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How does a pulse oximeter work?

Medicine & HealthLesson on requestFree to readयह पेज हिंदी में पढ़ें

Quick answer

A pulse oximeter clips onto a fingertip and shines red and infrared light through the skin. Oxygen-rich blood absorbs more infrared light, while oxygen-poor blood absorbs more red light. By comparing how much of each light passes through, the device calculates the percentage of haemoglobin carrying oxygen, called SpO2.

A pulse oximeter shines red and infrared light through a translucent body part like a fingertip, then measures how much of each wavelength is absorbed by blood. Because oxygen-rich and oxygen-poor haemoglobin absorb the two wavelengths differently, the device calculates the percentage of haemoglobin carrying oxygen.

Step by step

  1. The device emits two wavelengths of light—red and infrared—through a translucent body part like a fingertip.
  2. Oxygen-rich and oxygen-poor haemoglobin absorb the two wavelengths differently.
  3. A detector on the opposite side measures the light that passes through without being absorbed.
  4. The ratio of red to infrared absorption is calculated, focusing only on the pulsating arterial blood.
  5. The result is displayed as a percentage of oxygen saturation (SpO2) and a pulse rate.

Key things to understand

  1. Oxygen-rich haemoglobin absorbs more infrared light, while oxygen-poor haemoglobin absorbs more red light.
  2. The device shines both red and infrared light through a fingertip, earlobe, or toe.
  3. A light detector on the other side measures how much of each wavelength passes through without being absorbed.
  4. The ratio of red to infrared absorption is converted into an oxygen saturation percentage (SpO2).
  5. Pulsing blood flow creates a fluctuating signal, allowing the device to ignore static tissue and venous blood.

Labelled diagram

  1. Light emission

    Two LEDs emit red and infrared light into the fingertip.

  2. Light absorption

    Haemoglobin in blood absorbs some light; amount depends on oxygen level.

  3. Light detection

    A photodetector on the other side measures the light that passes through.

  4. Signal processing

    The device calculates the ratio of red to infrared absorption.

  5. Display

    Oxygen saturation (SpO2) and pulse rate appear on the screen.

How a pulse oximeter measures oxygen

Real-world examples

  • In hospitals, pulse oximeters are clipped to a patient's finger during surgery to monitor oxygen levels continuously.
  • Athletes at high altitudes use portable pulse oximeters to check how well their bodies are adapting to low oxygen.
  • People with asthma or COPD may use a home pulse oximeter to track their oxygen saturation between doctor visits.
  • During the COVID-19 pandemic, many people used pulse oximeters at home to detect silent hypoxia, a dangerously low oxygen level without obvious symptoms.

Common mistake

✗ Myth: A pulse oximeter directly measures the amount of oxygen in your blood.

✓ Fact: It actually measures the percentage of haemoglobin that is carrying oxygen, not the total oxygen content. Other factors like haemoglobin level and blood flow can affect oxygen delivery to tissues.

People also ask

What is a normal pulse oximeter reading?
A normal pulse oximeter reading for a healthy person at sea level is typically between 95% and 100%. Readings below 90% are considered low and may require medical attention. However, normal values can vary with altitude and individual health conditions, so always consult a doctor for interpretation.
How accurate is a pulse oximeter?
Pulse oximeters are generally accurate within 2% for readings between 90% and 100%, but accuracy drops at lower saturations. Factors like motion, poor circulation, dark skin pigmentation, and nail polish can reduce accuracy. For critical decisions, healthcare providers use calibrated devices and clinical judgment.
Why does a pulse oximeter show a pulse rate?
The pulse rate is derived from the pulsatile nature of arterial blood flow. As the heart beats, blood volume in the fingertip changes, causing a fluctuation in light absorption. The device detects this fluctuation and calculates the heart rate in beats per minute.
What do the numbers on a pulse oximeter mean?
The top number is oxygen saturation (SpO2), the percentage of haemoglobin carrying oxygen; normal is typically 95–100%. The bottom number is pulse rate in beats per minute. A low SpO2 may indicate breathing problems, so see a doctor if it persists.
Why does a pulse oximeter sometimes give a wrong reading?
Motion, cold hands, dark nail polish, or poor circulation can reduce blood flow or block light, causing inaccurate readings. Bright ambient light can also interfere. If the reading seems wrong, warm the hand, remove polish, and keep still before retrying.
Can a pulse oximeter detect carbon monoxide poisoning?
No. Standard pulse oximeters cannot distinguish between oxygen and carbon monoxide bound to haemoglobin, so they may show a falsely normal reading. Carbon monoxide poisoning requires a special co-oximeter blood test; seek emergency care if suspected.
Is a pulse oximeter reading of 92% dangerous?
A reading of 92% is below normal and may indicate low oxygen levels. It can be caused by lung or heart conditions. Consult a doctor promptly, especially if you have symptoms like shortness of breath, chest pain, or confusion.

Part of the guide: How Your Body Works →