3.2 12-Lead ECG Interpretation: Ischemia, Transmural Infarction & Conduction

3.2 12-Lead ECG Interpretation: Ischemia, Transmural Infarction & Conduction

In cardiac rehabilitation, the 12-lead electrocardiogram (ECG) is the primary non-invasive diagnostic tool used to monitor myocardial perfusion, detect subclinical ischemia, confirm anatomical infarction patterns, and evaluate intraventricular conduction delays. Clinicians must master lead orientation, axis determination, ischemic repolarization markers, and coronary artery localization.

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12-Lead Electrode Placement & Anatomical Orientation

The 12-lead ECG evaluates cardiac electrical forces in two perpendicular planes:

Frontal Plane (Limb Leads)

Comprises three bipolar limb leads (I, II, III) and three augmented unipolar leads (aVR, aVL, aVF). In cardiac rehabilitation telemetry, Mason-Likar torso electrode placement replaces wrist/ankle electrodes with infraclavicular and anterior iliac crest locations to eliminate skeletal muscle motion artifact during treadmill walking and cycle ergometry.

Horizontal Plane (Precordial Leads)

Six unipolar chest leads (V1 through V6) view transverse electrical vectors:

• $V_1$: 4th intercostal space (ICS) at the right sternal border (septal view).
• $V_2$: 4th ICS at the left sternal border (septal view).
• $V_3$: Midway between $V_2$ and $V_4$ diagonally (anterior view).
• $V_4$: 5th ICS in the left midclavicular line (apical/anterior view).
• $V_5$: 5th ICS in the left anterior axillary line, horizontal to $V_4$ (lateral view).
• $V_6$: 5th ICS in the left midaxillary line, horizontal to $V_4$ (low lateral view).

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Mean Electrical Axis Determination

The mean QRS electrical axis represents the net frontal plane vector of ventricular depolarization. It is evaluated rapidly using the Quadrant Method with Lead I ($0^\circ$) and Lead aVF ($+90^\circ$):

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Electrocardiographic Evolution: Ischemia, Injury & Infarction

Acute coronary artery disease produces a sequential triad of repolarization and depolarization abnormalities:

1. Myocardial Ischemia (Subendocardial)

Subendocardial ischemia delays repolarization without causing immediate necrosis:

• ST-Segment Depression: Measured at the ST 80 point (80 ms past the J-point) or at 60 ms when $HR > 130\text{ bpm}$:
  • Horizontal ST Depression: $\ge 1.0\text{ mm}$ ($0.10\text{ mV}$) flat depression; highly predictive of CAD.
  • Downsloping ST Depression: $\ge 1.0\text{ mm}$ downward-sloping depression; highest specificity for multivessel/left main CAD.
  • Upsloping ST Depression: Rapidly ascending depression; requires $\ge 1.5\text{ mm}$ measured 80 ms past J-point to be considered abnormal.
• T-Wave Inversion: Symmetrical, narrow, deeply inverted T waves ($\ge 1.0\text{ mm}$) in contiguous leads.

2. Myocardial Injury (Acute Transmural STEMI)

Transmural epicardial injury generates a systolic injury current manifesting as ST-Segment Elevation at the J-point in $\ge 2$ contiguous leads:

• Limb Leads (I, II, III, aVL, aVF) and $V_4\text{--}V_6$: J-point elevation $\ge 1.0\text{ mm}$ ($0.10\text{ mV}$).
• Leads $V_2$ and $V_3$: Men $\ge 40$ years: $\ge 2.0\text{ mm}$; Men $< 40$ years: $\ge 2.5\text{ mm}$; Women: $\ge 1.5\text{ mm}$.
• Reciprocal Depression: Coexisting ST depression in electrically opposing leads (e.g., inferior elevation with high lateral depression in I and aVL) differentiates acute STEMI from pericarditis.

3. Myocardial Infarction (Necrosis & Scar)

Necrotic myocardium creates an electrical void:

• Pathologic Q Waves: Duration $\ge 0.04\text{ seconds}$ (40 ms) or depth $> 25\%$ of ensuing R-wave amplitude in contiguous leads.
• Poor R-Wave Progression: Failure of the R wave to grow progressively across precordial leads ($R < 3\text{ mm}$ in $V_3$) indicates anterior or septal scar.

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Coronary Artery Localization Matrix

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Intraventricular Conduction Abnormalities: Bundle Branch Blocks

A widened QRS complex ($\ge 0.12\text{ seconds}$ / $120\text{ ms}$) indicates intraventricular conduction delay:

Right Bundle Branch Block (RBBB)

• Criteria: $QRS \ge 0.12\text{ s}$; broad, notched $R$ wave ($rsR'$, "bunny ears") in anterior leads $V_1$ and $V_2$; wide, slurred $S$ wave in lateral leads I, aVL, $V_5, V_6$.
• Exercise Testing: RBBB causes secondary ST-T changes in $V_1\text{--}V_3$, but does not invalidate ST interpretation in lateral leads ($V_5, V_6$, I, aVL). Lateral ST depression remains diagnostic for exercise-induced ischemia.

Left Bundle Branch Block (LBBB)

• Criteria: $QRS \ge 0.12\text{ s}$; broad, monophasic, notched $R$ waves in lateral leads I, aVL, $V_5, V_6$ with absent septal Q waves; deep, wide $QS$ complexes in leads $V_1$ and $V_2$.
• Exercise Testing: LBBB profoundly distorts repolarization across all 12 leads. Standard ST-segment depression or elevation cannot be used to diagnose myocardial ischemia. Graded exercise tests without imaging are non-diagnostic; patients require pharmacological or imaging stress modalities (stress echo or nuclear SPECT).