Everything You Need to Know About SpO2 Probe and Oxygen Saturation
SpO2 is simply regarded as peripheral capillary oxygen saturation. The degree of oxygen saturation is a critical medical consideration for patient care. The SpO2 probe or pulse oximeter probe is a medical device used in measuring oxygen saturation - the amount of oxygen-carrying hemoglobin in the blood in relation to the total hemoglobin content. SpO2, which is a measure of the saturation in the peripheral capillary, is the more commonly used parameter when compared to SaO2, which is the corresponding measurement in the arterial blood. Both measurements (in percentages) are well related. However, the SpO2 probe has revolutionized the measurement of oxygen levels in the blood by being a convenient, non-invasive, and cheap alternative to SaO2 measured from blood samples.
A pulse oximeter employs two kinds of light (red and infra-red) and the Beer-Lambert law of absorption in its measurement. The basis could be transmissive or less common reflectance.
History of Pulse Oximeters
The first attempt at oximetry was performed on the hand in 1874 by Karl von Vierordt. Profound improvements were recorded in the 1930s when Millikan and Wood developed a two-wavelength ear oximeter. This oximeter was modified in 1935 into an oxygen saturation meter, which uses red and green filters. Several scientists such as Squire, Goldie, and Wood made significant contributions to the preliminary knowledge before the emergence of pulse oximetry.
Pulse oximetry started in 1972. The conventional design was described by Takuo Aoyagi In Japan. The technique uses light absorption of infra-red and red light. The previous method from Wood was invasive, and the ‘pulse oximeter readings’ were derived by relating the ratio of measurements from incident light and transmitted light to the SaO2. Aoyagi was able to use the two different wavelength lights to discriminate oxygenated hemoglobin, giving more accurate results. He was also able to make the procedure non-invasive by describing that the SpO2 probe can be placed on any body part that has arterial pulsations. He modeled the techniques into a single device at his university and published the first paper on pulse oximetry shortly after.
The commercialization of the SpO2 probe began in 1977 by Minolta, a renowned Japanese lens maker. Today, many brands with technical advances are available for sale.
Indications to Use the Pulse Oximeter
The pulse oximeter measures parameters such as percentage oxygen saturation, pulse rate, and pulse strength displayed on the device. However, oxygen measurement is often the parameter of interest, as others can be easily measured through other means. These pulse oximeter readings help to:
- Monitor oxygen levels for numerous reasons
- Assess the efficacy of lung medications
- Assess the efficiency of ventilators
- To determine if someone has breathing problems
- To diagnose hypoxia, hypoxemia, sleep apnea
- To monitor parameters in anesthesia
- Monitor general vital signs
- Detect respiratory depression in cases like opioid medications
- Measure total hemoglobin, methemoglobin, and carboxyhemoglobin level (especially new advanced probes)
In doing all of the above, the oxygen saturation is simply measured and interpreted. At normal sea levels, values ranging from 96% to 100% are generally regarded as normal. Lower oxygen saturation occurs at higher sea elevations. SpO2 probes are designed to detect accurately between 70% to 100% with about 2% permissible error. At 70% oxygen saturation, a patient must have already developed serious clinical signs of hypoxemia and hypoxia. Hence, these devices are able to capture the clinical range of oxygen saturation and desaturation.
Several health conditions can interfere with oxygen circulation. It should also be noted that percentage oxygen saturation is critical to patient care and thus constitute a potential risk of an emergency. Pulse oximetry is also the minimum intra-operative care specified by recognized bodies. Medical conditions that require the use of SpO2 probe include anemia, asthma, pneumonia, chronic obstructive pulmonary disease (COPD), sleep and respiratory disorders, procedural sedation and anesthesia, lung cancer, heart failure, heart attack, endotracheal intubation, cyanotic heart diseases, acute respiratory distress syndrome (ARDS) and so on.
The use of SpO2 probes is well tolerated in neonates, children, adults, and so on. However, a few concerns can arise from:
- Risk of infection from reusable sensors
- Heating from the light rays or general safety reservations about frequent electrical exposure
A more interesting kind of complication is a device complication. Many entities can interfere with pulse oximeter readings. An example is a pigment. Pigments, in the form of nail polish, can prevent the sensor from picking any measurement. Similarly, the presence of dyes prevents the measurement of SaO2 from blood samples too. Other interfering factors are the presence of deoxyhemoglobins such as carboxyhemoglobin, light pollution around the sensor, and lack of pulsating blood at the testing site (maybe due to cold, hypotension, and so on).
