ECG criteria
- Net QRS is negative in lead I
- Net QRS is negative in lead aVF
- Isoelectric or close-to-isoelectric QRS amplitude in I and/or aVF requires precise angle calculation from other leads and exclusion of a technically indeterminate axis
- The calculated electrical axis angle falls into the sector from -90° to ±180°
- In lead II, the complex is usually negative; in III, the polarity is variable — it can be negative, isoelectric, or positive depending on the exact angle
- Exclude a technical cause: check correct placement of limb electrodes
- Correlate with clinical picture: rhythm (ventricular, VT), potassium, use of cardiotoxic drugs, structural heart disease
What it looks like on ECG
The electrical axis of the heart is evaluated by QRS polarity in six limb leads, but leads I and aVF are key for diagnosing northwest axis deviation.
| Lead | Typical QRS polarity |
|---|---|
| I | negative |
| aVF | negative |
| II | usually negative |
| III | variable: negative, isoelectric, or positive |
If the net-QRS in lead I is negative and in aVF is also negative, the axis cannot be classified as normal, left axis deviation, or right axis deviation: it lies in the sector from -90° to ±180°, which is located at the top of the Bayley hexaxial diagram and conventionally called the "northwest" axis. When calculated using formulas or tables, the angle is either sharply negative (closer to -90°…-120°) or approaches ±180°. Polarity in lead III is not constant and depends on how close the angle is to +180°: when the direction is close to this value, lead III may be positive, so orientation should be based on leads I and aVF rather than III. If QRS in I or aVF is close to isoelectric, visual estimation is unreliable and precise calculation from the remaining leads is required; in such situations, some sources refer to a technically indeterminate axis.
Mechanism
The direction of the resultant ventricular depolarization vector is determined by which myocardial regions are excited first and to what extent. In northwest axis deviation, the mean vector is directed predominantly superiorly, usually with a rightward component, which does not occur with the usual inferior-leftward depolarization direction; the exact direction within the sector depends on the specific angle. Such vector reorientation can occur with severe ventricular hypertrophy and dilation with aberrant depolarization pathways, with ventricular rhythm or ventricular tachycardia with morphology and propagation pathways producing such a direction (the specific focus location and conduction characteristics determine the final vector, not just the "inferior" origin of the impulse), with congenital anomalies of the conduction system, as well as with gross metabolic disturbances altering the speed and direction of impulse propagation in the myocardium. The axis direction alone does not allow for precise identification of the anatomical origin of the rhythm — analysis of QRS morphology and clinical context is required.
Causes
- Severe or atypical intraventricular conduction disturbance, combination of multiple fascicular/bundle blocks (isolated left anterior fascicular block by itself causes left axis deviation, usually within approximately -45°…-90°, rather than northwest)
- Dextrocardia and other severe congenital heart diseases
- Ventricular rhythm, accelerated idioventricular rhythm, ventricular tachycardia — the axis depends on the ectopic focus location and type of intraventricular conduction
- Severe hyperkalemia with marked intraventricular conduction disturbance
- Severe cardiotoxicity (sodium channel blocker poisoning, tricyclic antidepressants)
- Chronic obstructive pulmonary disease with emphysema and significant right heart strain
- Multiple myocardial infarctions with extensive scar tissue
- Technical cause — incorrect limb electrode placement (simulating axis deviation)
Differential diagnosis
An important and easily correctable cause of a spurious northwest axis is limb electrode reversal, which must be excluded along with rhythm analysis, QRS width, and clinical context. When the right and left arm electrodes are reversed, lead I is entirely inverted (negative P, QRS, and T). In normal sinus rhythm and absence of dextrocardia, a negative P wave in lead I strongly points to right/left arm reversal, but does not prove it by itself: a negative P in I is also possible with correct placement — in dextrocardia, situs inversus, and certain variants of ectopic atrial or junctional rhythms. Therefore, P waves in all limb leads and aVR should be evaluated, checking whether the rhythm is sinus, correlating with the clinical picture, and if in doubt, repeating the ECG with full electrode placement control. Left axis deviation differs in that QRS is negative in aVF, but positive in I (angle roughly from -30° to -90°). Right axis deviation is the mirror situation: I is negative, aVF is positive. Horizontal and vertical electrical axis positions lie within the normal range and do not produce simultaneous negativity in I and aVF.
