Left Axis Deviation on ECG: Causes, Criteria and Axis Calculator
Left axis deviation means the mean frontal QRS axis lies between -30 and -90 degrees: lead I is positive while leads II and aVF are mostly negative. Mild left axis deviation on an otherwise normal ECG is common and often benign; an axis beyond -45 degrees usually means left anterior fascicular block.
At a glance
- Normal adult axis: -30 to +90 degrees. Left axis deviation: -30 to -90. Right axis deviation: +90 to +180.
- Quick look: lead I positive and aVF negative puts the axis between 0 and -90 degrees. If lead II is mostly negative too, the axis is beyond -30: left axis deviation.
- An axis beyond -45 degrees with qR in aVL, rS in II, III and aVF and a QRS under 120 ms fits left anterior fascicular block, the most common cause.
- Other causes include inferior MI, left ventricular hypertrophy, pre-excitation, right ventricular pacing, hyperkalemia and some congenital heart defects.
- In children with left axis deviation and no other findings, heart disease was uncommon. An axis of -42 degrees or less, chamber enlargement on the ECG or an abnormal exam made it more likely.
Free calculator
ECG axis calculator
Enter the net QRS in any two or more limb leads: the height of the R wave minus the depth of the Q or S wave, in mm. Leads I and aVF are enough; more leads give a steadier answer.
Worked out by fitting the heart's electrical vector to every lead you enter, with the augmented leads scaled as in Goldberger's leads (aVF equals the average of II and III). With leads I and aVF alone this is exact; the quick two-lead shortcut can differ by up to 4 degrees.
Worked out in your browser. Nothing you enter is sent or saved.
Normal values
| Measure | Normal | Abnormal |
|---|---|---|
| Frontal QRS axis, adults | -30 to +90 degrees | Beyond -30: left axis deviation. Beyond +90: right axis deviation |
| Net QRS in leads I and aVF | Both mostly positive | I positive, aVF negative: axis between 0 and -90 degrees |
| Net QRS in lead II | Mostly positive | Mostly negative: axis beyond -30 degrees |
| Children | A wider range that shifts with age; newborns sit to the right | Use age-specific pediatric ranges |
Criteria
Left anterior fascicular block
Frontal axis between -45 and -90 degrees; qR pattern in aVL; R-peak time in aVL of 45 ms or more; QRS under 120 ms. The inferior leads usually show rS complexes.
Left posterior fascicular block, for contrast
Frontal axis between +90 and +180 degrees in an adult; rS in I and aVL; qR in III and aVF; QRS under 120 ms; other causes of right axis deviation excluded.
Bifascicular block
Right bundle branch block with left anterior or left posterior fascicular block.
What it looks like
Lead I: qR
aVL: qR
II: rS
aVF: rS
How to measure it
- Find the net QRS in lead I: the height of the R wave minus the depth of the Q and S waves, in mm.
- Do the same in aVF, and in lead II if the axis looks leftward.
- Lead I positive and aVF positive: normal axis, 0 to +90 degrees. Lead I positive and aVF negative: check lead II. Positive means a normal axis between 0 and -30 degrees; negative means left axis deviation.
- For a number, find the limb lead where the QRS is most nearly isoelectric: the axis lies at right angles to it, toward the leads where the QRS is positive. Or enter the net amplitudes in the calculator above.
- Compare with an old ECG. A new shift in the axis matters more than its exact value.
Causes
Conduction
- Left anterior fascicular block
- left bundle branch block with a leftward axis
- ventricular pacing from the right ventricular apex
Myocardium
- Old inferior myocardial infarction, with Q waves in II, III and aVF
- left ventricular hypertrophy
- cardiomyopathy
Other
- Pre-excitation through a posteroseptal accessory pathway
- hyperkalemia
- a raised diaphragm in pregnancy, ascites or obesity
Congenital heart disease
- Ostium primum atrial septal defect and other atrioventricular septal defects
- tricuspid atresia
Normal variant
- Common between -30 and -45 degrees in older adults and on otherwise normal ECGs
What to do
- Look for the cause on the same ECG: inferior Q waves, LVH voltage, a delta wave, pacing spikes, a wide QRS.
