B

Recording-2025-08-18T13:59:58.003Z

Inferior STEMI: overview and key considerations

  • Inferior category includes leads II, III, and AVF. These are contiguous in ECG territory because they anatomically correspond to the inferior wall of the heart which is supplied by the right coronary artery in most individuals.
  • Inferior STEMI accounts for about 40\%-50\% of STEMIs (not all MIs).
  • Prognosis: inferior STEMIs generally have a more favorable prognosis than anterior STEMIs, but there is a notable exception: about 40\% of inferior STEMIs have a concomitant right ventricular infarction (RVI).
  • Right ventricular infarction (RVI) implications:
    • Can cause severe hypotension after nitrates due to preload reduction; nitrates are often contraindicated or used with caution.
    • RVI is typically RV-dominant and can present with bradycardia and hypotension.
    • Screening for RVI: not always identified on a standard 12-lead; involves additional steps (see 15-lead). The minimum to assess RV involvement is V4R; the gold standard is a 15-lead ECG.
    • If you suspect RVI, you must adjust nitro use: avoid nitro if RVI suspected or if V4R elevation is present (see below).
  • Posterior involvement with inferior MI:
    • Inferior MI can be associated with posterior infarction, which is often underdiagnosed because standard 12-lead views only the anterior surface of the heart (~60% viewed).
    • Posterior MI can be better detected with 15-lead ECG that includes posterior leads (V7–V9) and by moving V4/V5/V6 to posterior positions (V8/V9) during the 15-lead view.
  • Practical approach for inferior MIs:
    • Primary steps: obtain 12-lead ECG, look for ST elevation in leads II, III, AVF; check reciprocal changes in other leads (e.g., ST depression in V1–V3 can accompany inferior STEMI).
    • If inferior STEMI is suspected, you should perform V4R (and possibly a 15-lead) to assess for RVI and posterior infarction.
    • The minimum standard is a V4R; the full assessment uses a 15-lead.
    • In practice, if ST elevation exists in II, III, AVF, and there is reciprocal depression, you should consider an inferior STEMI and prepare for cath lab activation if criteria are met.
  • Diagnostic cues on the 12-lead for inferior STEMI:
    • ST elevation in leads II, III, AVF is the hallmark.
    • Q waves may be present in II, III, AVF.
    • Reciprocal changes: when one region is elevated, another region tends to show depression (e.g., V1–V3 may show depression when II/III/AVF are elevated).
    • J-point: verify elevation relative to isoelectric baseline; subtle elevations can still indicate MI.
    • The presence of reciprocal changes supports genuine STEMI but their absence does not completely exclude MI.
  • Example patterns for inferior STEMI (described in the lecture):
    • Example 1: Subtle ST elevations in II, III, AVF (≈2 mm) with subtle depression in AVL (~1 mm) and small depression in VT or suggestive changes elsewhere; call MI if criteria met.
    • Example 2: Clear ST elevation in II, III, AVF (~3 mm) with corresponding reciprocal changes; easier to recognize on a 4-channel ECG (the extra channel helps view the underlying rhythm).
    • Example 3: A well-evolved inferior MI with noticeable elevation in II and III (≈3–4 mm) and additional depression in other leads; often accompanied by some ST elevation in V1 (or minimal), demonstrating posterior or RV involvement.
  • Important practical note: recognition may be aided by simple mnemonic and by counting millimeters across leads; however, computer readouts can assist with ST elevations, QRS duration, PR interval, etc.
  • Percussive notes on inferior MIs:
    • 12-lead alone may miss posterior/posterior RV involvement; 15-lead or V4R helps.
    • If an inferior MI has a suspected RVI, do not give nitrates if V4R ≥ 1\,\text{mm} elevation; if V4R is negative or < 1\,\text{mm}, nitro may be considered as per protocol.
    • Treat inferior RVIs as high-risk; ensure airway/ventilation is secure and monitor BP closely.

