A fingertip oxygen sensor, commonly called a pulse oximeter, is a non-invasive device used to monitor a person’s health, particularly during illnesses that affect breathing. This device clips onto a finger and provides a rapid estimation of the peripheral oxygen saturation (SpO2) in the blood. Oxygen saturation measures the percentage of hemoglobin molecules carrying oxygen. Monitoring this level assesses how effectively the lungs transfer oxygen into the bloodstream.
How the Sensor Measures Oxygen Levels
The pulse oximeter operates using the principle of light absorption, leveraging how oxygenated and deoxygenated blood interact with light. The device contains two light-emitting diodes (LEDs) and a photodetector positioned opposite them. One LED emits red light, and the other emits infrared light, which are the two wavelengths used for measurement.
Oxygen-carrying hemoglobin (oxyhemoglobin) absorbs more infrared light and allows more red light to pass through. Conversely, hemoglobin without oxygen (deoxyhemoglobin) absorbs more red light and lets more infrared light transmit through the finger. The light that passes through the tissue and blood vessels is then picked up by the photodetector.
The device’s internal processor calculates the ratio of light absorption at these two wavelengths. Since the amount of arterial blood changes with each heartbeat, the oximeter isolates the pulsatile signal to measure only the oxygen saturation in the arteries. This light absorption ratio is then converted into the displayed SpO2 reading.
Interpreting Oxygen Saturation and Pulse Rate
The pulse oximeter provides two main numerical readings: the SpO2 and the pulse rate. The SpO2 reading is displayed as a percentage and represents the estimated level of oxygen in the blood. For a healthy person, a normal oxygen saturation level ranges from 95% to 100%.
Readings below 95% may suggest hypoxemia, a reduction in oxygen available to the body’s tissues. Patients with chronic lung conditions, such as chronic obstructive pulmonary disease (COPD), may have a lower baseline saturation, sometimes between 88% and 92%, which is normal for them. The second number displayed is the pulse rate, measured in beats per minute (bpm). A resting heart rate for an adult is generally between 60 and 100 bpm.
Factors That Can Affect Reading Accuracy
Several external and physiological factors can interfere with the sensor’s ability to get an accurate reading. Dark nail polish colors (black, blue, or green) and artificial nails can block light transmission, leading to falsely low or erratic SpO2 readings. Poor circulation due to cold hands or low body temperature can also cause inaccurate readings because the device relies on a strong pulse signal.
Excessive movement, such as shivering, can distort the light signal and prevent the oximeter from calculating the pulse accurately, resulting in a motion artifact. To stabilize the reading, ensure the finger is warm and the hand is resting still at or below heart level. Bright ambient light, like direct sunlight, can also interfere with the light detection process, so readings should be taken in a moderately lit environment.
When to Seek Medical Help Based on Readings
A fingertip oxygen sensor serves as a screening tool, and readings should always be evaluated based on how a person feels. A persistent SpO2 reading below 92% is considered low and should prompt a call to a healthcare provider for advice. When the saturation level drops to 90% or lower, this is often a sign of clinically significant hypoxemia requiring medical evaluation.
An immediate medical emergency should be suspected if the reading falls to 88% or below, especially if accompanied by concerning symptoms. These symptoms include severe shortness of breath, a sudden change in mental status, or dizziness. The number on the screen is an estimate, and a rapid decline in readings, even if above the critical threshold, warrants attention from a medical professional.