A pulse oximeter is a non-invasive medical device that measures blood oxygen saturation (SpO2) as well as heart rate. It is commonly used in hospitals, clinics, and even at home to monitor the oxygenation of patients. Here's how it works:
1. Working principle:
The pulse oximeter is based on two principles: spectrophotometry (absorption of light) and plethysmography (variation of blood volume).
- Spectrophotometry: The device emits two wavelengths of light, one in the red (about 660 nm) and one in the infrared (about 940 nm). These lights pass through the tissue (usually a finger or earlobe) and are detected on the other side by a sensor.
- Plethysmography: This technique measures changes in blood volume in tissues, which helps determine the heartbeat.
2. Oxygen saturation measurement (SpO2):
- Light Absorption: Oxyhemoglobin (oxygen-bound hemoglobin) and deoxyhemoglobin (oxygen-free hemoglobin) absorb light differently. Oxyhemoglobin absorbs more infrared light, while deoxyhemoglobin absorbs more red light.
- SpO2 calculation: The oximeter compares the absorption levels of the two wavelengths to determine the percentage of oxygen saturation in the blood. This percentage represents the proportion of hemoglobin that is bound to oxygen in relation to the total amount of hemoglobin present in the blood.
3. Heart rate measurement:
- Pulsation detection: Each heartbeat causes a slight increase in blood volume in the capillaries, which changes the absorption of light by the tissue. The oximeter detects these periodic variations to calculate the heart rate.
4. Interpretation of the results:
- SpO2: A normal SpO2 level in a healthy person is usually between 95% and 100%. Lower levels may indicate hypoxemia (low oxygenation of the blood).
- Heart rate: This is normally between 60 and 100 beats per minute for a resting adult, but it can vary depending on age, physical condition and health status.
5. Usage:
The pulse oximeter is simple to use: simply place the sensor on a finger or earlobe. The device works continuously or on an ad hoc basis to monitor patients in a variety of settings, including :
- During surgeries: To ensure that oxygen levels remain stable.
- In patients with lung disease: Such as COPD or asthma.
- In intensive care: To monitor critically ill patients.
- At home: To monitor conditions like COVID-19 or sleep apnea.
6. Limitations:
- Insufficient perfusion: Low peripheral perfusion can cause readings to be inaccurate.
- Movements: Excessive movements can interfere with measurements.
- Pigmentation and nail polish: They can alter the accuracy of the results.
- Carboxyhemoglobin and methemoglobin: These abnormal forms of hemoglobin can skew SpO2 readings.
In summary, a pulse oximeter is a crucial tool for quickly and continuously assessing a patient's oxygenation status, although its use requires an understanding of its limitations and the factors that can affect its accuracy.