Anterior STEMI: patterns, prognosis, and patterns of involvement

  • Anterior STEMI usually results from occlusion of the left anterior descending (LAD) artery.
  • Prognosis: anterior MIs carry the worst prognosis due to large infarct size and loss of left ventricular forward pumping.
  • Key idea: anterior involvement is defined by ST elevation in leads V1–V6; the location and extent determine the specific subtype.
  • Anterior pattern categories based on which leads are involved:
    • Septal MI: ST elevation limited to V1–V2 (ventricle septum). Elevated ≥ 2\,\text{mm} in V1 and/or V2 defines septal MI.
    • Anterior MI: ST elevation from V2–V5; involves broad anterior wall (but not necessarily V1 or V6).
    • Anterior septal MI: elevation from V1–V4 (includes septal and anterior components).
    • Anterior and lateral MI: involvement from V1–V6 plus high-lateral leads (AVL, lead I); includes anterior components and lateral territories.
    • Extensive anterior MI: involves the entire anterior region (V1–V6) plus AVL and lead I; sometimes referred to as widowmaker due to high risk at the proximal LAD bifurcation.
  • Tombstoning morphology:
    • A tombstone ST elevation pattern indicates a very poor prognosis and significant myocardial injury; this morphology signifies a high-risk STEMI.
  • Lateral involvement in anterior MI:
    • Lateral patterns can accompany anterior MIs when the circumflex artery is involved or when LAD involvement extends laterally; patterns may include ST elevation in I, AVL, V5, V6 with reciprocal changes in III and AVF.
  • Practical approach:
    • The clinician’s goal is to determine STEMI vs non-STEMI and transport decisions, not necessarily to perfectly subtype (the treatment is largely similar for emergent care in most paramedic protocols).
  • Common variations:
    • Anterior and lateral involvement (circumflex or dual territory) is possible and may complicate ECG interpretation; still treated as STEMI if criteria are met.
  • Example scenarios described:
    • A basic anterior MI with ST elevation in V2–V4; possible accompanying elevation in V5 if lateral involvement; presence of first-degree AV block or PVCs may occur with larger infarcts.
    • An extensive anterior MI with ST elevation in V1–V6 plus I and AVL; tombstoning can occur in advanced stages.
  • Key takeaway for practice:
    • Identify whether anterior, lateral, or extensive involvement is present, but the treatment (nitro, aspirin, prompt cath lab) is guided by STEMI presence and hemodynamic status rather than precise subtyping.

Lateral STEMI: patterns and nuances

  • Lateral MI patterns involve leads I, AVL, V5, V6; no anterior component in classic lateral MI.
  • Reciprocal changes: often seen in leads III and AVF.
  • Variants:
    • Isolated lateral MI: ST elevation limited to I, AVL, V5, V6.
    • High lateral MI: prominent ST elevation in I and AVL with reciprocal changes in III and AVF.
    • Anterior-lateral involvement: can occur with LAD occlusion if the territory includes lateral aspects; may resemble anterior MI with added lateral involvement.
  • Practical guidance:
    • Look at leads I, AVL, V5, V6 for lateral involvement; check for reciprocal changes in leads III and AVF.
    • If lateral involvement coexists with inferior involvement, consider circumflex involvement and pursue 15-lead to assess posterior involvement if suspicion rises.
  • Example case described:
    • A high lateral MI shows elevation in I and AVL, with reciprocal depression in III and AVF.
  • Important caveat:
    • Lateral MIs can be less common than inferior or anterior MIs but carry significant prognostic weight depending on infarct size and the affected territory.

Posterior MI and the role of the 15-lead ECG

  • Posterior MI is often underdiagnosed with the standard 12-lead because the posterior wall is not directly visualized.
  • 15-lead ECG is used to visualize posterior and right-sided involvement:
    • Right-sided chest leads (V4R) view the right ventricle; posterior leads (V7–V9 or posterior placements of V8/V9) view the posterior wall.
  • Indications for a 15-lead ECG:
    • Any inferior involvement with ST depression in V1–V3 that could indicate a posterior component or a right-sided infarct; or when posterior infarction is suspected.
    • Suspected RV involvement in inferior STEMI; 15-lead helps confirm or rule out RVI.
  • How to perform the 15-lead in EMS practice (step-by-step as described):
    • Keep standard 12-lead in place.
    • Create a V4R lead to assess the right ventricle by moving the V4 electrode to the right side (V4R position).
    • Print a 12-lead with V4R included to evaluate for RV involvement (positive if ST elevation in V4R ≥ 1\,\text{mm}).
    • For posterior evaluation, move V5 and V6 posteriorly to form V8 and V9 (while printing/labeling appropriately).
    • The 15-lead view can reveal posterior MI (ST elevation in posterior leads) that is not visible in the standard 12-lead.
  • Key threshold for RVI on V4R:
    • If ST elevation in V4R ≥ 1\,\text{mm}, consider right ventricular infarct; nitroglycerin is typically avoided in RVI.
    • If V4R elevation is < 1\,\text{mm} or absent, nitro can be considered per protocol with caution.
  • Practical notes:
    • The 15-lead protocol views the posterior and right-sided aspects of the heart to avoid missing posterior or RV infarctions.
    • Label all modified placements (the computer readout may not reflect the changes).
    • The “boot” pattern (lower left boot-like ST elevations) might hint toward inferior MI with posterior involvement; V4R helps verify.

