A Guide for choosing Accurate Pulse Oximeter Spo2 sensor for blood Oxygen saturation Monitoring

This is Arunkumar , representing Biometric Cables, a company engaged in manufacturing of SpO₂ Sensors for the past 12 years. I am No scientist but have gained expertise from my manufacturing experience to genuinely differentiate between good Pulse Oximeter sensors and the bad ones.

Contents:

The blog throws light on 

1.What is Pulse Oximeter Sensor?

2. Working Principle of Pulse Oximeter Sensors

3. Uses of SpO2 Sensors

4. SpO2 Sensors for Covid Detection

5. How Accurate are Pulse Oximeter Sensors?

6. The Right Way to use the sensor

7. Functional Requirements of SpO2 Sensors

8. How does it affect clinical findings?

9. Myths in Choosing Perfect SpO2 Sensor

10. How to Choose the Right SpO2 Sensor

1. What is Pulse Oximeter Sensor?

A Pulse Oximeter Sensor (SpO2 Sensor) is used to measure the oxygen saturation in red blood cells.They are usually the small clip-like devices which can be attached to the fingers, toes, earlobes, etc.

"FDA and ISO 80601-2-61 describes Pulse Oximeters as a system (They comprise of the sensor and oximeter itself). Every new or modified system should be clinically validated for individual and combined properties unless adequate scientific justification can be provided in lieu of this testing".

 Life Span of a Pulse Oximeter is around 5-7 years, whereas the SpO₂ sensors will last not more than 1 to 1.5 years upon regular usage. In today’s scenario, more than 50% of the Patient Monitors and Pulse Oximeters users buy compatible sensors for the simple reason that they are cost-effective and have interchangeability. But do the users really know what is at stake? Take a look at it.

2. Working Principle of Pulse Oximeter Sensors

Before the invention of Pulse Oximeter, the practice was to draw blood from patients and analyse the samples at regular intervals in a day using large laboratory equipment such as blood gas analyser.

Blood gas analysers determine the partial pressure of Oxygen in the blood (pO2) by means of chemical sensors, and they directly measure the ratio of the oxygenated haemoglobin to the total haemoglobin in a sample of blood SaO2 (Arterial Oxygen Content).

 The Pulse Oximeter was invented in 1980s. They made use of the pulsatile arterial blood flow, generated by the heart, by emitting light rays of two different wavelengths, red at 660 nm and infrared at 940 nm. The small size of LEDs and photodiodes make it possible to mount the optical components directly on the sensor for use in patient non-invasively.

 The definition of SpO₂ can be stated by a simple equation

SpO2  = [HbO2] / [Hb][HbO2]

Where, HbO2 is concentration of oxygenated haemoglobin

Hb is concentration of haemoglobin

SpO2 is saturation of oxygen 

Infrared and Red Light LEDs in the SpO2 sensors measure the oxygenated and deoxygenated blood levels.

When the sensor is clipped onto the body application site, the light emitting diodes transmit the red and infrared light respectively through the body site (finger, toes, ear lobes) to the light sensitive photodetector.

While the tissue, bones and blood absorb some part of the light , the rest of light reaches the photo detector.

The amount of oxygen in blood is determined using the amount of light beams received by the detector.

The image illustrates the SpO₂ sensor working principle.

3. Uses of SpO2 Sensors

Apart from checking the blood oxygen/ SpO2 levels, these SpO2 sensors can be used in the following 

• Covid 19
• Heart Attacks
• Asthma
• Pneumonia
• Sleep Apnea
• Lung Cancer
• Respiratory syndrome

4. SpO2 Sensors for Covid Detection

Pulse Oximeter sensors (SpO₂ Sensors) have been always hitting the headlines ever since the emergence of Covid 19 pandemic.

So, Why is there an upsurge in demand for these Pulse Oximeter SpO₂ Sensors? Well, SpO₂ sensors play a pivotal role in measuring the heart rate and blood oxygenation levels in your body.

Be it a home, hospital, medical center or critical care setup, currently, Pulse oximeters are the massive game changers, being the non-invasive and fast method in the primary assessment of oxygenation levels in human blood.

When a person contracts Covid Pneumonia or silent pneumonia, there arises a mild to serious complication of Novel Covid virus infections around the lungs air sac.

This condition inturn causes low blood oxygen levels, even with or without difficulty in breathing.This condition is known as Silent Hypoxia which is quite commonly occurring symptom of Covid.

