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How does an ECG work?

Medicine & HealthLesson on requestFree to readयह पेज हिंदी में पढ़ें

Quick answer

An ECG works by detecting the small voltages that heart muscle cells produce as they depolarise and repolarise with every beat. Electrodes on the skin pick up these signals, a machine amplifies them, and a moving trace shows the timing and shape of each electrical wave.

An ECG records the heart's electrical activity by placing electrodes on the skin, which detect tiny voltage changes each time the heart muscle depolarises and repolarises. The resulting trace shows the timing and rhythm of each heartbeat, letting doctors spot abnormal rhythms, reduced blood supply or damaged muscle.

Step by step

  1. Electrodes are placed on the arms, legs and chest, where salty skin and body fluids conduct electricity.
  2. Each heartbeat starts in the sinoatrial node, which sends an electrical impulse across the atria.
  3. The impulse spreads to the ventricles, causing them to contract and pump blood out.
  4. The electrodes detect these voltage changes and send them to the ECG machine.
  5. The machine amplifies the signal and draws it as a series of waves on paper or a screen.
  6. A trained reader interprets the wave shapes, heights and spacing to assess heart function.

Key things to understand

  1. Heart muscle cells generate electrical impulses that spread through the heart in a fixed sequence with each beat.
  2. Electrodes stuck to the arms, legs and chest pick up these tiny voltages because salty body fluids conduct electricity.
  3. The machine amplifies the signal and draws it as a moving line of waves on paper or a screen.
  4. The P wave marks the electrical signal spreading through the atria, the QRS complex through the ventricles, and the T wave their electrical recovery.
  5. Doctors read the shape, spacing and height of these waves to judge heart rate, rhythm and possible damage.

Labelled diagram

  1. Electrodes

    Sticky pads on skin pick up tiny voltages from the heart.

  2. SA node

    Natural pacemaker fires and starts each heartbeat's electrical wave.

  3. Signal spread

    Impulse travels through atria and ventricles, making them contract.

  4. Amplifier

    Machine boosts the weak signal so it can be recorded clearly.

  5. Trace

    Moving line shows P, QRS and T waves for each beat.

How an ECG records a heartbeat

Real-world examples

  • A hospital emergency department runs an ECG within minutes of a patient arriving with chest pain.
  • A routine check-up may include an ECG to screen for irregular heart rhythms in older adults.
  • Athletes sometimes have an ECG before competing to check for hidden heart conditions.
  • A wearable heart monitor records a continuous ECG over a day to catch occasional rhythm problems.

Common mistake

✗ Myth: An ECG sends electricity into the body to shock the heart into beating.

✓ Fact: An ECG only records the heart's own electrical signals; it does not deliver any current. A defibrillator is the device that sends a controlled shock, and it is used separately.

People also ask

What does an ECG actually measure?
An ECG measures the electrical voltage changes produced by heart muscle cells as they depolarise and repolarise during each beat. It records the timing, strength and direction of these signals, which appear as waves on a trace. From this, doctors infer heart rate, rhythm and whether parts of the muscle are damaged or stressed.
How many electrodes are used in an ECG?
A standard resting ECG uses ten electrodes: four on the limbs and six across the chest. These ten electrodes combine to produce twelve different views, called leads, of the heart's electrical activity. Fewer electrodes are used in portable or wearable monitors that record over longer periods.
Why is an ECG done during exercise?
An exercise ECG records the heart while you walk or run on a treadmill or cycle, when the heart works harder and needs more oxygen. This can reveal narrowed arteries that cause no symptoms at rest. It helps doctors assess fitness, chest pain and heart disease risk under controlled conditions.
Why do you have to lie still during an ECG?
Muscle movement creates its own electrical signals that mix with the heart's tiny voltages, blurring the trace. Staying still and breathing normally keeps the recording clean so the waves can be measured accurately. Talking or shivering can also add noise.
Is an ECG painful or dangerous?
No. An ECG is completely painless and safe because the electrodes only listen to electrical signals; they do not send any current into the body. You may feel slight coolness when the gel or sticky pads are applied. The test takes only a few minutes.
What is the difference between an ECG and an echocardiogram?
An ECG records electrical activity, while an echocardiogram uses ultrasound to make pictures of the heart's structure and how blood moves through it. They answer different questions, so doctors often order both together. The similar names confuse many people.
Can an ECG detect a heart attack?
Yes, a heart attack changes the electrical pattern of damaged muscle, and an ECG can show this within minutes. Certain wave shapes and elevations help doctors identify which part of the heart is affected. A normal ECG does not always rule out a problem, so see a doctor if you have symptoms.

Part of the guide: How Your Body Works →