STEMI bypass protocol: criteria and decision-making

  • The STEMI bypass card/delegation is a guideline to activate PCI-capable centers when appropriate.
  • Key inclusion criteria (per the prompt card and companion guide):
    • Age: \ge 18 years.
    • Chest pain equivalent or symptoms consistent with cardiac ischemia or MI.
    • Time of onset within the last 12\text{ hours}.
    • 12-lead ECG criteria meeting STEMI thresholds (details below).
  • 12-lead criteria for STEMI activation (as simplified for this course):
    • Two contiguous leads with ST elevation meeting thresholds:
    • Within leads V1–V3: ST elevation of at least 2\,\text{mm} in two contiguous leads.
    • In other leads: ST elevation of at least 1\,\text{mm} in two contiguous leads.
  • Rationale for the thresholds:
    • In V1–V3, a baseline is often more variable; a higher threshold (2 mm) reduces false activations.
    • In other leads, 1 mm elevation in two contiguous leads is considered significant.
  • Activation steps once STEMI is identified:
    • Activate the cath lab and prepare for immediate transfer to a PCI center.
    • Even if the ECG normalizes later, activate the cath lab if criteria were initially met.
  • Important contraindications and reasons to withhold activation or modify management (simplified):
    • The patient is CTAS (triage) 1 or cannot secure airway/ventilate.
    • ECG is compatible with STEMI imitators: left bundle branch block (LBBB), ventricular paced rhythm, or other STEMI mimics.
    • Major contraindications to immediate PCI involvement or nitro protocols (see nitro section below).
    • Time-critical or hemodynamic instability that requires stabilization before cath lab activation.
  • Time targets and transport logistics:
    • Transport to a PCI center should occur within approximately 60\text{ minutes} of first medical contact when STEMI is identified.
    • If transport time is excessive, consider alternate strategies and consult the on-call cardiology/medical director.
  • Serial ECGs:
    • If the initial 12-lead does not clearly show STEMI but clinical suspicion remains high, perform serial ECGs to detect evolving STEMI changes.
  • Practical notes about document and process:
    • The companion document is a live guide; ED/EMS protocols may evolve and are intended to reflect common practice and safety.
    • If a patient cannot be quickly transferred to PCI, ensure appropriate stabilization and consider other destinations while continuing ECG monitoring.

Left bundle branch block (LBBB) and STEMI mimics: recognition and approach

  • LBBB is the most common STEMI mimic among bundle-branch patterns; it can obscure or imitate ischemic changes on ECG.
  • Right bundle branch block (RBBB) is less likely to mimic STEMI beyond V1, where RBBB may show some changes but typically not across all leads.
  • Diagnostic cues for LBBB (as described):
    • The key distinguishing feature used in this teaching is:
    • In V1, the deflection is markedly negative and wide (greater than 0.12\text{ s}, i.e., > 3 small squares).
    • This broad, predominantly negative deflection in V1 suggests LBBB if not clearly explained by prior ECGs.
    • In V1, LBBB typically presents with a wide QRS complex (> 0.12\text{ s}) and a negative deflection.
    • A typical RBBB pattern includes an RSR' in V1 (the classic “rabbit ears” pattern) and a wide QRS, but RBBB generally does not mimic STEMI to the same extent as LBBB.
  • Practical implications for EMS:
    • If there is a suspected LBBB on the 12-lead, it can complicate STEMI assessment; prioritize old ECGs if available to determine baseline pattern.
    • Do not rely solely on ST-elevation criteria in the presence of LBBB; use clinical judgement and consider serial ECGs and 15-lead assessment when appropriate.
  • The teaching emphasizes the following points:
    • LBBB can mask STEMI or indicate STEMI coexisting with conduction abnormality.
    • RBBB is easier to distinguish and is less likely to mimic STEMI across most leads.
  • Practical takeaways for paramedics:
    • If LBBB is present, consider consulting cardiology and use serial ECGs to detect evolving ischemia.
    • Ensure 15-lead assessment if clinical suspicion remains high or if inferior MI is suspected and 12-lead is inconclusive.

Pharmacologic management and contraindications in STEMI protocols

  • Aspirin vs nitroglycerin (nitro) in suspected MI:
    • Aspirin (ASA) is a critical, life-saving antiplatelet agent that reduces mortality when given early in MI.
    • Nitro provides vasodilation and symptom relief but does not address the underlying thrombotic process; early ASA administration has a greater impact on survival.
    • Practical note: The protocol emphasizes giving ASA as soon as possible and often contraindicates or limits nitro depending on hemodynamics (especially in inferior MI with possible RV involvement).
  • Heart rate window as a cue for right ventricular involvement:
    • Normal or stable heart rate range noted: 60\le HR \le 159\,\text{bpm} is used to screen for RVI risk; RV infarcts often present with hypotension and bradycardia.
  • Nitro contraindications and RVI: