# ECG Basics

> ECG waves, intervals, PR and QRS duration, Einthoven leads and rate calculation exam points for MBBS Physiology students.

- Canonical URL: https://prepelephant.com/topics/mbbs/physiology/ecg-basics
- 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: "ECG Basics", PrepElephant, https://prepelephant.com/topics/mbbs/physiology/ecg-basics

## Direct answer

The electrocardiogram, or ECG, is the graphic record of the electrical activity of the heart picked up from electrodes on the body surface. The P wave represents atrial depolarisation, the QRS complex ventricular depolarisation, and the T wave ventricular repolarisation; the atrial repolarisation wave is buried within the QRS. Each heartbeat therefore produces one coordinated sequence of waves whose shape and timing reveal rhythm, conduction and myocardial problems.

## What you must remember

- **Waves and complexes:** P — atrial depolarisation; QRS — ventricular depolarisation; T — ventricular repolarisation; the small U wave, when present, is attributed to repolarisation of the Purkinje fibres or papillary muscles.
- **Intervals:** PR interval 0.12 to 0.20 seconds, representing conduction from atria to ventricles with the AV nodal delay; QRS duration 0.08 to 0.10 seconds; QT interval varies with rate and roughly corresponds to ventricular systole.
- **Paper calibration:** standard speed 25 mm per second, so one small square is 0.04 seconds and one large square 0.2 seconds; 10 mm deflection equals 1 mV.
- **Leads:** bipolar limb leads I, II and III form Einthoven's triangle, in which lead II equals the sum of leads I and III; augmented unipolar leads aVR, aVL and aVF; and six precordial leads V1 to V6 over the chest.
- **Heart rate from the tracing:** divide 300 by the number of large squares between successive R waves, or 1500 by the number of small squares.
- **Axis and abnormalities:** a tall R in lead I with negative deflection in aVR suggests left axis deviation, the reverse a right axis; P mitrale and P pulmonale indicate atrial enlargement, and Q waves, ST changes and T inversion suggest infarction or ischaemia.

## Common confusion

Students equate the ECG with mechanical events — it records electrical activity only, and a normal ECG does not guarantee normal pumping. The waves are also mislabelled: repolarisation of the atria is hidden in the QRS, and it is the T wave, not a separate "atrial T", that ends ventricular systole electrically. Finally, do not confuse the PR interval (onset of P to onset of QRS) with the PR segment (end of P to onset of QRS), and remember that a short PR with slurred QRS upstroke suggests an accessory pathway as in Wolff-Parkinson-White syndrome.

## Exam-focused takeaway

In theory, write the genesis of waves with normal durations, Einthoven's triangle and its law, lead placement, and one line each on rate, rhythm and axis interpretation. In viva, expect why the ECG cannot measure contractility, the calibration values, and naming a first-degree heart block from a prolonged PR interval. In practicals and OSCE, record a rhythm strip, calculate the rate and R-R regularity, identify a normal sinus rhythm tracing, and demonstrate electrode placement for the limb and chest leads on a volunteer.

## Frequently asked questions

### What does each wave of the ECG represent?

The P wave is atrial depolarisation, the QRS complex is ventricular depolarisation, and the T wave is ventricular repolarisation. Atrial repolarisation is merged within the QRS complex.

### What is the normal PR interval and what does it reflect?

It lies between 0.12 and 0.20 seconds and measures conduction from the atria to the ventricles, chiefly the delay at the AV node. Prolongation indicates first-degree heart block.

### What is Einthoven's law?

Lead II equals the algebraic sum of leads I and III, because the three bipolar limb leads form a triangle around the heart. This relationship is used to check electrode placement errors.

### How do you calculate heart rate from an ECG?

Divide 300 by the number of large squares between two consecutive R waves when the rhythm is regular. For an irregular rhythm, count the R waves in a set length of the rhythm strip.