and thrombus formation that occludes the artery; the myocardium supplied by that vessel is starved of oxygen, and if flow is not restored, necrosis progresses from the subendocardium outward toward the epicardium over the following hours (StatPearls, “Acute Myocardial Infarction,” 2023; StatPearls, “Lateral Wall Myocardial Infarction,” 2024). Because the lateral wall’s blood supply is shared with adjacent territories — the LCx also frequently supplies the posterior and inferior walls, and the first diagonal is a branch of the LAD’s anterior territory — a lateral infarction commonly extends into a neighboring territory rather than staying isolated, most often producing a combined inferolateral or posterolateral pattern (Burns & Buttner, LITFL, “Lateral STEMI,” 2024; Burns & Buttner, LITFL, “Posterior Myocardial Infarction,” 2024).

Clinically, the LCx is the culprit vessel in roughly 15-20% of all myocardial infarctions, and lateral involvement (isolated or as an extension of another territory) is reported in about 13-23% of ST-elevation MIs (StatPearls, “Lateral Wall Myocardial Infarction,” 2024). Lateral MI is disproportionately likely to be underdiagnosed or diagnosed late: its ECG changes are frequently subtler than the more familiar anterior or inferior patterns, and the “high lateral” variant in particular can be easy to overlook because its ST elevation sits in only two, non-adjacent-looking leads (I and aVL) rather than a contiguous precordial block (Burns & Buttner, LITFL, “High Lateral STEMI,” 2023). When a lateral pattern extends into an adjacent territory, it signals a larger area of myocardium at risk and a correspondingly worse prognosis than an isolated, smaller infarct (Burns & Buttner, LITFL, “Lateral STEMI,” 2024).

Symptoms of lateral MI are the general symptoms of acute MI rather than anything territory-specific: deep, substernal, visceral chest pressure or pain, often radiating to the arm, jaw, neck, or back, together with dyspnea, diaphoresis, nausea, or vomiting (StatPearls, “Acute Myocardial Infarction,” 2023). Presentation is frequently atypical, particularly in women, older adults, and patients with diabetes — abdominal pain, dizziness, unusual fatigue, dyspnea, or confusion without classic chest pain are all recognized presentations in these groups (StatPearls, “Acute Myocardial Infarction,” 2023). Silent, painless ischemic episodes are common generally — most transient ischemic episodes on continuous monitoring produce no angina at all — and older patients with diabetes or a prior infarction are the group most likely to have a silent event go unrecognized until a later, incidental ECG (StatPearls, “Silent Myocardial Ischemia,” 2024).

Causes and risk factors mirror coronary artery disease generally rather than anything lateral-wall-specific: modifiable factors — smoking, an abnormal lipid profile, hypertension, diabetes, obesity, physical inactivity, and a poor diet — account for the large majority of cases, alongside non-modifiable factors of advancing age, male sex, and family history of premature coronary disease (StatPearls, “Acute Myocardial Infarction,” 2023).

Interpretation Guide

Key Features:

  • Rate: not a defining feature — a lateral infarction is a morphology and ST/T finding superimposed on whatever rate the underlying rhythm carries
  • Rhythm: not a defining feature — this finding describes the affected myocardial territory, not the rhythm’s origin or regularity
  • P waves: within normal limits; unaffected by the infarction itself
  • PR interval: within normal limits (0.12-0.20 s)
  • QRS complex: within normal limits acutely; an old or completed lateral infarction can leave a pathological Q wave (duration ≥40 ms and/or depth ≥25% of the following R wave) in the lateral leads I, aVL, V5, and V6, following the same general Q-wave criteria used for infarction elsewhere
  • ST segment: the defining acute feature — new ST-segment elevation at the J point in leads I, aVL, V5, and/or V6, generally read against a threshold around 0.1 mV (1 mm) in two or more contiguous leads; the “high lateral” variant shows elevation concentrated in I and aVL (± subtle V2) with only subtle or absent V5-V6 change, sometimes described as the “South African flag sign” pattern (Burns & Buttner, LITFL, “High Lateral STEMI,” 2023; StatPearls, “Lateral Wall Myocardial Infarction,” 2024) [CLINICAL REVIEW NEEDED: exact mV cutoffs for lateral-lead ST elevation vary slightly by source and this page uses the commonly cited general threshold rather than a single universally agreed figure]
  • T waves: T-wave inversion in the lateral leads can accompany or follow an evolving infarction; on an old, stable infarct the T waves are frequently normal
  • QT interval: not independently affected by this finding
  • Other findings: reciprocal ST-segment depression in the inferior leads (III and aVF especially) frequently accompanies acute lateral ST elevation, and may be the more obvious abnormality on the strip when the lateral elevation itself is subtle; because the lateral wall’s supply is often shared with the posterior wall, check leads V1-V3 for a mirror-image pattern (ST depression, tall/broad R waves, upright T wave) that can indicate posterior extension, which is not directly visualized by the standard 12-lead set (Burns & Buttner, LITFL, “Posterior Myocardial Infarction,” 2024)

