# Electrocardiogram Axis and Intervals

> ECG axis and intervals in MBBS Physiology: normal QRS axis, two-lead determination, PR, QRS and QTc values, rate calculation and hyperkalaemia sequence.

- Canonical URL: https://prepelephant.com/topics/mbbs/physiology/electrocardiogram-axis-and-intervals
- Exam / course: MBBS · Subject: Physiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Electrocardiogram Axis and Intervals", PrepElephant, https://prepelephant.com/topics/mbbs/physiology/electrocardiogram-axis-and-intervals

## Direct answer

The mean QRS axis normally lies between −30° and +110° (many textbooks quote −30° to +90°); left axis deviation points to left anterior hemiblock, inferior infarction or left ventricular hypertrophy, right axis deviation to right ventricular hypertrophy, pulmonary embolism or tall thin body habitus. The standard intervals on paper running 25 mm/s (one small square 0.04 seconds, one large square 0.20 seconds): PR 0.12-0.20 seconds, QRS up to 0.10 seconds (0.12 or more means complete bundle branch block), and rate-corrected QT (Bazett) under 0.44 seconds. Heart rate at the bedside equals 300 divided by the number of large squares between consecutive R waves.

## What you must remember

- **Axis by two leads:** lead I and aVF positive — normal axis; I positive, aVF negative — left axis (confirm with lead II, which must be positive in normal); I negative, aVF positive — right axis; both negative — extreme (north-west) axis.
- **Isoelectric lead method:** the axis runs perpendicular to the limb lead with the most equiphasic QRS, toward the lead that is positive next in Cabrera sequence — fast and examinable.
- **PR interval:** 0.12-0.20 seconds; short (under 0.12) in pre-excitation and junctional rhythms, long in first-degree block (0.20-0.40); PR lengthens progressively before the dropped beat of Wenckebach.
- **QRS duration:** under 0.10 seconds normal, 0.10-0.12 incomplete block, 0.12 or more complete block; hyperkalaemia widens QRS progressively by inactivating sodium channels.
- **QTc:** Bazett's QT divided by the square root of the R-R interval, normal under 0.44 seconds; prolonged by hypokalaemia, hypomagnesaemia, hypocalcaemia, class IA and III antiarrhythmics; short in hypercalcaemia.
- **Rate calculation:** 300 divided by large squares between R waves, or 1500 divided by small squares — at three large squares, the rate is 100.
- **U wave:** a small positive deflection after T, prominent in hypokalaemia and bradycardia.
- **Axis extremes of habitus:** tall asthenic frames hang the heart vertically (rightward), stocky frames push it horizontal (leftward) — a normal variant, not a disease.

## A worked interpretation: progressive hyperkalaemia

A dialysis patient misses two sessions and arrives with weakness; potassium is 7.2 mmol/L. The electrocardiogram tells the story in stages. Around 6-6.5, tall peaked T waves appear — the earliest and most feared sign missed at triage. By 7-7.5, the P wave flattens and the PR lengthens as atrial conduction slows. Above 8, P waves vanish (sinoventricular conduction) and the QRS begins to widen as sodium channel inactivation spreads. Left untreated, QRS merges with the T wave into a sine wave, and ventricular fibrillation or asystole follows. Physiology dictates therapy: intravenous calcium gluconate stabilises membranes within minutes (doing nothing to the potassium), insulin-dextrose shifts potassium into cells over half an hour, and dialysis removes it — sequence matters, and the ECG signs are the map.

Apply the same discipline to intervals in a patient on multiple QT-prolonging drugs: measure QT, correct with Bazett, and any QTc over 0.50 seconds puts torsades on the table.

## How the exam frames it

The trap axis question gives lead I negative and aVF positive and expects "right axis deviation — after checking lead II". If lead II is positive, the axis is right; if lead II is negative, the axis lies in the no-man's-land between −90° and −150° (extreme axis), seen in lead misplacement, ventricular rhythms and severe pulmonary disease with hyperinflation. The second favourite is PR shortening: a short PR with a delta wave and wide QRS defines pre-excitation (Wolff-Parkinson-White), whereas a short PR with a narrow QRS in a tachycardia suggests an AV-nodal re-entry or Lown-Ganong-Levine pattern. Third, expect the calculation question — always carry 300/large squares, because a completed R-R of four large squares is a rate of 75, and examiners set it that way.

## Frequently asked questions

### What is the normal QRS axis range and how is it quickly determined?

Between −30° and +110°; screen with leads I and aVF (both positive is normal), then refine with the lead showing the most equiphasic QRS — the axis lies perpendicular to it.

### What are the normal PR, QRS and corrected QT values?

PR 0.12-0.20 seconds, QRS under 0.10 seconds (0.12 or more indicates complete bundle branch block), and QTc under 0.44 seconds by Bazett's formula.

### How is heart rate calculated from the rhythm strip?

Divide 300 by the number of large squares between consecutive R waves (equivalently 1500 by small squares), since each large square is 0.20 seconds.

### What is the ECG sequence of progressive hyperkalaemia?

Peaked T waves (K around 6.5), then flat P waves with PR prolongation, then absent P waves with QRS widening (above 8), finally a sine wave pattern preceding ventricular fibrillation or asystole.

### What does a short PR interval with a delta wave indicate?

Pre-excitation from an accessory atrioventricular pathway (Wolff-Parkinson-White syndrome), with a risk of re-entrant supraventricular tachycardias.