What electrocardiogram (EKG) leads should be evaluated for a patient undergoing an EKG?

Standard 12-Lead ECG Evaluation

All 12 leads of the standard electrocardiogram should be systematically evaluated in every patient undergoing an EKG, including the 3 limb leads (I, II, III), 3 augmented limb leads (aVR, aVL, aVF), and 6 precordial leads (V1-V6). 1

Standard Lead Configuration

The complete 12-lead ECG consists of: 1

• Limb leads: I, II, and III
• Augmented limb leads: aVR, aVL, and aVF (using Goldberger modification)
• Precordial leads: V1 through V6 (using Wilson central terminal)

Precordial Lead Placement

The six chest electrodes must be placed at specific anatomic locations: 1

• V1: Fourth intercostal space at the right sternal border
• V2: Fourth intercostal space at the left sternal border
• V3: Midway between V2 and V4
• V4: Fifth intercostal space in the midclavicular line
• V5: Horizontal plane of V4 at the anterior axillary line (or midway between V4 and V6 if anterior axillary line is ambiguous)
• V6: Horizontal plane of V4 at the midaxillary line

Lead Nomenclature and Anatomic Correlation

Leads should be identified by their original nomenclature (I, II, III, aVR, aVL, aVF, V1-V6) rather than labeling them as "anterior," "inferior," or "lateral" leads. 1 This recommendation prevents the misconception that ST-segment abnormalities in a particular lead necessarily indicate ischemia in that anatomic region, when they may actually represent reciprocal changes from the opposite region. 1

Anatomically Contiguous Lead Groupings

For diagnosing acute ischemia/infarction, ST-segment elevation must be present in 2 or more contiguous leads: 1

• Chest leads: V1 through V6 are displayed in anatomically contiguous order from right anterior to left lateral
• Limb leads: Should ideally be displayed in the Cabrera format (aVL, I, -aVR, II, aVF, III) to show anatomic contiguity from left superior-basal to right inferior 1

Additional Leads for Specific Clinical Scenarios

Right-Sided Chest Leads

When ST elevation >0.1 mV occurs in leads II, III, and aVF (indicating inferior wall ischemia/infarction), right-sided chest leads V3R and V4R should be recorded immediately. 1 These leads are essential for: 1

• Diagnosing right ventricular involvement in inferior wall infarction
• Distinguishing between right coronary artery (RCA) and left circumflex (LCx) occlusion
• Differentiating proximal from distal RCA occlusion

Critical timing consideration: ST elevation in right-sided chest leads associated with right ventricular infarction persists for a much shorter period than inferior wall ST elevation, so V3R and V4R must be recorded as rapidly as possible after chest pain onset. 1

Reduced Lead Systems (Monitoring Context)

While the standard 12-lead ECG requires all leads, certain monitoring situations may use reduced lead systems: 1

• 6-electrode systems: Use Mason-Likar limb leads plus V1 and V5 (or V2 and V5), from which remaining precordial leads are mathematically derived 1
• Derived 12-lead ECG: Can be synthesized from Frank X, Y, Z leads, though this is an approximation and may differ from standard 12-lead recordings 1

Important caveat: When comparing serial ECGs, caution must be exercised if combining standard 12-lead recordings with derived or reduced lead system recordings, as QRS, ST, and T waves may differ between systems. 1

Systematic Evaluation Approach

When evaluating the 12-lead ECG, assess: 1

• Rate and rhythm: Document in all leads
• Conduction intervals: PR interval, QRS duration, QT/QTc interval
• Axis: Frontal plane axis determination using limb leads
• Morphology: QRS morphology for bundle branch blocks, fascicular blocks, or hypertrophy patterns
• ST-segment changes: Using age- and gender-specific thresholds 1
• Screening: For structural heart disease or systemic illness 1

ST-Segment Elevation Thresholds

Age- and gender-specific thresholds for abnormal J-point elevation: 1

• Men ≥40 years: 0.2 mV (2 mm) in V2-V3; 0.1 mV (1 mm) in all other leads
• Men <40 years: 0.25 mV (2.5 mm) in V2-V3; 0.1 mV (1 mm) in all other leads
• Women: 0.15 mV (1.5 mm) in V2-V3; 0.1 mV (1 mm) in all other leads
• Both genders: 0.05 mV (0.5 mm) in V4-V5 (except males <30 years: 0.1 mV)

Common Pitfalls to Avoid

Electrode placement errors are common and can lead to misdiagnosis—proper skin preparation with cleaning and gentle abrasion before electrode application reduces noise and improves ECG quality. 1 Variation in limb electrode placement, particularly the left arm electrode, can alter the ECG recording. 1

Reciprocal changes: ST-segment depression in leads V1-V3 during inferior wall infarction may represent posterior/lateral ischemia from either RCA or LCx occlusion, not anterior wall pathology. 1