ECG criteria
- Find the lead with the most isoelectric (biphasic, sum of waves ≈ 0) QRS complex
- The ECG axis vector is directed perpendicularly to this lead — determine the direction based on the positivity/negativity of the net QRS in one of the perpendicular leads
- Assess QRS in I and aVF: both positive — normal axis (0°…+90°); I positive, aVF negative — possible left axis deviation
- With left axis deviation (alpha angle < −30°), QRS in II and III is predominantly negative
- With right axis deviation (alpha angle > +90°, according to some guidelines > +100°), QRS in I is negative; moderate right axis deviation is usually accompanied by a positive QRS in III, while marked deviation requires assessing II, aVF, and the magnitude of the angle
- If I and aVF are both negative — extreme (northwest) axis; use II, III, aVR, aVL and QRS amplitudes for clarification; if QRS is negative in all limb leads, first rule out a technical issue and low voltage
- To obtain the precise angle, use the hexaxial Bailey's system (a scale marked every 30°, with axes I, II, III, aVR, aVL, and aVF positioned at 0°, +60°, +120°, −150°, −30°, and +90°, respectively) or tables/calculators based on wave amplitudes
What it looks like on ECG
The ECG axis is evaluated solely using the six limb leads — I, II, III, aVR, aVL, aVF, located in the frontal plane. In the hexaxial Bailey's system, their axes lie at the following angles: I — 0°, II — +60°, III — +120°, aVR — −150°, aVL — −30°, aVF — +90°; the scale itself is marked in 30° increments, but this does not mean the leads are evenly spaced one after another every 30° around the circle. For each lead, the algebraic sum of the QRS waves is calculated (R minus Q and S in mm), which gives the projection of the net vector onto the lead axis.
Quick method: look for the lead with a wave sum close to zero (isoelectric, biphasic complex) — the ECG axis vector is perpendicular to this axis. After finding the most isoelectric lead, choose one of the two perpendicular directions and determine it by the positivity/negativity of the net QRS in a suitable lead, taking into account the algebraic sum of all deflections.
Simplified two-lead reference guide:
| I | aVF | Conclusion |
|---|---|---|
| + | + | normal axis (0°…+90°) |
| + | − | possible left axis deviation (requires II/III) |
| − | + | right axis deviation |
| − | − | extreme (northwest) axis |
If an exact alpha angle is needed, amplitudes from two non-perpendicular leads (e.g., I and III) are plotted on the Bailey's coordinate system to find the resultant vector; this is the basis of manual plotting and automated ECG machine algorithms.
Mechanism
The ECG axis reflects the net instantaneous vector of ventricular depolarization, averaged over the entire QRS. In a healthy individual, the mass of the left ventricle is greater than that of the right, and excitation spreads predominantly to the left and inferiorly; therefore, normally, the mean QRS vector in the frontal plane is directed predominantly inferiorly and to the left. Any change in myocardial mass, conduction delay in one of the bundle branches, or alteration of the heart's position in the thoracic cavity shifts the vector direction and, consequently, the alpha angle.
Normal Variants
- Normal ECG axis position: alpha angle from −30° to +90° (simplified normal range 0°…+90°, more frequently cited as +30°…+69°)
- Horizontal position: closer to 0°, typically seen in individuals with a broad chest, hypersthenics
- Vertical position: closer to +90°, in asthenics, tall thin individuals, children, and adolescents
- In newborns and infants, right axis deviation is physiological due to the relatively greater mass of the right ventricle; in older children and adolescents, age-specific ECG axis ranges are used and the clinical context is taken into account
Pathological changes
- Left axis deviation (alpha angle < −30°): left anterior fascicular block, left ventricular hypertrophy and overload, inferior myocardial infarction, certain intraventricular conduction disturbances
- Right axis deviation (alpha angle > +90°, sometimes a threshold of +100°): right ventricular hypertrophy and overload, left posterior fascicular block, lateral myocardial infarction, pulmonary embolism with acute right heart overload, chronic cor pulmonale
- Extreme / northwest axis (I− and aVF−): ventricular rhythms and tachycardia, combined hypertrophy or structural heart disease, certain intraventricular conduction disturbances; when in doubt, rule out technical errors and assess the axis using a reliable averaged QRS
- Severe hyperkalemia specifically: causes QRS widening, flattening and disappearance of the P wave, and complex fusion; against this background, the axis may become extreme or virtually indiscernible, but this is not a typical cause of isolated left axis deviation.