Clinical significance and management
The northwest axis per se is not a diagnosis, but an unusual finding that requires confirmation and search for the cause; the urgency and likelihood of dangerous pathology are determined not by the magnitude of the axis as such, but by the rhythm, QRS width, clinical presentation, electrolytes, and structural heart disease data. The physician should: 1) repeat the ECG and ensure correct limb electrode placement; 2) evaluate rhythm and QRS width — rule out ventricular or idioventricular rhythm, ventricular tachycardia; in wide-complex tachycardia, immediately assess hemodynamics — unstable VT requires emergency electrical therapy according to clinical algorithms, stable wide-complex tachycardia is managed according to appropriate diagnostic and therapeutic algorithms and, until proven otherwise, managed as VT; 3) check potassium and other electrolytes, obtain a history of medications and potential cardiotoxic drug poisoning; 4) evaluate cardiac structure (echocardiography) if severe hypertrophy, congenital defect, or scar tissue from prior infarctions is suspected. A true northwest axis may accompany serious organic pathology or metabolic disorder, as well as isolated intraventricular conduction disturbance or ventricular rhythm without proven severe structural disease — the axis alone does not allow assessment of the severity of the cause, so the finding requires an active diagnostic workup rather than merely noting it in the conclusion.
Common interpretation errors
- Failing to check electrode placement and mistaking an artifact for true axis deviation
- Failing to evaluate rhythm and QRS width — mistaking ventricular extrasystoles or VT for northwest axis without analyzing morphology and heart rate, and failing to timely assess hemodynamics in wide tachycardia
- Failing to check the P wave in lead I as a marker for reversed arms, even though it does not rule out all incorrect connection variants
- Describing the finding as an unconditional "normal variant" although it is unusual and almost always requires searching for a cause
- Confusing it with marked left axis deviation without calculating the angle from I and aVF simultaneously, and relying on lead III polarity, which is variable
1. What combination of QRS polarity is characteristic of the northwest (extreme) axis?
The northwest axis sector (-90°…±180°) produces a negative net-QRS simultaneously in both leads I and aVF.
2. What cause is important to exclude when detecting a northwest axis?
Technical artifact from incorrect electrode placement is an important and easily correctable cause of spurious extreme axis, which should be checked alongside rhythm and clinical analysis.
3. Which of the following makes right and less arm electrode reversal a less likely cause of extreme axis?
In sinus rhythm and correct electrode placement, the P wave in lead I is usually positive; its positivity reduces the likelihood of specifically RA–LA reversal, though it does not rule out other connection errors.
Frequently asked questions
What does a northwest axis on the ECG mean?
This is an extreme variant of electrical axis deviation in which the net QRS is negative in both lead I and aVF. The axis angle falls into the sector from -90° to ±180°, which is almost never encountered physiologically, so the finding warrants investigating the cause. Separately, literature sometimes distinguishes a technically undetermined axis—when the calculation is unreliable due to low-voltage QRS complexes.
Can a northwest axis be a normal variant?
This is an unusual finding that typically requires confirmation and investigation of the cause. Most important are re-checking the electrodes and ruling out wide QRS complex, ventricular rhythm, conduction disturbances, electrolyte imbalances, and structural causes; the severity is determined by the clinical context rather than the magnitude of the axis itself.
How to distinguish a northwest axis from swapped electrodes?
When the right and left arm electrodes are swapped, the P wave in lead I during sinus rhythm usually becomes negative. A negative P wave in lead I strongly points to this transposition, while a positive P wave makes it less likely, but does not rule out other limb lead misconnection variants. When in doubt, the P wave is assessed in all limb leads, and the ECG is repeated with verified electrode placement.
Is the northwest axis dangerous by itself?
The finding itself is not dangerous; the underlying cause that triggered it is. It is necessary to rule out ventricular rhythm or tachycardia (in wide tachycardia, urgently assess hemodynamics), severe hyperkalemia, cardiotoxic drug overdose, and gross structural heart disease.
How to calculate the angle with a northwest axis?
The same methods used for routine electrical axis calculation are employed—based on QRS polarity in limb leads or using tables and angle calculators, provided there is sufficient QRS amplitude. If complexes in most leads are nearly isoelectric, the calculation may be technically unreliable, so the tracing is repeated and all leads are evaluated.
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Related terms
More in the section «Basics: waves, intervals, axes»
Sources
- Goldberger's Clinical Electrocardiography, latest edition
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- Orlov V.N. Guide to Electrocardiography
- AHA/ACCF/HRS Recommendations for the Standardization of ECG, 2009
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.