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ECG Axis Calculator

Calculate the heart's electrical axis using ECG lead values. This tool helps determine cardiac axis deviation from normal ranges.

Enter the QRS amplitude in lead I (in millimeters)
Enter the QRS amplitude in lead aVF (in millimeters)
Enter the QRS amplitude in lead III for verification (in millimeters)

How to Use This Calculator

  1. Measure the QRS amplitude in lead I (from the start of Q to the end of S)
  2. Measure the QRS amplitude in lead aVF (from the start of Q to the end of S)
  3. Optionally, measure the QRS amplitude in lead III for verification
  4. Select your preferred calculation method (Quadrant or Precise)
  5. Click Calculate to determine the cardiac axis and interpret the results

Formula Used

Axis Angle = arctan(Lead aVF / Lead I)

Where:

  • Lead I = QRS amplitude in lead I (in millimeters)
  • Lead aVF = QRS amplitude in lead aVF (in millimeters)
  • arctan = Inverse tangent function

Axis Interpretation:

  • Normal: -30° to +90°
  • Left Axis Deviation: -30° to -90°
  • Right Axis Deviation: +90° to +180°
  • Extreme Axis Deviation: -90° to -180°

Example Calculation

Real-World Scenario:

A 65-year-old patient with a routine ECG showing the following QRS amplitudes: Lead I = +7mm, Lead aVF = +5mm

Given:

  • Lead I = +7mm
  • Lead aVF = +5mm

Calculation:

Axis Angle = arctan(Lead aVF / Lead I) = arctan(5/7) = arctan(0.714) = 35.5°

Result: The cardiac axis is 35.5°, which falls within the normal range (-30° to +90°).

Why This Calculation Matters

Clinical Applications

  • Diagnosing ventricular hypertrophy
  • Identifying conduction abnormalities
  • Detecting myocardial infarction patterns
  • Evaluating chamber enlargement

Key Benefits

  • Quick assessment of cardiac electrical activity
  • Helps identify underlying cardiac pathology
  • Provides objective measurement of axis deviation
  • Supports clinical decision-making

Common Mistakes & Tips

Always measure from the beginning of the Q wave to the end of the S wave. If there's no Q wave, start from the beginning of the R wave. If there's no S wave, end at the end of the R wave. Ensure you're measuring the net deflection (positive minus negative).

Lead III can be used to verify your calculation. In a normal ECG, the sum of QRS amplitudes in leads I, II, and III should equal zero (I + III = II). If this relationship doesn't hold, recheck your measurements for accuracy.

Frequently Asked Questions

The cardiac axis represents the average direction of electrical depolarization through the ventricles. In a normal heart, this flows downward and to the left (between -30° and +90°). Deviations from this range can indicate various cardiac conditions.

Left axis deviation (-30° to -90°) can be caused by left anterior fascicular block, left ventricular hypertrophy, inferior myocardial infarction, Wolff-Parkinson-White syndrome, or certain congenital heart diseases.

Right axis deviation (+90° to +180°) can be caused by right ventricular hypertrophy, left posterior fascicular block, anterolateral myocardial infarction, chronic lung disease, pulmonary embolism, or certain congenital heart conditions like tetralogy of Fallot.

References & Disclaimer

Medical Disclaimer

This calculator is for educational purposes only and should not replace professional medical judgment. Always consult with a qualified healthcare provider for diagnosis and treatment of medical conditions.

References

Accuracy Notice

This calculator provides estimates based on standard ECG axis calculation methods. Accuracy depends on correct measurement of QRS amplitudes and proper lead placement. For clinical decision-making, always confirm with a qualified healthcare professional.

About the Author

Kumaravel Madhavan

Web developer and data researcher creating accurate, easy-to-use calculators across health, finance, education, and construction and more. Works with subject-matter experts to ensure formulas meet trusted standards like WHO, NIH, and ISO.

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