A pulse oximeter is a common device used to non-invasively monitor a person’s oxygen levels, typically by clipping a sensor onto a finger or earlobe. The measurement provided is the saturation of peripheral oxygen (SpO2), which indicates the concentration of oxygen in the blood circulating through the extremities. To obtain this reading, the device relies on plethysmography, often called “pleth,” which tracks changes in blood volume. SpO2 and plethysmography offer a continuous assessment of how effectively the lungs transfer oxygen into the bloodstream.
Understanding Peripheral Oxygen Saturation (SpO2)
Peripheral oxygen saturation (SpO2) represents the percentage of hemoglobin bound to oxygen within the red blood cells. Hemoglobin is a protein that transports oxygen from the lungs to the rest of the body. The SpO2 value is a ratio comparing the amount of oxygen-carrying hemoglobin to the total amount available. This non-invasive measurement serves as a proxy for arterial oxygen saturation (SaO2), which requires a direct blood draw.
Maintaining adequate oxygen saturation is necessary for cellular respiration and organ health. When oxygen levels drop too low, hypoxemia can occur, potentially damaging tissues. Since the brain and heart are sensitive to drops in oxygen concentration, the SpO2 reading provides an immediate indicator of respiratory and circulatory performance.
The Mechanism of Plethysmography
Plethysmography broadly refers to measuring volume changes within a body part. In pulse oximetry, photoplethysmography uses optical techniques to detect blood volume changes. The device shines two light wavelengths—red light and infrared light—through the tissue. Hemoglobin’s light absorption properties change depending on whether it is bound to oxygen.
Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through, while deoxygenated hemoglobin does the opposite. By measuring the amount of each light passing through the pulsing tissue, the oximeter calculates the ratio of oxygenated to deoxygenated blood. This calculation focuses only on arterial blood, identified by its pulsatile flow. The changes in light absorption occurring with each heartbeat are recorded as the photoplethysmographic waveform, or “pleth wave,” which confirms the presence of pulsatile blood flow required for a reliable SpO2 calculation.
Interpreting SpO2 Values and Clinical Relevance
The final SpO2 number offers significant insight into a person’s respiratory status. For a healthy adult, a reading falls within the range of 95% to 100%. A reading consistently below 92% is considered a cause for concern, indicating insufficient oxygen delivery to the tissues. Physicians may set different target ranges for individuals with chronic conditions, such as Chronic Obstructive Pulmonary Disease (COPD), where 88% to 92% may be acceptable.
Pulse oximetry is a standard tool in medical settings due to its ease of use and continuous monitoring capability. It is routinely used during surgical procedures to monitor oxygenation under anesthesia. It is also valuable for screening patients in sleep studies, where drops in saturation can indicate obstructive sleep apnea, and for home monitoring of people managing respiratory or cardiac conditions.
Factors Affecting Measurement Accuracy
While pulse oximetry is reliable, several physiological and external factors can interfere with light absorption and lead to inaccurate readings. Patient movement (motion artifact) is a common issue because it disrupts the sensor’s ability to isolate the arterial pulse signal. Low peripheral blood flow, known as poor perfusion, also distorts the measurement, often occurring with cold extremities or low blood pressure.
External substances on the skin can block or alter the light path. Dark nail polish or artificial nails, for instance, can cause falsely low readings by absorbing light. Dark skin pigmentation can sometimes cause the oximeter to overestimate actual oxygen saturation. Ambient light, if allowed to enter the sensor, can also contaminate the signal, making the final SpO2 calculation unreliable.