All About Pulse Oximetry
In recent years, the use of optical sensor technology in medical procedures has increased tremendously. This can be attributed to their intrinsic safety compared to some of the traditional treatment methodologies. Among their main benefits is how they have eliminated the dangerous use of invasive diagnostic procedures. Another major benefit is the fact that there is no electrical contact between the patient and the equipment and thus there is less magnetic interference. These benefits have given rise to the numerous optical techniques to monitor physiological parameters such as Pulse Oximetry among many others that have changed the way medical arena used to operate. We’re going to discuss in details Pulse Oximetry as one of the primary optical procedures in respiratory monitoring.
What is Pulse Oximetry?
Pulse Oximetry is a respiratory monitoring test used to measure the level of oxygen in the blood. It’s a simple, relatively cheap, and non-invasive method for monitoring the amount of oxygen your blood is carrying, or how well oxygen is being sent to parts of your heart such as the arms and legs furthest. This is called oxygen saturation. According to medical experts, oxygen saturation for a healthy person should always be above 95%, although patients suffering from a respiratory disease or congenital heart disease may experience a lower percentage depending on the severity of the disease. But if the level goes below 90%, it’s considered a health risk.
Pulse Oximetry is applied in two different ways: transmissive and reflectance-types. The transmissive application is the most common, and it’s a very simple method to carry out. It works with a sensor device. With this method, it requires that the measuring device (Pulse Oximeter) be placed on a thin body part such as an earlobe, fingertip or in the case of infants, it’s recommended to be placed across a foot. On the other hand, the reflectance is less popular, and it’s only used as an alternative to the transmissive pulse oximetry. This method doesn’t require a thin section of the body making it great for universal applications such as chest, feet, and forehead, yet there are a number limitations that make it less popular.
Pulse Oximetry is measured by a device known as the pulse oximeter. The reading is obtained in either of the two numerical values, i.e., SpO2 or SaO2, but the two are not the same and are correlated well enough that this method is accurate in measuring the oxygen saturation in clinical use.
Principle of Oximetry
As early as 1860, it was discovered that the coloured substance in the blood, haemoglobin, was also responsible for carrying oxygen. It was also observed that the absorption of visible light by the haemoglobin (protein bound to the red blood cells) solution varied with oxygen. The reason is that the two molecules, oxidised and reduced haemoglobin, have different optical spectrum in the wavelength range. That’s why the oximeters work by the principle spectrophotometry to determine oxygen saturation. Actually, the measurement is usually the ratio of oxyhaemoglobin (HbO2) to the total haemoglobin concentration present in the blood. This provides the percentage that shows the rate at which oxygen is supplied to the body.
The technique is based on two physical principles: (a) The fact that both oxyhaemoglobin (O2Hb) and reduced haemoglobin (Hb) have varied absorption spectra and (b) is the pulsatile signal generated by arterial blood independent of non-pulsatile capillary, venous and arterial blood. The current oximeters make use of two light-emitting diodes that emit light at the 660nm (red) and the 940 nm (infrared) wavelengths. The two wavelengths are used simply because both O2Hb and Hb have varied absorption spectra. In the red region of the spectra, the oxyhaemoglobin absorbs less light compared to the reduced haemoglobin and vice versa.
This recent development of the technique has been accepted as one of the best non-invasive technique methods for the measure of oxygen saturation or arterial SaO2. But it’s important to note that the intensity of the light across the body part varies. The pulsating signal strength depends on the heartbeat, and that’s why the actual graph if the test is sinusoidal. The technique works by splitting the light attenuation of the body segment into three components: arterial blood, tissue and venous blood.
Brief History of Oximetry
The technology has been developed over a long period of time. In fact, the first devices to measure oxygen saturation in human blood by trans-illumination of loured light were built in the 1930’s. In 1935, a German physician, Karl Matthes, designed the first 2-wavelength ear oxygen saturation meter with a red and green filter. Later on, he switched to red and infrared filters. He is actually the father of this technology. But the original oximeter was designed and made by Glenn Allan Millikan in the 1940s. Glenn was able to come up with a device that could get the test done more accurately compared to the previous releases.