- Isolated left axis deviation or left anterior fascicular block, in an adult without symptoms and with a normal exam, rarely needs testing. Get an echocardiogram when there are symptoms, a murmur, other ECG abnormalities or other signs of structural heart disease.
- In children, consider an echocardiogram when the axis is -42 degrees or less, the ECG shows chamber enlargement or hypertrophy, or the cardiac exam is abnormal.
- Bifascicular block with syncope needs a search for intermittent high-grade AV block: ambulatory monitoring and, if that is unrevealing, an electrophysiology study.
Pitfalls and mimics
- Reversed arm electrodes invert lead I (negative P, QRS and T waves) and fake a right or extreme axis.
- Cutoffs vary a little between references; some call -30 to -45 degrees a leftward normal axis.
- Adult ranges don't apply to infants and young children.
- An inferior MI and left anterior fascicular block can look alike: in fascicular block the inferior leads start with a small r wave (rS), while an old inferior MI leaves Q waves.
Key studies
Schneider 2018, Pediatrics
Of 296 children with left axis deviation and no known heart disease, 158 had an echocardiogram and 24 of them (15%) had heart disease. Disease was more likely with an axis of -42 degrees or less, chamber enlargement or hypertrophy on the ECG, or abnormal cardiac findings on exam.
Surawicz 2009, AHA/ACCF/HRS ECG standards, part III
Sets the ECG criteria for fascicular and bundle branch blocks, including an axis between -45 and -90 degrees for left anterior fascicular block.
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Questions clinicians ask
What is left axis deviation on an ECG?
A mean frontal QRS axis between -30 and -90 degrees. On the tracing, lead I is positive and leads II and aVF are mostly negative.
Is left axis deviation dangerous?
Usually not on its own. It most often reflects left anterior fascicular block or a normal variant. It matters when it comes with symptoms, is new compared with an old ECG, or appears with other findings such as bundle branch block, Q waves or signs of structural heart disease.
What causes left axis deviation?
Left anterior fascicular block is the most common cause. Others include old inferior MI, left ventricular hypertrophy, pre-excitation, right ventricular pacing, hyperkalemia, a raised diaphragm and some congenital heart defects such as an ostium primum atrial septal defect.
How do you calculate the ECG axis?
Use the net QRS in leads I and aVF, plus lead II for the -30 degree line. For a number, the axis lies at right angles to the most nearly isoelectric limb lead, toward the positive leads. The calculator on this page fits every limb lead you enter.
What does LAD mean on an ECG report?
On an ECG report, LAD usually means left axis deviation. In an angiography or cardiology report it can also mean the left anterior descending artery, so read it in context.
Sources
- Surawicz B, Childers R, Deal BJ, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part III: intraventricular conduction disturbances. Circulation. 2009;119(10):e235-e240. doi.org/10.1161/CIRCULATIONAHA.108.191095
- Kligfield P, Gettes LS, Bailey JJ, et al. Recommendations for the standardization and interpretation of the electrocardiogram: part I: the electrocardiogram and its technology. Circulation. 2007;115(10):1306-1324. doi.org/10.1161/CIRCULATIONAHA.106.180200
- Mason JW, Hancock EW, Gettes LS. Recommendations for the standardization and interpretation of the electrocardiogram: part II: electrocardiography diagnostic statement list. Circulation. 2007;115(10):1325-1332. doi.org/10.1161/CIRCULATIONAHA.106.180201
- Schneider AE, Cannon BC, Johnson JN, Ackerman MJ, Wackel PL. Left axis deviation in children without previously known heart disease. Pediatrics. 2018;141(3):e20171970. doi.org/10.1542/peds.2017-1970
- Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay. Circulation. 2019;140(8):e382-e482. doi.org/10.1161/CIR.0000000000000628
Written in our own words by 2 Minute Medicine's physician editors from the sources listed here. The diagrams are schematic drawings made for 2MM, not patient tracings. This page supports, and does not replace, a full ECG reading and clinical judgment.
For health care professionals. Values are for adults unless a page says otherwise.
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