 Here’s where these sensors come into the picture to throw the spotlight on your oxygen level metrics even before your realisation on covid contraction. 

A Good SpO₂ sensor signals the abnormalities and warning risks of the covid patients. On the other hand, the detection of abnormal blood oxygen levels in Adults, Pediatrics, neonates and premature-neonates who have been affected by covid can be done using good pulse oximetry systems.

 The next question pops in minds “How accurate are SpO₂ sensors"?

It is often noted that medical grade SpO₂ sensors are generally more efficient than the home portable pulse oximeters. In Spite of pulse oximeter sensors undergoing rigorous medical testing and certifications, the clinical results may vary occasionally as per WHO report. 

The article provides you the useful insights of choosing the accurate sensors for blood oxygen saturation monitoring. The fairly accurate SpO₂ readings hold true specifically when best quality sensors are employed in any settings. 

The Pulse Oximetry Training Manual by World Health Organisation states that once the oximeter is clipped on the fingers, it must be given a 10 seconds time interval to obtain the best accurate readings, thus handling with utmost patience.

Can SpO₂ sensors give false reading ?

Well, yes. The accuracy is greatly influenced by many factors.

So, What are the factors that cause false reading on Pulse Oximeter sensor?

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 1.Body Peripheral application site

Normally the SpO₂ levels are measured at the peripheral sites of the human body mainly toes, fingers, ear and forehead. While one is facing breathing problems or suspecting Covid, to level up the chance of accurate readings, the sensors can be used at the peripheral site that are sufficient for blood perfusion.

The fingers are the viable options to be used in the medical grade sensors in order to get precise readings due to the less shaky movements and better blood perfusion. 

According to the study, the middle finger of the dominant hand is useful in displaying significant SpO₂ results. However, poor blood perfusion sites, extreme cold conditions of the patient and Vasoconstriction (limits the blood flow to the peripheral sites) cause failure in recording the levels. 

2. Proper Application of the SpO₂ sensor

Usage of correct SpO₂ sensors are extremely essential to achieve precision in the readings. Make sure the sensor properly fits the application site. Loose fitting sensors can result in preventing the passage of light from emitter to the site (optical bypass).

Whereas, Tight fitting sensors can cause venous pulsation (CPR). Both scenarios can provide discrepancy in the blood oxygen levels. To avoid pseudo reading, the sensor probes should be aligned at the heart level.

3. Role of Motion artefacts

Conversely, psychological factors such as shock, fear and anxiety may produce false  SpO₂ sensor alarm readings.Moreover, in low blood pressure cases due to low cardiac output, the sensors fail to detect the weak pulse signals.

Poor signals can be detected when mechanical interference with blood circulation (restricted clothes, blood pressure cuffs), thus causing inaccuracy results. 

Normally, pulse oximeters attached to the person who is sitting upright has the chance of getting high oxygenated saturation levels compared to the other one in a reclining/ laying down position. Utilising the benefits of a motion tolerant sensor can gain significant clinical performance.

4. Nail Polish/ Art

5. Effect of Lighting 

6. Skin Pigmentation 

Though skin pigment Melanin can absorb a significant amount of light radiation, the report by Food Drug Administration depicts that SpO₂ accuracy levels differ between the dark skin and white skin pigmentations.

The accuracy difference readings are usually slightly smaller when the oxygenated saturation levels are above 80% and greater while the oxygenated levels are below 80%.

Further scientific evidence should be backed up to address the accuracy and reliability factors. Erroneous oxygenated levels might also arise due to the skin thickness and poor blood circulation.

7. Smoking, Medication

Carbon monoxide levels increases right after the person smokes a cigarette or was on the smoky fire environment.The carbon monoxide from the smoke combines with the haemoglobin and oxygen displacement happens to form carboxyhemoglobin.

Carbon monoxide poisoning can lead to flawed readings of oxygenated levels.Besides, certain medications such as nerve blockers can impede the accuracy results of the clinical performance. 

6. Correct Placement Of pulse oximeter sensor:

Here's a video on the correct placement of pulse oximeter sensor and the consequence of improper placement of SpO2 sensor or usage of wrong SpO2 sensors types.

