A pulse oximeter is a non-invasive medical device that provides a rapid, indirect measurement of oxygen levels in the blood. This small, clip-like instrument functions by emitting light through a translucent part of the body, typically a fingertip. The device measures the differential absorption of this light by oxygenated and deoxygenated hemoglobin in the arterial blood flow. The resulting readout displays two primary metrics: peripheral oxygen saturation (SpO2), expressed as a percentage, and the pulse rate, measured in beats per minute (BPM).
Preparing for an Accurate Reading
Achieving a reliable reading begins before the device is even turned on, requiring the user to be in a calm, resting state. Physical activity immediately prior to a measurement can temporarily elevate the heart rate and affect blood flow, potentially skewing the results. The chosen measurement site, usually a finger, must be clean, warm, and relaxed to ensure adequate circulation, or perfusion, which is necessary for the device’s light sensors to work correctly.
A common interference is the presence of color on the nail. Dark nail polish, artificial, or acrylic nails must be completely removed from the finger being used. These materials can block the light path, leading to falsely low or unstable SpO2 readings. Ensuring the finger is warm is also important because poor peripheral circulation from cold hands can significantly diminish the quality of the signal.
Step-by-Step Guide to Taking a Measurement
Once preparatory steps are complete, the device can be activated and applied to the finger. The middle or index finger is generally the preferred choice, as they often provide the most consistent and strongest signal. The clip should be opened and placed completely over the fingertip, ensuring the nail faces upward toward the light source and the finger pad rests flat on the sensor.
Keep the hand and finger completely still while the oximeter captures the light absorption data. After clipping the device on, the user must wait for the displayed values to stabilize, which typically takes between 10 and 30 seconds. The numbers often fluctuate initially, but a steady reading is required for an accurate determination of the SpO2 and pulse rate. The final, steady reading should then be recorded, along with the time of the measurement, for tracking purposes.
Interpreting the Results
The two numbers displayed represent the percentage of hemoglobin saturated with oxygen (SpO2) and the pulse rate (BPM). For a healthy adult at rest, a normal SpO2 reading is between 95% and 100%. The pulse rate should fall within the range of 60 to 100 beats per minute for most adults.
The SpO2 reading determines respiratory function, and a reading below 95% may warrant attention. Readings between 90% and 94% are considered low and may indicate mild hypoxemia (low blood oxygen). If the SpO2 measurement consistently drops to 92% or below, this is a sign of concern and an indication to contact a healthcare provider promptly.
A sustained SpO2 reading below 90% is considered a medical emergency. Individuals with chronic lung conditions, such as Chronic Obstructive Pulmonary Disease (COPD), may have a different target range. They sometimes accept a baseline SpO2 between 88% and 92% as normal for their condition. It is important to discuss target ranges with a physician if a pre-existing respiratory issue is present.
Common Causes of Inaccurate Readings
Several factors can lead to a falsely low or unreliable reading. Excessive movement, such as shivering or trembling, can disrupt the light signal and cause the reading to fluctuate or display an error. The accuracy relies on a strong pulsatile signal from the arterial blood flow. Poor circulation due to cold hands or low blood pressure (low perfusion) will weaken this signal.
Other environmental factors can interfere with the sensor’s ability to measure light transmission accurately. Intense ambient light, such as direct sunlight or bright overhead lamps, can sometimes be detected by the sensor, corrupting the true reading. Certain medical conditions can also introduce inaccuracies. For example, carbon monoxide poisoning causes the device to display a falsely high SpO2 because the oximeter cannot distinguish between oxygen and carbon monoxide bound to the hemoglobin.