ECG Electrocardiogram: Reading Heart Electrical Activity

An ECG, or electrocardiogram, is a graph of the electrical activity of the heart recorded from the skin. With every heartbeat, a wave of electrical signals spreads through the heart muscle, and electrodes stuck on the chest, arms and legs pick up these tiny signals.

Doctors rely on the ECG because many heart problems are electrical problems first. A blocked artery, an irregular rhythm or an enlarged chamber changes the shape of the waves in recognisable ways, often before the patient feels anything serious.

Key Concepts

• P wave is a small upward bump showing electrical excitation (depolarisation) of the atria, just before they contract.
• QRS complex is the tall spike showing depolarisation of the ventricles; it hides the repolarisation of atria inside it.
• T wave is a rounded wave showing the ventricles recovering (repolarisation) and relaxing.
• A standard ECG uses 12 leads — different viewpoints of the same electrical activity.
• On ECG paper, speed is usually 25 mm per second, so each 1 mm small box equals 0.04 seconds.
• Counting QRS complexes gives heart rate directly, since each complex is one ventricular beat.

How It Works

1. The SAN fires and the signal spreads over the atria, drawing the P wave.
2. The signal pauses at the AVN, creating a short flat gap that lets ventricles fill.
3. The impulse races down special fibres and sweeps through the ventricles, drawing the sharp QRS complex.
4. Ventricles contract and pump blood while the trace briefly flattens.
5. The ventricles recharge electrically, drawing the T wave, and the cycle repeats.

Worked Example

On an ECG strip, suppose consecutive QRS peaks are 20 mm apart at a paper speed of 25 mm per second. Time between beats = 20 mm divided by 25 mm per second = 0.8 seconds. Heart rate = 60 divided by 0.8 = 75 beats per minute. If the gap shrank to 12 mm, the interval would be 12/25 = 0.48 s and the rate would be 60/0.48 = 125 beats per minute — a fast rate (tachycardia) needing explanation, such as fever, anxiety or a rhythm disorder.

Common Misconceptions

• "ECG measures blood flow." Wrong. It measures electrical signals only; blood flow tests are different (like echocardiography).
• "A normal ECG means the heart can never have a problem." Wrong. Some blockages appear only during exertion, so stress tests are also used.
• "The QRS spike is the heart contracting." Not exactly — it is the electrical trigger just before contraction; the muscle movement follows the signal.

Real-World Applications

Hospitals use ECG to diagnose heart attacks within minutes of chest pain, since a raised ST segment is a classic sign. Small portable ECG monitors record rhythm for 24 hours to catch occasional irregular beats. Fitness watches use a simplified one-lead ECG to estimate heart rate and warn about irregular rhythm. Before surgery, an ECG is a routine safety check.

The ECG turns invisible heart electricity into a readable graph: P for atria, QRS for ventricles, T for recovery. Measuring its waves and gaps lets doctors read the heart's health from the skin's surface.