7. Functional Requirements of SpO2 Sensors

According to FDA 510 k and ISO 80601-2-61, here are the prerequisites for basic safety and essential performance of Pulse Oximeter equipment - “SpO₂ Sensors should be validated for affected combined properties and at least include high Accuracy, EMC, electrical safety, and protection against excessive temperature and separately testable properties for sensors, like bio-compatibility”

8. How does it affect clinical findings?

 The Pulse Oximeter reading is one of the minimum requirements for monitoring during anaesthesia. Pulse Oximeters are often used to guide therapeutic interventions.

The British Thoracic Society guidelines on the use of emergency oxygen in adults state that treatment should be targeted to oxygen saturation. Hence, it is important for pulse oximeter probes to be accurate in order adapt right procedures.

Any Pulse oximeter probe with inappropriate design of LEDs, photo sensors, cables, connectors, housings will deter EMC, electrical safety of the pulse oximeter itself.  An amateurish sensor may inflict allergies to patients if not tested for its bio-compatibility.

9. Myths in choosing a perfect SpO₂ Sensor

Most Buyers tend to choose the right SpO₂ Sensors for themselves based on the following factors.

1. Clinical performance using functional generators
2. Price of SpO₂ sensors
3. Reliability

1. Clinical performance of SpO₂ sensors

 Myth: A SpO₂ sensor is deemed good if it passes the functional generator test.

 Clinical performance of a SpO₂ sensor is tested with the help of functional testers (Simulators or Calibrators) which is not recommended by ISO 80601-2-61 for a simple reason for its limited ability to determine whether the pulse oximeter probe is performing as the manufacturer designed it to perform and can never determine whether the design was correct.

Detailed Elaboration on this point can be found in ISO 80601-2-61 Clause FF

2. Price of SpO₂ Sensors

 Myth: OEM sensors and compatible sensors perform the same intended function; then why pay the high price.

In present scenario, OEM sensors costs almost 7-8 times the price of compatible SpO₂ sensors, justifying their pricing by adhering to the regulatory requirements. However, almost 90-95% of the compatible SpO₂ sensor manufacturers present in today’s market lack the basic requirements of ISO 80601-2-61.

Absence of regulations in certain countries helps manufacturers like these to create confusion among the customers and prospective buyers about the price.

They use this gap of “absence of regulation” combined with half-truths of “testing the SpO₂ sensors for accuracy with functional generators”, to sell a sub-standard, clinically unworthy products.

 Are all the compatible SpO₂ manufacturers unfit to use?

 NO, there are good manufacturers who do a really good product by adhering to regulatory norms and justified pricing.

3. Reliability of SpO₂ Sensors

 The term reliability is defined as “The ability of a SpO₂ sensor to consistently perform its intended clinical function on demand and without degradation or failure over the stated period”

It is highly dependent of the following parameters

1. Manufacturing processes
2. Raw materials used

 Hence, it is ideal to directly buy the sensor from the manufacturer who practises a valid ISO 13485 than to buy it from a trader who may not be able submit necessary testimonials.

 A short video that depicts the integral parts of a SpO₂ sensor that helps to identify the reliable parts versus the unreliable ones.

10. How to choose the right one?

As mentioned earlier, the accuracy of a pulse oximetry readings are combination of Pulse oximeter and its sensors, if the customer wants to change to compatible sensors instead of OEM sensors, the following should be verified.

1. Accuracy statement and method claimed by the manufacturer of the sensor
2. Bio-compatibility requirements
3. Certifications and standards are adhered to

 Summary: The market is flooded with many types of SpO₂ sensors from different manufacturers. Therefore, it is the responsibility of a physicians, biomedical engineers, and competent purchase officers to identify the right ones in order to save the life of their patients, who they have been entrusted with.

Even though the calibration of pulse oximeters sensors are the integral part of standardised procedures, it is really the need of the hour to realise that working of SpO₂ sensors in clinical setup must adhere to the manufacturer’s accuracy specifications. 

Wise decisions can be made by buying oxygen saturation sensors from a manufacturer, no matter be it a compatible manufacturer or OEM, who complies with EN ISO 80601-2-61. 

It is highly crucial to purchase from a reliable manufacturer with a track record of transparency in the materials used in the manufacturing process and also with a solid proof of positive ratings.

In addition, during the current pandemic, getting yourself associated with good manufacturers who do not overcharge the sensors price is certainly an essential factor to be considered.

Let’s not fall into the prey of non-approved, unreliable and inaccurate clinical SpO₂ sensors /Pulse Oximeters.