Reciprocal ST depression in the inferior leads is a genuinely useful confirmatory clue for lateral involvement — when the lateral changes themselves are subtle, that reciprocal pattern is sometimes the first thing to catch the eye, and its presence should prompt a deliberate look at I, aVL, V5, and V6 rather than being dismissed as an isolated inferior finding.

Key Leads

  • Leads I, aVL, V5, V6 – the primary lateral lead group; the defining ST-elevation and (on an old infarct) Q-wave territory
  • Leads I, aVL (± V2) – the “high lateral” subset; ST elevation confined largely to these leads, with only subtle V5-V6 change, points toward a first-diagonal (LAD branch) culprit rather than the broader LCx-territory pattern
  • Leads III, aVF – reciprocal ST depression here often accompanies acute lateral elevation and can be the more visually obvious abnormality on the strip
  • Leads V1-V3 (indirect) – no standard 12-lead electrode directly faces the posterior wall, so posterior extension of a lateral infarct shows as a mirror-image pattern here (ST depression, tall R waves) rather than ST elevation

Differential Diagnosis

  • Abnormal Q Wave (AQW) — a pathological Q wave confined to the lateral leads (I, aVL, V5-V6) is one of this dataset’s general, non-territory-specific findings; supporting ischemic history, accompanying ST/T changes, or a documented event favor the more specific lateral-infarction label over the general Q-wave finding alone
  • Anterior Myocardial Infarction (AnMI) — a different LAD-territory infarction; ST elevation and Q waves confined to the anterior precordial leads (V1-V4) rather than I, aVL, V5-V6 point toward the anterior label instead
  • Acute Myocardial Infarction (AMI) — this dataset’s general acute-MI label; when ST elevation and reciprocal changes are not clearly confined to the lateral lead group, or territory cannot be localized from the strip alone, the general acute-MI finding is the more defensible read than a specific wall assignment
  • R Wave Abnormal (RWAb) — a tall, broad R wave with an R/S ratio ≥1 in V1-V2 is the classic mirror-image marker of posterior involvement, which frequently co-occurs with lateral infarction because the two territories often share a culprit vessel; its presence alongside lateral changes suggests posterolateral extension rather than an isolated lateral event
  • Left Ventricle Hypertrophy (LVH) — a lateral “strain” pattern (ST depression and T-wave inversion in I, aVL, V5-V6) can mimic ischemic lateral change; voltage criteria for hypertrophy and the absence of reciprocal inferior ST depression point toward strain rather than a true lateral infarction

Treatment Brief

New ST-segment elevation confined to I, aVL, V5, and/or V6 — or to I and aVL alone with reciprocal inferior depression — is a time-critical, STEMI-equivalent finding regardless of how subtle it looks, and should be treated with the same urgency as a more obvious anterior pattern.

  • Notify the provider immediately for any new lateral ST elevation, and do not let a “high lateral” pattern’s subtlety (elevation in only I and aVL) delay escalation the way a broader, more visually obvious pattern would not.
  • Confirm lead placement and repeat the strip if the pattern is isolated or unexpected, since technical artifact can mimic or mask a true lateral change.
  • Obtain serial 12-lead strips and flag the patient for serial troponin measurement; a single normal or equivocal strip does not exclude an evolving infarction.
  • Scan the inferior leads (III, aVF) for reciprocal ST depression and the anterior precordial leads (V1-V3) for a posterior mirror-image pattern — either supports lateral involvement and can help confirm a subtle presentation.
  • Maintain continuous rhythm monitoring and report new ectopy or conduction disturbance promptly, as with any acute MI event.
  • An old, stable Q-wave or T-wave pattern in these same leads, without new ST changes and with a known prior cardiac history, is not a STEMI-equivalent emergency but still warrants documentation and routine provider follow-up.

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