Pulse oximetry is a noninvasive method for monitoring a person’s oxygen saturation. Peripheral oxygen saturation (SpO2) readings are typically within 2% accuracy (within 4% accuracy in the worst 5% of cases) of the more desirable (and invasive) reading of arterial oxygen saturation (SaO2) from arterial blood gas analysis.[rx] But the two are correlated well enough that the safe, convenient, noninvasive, inexpensive pulse oximetry method is valuable for measuring oxygen saturation in clinical use.

Pulse oximetry – is a non-invasive monitor that measures the oxygen saturation in the blood by shining light at specific wavelengths through tissue (most commonly the fingernail bed). Deoxygenated and oxygenated hemoglobin absorb light at different wavelengths (660 nm and 940 nm respectively), and the absorbed light is processed by a proprietary algorithm in the pulse oximeter to display a saturation value. It is a standard monitor for all anesthesia cases in most developed countries  and also used in emergency departments, hospital wards, and ambulances to assess blood oxygenation in patients with respiratory difficulties or monitor for respiratory depressant effects of pain medications. Since its widespread use in hospitals, the incidence of unrecognized desaturations has decreased significantly .

Pulse oximetry – is a non-invasive and painless test that measures your oxygen saturation level or the oxygen levels in your blood. It can rapidly detect even small changes in how efficiently oxygen is being carried to the extremities furthest from the heart, including the legs and the arms.

Pulse oximetry –  has revolutionized the ability to monitor oxygenation in a continuous, accurate, and non-invasive fashion. Despite its ubiquitous use, it is our impression and supported by studies that many providers do not know the basic principles behind its mechanism of function

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Dyshemoglobinemias, such as carboxyhemoglobin, methemoglobin, and others, will change the color and absorption spectrum of blood and give false readings. In that case, a confirmation with a Co-oximeter should be obtained. Some newer pulse oximeters using multiple wavelengths can display some methemoglobinemia.

Light pollution into the sensor part of the probe can be another interfering factor to an accurate reading, such as certain ambient light or other probes emitting light in a similar spectrum in the vicinity of the pulse oximeter probe. It should be avoided by covering the site or the probe.

Pulsating blood is another prerequisite for an accurate reading. The pulse amplitude in tissue beds is very small and accounts for about only 5% of the pulse oximeter signals to be available for analysis. Any further decrement in pulse wave amplitude such as severe hypotension, cold extremities, Raynaud disease, or other factors such as excessive motion may interfere with an accurate reading. Hospital-grade pulse oximeters usually can read through perfusing cardiac arrhythmias such as atrial fibrillation and premature atrial or ventricular contractions. Most pulse oximeters display the plethysmographic waveform in addition to the saturation number to give the clinician another parameter to interpret the saturation number.

Types of

There are two main types of home pulse oximeters:

  • Over-the-counter oximeters. This is the most common type for home use. You can buy them online or in stores. Some link to smartphone apps as well. The FDA doesn’t review these devices and recommends against using them for medical purposes.
  • Prescription oximeters. These are the same as those used by hospitals and doctor offices. You can get one with a prescription from your doctor. The FDA reviews these devices to make sure they fall within acceptable ranges of accuracy.

Follow guidance from your doctor and the device manufacturer on how and when to take a reading. You can help improve your chances of accurate and useful readings if you:

  • Make sure your hand is relaxed, warm, and below heart level.
  • Remove any fingernail polish on the finger you’re reading.
  • Place the device exactly how the manufacturer’s instructions suggest.
  • Look for one steady number. (Numbers might jump around for a few seconds.)
  • Keep track of your levels along with the date and time to give your doctor a sense for how your numbers change over time. (Levels that get lower over time could suggest a serious health issue).

What Do the Results Mean?

A blood oxygen level lower than 89% means you may not have enough oxygen in your blood to meet your body’s needs. This could be because there’s a problem with your heart or lungs. If your levels are low, you may need to breathe in extra oxygen through a tube.

But a pulse oximeter reading is simply an estimate. For example, a reading of 90% oxygen saturation on an FDA-approved prescription machine could mean anything from 86% to 94%. In addition, a number of other things can affect the accuracy of the reading, including:

  • Bad circulation
  • Fingernail polish
  • Long or dirty fingernails
  • Tobacco use
  • Different pulse oximeter sensors (finger clip vs. adhesive)
  • Skin thickness
  • Skin temperature
  • Skin color: One study shows that dark skin (in African-American people, for example) may get a less accurate measurements on current devices.

