An electrocardiogram (EKG or ECG) measures electrical activity of the heart, and its interpretation involves assessing various components with normal ranges. Below is a detailed breakdown of normal EKG values, the time represented by small and large boxes on EKG paper, and the anatomical regions associated with each lead.

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Normal EKG Values

These values represent typical ranges for a healthy adult at rest. Variations may occur based on age, sex, and clinical context.

1. Heart Rate (HR):
   • Normal: 60–100 beats per minute (bpm)
   • Bradycardia: <60 bpm
   • Tachycardia: >100 bpm
   • Calculation: 300 ÷ number of large boxes between R waves (or 1500 ÷ number of small boxes)
2. Rhythm:
   • Normal: Sinus rhythm (P wave precedes each QRS complex, consistent PR interval)
3. P Wave:
   • Duration: 80–120 ms (2–3 small boxes)
   • Amplitude: <2.5 mm in limb leads, <1.5 mm in precordial leads
   • Morphology: Upright in leads I, II, aVF; may be biphasic in V1
   • Represents: Atrial depolarization
4. PR Interval:
   • Duration: 120–200 ms (3–5 small boxes)
   • Measured: From start of P wave to start of QRS complex
   • Represents: Time from atrial depolarization to ventricular depolarization (includes AV node conduction)
5. QRS Complex:
   • Duration: 80–120 ms (2–3 small boxes)
   • Amplitude: Varies by lead; typically <25 mm in precordial leads
   • Morphology: Narrow, upright in most leads (except aVR, V1)
   • Represents: Ventricular depolarization
   • Components:
     • Q Wave: Small (<40 ms, <25% of R wave height); pathological if >40 ms or >25% R wave
     • R Wave: Progression increases from V1 to V5/V6
     • S Wave: Deep in right precordial leads (V1–V3)
6. ST Segment:
   • Duration: Not typically measured independently
   • Morphology: Isoelectric (at baseline); deviation <1 mm in limb leads, <2 mm in precordial leads
   • Represents: Time between ventricular depolarization and repolarization
7. T Wave:
   • Morphology: Upright in most leads (I, II, V3–V6); may be inverted in V1, aVR
   • Amplitude: <5 mm in limb leads, <10 mm in precordial leads
   • Represents: Ventricular repolarization
8. QT Interval:
   • Duration: 350–440 ms (varies with heart rate)
   • Corrected QT (QTc): ≤440 ms (men), ≤460 ms (women)
     • Formula: Bazett’s: QTc = QT / √RR (RR in seconds)
   • Measured: From start of QRS to end of T wave
   • Represents: Total time of ventricular depolarization and repolarization
9. Cardiac Axis:
   • Normal: -30° to +90°
   • Left axis deviation: <-30°
   • Right axis deviation: >+90°
   • Determined by: QRS deflection in leads I and aVF

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EKG Paper: Small and Large Boxes

EKG paper is standardized with a grid where time is measured horizontally and voltage vertically.

• Small Box (1 mm):
  • Time: 0.04 seconds (40 ms)
  • Voltage: 0.1 mV
• Large Box (5 mm):
  • Time: 0.2 seconds (200 ms) (5 small boxes)
  • Voltage: 0.5 mV
• Standard Calibration:
  • Speed: 25 mm/second
  • Voltage: 10 mm = 1 mV
  • 5 large boxes = 1 second

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EKG Leads and Corresponding Body Regions

A standard 12-lead EKG includes 6 limb leads and 6 precordial (chest) leads, each viewing the heart from different angles.

1. Limb Leads (6):
   • Bipolar Leads (measure voltage between two electrodes):
     • Lead I: Left arm (+) vs. Right arm (-) → Views lateral wall (left ventricle)
     • Lead II: Left leg (+) vs. Right arm (-) → Views inferior wall
     • Lead III: Left leg (+) vs. Left arm (-) → Views inferior wall
   • Augmented Unipolar Leads (measure voltage relative to a reference point):
     • aVR: Right arm (+) → Views right heart, often inverted
     • aVL: Left arm (+) → Views lateral wall
     • aVF: Left leg (+) → Views inferior wall
2. Precordial (Chest) Leads (6):
   • Unipolar leads placed on the chest, viewing the heart in the horizontal plane:
     • V1: 4th intercostal space, right sternal border → Views right ventricle, septum
     • V2: 4th intercostal space, left sternal border → Views septum
     • V3: Midway between V2 and V4 → Views anterior wall
     • V4: 5th intercostal space, midclavicular line → Views anterior wall, apex
     • V5: 5th intercostal space, anterior axillary line → Views lateral wall
     • V6: 5th intercostal space, midaxillary line → Views lateral wall
3. Electrode Placement:
   • Right Arm (RA): Upper right arm or shoulder
   • Left Arm (LA): Upper left arm or shoulder
   • Left Leg (LL): Lower left leg or abdomen
   • Right Leg (RL): Lower right leg (ground electrode)
   • Chest Leads (V1–V6): Specific chest positions as described above

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Additional Notes

• Anatomical Correlations:
  • Inferior wall: Leads II, III, aVF (often right coronary artery)
  • Lateral wall: Leads I, aVL, V5–V6 (often left circumflex artery)
  • Anterior wall: Leads V3–V4 (often left anterior descending artery)
  • Septum: Leads V1–V2
• EKG Variants: Normal variants (e.g., early repolarization) or pathological findings (e.g., bundle branch blocks) may alter measurements.
• Clinical Context: Always interpret EKG values in the context of patient history, symptoms, and other findings.