Common interpretation errors
- Do not assess the ECG axis using chest leads — it is determined solely by limb leads
- Do not confuse the ECG axis with the transition zone in chest leads — these are different concepts
- Do not forget to check for swapped limb electrodes: a sudden unexpected axis deviation is frequently explained by this exact artifact rather than pathology
- Do not conclude there is a deviation based on a single lead (I or aVF) without confirmation from II and III
- Do not automatically assign clinical significance to any deviation without considering age, body habitus, and the clinical picture
- Do not consider an "indeterminate axis" as a separate range from the extreme/northwest axis: I− and aVF− correspond specifically to the extreme (northwest) axis, which is referred to in some sources as an indeterminate axis; a truly unresolvable axis occurs when the net QRS amplitude is near zero in all limb leads
"Thumbs up, thumbs down": if I and II are positive, the axis is definitely normal (a simplified rule for a quick estimation).
1. QRS in lead I is positive, in aVF negative, in II positive. How should the ECG axis be interpreted?
A positive II confirms that the axis has not gone beyond −30°, meaning it remains within the normal range despite a negative aVF.
2. Which alpha angle range corresponds to the expanded normal position of the ECG axis?
The expanded normal range of the ECG axis is from −30° to +90°; the simplified normal range of 0°…+90° is also found in guidelines, but −30°…+90° accounts for individual variants.
3. QRS in I and aVF are both negative. What is the ECG axis position called?
I− and aVF− indicate an extreme (northwest) axis; II, III, aVR, aVL, and QRS amplitudes are used for clarification. The axis is considered truly unresolvable only when there is a near-zero net QRS amplitude across all limb leads.
Frequently asked questions
How to quickly determine the QRS axis by eye without calculating the alpha angle?
Look at the QRS in leads I and aVF: both positive — the axis is normal. To clarify left or right axis deviation, look at leads II and III. An exact angle is rarely needed, mostly for research and borderline cases.
Is QRS axis deviation dangerous on its own?
Axis deviation itself is not a life-threatening condition; it is a descriptive ECG sign. The danger is determined by the cause of the deviation — for example, ventricular hypertrophy, bundle branch block, or acute right heart overload in pulmonary embolism.
How does a normal QRS axis position differ from vertical and horizontal positions?
All three variants lie within the normal range of the alpha angle (from −30° to +90°); the difference is purely descriptive and related to body habitus and age: a horizontal position is closer to 0°, while a vertical position is closer to +90°.
Why is the QRS axis often deviated to the right in children?
In newborns and infants, the right ventricle is relatively larger than the left, which makes the rightward shift of the depolarization vector physiological. With age, as the left ventricular mass increases, the axis shifts toward the adult norm. In older children, age-specific QRS axis norms are used.
Is it necessary to calculate the exact alpha angle in routine practice?
In most cases, a qualitative assessment using I, II, III, and aVF leads is sufficient to classify the axis as normal, left deviation, right deviation, or extreme axis. Precise angle calculation is used for borderline values or research purposes.
ECG Course
Interpretation from basics to myocardial infarction — learn to read any ECG using a systematic algorithm.
- Video lessons and theory for every topic
- Simulator case studies under every lesson
- Unlimited ECG simulator practice
- Study guide: "All ECGs in 25 minutes"
ECG Strip Simulator
ECG, radiology, histology, pathomorphology, pharmacology — thousands of flashcards with detailed explanations.
- Real ECG strips, not textbook drawings
- Instant answers and explanations
- Streaks and progress tracking in your dashboard
Related terms
More in the section «Basics: waves, intervals, axes»
Sources
- Goldberger A.L. Clinical Electrocardiography: A Simplified Approach
- Hampton J.R. The ECG Made Easy
- Orlov V.N. Guide to Electrocardiography
- Surawicz B., Knilans T.K. Chou's Electrocardiography in Clinical Practice
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.