Principles of pulse oximetry

The technique of pulse oximetry has been previously described []. Using the spectrophotometric methodology, pulse oximetry measures oxygen saturation by illuminating the skin and measuring changes in light absorption of oxygenated (oxyhemoglobin) and deoxygenated blood (reduced hemoglobin) using two light wavelengths: 660 nm (red) and 940 nm (infrared) [,rx]. The ratio of absorbance at these wavelengths is calculated and calibrated against direct measurements of arterial oxygen saturation (SaO2) to establish the pulse oximeter’s measure of arterial saturation (SpO2). The waveform, which is available on most pulse oximeters, assists clinicians in distinguishing an artifact from the true signal.

Indications

Pulse oximetry is one of the standard American Society of Anesthesiologists monitor parameters for all anesthesia cases. In addition, it should be used for hospitalized patients that receive medications that may impair their respiration (mostly opioids). All patients with acute respiratory problems should be monitored with pulse oximetry whether in an emergency room, intensive care unit, general hospital or pre-hospital ambulance setting. Pulse oximetry is not only used to rapidly diagnose hypoxia but also to titrate the treatment for hypoxia such as ventilator support parameters and supplemental oxygen in order to avoid hyperoxia, which can be detrimental in neonates but as recent publications also suggest in adults suffering from myocardial infarction and possibly other diseases.

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Indications for pulse oximetry include any clinical setting where hypoxemia may occur. These settings include patient monitoring in emergency departments, operating rooms, emergency medical services systems, postoperative recovery areas, endoscopy suites, sleep and exercise laboratories, oral surgery suites, cardiac catheterization suites, facilities that perform conscious sedation, labor and delivery wards, interfacility patient transfer units, altitude facilities, aerospace medicine facilities, and even patients’ homes.

Procedure steps

Pulse oximetry may be used in both inpatient and outpatient settings. In some cases, your doctor may recommend that you have a pulse oximeter for home use.

The pulse oximetry process is as follows:

  • Most commonly, a clip-like device will be placed on your finger, earlobe, or toe. You may feel a small amount of pressure, but there is no pain or pinching. In some cases, a small probe may be placed on your finger or forehead with a sticky adhesive. You may be asked to remove your fingernail polish if it’s being attached to a finger.
  • You’ll keep the probe on for as long as needed to monitor your pulse and oxygen saturation. When monitoring physical activity capabilities, this will be during the extent of the exercise and during the recovery period. During surgery, the probe will be attached beforehand and removed once you’re awake and no longer under supervision. Sometimes, it will only be used to take a single reading very quickly.
  • Once the test is over, the clip or probe will be removed.

How it works

Pulse oximeters are clip-on devices that measure oxygen saturation. The device may be attached to a finger, a wrist, a foot, or any other area where the device can read blood flow.

Oxygen saturation can drop for many reasons, including:

  • suffocation
  • choking
  • infections, such as pneumonia
  • drowning
  • diseases, such as emphysema, lung cancer, and lung infections
  • inhaling poisonous chemicals
  • heart failure or a history of heart attacks
  • allergic reactions
  • general anesthesia
  • sleep apnea

Pulse oximeters work by shining a light through a relatively transparent area of the skin. The light shines through to a detector positioned on the other side of the skin.

For example, when a pulse oximeter is clipped onto a finger, one side of the clip shines the light, and the other detects it. The amount of light absorbed by the blood indicates oxygen saturation. A pulse oximeter does not directly measure oxygen saturation but instead uses a complex equation and other data to estimate the exact level.

Use of pulse oximeters

The importance of pulse oximeters was quickly recognized in many different clinical areas, particularly in intensive care, anesthesia, respiratory medicine, pediatric and emergency care. Since the late 1980s, they have been introduced into all clinical areas, including home therapy.

In addition to monitoring for hypoxemia, pulse oximeters can be used to ensure the most efficient use of oxygen therapy, which is especially important in resource-limited settings, where oxygen may be scarce. Furthermore, oximetry is considered essential in neonatal practice to prevent the devastating complication of blindness due to retrolental fibroplasia, which is caused by excess use of oxygen.

Pulse oximetry for anesthesia and surgery

For more than 20 years, the use of pulse oximetry for anesthesia monitoring during surgery has been a standard of care in the developed world. Pulse oximeters are used in nearly every procedure that involves anesthesia or sedation. The safety of anesthesia improved dramatically following the availability of routine oximetry, and the risk of mortality from anesthesia reduced from around. Other developments happening around the same time, including capnography, training, drugs, and equipment, also contributed to improving patient safety during anesthesia. However, these safety practices have not been routinely instigated in the developing world. Estimates suggest that more than half of operating rooms are not equipped with pulse oximeters.