But Glen had missed something that would improve the performance of the oximeter—that’s the pressure to squeeze out blood. But that feature was added by Wood in 1949 by designing pressure capsules to squeeze blood of the ear so as to obtain absolute oxygen saturation value. The concept is still relevant in the modern pulse oximetry. It was a challenge to implement the concept due to unstable photocells and light sources, and thus it was not used widely. But in 1964, there was a kind of breakthrough in this technology. That was the year that Shaw assembled, what is referred to as, the first complete ear oximeter using eight wavelengths of light. The Hewlett-Packard commercialized the product, but due to cost and size, its use was limited to sleep laboratories and pulmonary functions.
Pulse oximetry was fully developed in 1972 by a duo of bio engineers, Michio Kishi and Takuo Aoyagi, at Nihon Kohden. They used the ratio of red to infrared light absorption of the pulsating components at the measuring site. The device by the duo was first tested in 1975 and found to work incredibly well, and the results were profoundly accurately. It was then commercialised by Biox and Nellcor in 1981 and 1983 respectively. Although the Biox first focused on the respiratory care, later on, they focused on operation rooms in 1982. Nellcor also started supplying oximeters to the operating rooms in 1983.
The technique (pulse oximetry) was boosted in 1987 when it became one of the standards of care for the administration of general anaesthetic. From operating, the use of this technique spread throughout various hospital departments including recovery rooms and intensive care units. The technique was of particular value in the hospitals’ neonatal unit where an adequate level of oxygen is critical for the survival of the patient. It’s important to note that too much oxygen or fluctuation of the same can be dangerous to the health of the patient. In fact, it is known to cause vision impairment or blindness. Furthermore, obtaining an arterial blood gas from such patients is painful and a major cause of severe medical conditions such as anaemia.
That’s why the device became crucial in the neonatal units of most hospitals. But one problem that experts found difficult to figure out is false alarms resulting from motion. The device could not differentiate between moving venous blood and pulsating arterial blood. The problem was solved in 1995 by Masimo, who introduced SET(Signal Extraction Technology) which was able to provide accurate data during patient moving by separating arterial signal from the rest such as a venous signal. That’s what made the modern Oximeter complete, and until today, very few significant improvements have been done on the Masimo gadget.
That was a brief history of pulse oximetry. It’s a technique that has been developed over decades to what we have today. It’s one of the most crucial practices used in the majority of medical processes in modern treatment particular where the level of oxygen is crucial. In most surgeries and other treatment programs, Pulse oximetry is a must.
Knowing the Pulse Oximeter
A pulse oximeter is a small medical device that directly monitors the level of oxygen in your blood. The device has been developed over the years to what we have as the modern oximeter today. The first device was designed in 1930, but the absolute oximeter was developed in early 1970’s. Over the years, the device has been improved to enhance its performance, bulkiness and affordability. The modern oximeter is quite simple yet very effective in delivering incredibly accurate results. In fact, it comes with numerous advantages that make it the number one choice over other testing methodologies for the oxygen saturation such as blood gas.
The main advantages of pulse oximeter over the alternative methodologies are the speed and cost. For instance, while some of the alternative methodologies may require a laboratory test to determine oxygen saturation, an oximeter will take a matter of seconds to provide similar readings. That’s how quick the technique is. With an error of 1% to 2% depending on various conditions, this is definitely sufficient for most situations. But the accuracy level depends on the quality of the device and the age of the patient. In fact, that’s an error that does not make any significant changes making the oximeter one of the most accurate in regards to the pace of result delivery. There are numerous types of pulse oximeters, but the fingertip pulse oximeter is the most common.
How Does the Pulse Oximeter Work?
The device comes in a small size that is either with a built-in toe/finger clip, or a small hand-held device with a wire probe that can attach to your earlobe, finger or toe. The modern oximeter is less expensive and more practical for home use especially for patients with chronic obstructive pulmonary disease (COPD), COAD (chronic obstructive airway disease) and asthma among other respiratory conditions. The oximeter probe has two parts: two light emitting diodes (LEDs) and a light detector or photodetector. It uses two LEDs generating red and infrared lights that pass through the translucent part of the body. The tissue, bone, venous vessels and the pigmentation absorbs a constant amount of light while the O2Hb and the Hb show different absorption patterns. This makes it easy for the device read or measure oxygen saturation.