Other device complications and solutions are discussed below:
- When SpO2 probe values differ from SaO2 - In this situation, it is vital to confirm time and location proximity. Both procedures should be performed at the same location and around the same time, with minimal intra-location transport. Verify the blood drawing technique too.
- Alternating pulse oximeter readings with baseline wander – Motion movements around the SpO2 probe should be reduced. Another kind of probe with motion artifact reduction can also be used.
- Alternating pulse oximeter readings with low waveform – This indicates low perfusion. A more perfused site or a SpO2 probe with better sensitivity can be used.
- Low pulse oximeter readings with good waveform – This indicates the presence of abnormal oxyhemoglobin or venous pulsations. Another instrument should be used to measure oxygen saturation.
- No readings – SpO2 probe or cable is faulty. Replace the device.
There are not any alternatives to the use of a pulse oximeter. The invasive CO oximetry is not routinely used except when needed. Other non-invasive options are the reflectance pulse oximeter and a temporal lobe method, which are not commonly used.
It should be noted some general specifications differentiate a pulse oximeter. This could be the kind of probe which includes fingertip, auricular, toe, foot, forehead, paw (for veterinary), rectum, and so on. It could also be the use of terms such as a reusable, washable, disposable, adhesive probe, or MRI compatible.
However, the technical specifications of a basic SpO2 probe are highlighted below.
- SpO2 measurement- A range of 35% - 100% can be measured but the trusted accuracy range exits at 70% - 100% ± 2%. The resolution is 1>#/p###
- Pulse rate- measurement range is 30-250 bpm ± 2 bpm. Resolution is 1 bpm
- Perfusion index performance – 0.4-20 %. Resolution is 0.1>#/p###
- Default alarm settings – When SpO2 is 90% or pulse rate is ≥ 120 bpm or drops to ≤ 50 bpm
- Use temperature and humidity are 5 – 40 ℃ and 15 - 95 %, respectively.
- Storage temperature and humidity are 20-40 ℃ and 10 – 95% respectively
- Power voltage of 2.5V – 3V and < 30mA work current.
How to Use the SpO2 Probe Device
The SpO2 probe is a clip-like device used on the fingers, ear, etc. The probes are fragile and should be used carefully and correctly. The probes shine a light on the site of use (mostly fingers) and get the measurement done. There are pediatric probes for those little fingers.
A finger should be placed inside the probe correctly, not too tight or loose. It is not advisable to use the thumb as it is often too big. This procedure is painless, although some pressure can be felt. The beam of light from the Sp02 probe passes through the blood, and the oxygen saturation is measured. This is recorded from the device indicator. This measurement can be an intermittent procedure, or the probe can be left on the finger for continuous supervision of oxygen levels and pulse rate till a patient is deemed fine without the need for monitoring.
Ear probes are the second most common ones. They are used on the ear lobe. They could also be used on the cheek (through the inner and outer part of the mouth) in the pediatrics population. Nearly all health practitioners are permitted and capable of using this device, and they are even given to some patients for personal home monitoring.
This device has a life expectancy of about 7-10 years. The probe and cable are routinely changed every 6-12 months except for disposable single-use probes.
There are numerous brands available in the market today. The following are prominent players in this space:
- Envitec (New Jersey, USA)
- Bio Medical Technologies (South Korea)
- Mecun (Shenzhen Mecun Medical Supply Ltd based in South China)
- Berry (Shanghai Berry Electronic Technology Ltd in China)
- Orantech (China)
- Smiths Medical
- Heal Force
Pulse oximeter readings continue to be a paramount monitoring parameter in a surgical, emergency, and general medicine. The SpO2 probes are arguably one of the medical devices with the highest market share. They also show relevance in the current global pandemic by helping with early diagnosis of COVID-19 infection. They can detect little changes in oxygen saturation and hypoxia.
The development of non-invasive oxygen saturation probes remains an invaluable achievement in medicine. The device enables oxygen saturation to be measured at the point of care. This is remarkable as a medical response to such health problems is needed to be swift and immediate. The invasive CO oximetry cannot afford this advantage.