It has been estimated that over 230 million surgical procedures are performed around the world each year [rx]. In the developed world, 3–16% of hospitalized surgical patients have major complications, and nearly 1% experience permanent disability or death as a result of their operation [rx, rx], If these numbers are extrapolated globally, at least 7 million people develop serious complications and 1 million die; 50% of these complications are thought to be preventable [rx].

Due to substantial differences in the safety of surgery between developed and developing countries, a disproportionate number of complications and deaths are likely to occur in resource-limited settings. Anesthesia death rates in these settings are reportedly 100- to 1,000-times higher than in the developed world [rx]. For example, a mortality rate of 1:133 has been recorded in Togo, mostly hypoxia-related [rx].

Central cyanosis, the traditional clinical sign of hypoxemia, is an insensitive marker occurring only at 75-80% saturation. Consequently, pulse oximetry has a wide range of applications including:

  • Individual pulse oximetry readings – can be invaluable in clinical situations where hypoxemia may be a factor – for example, in a confused elderly person.
  • Continuous recording – can be used during anesthesia or sedation, or to assess hypoxemia during sleep studies to diagnose obstructive sleep apnoea. Peri-operative monitoring has not, however, been shown to improve surgical outcomes[rx].
  • Pulse oximetry can replace blood gas analysis in many clinical situations unless PaCO2 or acid-base state is needed. It is cheaper, easier to perform, less painful, and can be more accurate where the patient is conscious (hyperventilation at the prospect of pain raises PaO2).
  • Pulse oximetry allows accurate use of O2 and avoids wastage. For example, in patients with respiratory failure, rather than limit the use of O2 to maintain hypoxic ventilatory drive, it can be adjusted to saturation of ~90% which is clinically acceptable.
  • Neonatal care – the safety limits for oxygen saturations are higher and narrower (95-97%) compared to those for adults[5]. Pulse oximetry is not yet a standard of care in the screening of neonates for asymptomatic congenital heart disease but may become so. A UK pilot concluded that many babies found to have a low oximetry reading were either normal or had a non-cardiac cause for their low oxygen level. Further evaluation is required[rx].
  • Intrapartum fetal monitoring – the use of fetal pulse oximetry in combination with routine cardiotocography (CTG) monitoring has been studied and found not to reduce the operative delivery rate[rx].
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Pulse oximeters are now used routinely in critical care, anesthesiology, and A&E departments, and are often found in ambulances. They are an increasingly common part of a GP’s kit. Pulse oximetry’s role in primary care may include:

  • Diagnosing and managing a severe exacerbation of chronic obstructive pulmonary disease (COPD) in the community.
  • Grading the severity of an asthma attack. Where oxygen saturations are less than 92% in air, consider the attack potentially life-threatening[rx].
  • Assessing severity and oxygen requirements for patients with community-acquired pneumonia[rx].
  • Assessing severity and determining management in infants with bronchiolitis.
General pointers to the management of hypoxaemia
Oxyhaemoglobin saturation Management
90-95% Measure regularly and especially at night. Review trends. Where value is unexpected, check signal quality and probe.
80-90% As above, continuous monitoring and give oxygen until saturations above 90%.
<80% As above and consider ventilatory support.
  • Resting readings should be taken for at least five minutes.
  • Poor perfusion (due to cold or hypotension) is the main cause of an inadequate pulse wave. A sharp waveform with a dicrotic notch indicates good perfusion whilst a sine wave-like waveform suggests poor perfusion.
  • If a finger probe is used, the hand should be rested on the chest at the level of the heart rather than the affixed digit held in the air (as patients commonly do) in order to minimize motion artifact.
  • Checking that the displayed heart rate correlates to a manually checked heart rate (within 5 beats per minute) generally rules out significant motion artifacts.
  • Emitters and detectors must oppose one another and light should not reach the detector except through the tissue. Ensure the digit is inserted fully into the probe and that flexible probes are attached correctly. Appropriately sized probes should be used for children and infants.
  • Oximeter accuracy should be checked by obtaining at least one simultaneous blood gas, although this rarely happens. Oximeters may correct average oximeter bias based on pooled data but this does not eliminate the possibility of larger individual biases.
  • Pulse oximetry cannot differentiate between different forms of hemoglobin. Carboxyhaemoglobin is registered as 90% oxygenated hemoglobin and 10% desaturated hemoglobin, thereby causing an overestimation of true saturation levels.
  • Significant venous pulsation such as occurs in tricuspid incompetence and venous congestion.
  • Environmental interference: vibration at 0.5-3.5 Hz and excessive movement. Ambient light – including infrared heat lamps – makes a difference of less than 5%[10].
  • Cold hands – warm extremity if local poor perfusion.
  • Nail polish should be removed, as it may cause false readings[10].
  • Intravascular dyes, such as methylthioninium chloride, may also temporarily falsely reduce saturation readings.