The reading on the device provides the percentage of your blood that’s carrying oxygen and the heart rate (pulse) reading at the same time. But it’s advisable to compare the device’s pulse with your own count to ensure that you are getting the right signal. If they’re the same, then you are getting a good signal. Additionally, the device’s use is another factor that can affect the accuracy of the reading. Ensure that you’ve followed the instructions correctly. Remember that it will only provide an accurate reading if the probe attached to your toe, earlobe or finger. The following are simple steps for practical use of the pulse oximeter:
- Turn the pulse oximeter on. It will automatically undergo internal calibration and checks.
- Select the appropriate probe depending on where it will go—finger, ear or toe. If it is to be used on a finger, make sure that the part is clean and also remove any nail varnish.
- Connect the probe to your oximeter.
- Have the probe positioned properly and carefully. It shouldn’t be too loose or too tight.
- It’s always advisable to avoid the arm that’s used for blood pressure monitoring for better results.
- Give the oximeter some time. Allowing several seconds for the device to detect the pulse is advisable for better results.
- Look if the display is indicating that the device has detected a pulse. If the machine has not detected a pulse, any readings are innacurate.
- If or once the oximeter detects a pulse, the machine will display oxygen saturation and pulse rate.
- Always make sure that alarms are on.
Should you get a Pulse Oximeter?
Well, not everyone needs a pulse oximeter. The device is subscribed to people with specific medical conditions or in situations that could cause them to experience periods of low oxygen. A good example is when you’re exercising or travelling to high altitudes. While in such conditions, having the device will allow you to monitor the oxygen level in your blood so as to know when to adjust your supplement. All you need to know is the percentage of oxygen that you need to maintain. If you must take the device home, read the prescription properly.
How Accurate Is the Pulse Oximeter?
The reading from a pulse oximeter is reasonably accurate. In fact, its accuracy cannot be compared to most of the alternative methodologies used. Most of the devices provide a reading 2% over or 2% under what your actual oxygen saturation would be if arterial blood gas were used. For example, if the pulse meter reads 92%, the actual oxygen saturation in the blood would be anywhere from 90 to 94%. But why would a pulse oximeter read errors? If the patient is wearing nail polish, cold hands, artificial nails or has poor circulation. The device may also be less accurate with darker skin and very low oxygen saturation levels.
When Should You Use a Pulse Oximeter?
This is a question for your healthcare provider if you are prescribed to a pulse oximeter. Ask them when they want you to monitor the oxygen level in your blood to be on the safe side. Ask, at what reading it will be necessary to change the flow rate of your supplemental oxygen. You should also learn what reading should alert medical attention. But what times would oximeter reading be helpful? Here are some of the main times when you need to check the device’s reading:
- The first time you are prescribed to oxygen, you need to have the oximeter reading performed. This helps your healthcare provider to understand when your oxygen saturation levels change. Is it while doing activities at home? Is it while in motion? Is it while exercising? Activities similar to these. This will give them an idea of when you should be checking your oxygen levels.
- During or just after exercising. While the body is active, it requires more oxygen. Remember that oxygen is like the fuel that keeps the car running. Consequently, it’s important to have the oximeter reading done so as to ensure that oxygen levels don’t go too low to a point compromising your health.
- While travelling or flying to a high altitude location, the level of natural oxygen will fall and this might cause low oxygen saturation. If you have Oxygen problems, it’s advisable to take the oximeter reading to know when you need to increase your supplemental oxygen needs.
How can you get the best reading from a pulse oximeter?
Your oximeter is designed to measure the percentage of blood carrying oxygen or your oxygen saturation. Lack of adequate oxygen or too much oxygen can cause severe health complications and thus the need to have the levels controlled, especially if you have oxygen problems. There are a few things that you need to do so as to ensure that you’ve gotten the correct and the best reading from the device. First, you have to make sure that enough blood is flowing to the hand and the finger wearing the device. Secondly, your hand must be relaxed, warm and below the level of your heart.
