Pulse Oximeters for Detecting Hypoxaemia and Monitoring Oxygen
What Are Pulse Oximeters and Why Are They Important?
Pulse oximetry is the most accurate noninvasive method for detecting and monitoring hypoxaemia, or abnormally low oxygen levels in blood. Combined with appropriate and sufficient oxygen supply, pulse oximetry is necessary for the safety and efficiency of oxygen therapy, and can be life-saving in managing respiratory distress, pneumonia, sepsis, trauma, and anesthesia for safe surgery. Pulse oximeters (Blood gas analysis is more complicated and invasive) are the widely accepted global standard for detecting and monitoring hypoxaemia.
Pulse Oximeters are devices used to measure pulse rate and the percentage of oxygenated hemoglobin in arterial blood (SpO2), using differential light absorption. Pulse oximeters have a sensor probe, which attaches to a patient’s finger, toe, or earlobe and then connects with the computerized unit with the user interface. The sensor (probe) transmits different wavelengths of light from light-emitting diodes (LEDs) through the skin and into the tissue. These wavelengths are differentially absorbed by the blood’s oxyhemoglobin (HbO2), which is red, and deoxyhemoglobin, which is blue. A photodetector in the sensor (opposite to the LED) converts the transmitted light into electrical signals, then the computerized unit’s microprocessor processes these signals and produces a SpO2 reading. Additionally, pulse oximeters produce an audible signal for each pulse beat, a pulse rate and, in some models, a graphical display of the blood flow past the probe, also known as the plethysmographic (pulse) wave.
Pulse oximeters can be used for either spot checking, in which a single Sp02 reading is taken, or for continuous monitoring in which the probe remains attached to the patient for a continuous reading. (1) Clinical signs (visible or palpable) often either miss or misdiagnose hypoxaemia, and patients with hypoxaemia may not even show clinical symptoms. Pulse oximetry, when used correctly and with a quality device, allows for more reliable monitoring and detection of low oxygen levels, and can be a valuable screening or “triage” tool in facilities where there are patients presenting, or admitted, with respiratory illness, emergency signs or for premature babies. Pulse oximetry also has the benefit of management of oxygen administration, and ensuring that this resource which can sometimes be expensive, is managed as efficiently and effectively as possible.
Types of Pulse Oximeters
There are different types of pulse oximeters, each with unique advantages and drawbacks, and specifics for intended clinical use. It is important for any user or buyer to understand how the device would be used, and on what patient population prior to selecting.
Considerations when Selecting a Pulse Oximeter
Pulse oximeters come in different types and features, and it is best to know which one will be most appropriate for clinical environment and patient use.
- Accuracy. Pulse oximetry should be an accurate test, depending on the quality of the device. Devices should indicate if they are accurate for adults, children, and neonates and for all skin pigmentations, and ideally would indicate a weight range for different patient categories.
Sp02 accuracy should be at least within + 3% for all patients even with perfusion and movement artifacts, and within + 2% for ideal conditions (less movement). Sp02 detection range should be 70-99% for fingertip oximeters, and 70%-100% for portable handheld and tabletop
Pulse rate accuracy should be within + 3 beats per minute (bpm) and pulse rate range should be 30-240 bpm
- Probes (sensors). Probes for pulse oximetry devices can present a significant recurring cost. Probes are either disposable (single-use), or reusable, both with unique considerations, and both with annual costs that are likely to exceed the cost of the device itself. Cables and connectors are susceptible to wear and tear and may also require frequent replacement. It is very important to have the correct probe sizes for the clinical application of pulse oximetry, if the device will be used with children or neonates. Neonate probe are often flexible bands for use on the foot, wrist, or palm; and pediatric sizes range by weight and may not be standard across manufacturers. Probes for pulse oximeters attach to finger, nose, earlobe, or forehead as those areas have high vascular density more appropriate for reading Sp02.
- Disposable probes, may be necessary for proper fitting with very small or premature babies, or in highly infectious disease scenarios, but need to be replaced after each patient. Reusable probes come in spring-clip (hard), more suitable for spot-checks so they don’t lead to skin irritation, and softer more flexible designs more appropriate for continuous monitoring. Y-shaped probes allow for multi-site monitoring, and include attachment wraps (adhesive or non-adhesive) for placement of sensors.
- Compatibility is a very important consideration when selecting devices and probes; generic probes may be available but manufacturers might only assure accuracy when used with their proprietary probes. For that reason, VIA Global Health recommends using sensors specified or provided by the manufacturer of the device.
- Ongoing Costs. Probes, sensor wraps or adhesives (for reusable probes), and batteries (single use or rechargeable) should all be factored in to the cost of ownership of the life of the device.
- Electrical, Power Performance. For all pulse oximeters, the power source and battery life are critical specifications to ensure uninterrupted and optimal performance. See the Table for relevant specifications on each device.
- User Interface (Display, Alerts, Alarms) Pulse oximeters should, at the minimum, show SpO2, pulse rate, signal quality, sensor error or power/battery status. Portable handhelds and tabletop devices should also show plethysmographic waveform and have audible and visual alarms for abnormal Sp02 and pulse rates (thresholds to be set by users) and for device performance and errors.
Special Considerations for Newborn Patients
Pulse oximeters are important devices to indicate if an infant is in respiratory distress, and in the need for immediate interventions or for monitoring during supplemental oxygen therapy or continuous positive airway pressure (CPAP) therapy to make sure they are not receiving too little or too much oxygen, both of which can present serious health risks.
Newborn infants, especially preterm infants, have lower than normal oxygen saturation during the first few hours after delivery; normal levels are > 88% and it may take a few hours to reach 90%.
Pulse oximetry is very important for preterm infants born < 32 weeks’ gestation SpO2 should be maintained between 88% and 95%, and not above 95%,
Devices that have probes specifically for neonate populations that can be attached safely and firmly are important in order to have accurate readings and not damage the delicate skin
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References and Acknowledgments
(1) ECRI Institute. Pulse oximeters product comparison. Plymouth Meeting (PA): Emergency Care Research Institute; 4/1/2018.
(2) “WHO-UNICEF technical specifications and guidance for oxygen therapy devices.” 2019
(3) WHO- Essential COVID Equipment and Supplies, 2020