ECG Axis, Intervals and Chamber Hypertrophy
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Direct answer
The ECG axis is the overall direction of ventricular depolarisation, normally between minus 30 and plus 90 degrees, read from leads I and aVF. PR is 120 to 200 milliseconds, QRS under 120 milliseconds and QTc below about 450 milliseconds in men. Left ventricular hypertrophy is suggested by voltage criteria, while right ventricular hypertrophy shows right axis deviation with a dominant R wave in V1.
What you must remember
- Axis reading: positive lead I and positive aVF give a normal axis, positive lead I with negative aVF gives left axis deviation (a true deviation beyond minus 30 degrees also needs a negative lead II), and negative lead I with positive aVF gives right axis deviation.
- Extreme axis: negative leads I and aVF together give a northwest axis, which suggests a ventricular origin or severe disease.
- Intervals: PR 120 to 200 milliseconds, QRS under 120 milliseconds, and QTc under about 450 milliseconds in men and 460 milliseconds in women.
- Short PR: a PR under 120 milliseconds with a delta wave indicates pre-excitation, as in Wolff-Parkinson-White syndrome.
- Sokolow-Lyon: an S wave in V1 plus an R wave in V5 or V6 of 35 mm or more suggests left ventricular hypertrophy.
- Cornell voltage: an R wave in aVL plus an S wave in V3 above 28 mm in men or 20 mm in women.
- Right ventricular hypertrophy: right axis deviation, a dominant R wave in V1 and a strain pattern of ST depression and T inversion in V1 to V3.
- P wave clues: P pulmonale, a tall peaked P wave above about 2.5 mm in lead II, indicates right atrial enlargement, while P mitrale, a broad notched P wave over 120 milliseconds in lead II, indicates left atrial enlargement.
A fixed order for reading an ECG
A 56-year-old hypertensive man has an ECG for breathlessness, and the report mentions left axis deviation with voltage criteria for hypertrophy. The steps below show how to read the tracing in a fixed order so that nothing is missed.
- Rate and rhythm: Count the rate from the R to R interval, using 300 divided by the number of large squares between beats, and check that every P wave is followed by a QRS complex.
- Axis: Read leads I and aVF and classify the axis as normal, left or right deviation, or extreme, since the axis narrows the differential diagnosis.
- Intervals: Measure the PR interval, the QRS duration and the corrected QT, and note any short PR with a delta wave or a prolonged QTc.
- Voltage criteria: Apply Sokolow-Lyon and Cornell criteria for left ventricular hypertrophy, and look for dominant R waves in V1 for right ventricular hypertrophy. Check for a strain pattern, because strain supports true hypertrophy over a normal variant.
- Atria and the overall picture: Examine the P waves in lead II and V1 for P pulmonale or P mitrale, and correlate with echocardiography to confirm chamber enlargement.
Where the examiner sets the traps
ECG questions usually test whether you read one feature in the context of the others.
- Trap: reading left axis deviation as an automatic sign of hypertrophy Correct: left axis deviation also occurs with left anterior fascicular block and inferior infarction, so the axis alone does not diagnose hypertrophy.
- Trap: calling a short PR normal in a palpitating young patient Correct: a PR under 120 milliseconds with a delta wave suggests pre-excitation, which carries an arrhythmia risk and needs electrophysiology review.
- Trap: diagnosing hypertrophy from voltage alone in a trained athlete Correct: athletic training raises voltage, so strain patterns and echocardiography are needed before labelling disease.
Frequently asked questions
How is the ECG axis determined from leads I and aVF?
The ECG axis is determined from leads I and aVF, where a normal axis gives positive deflections in both leads. Left axis deviation gives positive lead I with negative aVF, and right axis deviation gives negative lead I with positive aVF, while negative leads I and aVF together indicate an extreme axis.
What are the normal ECG intervals in adults?
The normal ECG intervals in adults are a PR interval of 120 to 200 milliseconds and a QRS duration under 120 milliseconds. The corrected QT should be below about 450 milliseconds in men and 460 milliseconds in women.
Which voltage criteria diagnose left ventricular hypertrophy on ECG?
The Sokolow-Lyon criterion for left ventricular hypertrophy is an S wave in V1 plus an R wave in V5 or V6 of 35 mm or more. The Cornell voltage, with an R wave in aVL plus an S wave in V3 above 28 mm in men or 20 mm in women, is an alternative widely used in ECG reports.
What ECG features suggest right ventricular hypertrophy?
ECG features of right ventricular hypertrophy include right axis deviation, a dominant R wave in V1 and a right ventricular strain pattern. Strain describes ST depression and T wave inversion in V1 to V3, and voltage criteria alone are less reliable than the combination.