Pulse Oximeter

Every cell in your body requires oxygen to function. Your red blood cells pick up oxygen molecules from your lungs and transport them even to the farthest corners of your body, where they hand them over to the cells. Measuring blood oxygen levels shows how much of the red blood cells' possible oxygen-carrying capacity is being used.

The higher the value, the better. A reading of 98 means your body is operating at 98% of its potential oxygen transport capacity. A value of 50 would indicate only half of that capacity.

• The maximum oxygen saturation value is 100% (there is no such thing as 105% saturation).
• A value between 96-100% is healthy and normal; no action is needed.
• Values between 90-95% indicate a decrease in oxygen supply. Consult your general practitioner for a comprehensive heart-lung examination. No need to rush, but it is advisable to do so soon.
• If your value is 88% or lower, seek immediate medical advice, or if caused by a known condition, start oxygen therapy as per your doctor's instructions.

Measuring blood oxygen levels is very important in many medical conditions

In certain diseases, insufficient oxygen reaches your cells, which suffer from oxygen deficiency, preventing vital cellular metabolic processes like energy production. Your body’s functions progressively deteriorate. The first priority is to identify and treat the cause of the problem.

Once the cause is known and treatment begins, regular monitoring of blood oxygen levels and oxygen transport capacity is necessary. This is ensured by a simple device, the pulse oximeter.

Your blood oxygen level can decrease due to the following reasons

• Your heart cannot effectively circulate blood,
• Your lungs do not receive enough oxygen,
• Your red blood cells do not function properly and thus cannot absorb and/or transport sufficient oxygen.

Blood oxygen level measurement is recommended for

Respiratory diseases

• Chronic bronchitis
• Obstructive pulmonary disease (COPD)
• Asthma (during attacks)
• Allergy (during attacks)
• Lung tumor
• Pneumonia, etc.
• A recent American publication reports that in some patients during early stages of Covid-19 infection, oxygen saturation may drop below 80% while they remain asymptomatic. In these cases, the pulse oximeter can provide early warning of illness.

Heart diseases

• Heart failure,
• Cardiomyopathy,
• Myocarditis,
• Atrial fibrillation - absolute arrhythmia,
• Paroxysmal arrhythmias, etc.

Diseases of red blood cells:

• Low red blood cell count or malfunction
• Hemoglobin problems (this molecule is responsible for oxygen transport)
• Deficiency of certain substances (iron, vitamin B12, folic acid) leading to insufficient or dysfunctional red blood cells.

Pulse Oximeter – the tool for measuring blood oxygen level

Main types of pulse oximeters:

• Finger clip-on
• Handheld
• Table (clinical).

All pulse oximeters operate on the same principle and offer nearly identical accuracy in measuring blood oxygen levels. For home use, devices priced between 10,000 to 20,000 HUF are generally sufficient. Devices over 50,000 HUF usually offer additional features mainly useful for medical professionals; for home use, especially for patients who cannot care for themselves, clinical devices may be considered. For home use, I recommend finger clip-on and optionally handheld devices, especially if an alarm feature is needed.

Always measure at rest!

WARNING! Finger clip-on pulse oximeters costing around 10,000-20,000 HUF are ONLY suitable for measuring resting blood oxygen levels! They can show erratic values during movement.

For measurements during movement, a pulse oximeter with a motion-tolerant algorithm is needed. These are for clinical use and are very expensive, often costing several hundred thousand forints.

If you have a simple finger clip-on device, do not move or walk during measurement! Keep the hand with the pulse oximeter perfectly still. This way, you get accurate results.

When you place the device on your finger and turn it on, it will initially show nothing and then quickly fluctuate between numbers. This is normal, do not worry! The device needs about 20 seconds to determine the average value. Only values shown 20-30 seconds after turning on have meaningful significance.

Watch Dr. Zsolt Zátrok's video about blood oxygen level measurement, including how to use the M70 finger clip pulse oximeter

Factors influencing blood oxygen levels

Inexpensive pulse oximeters do not provide accurate data during movement. Motion-tolerant algorithms are included only in clinical (and expensive) devices. So if you walk around, talk, or gesture during measurement, the device cannot detect pulse waves properly and either does not show a value or displays erratic, fluctuating values. As mentioned, measure while seated, resting your arm on a table and stay still. 50-60 seconds per measurement is enough. Sporting or exercising with a 10,000 HUF finger clip device is not feasible.

Heart disease (cardiomyopathy, heart failure) leads to poor peripheral blood circulation — your heart is too weak to "push" blood to your finger tips. The tips of your fingers, your nails, and even your lips may appear bluish, indicating insufficient fresh blood supply. Cheap pulse oximeters are not accurate under low blood flow conditions.

In arrhythmias (atrial fibrillation, absolute arrhythmic heartbeat), your heartbeat is very irregular, causing unpredictable pulse strength variations—sometimes very weak or absent, sometimes strong. Cheap devices struggle here as well, often not showing any value or displaying erratic and nonsensical readings.

Body posture affects the reading. A medical study examined the relationship between body position and blood oxygen saturation. They found readings to be higher when sitting than lying down. A 3-5 point higher reading when sitting is completely normal. This is because posture changes blood circulation — when lying down, pulse waves flatten, while when sitting, gravity enhances them. Lung ventilation also changes; lying down reduces lung expansion and oxygen intake. Always perform measurements seated and compare only with other seated readings. Apples to apples!

Occasionally, readings differ between hands, possibly due to developmental abnormalities, narrowing or dilation of arteries on one side. Generally, I recommend always measuring the same way, for example on the middle or index finger of your left hand. When comparing, use measurements taken in the same position on the same finger.

How does a pulse oximeter work?

The device has a small light source and a sensor placed opposite each other.
The light source emits two beams (red and infrared wavelengths). Light passes through your finger where you place the sensor. The sensor detects how much of the emitted light is absorbed by the cells and blood flowing within your finger. From this, it determines what percentage of the total oxygen-carrying capacity is bound.
Simply put, if every oxygen-binding site on the red blood cells is occupied by oxygen, oxygen saturation is 100%. If only half are carrying oxygen, the device shows 50%.