If you’re a smoker, then you might not get the correct reading from your pulse oximeter—the readings are likely to be higher than the actual oxygen saturation. This is because cigarette smoking increases the amount of carbon monoxide in the blood and the device will be unable to distinguish oxygen from carbon monoxide. If you smoke, seek advice from a healthcare provider on how to correctly measure your oxygen saturation. Sometimes, the problems error could be coming from the device itself. Make sure that you have the probe set right and if the problems persist, bring your device to the healthcare provider or the equipment company to have it checked for accuracy against theirs.
Other sources of errors are hypotension, a cardiac failure that usually results in no reading, cold weather, hypovolaemic shock, wrong probe size, or carbon monoxide poisoning, pulse detection amongst many others. These are just a few of the tips that can help you get the reading of your oximeter correctly.
Limitations of Oximeters
Although oximeters are popular in testing and measuring the oxygen saturation, there are a number of limitations that may lead to inaccurate reading. Here are some of the main limitations:
- The device measures SaO2 that’s physiologically related to arterial oxygen tension (PaO2) according to the O2Hb dissociation. Because the dissociation curve shape has a sigmoid shape, oximetry is usually insensitive to detecting conditions such as hypokemia.
- The device can only utilise two wavelengths of light. This means that it can only distinguish two substances O2Hb and Hb. If the patient is a smoker, the oximeter is unable to detect carboxyhaemoglobin and methemoglobin, and this is what results in errors.
- Intravenous dyes such as indigo carmine, methylene blue and indocyanine green can also cause false results. Nail polish particularly if it’s blue, black or green can also cause false results.
Understanding the Oxygen Physiology Transport
To understand pulse oximetry better, it’s imperative to understand the physiology of Oxygen in the body and how it’s transported throughout the body. First, it’s important to note that humans depend on oxygen for life. The gas crucial to all organs since it supports metabolism but the heart and the brain are very sensitive to changes in oxygen, especially if it’s a drop or what is referred to as the hypoxia. A serious drop can cause devastating effects on the health of the patient in just a matter of minutes. That’s why it’s necessary for patients with oxygen problems to have their oxygen saturation levels checked, and pulse oximetry happens to be the best choice of measurement due to its instant results.
During operations such as surgery, patients are put under anaesthesia, and their airways may become obstructed. Their breathing may become depressed, and the oxygen circulation may be affected by the loss of blood. Their heart rhythm may also be affected by the operation. All these factors will result in low and unstable oxygen supply, and this can lead to injuries or death. That’s why the use of pulse oximetry is an essential requirement in the operation room today.
Transport of oxygen to the Tissue
Oxygen is transported throughout the body attached to the haemoglobin, iron-containing protein, contained in red blood cells. The gas is breathed into the lungs where it combines with haemoglobin as they pass through the pulmonary capillaries. The heart pumps the blood continuously so as to deliver the oxygen to the tissue. So, five things must happen for adequate blood to be delivered to the tissues. First, oxygen must be breathed into the lungs naturally from the air or the anaesthesia circuit into the lungs. Secondly, the alveolar gas exchange must happen—this is when oxygen passes into the alveoli to the blood. Thirdly, there must be enough haemoglobin in the blood to allow for maximum transport of the oxygen to the tissues.
Fourth, the heart must be strong enough to pump sufficient blood to the tissues to meet the patient’s oxygen needs. Otherwise, without enough blood, it will be impossible to meet the quantity of oxygen required by the body tissue. Fifth and the last is the volume of the blood that’s in the circulation. It must be adequate to ensure that the oxygenated blood is distributed to all the tissues. If any of these five factors is not met, then it will be difficult for the patient to remain healthy.
The Amount of Oxygen That Blood Carries
When it comes to how much oxygen the blood carries, few facts affect this. There are a few things that determine the amount of blood in the blood. Things like partial pressure, amount of haemoglobin and the health of the patients, amongst other factors, are crucial considerations. In a healthy patient;
- The amount of haemoglobin in the blood is crucial. For each gram of haemoglobin, it combines with 1.34ml of oxygen. So, for a blood with a normal concentration of haemoglobin of 15g/dl, 100 ml of blood carries around 20 ml of oxygen plus haemoglobin. In addition, a small but significant quantity of oxygen is dissolved in the blood.
