A pulse oximeter is a non-invasive medical device used to estimate the saturation of oxygen in the blood, commonly referred to as SpO2. This measurement reflects the percentage of hemoglobin carrying oxygen within the arterial blood supply. Cold temperatures significantly interfere with the device’s ability to detect the necessary signal for a reliable reading. This interference is a common problem because the device relies on detecting a strong, rhythmic blood flow, which is drastically altered when the extremities are cold.
How Pulse Oximeters Measure Oxygen Saturation
The basic science of pulse oximetry involves shining two different wavelengths of light through a translucent part of the body, usually a fingertip. The device emits a red light (around 660 nanometers) and an infrared light (near 940 nanometers). Oxygenated and deoxygenated hemoglobin absorb these two wavelengths of light differently. Oxygen-rich blood absorbs more infrared light and allows more red light to pass through.
The oximeter measures the ratio of light absorption at these two specific wavelengths to calculate the oxygen saturation percentage. The device must isolate the signal from arterial blood, which is pulsing with each heartbeat, from the non-pulsing background signals caused by venous blood and surrounding tissue. It achieves this by focusing only on the pulsatile component of the light signal, which corresponds to blood volume changes occurring with each cardiac cycle. This requirement for a strong, clear pulse is why the reading is called pulse oximetry.
The Physiological Impact of Cold on Readings
Cold temperatures trigger a natural physiological defense mechanism called peripheral vasoconstriction. This response causes the small arteries and arterioles in the extremities, such as the fingers and toes, to narrow significantly. The purpose of this narrowing is to reduce blood flow to the skin’s surface, thereby conserving core body heat.
The resulting reduction in blood flow, known as low perfusion, severely impacts the pulse oximeter’s performance. Because the blood vessels are constricted, the volume of pulsatile arterial blood reaching the capillary bed is greatly diminished. When the device attempts to measure light absorption, it receives a much weaker, erratic signal, often showing a reduction in the normalized pulse amplitude.
A weak pulsatile signal makes it difficult for the sensor to distinguish the arterial pulse from the surrounding tissue noise. This lack of a clear signal can cause the device to display error messages, show a non-existent reading, or return a reading that is falsely low. The device’s internal algorithms struggle to process the poor quality data, leading to an inaccurate result that does not reflect the patient’s actual systemic oxygen saturation.
Steps for Obtaining Accurate Readings
The most effective action to mitigate the effect of cold is to warm the finger before attempting a measurement. Simple friction methods, such as rubbing the hands together briskly, can help stimulate peripheral blood circulation. A more direct technique involves soaking the hand in warm water or placing the entire hand inside a warm towel or blanket.
Once the hand is warm, ensure the finger is placed correctly within the oximeter, often choosing the middle or index finger which typically has a more robust arterial supply. The hand should be kept still and relaxed, ideally resting on a surface at or slightly below heart level to optimize blood flow. Movement, even slight tremors, can create motion artifacts that mimic a poor signal, leading to an inaccurate reading.
Users should also be aware of other factors that interfere with light transmission. Dark or thick nail polish, particularly black or blue shades, can block the light and must be removed. Acrylic or artificial nails can also interfere with light penetration. If the reading is persistently low or erratic, wait a full minute for the reading to stabilize and try a different finger.