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Radiology · Additional topics

Determination of ECG axis

The electrical axis of the heart (EAO) is a vector representing the main direction of electrical impulse propagation in the myocardium. Assessing it is a mandatory second step in the standard algorithm for interpreting any ECG.

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Normal EAOFrom +30° to +70° (vertical vector position)
Right axis deviationAngle greater than +70°, clockwise vector rotation
Left axis deviationAngle less than +30° (down to negative values), counterclockwise rotation
Cause of displacementThe vector always deviates toward the predominant myocardial muscle mass
3 min readFor medical studentsUpdated 10.10.2026
Material prepared by WhiteCoat Pro editorial team based on platform study guidesPending instructor review

Place of the EAO in the overall ECG interpretation plan

ECG analysis requires a strict sequence. Assessing the axis position is just one of five key stages:

  1. Assessment of rhythm and conduction. Includes checking contraction regularity, calculating heart rate (HR), searching for the main pacemaker, and checking the conduction system.
  2. Determination of the electrical axis of the heart. Analysis of the vector direction in the frontal plane.
  3. Study of the atrial component. Detailed breakdown of P wave characteristics.
  4. Analysis of the ventricular complex (QRST). Assessment of the QRS complex, ST segment, T wave, and Q-T interval.
  5. Formulating the final conclusion. Combining all identified findings into a single summary.

How the electrical axis forms and shifts

The electrical axis reflects the net flow of electrical activity passing through the heart muscle. Normally, this vector ranges from +30° to +70°, which corresponds to a vertical position.

The main principle of EAO changes is that the electrical impulse "pulls" toward the larger tissue volume. The axis shifts toward the side where the myocardial muscle mass increases, because exciting a larger muscle volume requires a stronger impulse.

To precisely calculate the angle and determine axis position, the Einthoven triangle is traditionally used. The physician analyzes and compares wave amplitudes in standard leads.

Test your knowledge on the topicTasks from the "Diagnostic Radiology" course bank — instant answers and explanations.

Basic parameters of rhythm and the QRS complex

Proper interpretation of the EAO requires understanding the context of the entire ECG, specifically the characteristics of the rhythm and ventricular complexes.

Assessment of rhythmicity and the source of excitation:

  • Sinus rhythm is considered normal. With this rhythm, a clear, undeformed P wave is recorded before each QRS complex. The R-R interval is stable (a ±10% margin of error is allowed).
  • Non-sinus rhythms occur when the impulse is generated outside the sinus node. If the P wave is altered (flattened, notched), the rhythm is atrial. If the P wave is completely absent, the rhythm is ventricular.
  • With marked fluctuations in R-R intervals (greater than 10%), arrhythmia is diagnosed.

Heart rate calculation: The formula depends on the paper speed:

  • At a speed of 50 mm/s: 600 is divided by the number of large squares between R waves.
  • At a speed of 25 mm/s: 300 is divided by the number of large squares between R-R.

At rest, a normal heart rate is about 60–80 beats per minute. During sleep, the rate may physiologically drop to 45 beats per minute.

Analysis of the QRS complex itself (its rate, duration, and amplitude) helps detect pathological changes such as hypertrophy or blocks, which directly affect the shift of the electrical axis.

How to memorize

The mass rule: The EAO acts like a scale needle—it always deviates toward the side where the heart muscle is "heavier" (more massive).

Class and exam questions

What is the normal range for the electrical axis of the heart?

The normal position of the EAO in the frontal plane ranges from +30° to +70°, which is characterized as a vertical position.

How to quickly calculate heart rate if the ECG recording speed is 50 mm/s?

It is necessary to divide the number 600 by the number of large squares fitting between two adjacent R waves.

What does it mean if there is no P wave on the ECG?

The absence of a P wave indicates a non-sinus, specifically ventricular rhythm—the source of excitation is located outside the sinus node.

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What else to study on this topic

  • Detailed assessment of the atrial component (P wave)
  • Changes in the ST segment and T wave in pathologies
  • Signs of cardiac conduction disturbances
  • Methods of working with the Einthoven triangle
  • Determination of QRS complex pathologies (blocks, hypertrophy)

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  • Complete study guide on "Determining the ECG Axis Position" and 2,199 more subject pages
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Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.