The WHO Surgical Safety Checklist

To improve the safety of surgery, WHO launched the Safe Surgery Saves Lives program in 2007 [rx]. One goal was to define a minimum set of surgical safety standards that could be applied in all countries and hospital settings. The result was the WHO surgical safety checklist, which was launched in June 2008 [rx]. The checklist is simple and can be completed in less than two minutes.

Over 280 professional bodies, hospitals, and ministries of health have approved the checklist, which includes a set of basic steps to follow before, during, and after surgery. Examples of the steps include: confirming patient identity, recording medication allergies, administering antibiotics on time, counting instruments, sponges, and needles, and ensuring that a pulse oximeter is on the patient and functioning.

At the time of the checklist launch, WHO released preliminary results from over 1,000 patients in eight pilot hospitals across the world. The checklist nearly doubled the chance that patients would receive proven standards of surgical care and substantially reduced complications and deaths [rx].

The WHO decided that pulse oximetry should become a standard of care and feature as a requirement on the checklist. Following the publication of the checklist, the WHO and the World Federation of Societies of Anesthesiologists (WFSA) confirmed oximetry as a standard of care for all patients undergoing anaesthesia and surgery [r, rx].

Despite the establishment of pulse oximetry as a monitoring standard, the technology is still not currently available in many operating rooms around the world. WHO started the Global Pulse Oximetry Project, an initiative to improve the availability of pulse oximeters in every operating room in the world.

Although it has been estimated that around 77,000 operating rooms need pulse oximeters in resource-poor settings, the overall figure is much higher. If all clinical areas are taken into account, the number of oximeters required has been estimated at over 1,000,000 [rx].

Barriers to achieving global pulse oximetry

The relatively high initial cost of pulse oximetry technology has been a significant barrier in many developing world settings. Markets in resource-poor areas are small and manufacturers do not find it profitable to make the investment in developing a sales infrastructure in many economies.

Operating theatres have to compete for funds with other parts of the hospitals and capital investment is often low, limiting the availability of all equipment that might not be considered absolutely essential.

Where pulse oximeters have been purchased, the costs are generally higher than in well-resourced settings due to the difficulty of market creation and resultant small numbers of orders.

The fragility of the probe is another problem; in many settings, donated oximeters lie unused in cupboards due to broken probes, which have not been, or cannot be replaced. Spare probes may be prohibitively expensive in this setting, as a single import.

Affordability is not the only factor affecting the accessibility of pulse oximetry in low-resource settings. As with all equipment, challenges of distribution within-country including importation and taxation costs, coupled with inadequate supply chains pose difficulties. An absence of a reliable electricity supply is common, requiring battery function.

Many countries have a serious lack of trained healthcare workers. This difficulty is greatest in sub-Saharan Africa, which has 11% of the world’s population and 24% of the world’s disease burden, but only 3% of the world’s healthcare workers. Although pulse oximetry is a simple technique, there is still a requirement for interpretation by someone trained in their use. 75% might be considered an excellent exam result, but it is dangerously low oxygen saturation. Training is required to understand the implications!

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The realities of workforce and training have been highlighted in anesthesia. In 2007, Uganda had around 15 physician anesthetists for a population of 27 million. The UK has 12,000 physician anesthetists for a population of 60 million. Due to the severe shortage of physician anesthetists in Uganda, most anesthetics are administered by 350 anesthetic officers in the country who have received only 1–2 years of anesthesia training. Electrical supplies were constant for only 20% of providers and 10% of anaesthetic providers worked without an oxygen supply. The item most frequently unavailable for 75% of providers was a pulse oximeter [rx].

Benefits

Pulse oximeters are useful for people who have conditions that affect oxygen saturation. For example, a sleep specialist might recommend a pulse oximeter to monitor the nighttime oxygen saturation level of someone with suspected sleep apnea or severe snoring.

Pulse oximetry can also provide feedback about the effectiveness of breathing interventions, such as oxygen therapy and ventilators.

Some doctors use pulse oximetry to assess the safety of physical activity in people with cardiovascular or respiratory problems or may recommend that a person wears a pulse oximeter while exercising. A doctor may also use pulse oximetry as part of a stress test.