- The heart of an average sized adult normally pumps approximately 5000ml of blood per minute to the tissue. This simply means that it delivers about 1000ml of oxygen to the tissue per minute.
- The main reason why tissue cells required oxygen is for the metabolism purposes. The entire body will require around 250ml of oxygen per minute. That simply means that the body has enough oxygen for three minutes if there is no oxygen exchange in the lungs. That’s why suffocation kills fast within a few minutes. Note that only 75% of the oxygen is available to the tissue.
- Prior to the induction of anaesthesia, the patient should be administered with 100% oxygen to increase the oxygen stores in the lung. This helps in case the patient stops breathing so that the oxygen already in the body can take them longer. The patient should be given pure oxygen for several minutes prior to induction of anaesthesia to increase oxygen reservoirs thus adding potential life-saving minutes. There are many other situations where this may also be important. A good example is the expectant mother where the uterus may enlarge thus reducing lung volume. The metabolism demands by developing foetus may also require the mother to receive more oxygen.
- Another case that may require exceptional levels of oxygen is the anaemic patients. These patients have lower levels of haemoglobin and thus are unable to deliver adequate oxygen to the tissues naturally. This fails to meet their metabolic demands and can be worse for if not controlled. For instance, patients who suffer major blood loss during surgery, they become acutely anaemic and should be supplied with 100% oxygen to breathe. This helps to boost the amount of oxygen dissolved in their blood thus improving the tissue oxygen delivery. Such patients need a pulse oximeter to help monitor their oxygen level.
With this information, it’s clear that pulse oximetry is crucial for different situations. It shows that you don’t need to be sick to be prescribed an oximeter. It’s also clear why regular monitoring of the oxygen saturation is crucial, particularly for people living with conditions that affect the volumes of blood and red blood cells or the haemoglobin. If you’re a climber that goes to high altitude, it’s advisable to bring this device to ensure your oxygen level is maintained.
Application of Pulse Oximetry
Today, pulse oximetry is widely used in different medical operations. In fact, it is one of the essential medical devices in recovery and operation rooms among many others. Here are key areas where the technique is widely used:
- Individual pulse oximetry readings- this technique is extremely useful in clinical situations where low oxygen (hypoxaemia) may be a factor. A good example is dealing with confused elderly persons.
- Neonatal care- for the new-borns, the limits for oxygen saturation is higher compared to that of adults. They require about 95-97% and that’s why they require to be monitored closely and oximeter provides instant results. It’s more reliable than the clinical methods alone.
- Intrapartum foetal monitoring- this has been a developing interest in recent years. The use of foetal pulse oximetry in combination with routine CTC (cardiotocography). Although the use does not reduce the operative rate.
- Continuous recording- the technique can be used during anaesthesia, sedation or just assess the low oxygen during sleep study to diagnose obstructive sleep apnoea. This is to ensure that the patient is not injured in the process.
- In many clinical situations, the technique can replace blood gas analysis unless where PaCO2 or acid-base state is required. It’s a cheaper and easy to perform alternative. It’s less painful and can be more accurate where the patient is conscious.
- Pulse oximetry is also used as a way of controlling the use of oxygen. The technique provides you with your oxygen saturation to allow control the right amount of supplemental oxygen that you need to minimises wastage.
Application of Pulse Oximeter in Diagnosing Medical Conditions
Pulse oximetry is now used routinely to diagnose various medical conditions. Here are the medical conditions that the technique is used to diagnose:
- Diagnosing exacerbation of chronic obstructive pulmonary disease (COPD). COPD is a group of lung diseases that interfere with normal breathing. It’s described as one of the leading causes of death in Australia. It can be used to assess the severity and determining management in infants with chronic bronchitis and bronchiolitis.
- Anaemia- this is simply a condition that is caused by fewer red blood cells than what’s required. This results in the low concentration of haemoglobin in the blood.
- Heart attack- this medical condition occurs when one or more muscles of the heart experiences prolonged lack of oxygen. With this device, you can instantly get oxygen saturation to determine the cause of the attack.
