1. What is P Axis?

The P axis represents the net direction of atrial depolarization on the ECG. It's calculated from the limb leads (I, II, III, aVR, aVL, aVF) and normally ranges between 0° and +75°.

2. How is P Axis Calculated?

The calculator uses the following formula:

Where:

• Lead aVF — Vertical component of P wave (mV)
• Lead I — Horizontal component of P wave (mV)

Explanation: The calculation determines the angle of the mean P wave vector in the frontal plane using the hexaxial reference system.

3. Clinical Significance

Details: Abnormal P axis may indicate conditions like:

• Left atrial enlargement (P axis > +75°)
• Right atrial enlargement (P axis > +75° or < 0°)
• Dextrocardia (P axis between +90° and +180°)
• Ectopic atrial rhythms

4. Using the Calculator

Tips:

• Enter the net amplitude of the P wave in each lead (positive or negative)
• Measure from baseline to peak (or nadir for negative P waves)
• Normal P axis typically between 0° and +75°

5. Frequently Asked Questions (FAQ)

Q1: What is a normal P axis range?
A: The normal P axis is between 0° and +75°. Values outside this range may indicate abnormal atrial depolarization.

Q2: How does lead I affect P axis?
A: Lead I represents the horizontal component (right to left). Positive deflection in lead I means the vector is moving toward the left.

Q3: How does lead aVF affect P axis?
A: Lead aVF represents the vertical component (superior to inferior). Positive deflection in aVF means the vector is moving downward.

Q4: What does a negative P axis mean?
A: A negative P axis (between -90° and 0°) suggests superiorly directed atrial activation, seen in conditions like ectopic atrial rhythms.

Q5: Can P axis help diagnose atrial enlargement?
A: Yes, right atrial enlargement often causes P axis > +75°, while left atrial enlargement may shift it leftward (though less reliably).