A pulse oximeter is a small, non-invasive device that estimates the amount of oxygen in a person’s blood, known as oxygen saturation (SpO2). This tool clips onto a body part, most commonly a fingertip, providing a quick and painless way to assess respiratory and circulatory health. It is a common monitoring device in healthcare settings and for home use.
Understanding Pulse Oximetry
A pulse oximeter operates by emitting two different wavelengths of light, red and infrared, through the skin. These light beams pass through the blood in the finger, and a sensor on the other side detects how much light is absorbed. Oxygenated hemoglobin, which carries oxygen, absorbs more infrared light, while deoxygenated hemoglobin absorbs more red light. By measuring these absorption differences, the device calculates the percentage of hemoglobin saturated with oxygen in the pulsatile flow of arterial blood, providing the SpO2 reading along with the heart rate.
Common Factors Affecting Accuracy
Several factors can lead to inaccurate pulse oximetry readings, potentially misleading users about their true oxygen levels. These interferences can stem from external obstructions, compromised blood flow, or changes in blood composition.
External obstructions can interfere with the light signal the oximeter relies upon. Dark nail polish, artificial nails, or dirt on the finger can block or distort the light, leading to unreliable readings. Excessive motion during measurement, such as shivering or shaking, also disrupts the signal. Additionally, bright ambient light shining directly on the sensor can dilute the emitted light, making it difficult for the device to accurately detect absorption.
Impaired blood flow to the measurement site can cause inaccurate readings because the device relies on a strong pulsatile signal. Conditions that reduce circulation, such as low blood pressure (hypotension) or cold fingers, can diminish the blood flow needed for a proper reading. Other issues like shock or peripheral artery disease also hinder adequate blood perfusion, affecting the oximeter’s ability to pick up a consistent signal.
Altered blood composition can mislead readings. Carbon monoxide (CO) poisoning is a concerning example, as CO binds to hemoglobin and absorbs light similarly to oxygenated hemoglobin. This can result in a falsely high or normal SpO2 reading, even when the body is dangerously deprived of oxygen. Certain intravenous dyes, such as methylene blue, can also interfere, causing falsely low SpO2 readings.
Ensuring Reliable Readings
Taking steps to minimize interference can improve the accuracy of pulse oximeter readings. Before measurement, remove any nail polish or artificial nails, as they can block the light signals. Warming cold fingers can improve blood circulation, ensuring a stronger pulsatile signal for the device to detect.
Proper probe placement is important; ensure the sensor is placed firmly on a clean, dry finger and is not too loose or tight. Remaining still during the measurement helps prevent motion artifacts that can distort the reading. If readings appear inconsistent, try taking multiple measurements on different fingers, allowing the device time to stabilize before recording the result.
When to Seek Medical Advice
While pulse oximeters are monitoring tools, they are not a substitute for medical diagnosis. It is important to seek medical attention if SpO2 readings are consistently low, below 90-92%, or if they fall below the level recommended by a healthcare provider.
Seek immediate medical attention if symptoms of low oxygen are present, regardless of the oximeter reading. Symptoms include severe shortness of breath, confusion, or a bluish tint to the lips or fingernails. If carbon monoxide poisoning is suspected, seek emergency care, as pulse oximeters can give misleadingly high readings in such cases. Always consult a doctor if readings are inconsistent or unreliable despite following best practices, or if there are concerns about overall health.