- The technique can also be used to detect the presence of lung cancer. This is a condition that affects normal breathing and mostly the level of oxygen absorbed by the blood. If the device detects low oxygen saturation over an extended period of time, then it would probably mean that the patient should get tested for lung cancer.
- Other conditions that the pulse oximetry can be used to detect and manage include asthma, pneumonia, diabetes and heart failure amongst other conditions. Your health care provider will guide you on how to use the device for any of the mentioned conditions.
Advantages and Limitations of Pulse Oximetry
But just like any other medical procedure, the technique has advantages and shortcomings. Although pulse oximetry is one of the most celebrated procedures, due to its numerous advantages over the available alternatives, there are some limitations associated with it. The following are the pros and cons of this technique:
Pulse Oximetry Advantages
Completely non-invasive- This is one of the most popular advantages that make pulse oximetry stands out from the rest of the available alternatives. The technique is very convenient for the non-invasive continuous monitoring and measurement of oxygen saturation. Compared to the alternatives such as blood gas tests that are determined in a laboratory on a drawn sample of the blood from the patients, this is done without drawing any blood from the patients. That means that it can be repeated as often as desired since it does not disturb physiology of your body.
Wide application and high versatility- this is another advantage that has seen the use of pulse oximetry widen in the recent years. The technique can be used in any setting where the patient may need their oxygen level monitored. The technique can be used in unstable situations such as intensive care, hospital ward settings, operating and recovery rooms. The device can be used outdoors for a patient who needs to monitor oxygen saturation levels regularly. It has also been a must for safety and in medical kits for pilots in unpressurised air crafts and high-altitude mountain climbers to help them assess their oxygenation.
Simplicity- Over the years, the pulse oximeter has been improved to enhance its performance. In 1970, the device was too large for practical use and it had to be carried in a cart. It was very complicated to operate, and it required a specialist to operate. But the modern oximeter has been simplified into a small device that you can comfortably carry around. In fact, it can comfortably fit in your small bag or safety kit. They are easy to operate, and you only need to follow a few simple instructions to operate. In fact, you don’t need to be a specialist to operate the device.
Detect and manage numerous medical conditions- Pulse oximetry has been used to identify and manage various medical conditions that affect blood oxygen levels. Among the conditions that the technique can be used for includes COPD and other breathing conditions. The device can also be used for diagnosis of some sleeping disorders such as hypopnea and apnea.
Inexpensive and accurate- Compared to most of the alternatives available in the market, it costs less to buy and to maintain. The cost of the hardware is very low, and that’s what makes it less expensive. There are no service contracts, and the device does not include consumable items such as cal fluid/gases, syringes, anaesthesia, surgical supplies and catheter care material amongst many others needed in its alternatives.
Pulse Oximetry Limitations
Pulse oximetry only measures haemoglobin saturation, and thus it’s not a complete measure of respiratory sufficiency. In fact, it’s not an ideal substitution for blood gas tests in laboratories because it does not give carbon dioxide levels, bicarbonate concentration, base deficit and blood pH. While the metabolism of oxygen can be measured by monitoring expired CO2, the reading on the pulse-ox does not give information about blood oxygen content. A good example is diagnosing anaemia where the blood carries less oxygen due to less red blood cells; the device cannot detect or diagnose it since it will always show that haemoglobin is 100% saturation.
There is a possibility of reading errors due to hypoperfusion caused by either the limb being cold or vasoconstriction. Movements such as shivering, highly calloused skin and incorrect sensor application may be the cause of incorrect reading. These are challenges not experienced with other alternatives such as blood gas test. This technique requires the user to be very careful particularly while placing the sensor or the probe for accurate results.
Pulse oximetry is probably one of the most significant advances in respiratory monitoring. Over the last two decades, numerous studies have been focused on the technical aspects of the device to improve its performance, ease of operation and the degree of accuracy. It’s an essential technique in almost all the medical operations and almost in every department of the hospital. Its versatility and ease of operation make it the ultimate choice for testing oxygen saturation. The margin of error is negligible given that the technique has a reasonably high degree of accuracy.