Some hospitals also use pulse oximeters for particularly vulnerable patients. For instance, infants in neonatal intensive care units may wear pulse oximeters, which can alert staff of a drop in oxygen saturation.

Some benefits of pulse oximetry include:

  • monitoring oxygen saturation over time
  • alerting to dangerously low oxygen levels, particularly in newborns
  • offering peace of mind to people with chronic respiratory or cardiovascular conditions
  • assessing the need for supplemental oxygen
  • monitoring oxygen saturation levels in people under anesthesia
  • indicating dangerous side effects in people taking drugs that affect breathing or oxygen saturation

Some companies now market pulse oximeters to parents of young infants. These devices promise peace of mind to parents concerned about sudden infant death syndrome (SIDS) and sleeping accidents, but no research supports the claim that they can prevent SIDS or accidents.

Function

There are multiple current clinical uses of pulse oximetry in primary care. In stable underlying lung disease patients, pulse oximetry (SpO2) is helpful for the following:

  • To establish a baseline value.
  • Monitoring the patients with exercise-related dyspnea.
  • A screening tool to identify patients with underlying lung disease (with SpO2 less than 92%)
  • Stable underlying lung disease or recovering from an exacerbation; a SpO2 88% or less qualifies for oxygen therapy
  • Titrating the rate of flow of oxygen in patients on long-term oxygen therapy – the target is resting and ambulatory pulse oximetry around 88% to 92%

During a chronic obstructive pulmonary disease (COPD) or an asthma exacerbation, pulse oximetry is helpful to:

  • Evaluate patients with severe disease (defined as forced expiratory volume in 1 sec (FEV1) of less than 50% predicted), cyanosis, or cor pulmonale for possible respiratory insufficiency/failure
  • Assess patients with acutely worsening symptoms, especially shortness of breath
  • Determine how severe is the exacerbation based on pulse oximetry and partial pressure of oxygen in an arterial blood gas – based on this physician can determine whether the patient is treatable on an outpatient basis or needs in-hospital treatment
  • Titrating oxygen therapy during exacerbation of any underlying lung disease – COPD, asthma, interstitial lung diseases, pulmonary fibrosis, etc.

Clinical uses of Capnography:

  • Confirmation of tracheal intubation
  • Assessing tracheal tube and tracheostomy patency and position
  • Monitor adequate ventilator support
  • During percutaneous tracheostomy placement
  • Monitoring patients with raised intracranial pressure
  • Monitoring response to treatment of bronchospasm
  • Estimation of cardiac output
  • Use during cardiac arrest
  • Measures kinetics of carbon dioxide (CO2) elimination on a breath-by-breath basis

Complications

Complications from using a pulse oximeter are rare. However, it is necessary to be aware of the probe site as blisters or nail damage may occur with extended use. Tissue injury may also occur in the setting of incompatible probes or during a substitution in the form of electrical shock or burns. It is also essential to know how to improve the measurements of pulse oximeters.

Possible ways to improve pulse oximeter signals include:

  • Warm and rub the skin
  • Apply a topical vasodilator
  • Try a different probe site, especially the ear
  • Try a different probe
  • Use a different machine

Factors that may reduce the accuracy of pulse oximeter signals include:

  • Nail Polish
  • Pigmentation of the skin
  • High-intensity ambient lighting
  • Excessive patient movement, or motion artifacts
  • Decreased perfusion
  • Presence of abnormal hemoglobin, carboxyhemoglobin
  • Intravascular dyes
  • Reduced accuracy with saturations below 83%

One significant risk of using a pulse oximeter is the possibility of treating an incorrect reading as accurate. False-negative results for hypoxemia and false-positive results for normoxia or hypoxemia can occur. In these situations, a patient may receive inappropriate treatment, leading to harm.

False normal or high readings can occur in multiple different settings. Carboxyhemoglobin absorbs light at 660 nanometers, which is roughly the same as oxyhemoglobin. Thus, in situations where carboxyhemoglobin is high, a false normal reading may occur. When glycohemoglobin A1c levels are greater than 7%, such as in patients with type 2 diabetes, an overestimation of arterial oxygen saturation may occur. These situations may require an arterial blood gas to determine oxygen saturation accurately. It is also necessary to consider the clinical diagnosis when evaluating a patient with hypoxemic symptoms, as in the case of carbon monoxide toxicity.

False low readings can also occur in multiple settings. Below are some situations that may cause falsely low readings to occur.

  • Methemoglobinemia
  • Sulfhemoglobinemia
  • Sickle hemoglobin
  • Abnormal inherited forms of hemoglobin
  • Severe anemia
  • Venous congestion

References

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