A pulse oximeter is a non-invasive medical device used to estimate the saturation of oxygen in a person’s arterial blood (SpO2) and their pulse rate. This portable technology is commonly used for home monitoring to track general health and respiratory function.
How the Pulse Oximeter Measures Oxygen
The pulse oximeter operates using the Beer-Lambert Law and spectrophotometry, which relates a substance’s concentration to its light absorption properties. The device shines two types of light—red light (660 nm) and infrared light (940 nm)—through a translucent body part, typically a fingertip.
Oxygenated and deoxygenated hemoglobin absorb these two wavelengths of light differently. Oxygen-carrying hemoglobin absorbs more infrared light and allows more red light to pass through, while deoxygenated hemoglobin does the opposite. The oximeter measures the ratio of light transmitted through the tissue to calculate the percentage of saturated hemoglobin.
To ensure the reading reflects only arterial blood, the device uses plethysmography. With each heartbeat, the volume of arterial blood in the finger momentarily increases, creating a measurable pulse. By focusing only on this pulsatile component, the oximeter isolates the arterial blood reading from the constant absorption caused by venous blood, skin, and bone. This reliance on a clear, stable pulsatile signal causes fluctuations when the light path or blood flow is disrupted.
Common Application and User Errors
Fluctuations often stem from errors in how the device is applied or used, rather than actual changes in the body’s oxygen level. A primary cause is motion artifact, where any movement, such as tremors or shivering, disrupts the light signal. The device struggles to distinguish the true pulse of blood flow from the erratic signal caused by the sensor moving on the finger. This can cause the SpO2 reading to falsely skew by 5 to 10 percentage points, or the device may fail to provide a reading.
Improper placement is another frequent application error that introduces signal noise. The probe must be placed snugly on a finger, typically the index or middle finger, and inserted completely into the device chamber. A loose fit allows excessive ambient light to enter or permits slight movements interpreted as a poor signal.
External barriers on the fingertip can directly interfere with the light transmission required for measurement. Dark-colored nail polish, such as black or deep red, or artificial nails can absorb the red and infrared light, making it difficult for the photodetector to receive a clear signal. This obstruction can lead to falsely low or fluctuating readings.
Cold extremities or poor peripheral circulation reduce signal quality. When the hands are cold, blood vessels constrict (vasoconstriction), significantly lowering blood flow to the fingertip. Since the oximeter depends on strong, pulsatile blood flow, low perfusion results in a weak and unstable signal. Warming the hand before measurement often corrects this issue.
Physiological Causes of Variation
Fluctuations not caused by user error often originate from the user’s internal physiological state, affecting the quality of the pulse signal. Low peripheral perfusion, where blood flow to the extremities is compromised, is a major factor. Conditions such as shock, severe hypotension, or hypothermia cause the body to restrict blood flow to the fingers and toes to prioritize core organs.
This systemic vasoconstriction leads to a weaker pulsatile signal, making it difficult for the oximeter to isolate the arterial component. In these scenarios, even high-quality devices may display an unreliable reading because the signal strength is too low to process accurately.
Cardiac arrhythmias, or irregular heart rhythms, can also cause the displayed pulse rate and SpO2 readings to fluctuate. The device relies on a regular, predictable pulse to calculate saturation. If the heart rate is highly irregular, the inconsistent intervals between pulses confuse the oximeter’s processing algorithms, leading to inconsistent values.
Environmental Interference and Device Limitations
External environmental factors can introduce noise into the oximeter’s sensitive light detection process. Bright ambient light, particularly direct sunlight or certain medical lights, can overwhelm the sensor’s ability to measure the light emitted by the device. This interference can be misinterpreted, resulting in an unstable or inaccurate reading. Shielding the sensor from bright light by covering the probe often resolves this issue.
The inherent quality and maintenance of the device itself also lead to unstable readings. Hardware issues such as a low battery level or a dirty sensor can compromise the integrity of the light signal, leading to erratic performance. Cheaper, non-clinical grade devices often lack the advanced signal processing algorithms necessary to filter out common interference, such as motion artifact or poor perfusion.
These lower-quality devices are more susceptible to fluctuations and may show errors of five percentage points or more in challenging conditions. Poor calibration or faulty sensors can cause the device to drift or fail to maintain a stable reading even when the user and environment are optimal.