A pulse oximeter is a non-invasive tool that provides an estimate of the oxygen saturation level in the blood, known as SpO2, along with the heart rate. While the fingertip is the standard and most common site for this device, alternative locations are often needed when a finger is unavailable or unsuitable. The question of whether the toe is a viable option for accurate monitoring is a frequent one for users in both home and clinical settings.
What Pulse Oximeters Measure
The device provides two primary readings: the blood oxygen saturation (SpO2) and the pulse rate, measured in beats per minute (BPM). The SpO2 reading is an estimate of the percentage of hemoglobin molecules in the arterial blood carrying oxygen. This measurement is accomplished by passing two different wavelengths of light, one red and one infrared, through the tissue.
Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through, while deoxygenated hemoglobin does the opposite. The sensor measures the amount of light transmitted, and a processor calculates the ratio of the two light absorptions to determine the SpO2 percentage. The device simultaneously measures the pulse rate by detecting the change in blood volume that occurs with each heartbeat.
Using the Device on the Toe
Yes, the toe is an acceptable alternative site for pulse oximetry, but it requires the correct equipment and technique for a reliable reading. Standard finger-clip oximeters may not fit correctly on a toe, especially the big toe, often being too tight or too loose, which compromises accuracy. Specialized sensors are often necessary, such as flexible adhesive probes or pediatric wrap sensors, designed to fit various body parts.
For successful placement, the sensor must be positioned so the light emitter and the detector are directly opposite each other, ensuring the light passes through the pulsatile blood flow. The cable should ideally run along the sole of the foot to minimize the chance of accidental displacement. Ensure the sensor is not wrapped too tightly, as this could restrict local blood flow and yield a falsely low or inconsistent reading.
Why Alternative Sites Are Sometimes Necessary
The need to use a toe instead of a finger often arises from specific clinical or physical constraints. Conditions that cause poor blood flow to the hands, such as Raynaud’s phenomenon or severe cold, can make it difficult for the oximeter to detect a strong pulse signal in the fingers. Local trauma, burns, or amputation of a finger also necessitate finding an alternate location.
For infants and small children, a finger may be too small for a standard sensor, making the toe or foot a more suitable site for a pediatric-sized wrap sensor. Placing the sensor on the toe, often under a sock, can also prevent confused or restless patients from constantly pulling the device off, allowing for continuous monitoring.
Ensuring Accurate Readings
Accuracy is paramount, and several factors can compromise the readings, especially when using the toe. The most common issue at any site is patient movement, known as motion artifact, which is particularly challenging on the foot or toe. Any movement can cause the sensor to shift, disrupting the alignment of the light and the detector, leading to fluctuating or unreliable numbers. The patient must remain as still as possible during the measurement to get a stable reading.
Ambient light, such as bright sunlight or high-intensity lamps, can also interfere with the sensor’s ability to read the transmitted light accurately. Extreme cold causes peripheral vasoconstriction, which reduces blood flow to the toe, weakening the pulse signal and making the SpO2 reading less reliable. Additionally, dark nail polish or artificial nails on the toe can block or absorb the light emitted by the sensor, leading to a falsely low reading or no reading at all. If the reading is inconsistent with the person’s symptoms, such as a low SpO2 without any signs of breathlessness, it is advisable to remove all interfering factors, reposition the sensor, and recheck the reading.