Pulse Oximeters: How They Work and How to Use One

How a pulse oximeter works

Blood that is rich in oxygen is bright red, and blood with less oxygen is darker. The oximeter clip has two small lights, one red and one infrared, on one side, and a sensor on the other. The lights shine through the fingertip, and the sensor measures how much light is absorbed. Because the amount absorbed changes with each pulse of blood, the device can separate arterial blood from other tissue and estimate the percentage of haemoglobin that is carrying oxygen. This percentage is called oxygen saturation, shown as SpO2. The device also counts the pulse.

The reading is an estimate. It is not the same as a blood gas test done in hospital, though it is often close in healthy people.

Who may need an oximeter at home

• People with chronic lung disease such as COPD or pulmonary fibrosis, as advised by their doctor.
• People on home oxygen, or recovering from pneumonia or a severe infection. See home oxygen during recovery from pneumonia or severe infection.
• People with heart failure whose breathing changes.
• Those with sleep apnoea or other conditions as advised.
• Older adults with an acute chest illness when a doctor wants home checks.

For a healthy older adult with no symptoms, daily oximeter checks are usually unnecessary and may cause worry.

Choosing an oximeter

• Display: large, bright digits that rotate or can be read from any angle. Models with a graph or pulse waveform help show whether the reading is good.
• Fit: a clip that holds firmly without pinching, for thin or thick fingers.
• Battery: common AAA batteries are easy to replace; keep spares.
• Quality: buy from a trustworthy source; cheap devices of unknown origin vary in accuracy. Ask a doctor or pharmacist for a trusted brand. See health device budgets and quality checks in India.
• Alarms: some models sound an alert for low saturation or pulse limits.
• Memory or connectivity: optional, helpful in some cases.

Remember that a fingertip oximeter used at home is a screening aid, not a medical-grade device.

How to use it step by step

1. Rest first. Sit quietly for about five minutes, back supported, with the hand resting on the chest or a table at heart level, and relaxed. 2. Warm the hand. If cold, warm hands by rubbing or soaking in warm water and dry them thoroughly. 3. Remove nail polish and artificial nails, at least from the finger being used. Dark polish, gel and acrylic can alter readings. See pulse oximeter accuracy: cold fingers, nail polish and skin tone. 4. Check the finger. Choose the middle or index finger. The finger should be clean, dry and free of henna, dirt or lotion. 5. Place the clip. Insert the finger fully, with the nail facing upward and the fingertip touching the stop inside. Do not squeeze. 6. Keep still and quiet. Avoid talking or moving the hand. 7. Wait for a stable number. Watch for 30 seconds to a minute until the number stops changing. Many devices show a wave pattern; a steady, regular wave suggests a good signal. 8. Record the highest steady value you see, along with the pulse, the date and time, and how your parent feels. Do not note a number that appears in the first seconds or flickers. 9. Repeat if the reading seems odd, on another finger or after warming the hand.

What the readings usually mean

These are general figures, and a doctor decides what is right for each person.

• In healthy adults at rest, SpO2 is usually about 95 to 100 per cent.
• A reading of 94 per cent or lower at rest, if it persists and is new, deserves a call to the doctor, particularly if your parent has breathlessness or other symptoms.
• A reading of 92 per cent or lower with symptoms is often treated as needing prompt medical review, depending on the person.
• A reading below about 90 per cent is usually a reason to seek urgent medical advice.
• Some people with chronic lung disease normally run lower, such as 88 to 92 per cent, and their doctor may set a personal target. See when oxygen saturation is low: what to do.

A normal reading does not mean everything is fine. If your parent is struggling to breathe, confused, grey or blue around the lips or very drowsy, get help regardless of the number.

Pulse rate

The oximeter also shows pulse rate. A normal resting pulse for most adults is roughly 60 to 100 beats a minute, though athletes and people on certain medicines may be lower. A pulse that is very fast, very slow or irregular, particularly with dizziness, chest pain or breathlessness, needs medical attention. Fingertip pulse readings can be unreliable if the rhythm is irregular, as in atrial fibrillation.

When to check

• When your parent has breathlessness, cough, fever or a chest infection.
• When a doctor has advised scheduled checks, for example daily during recovery.
• When your parent seems unusually sleepy, confused or weak, as low oxygen can show up this way.
• Not every hour: repeated checking causes anxiety and rarely helps.

For more about frequency, see how often should an older adult check vitals at home.

Limits and common causes of wrong readings

• Cold fingers and poor circulation.
• Nail polish, gel, henna or dirt.
• Movement and tremor.
• Bright light, such as direct sunlight.
• Tight or ill-fitting clip.
• Low blood pressure or shock.
• Carbon monoxide poisoning, where the reading may look normal despite danger.
• Anaemia and some other blood conditions.
• Skin pigmentation. Studies have found that some oximeters may overestimate oxygen levels in people with darker skin, so symptoms matter even more.
• Cheap or uncertified devices.

Always combine the number with how your parent looks and feels.

Care and storage

Wipe the clip with a soft cloth lightly dampened with an alcohol wipe or mild disinfectant, and let it dry. Do not immerse it. Store it in a cool, dry place, away from direct sun, humidity and the bathroom, ideally in its case. Replace batteries when the display dims, and keep spare batteries. In humid monsoon months, add a silica gel packet to the storage box. See calibration, batteries and replacement of home health devices.

Using the oximeter with a thermometer and log

When your parent is unwell, record temperature, pulse, oxygen saturation and symptoms together, with the time. This helps the doctor decide whether to treat at home or visit. See digital thermometers for older people and monitoring an older adult during an acute illness at home.

When to ask a professional

Ask a doctor or nurse when an oximeter is appropriate for your parent, which target range to use and when to call. Take the device to the clinic for a comparison with the clinic's own. Call the doctor the same day if the reading is below the target or lower than usual, or if breathlessness is worsening. Call 112 if your parent is struggling to breathe, cannot speak a full sentence, has blue or grey lips, chest pain, new confusion, or the reading is below about 90 per cent and not rising with rest.