Pulse oximetry has been used in the assessment and management of patients in pre-hospital and primary care for over 30 years (Silverston, 1989; 1991). The use of pulse oximetry in primary care has increased steadily over that time, partly due to the availability of inexpensive, finger clip pulse oximeters and also in response to the incorporation of oxygen saturation readings (SpO2) into the national guidelines for the assessment and management of patients with a wide variety of respiratory and non-respiratory illnesses (Ingram and Munro, 2005; Potter, 2007; Plüddemann et al, 2011). In 2020, the emergence of a new respiratory illness, COVID-19, led to pulse oximetry being used routinely in the assessment and management of large numbers of patients with COVID-19 respiratory illness. Pulse oximetry was seen as a non-invasive, simple to use and inexpensive technology that would enable clinicians working in primary care to identify patients with the respiratory form of COVID-19 disease and to categorise that illness as being either mild, moderate or severe (NHS England and Wales, 2020). National guidelines for the assessment and management of patients with COVID-19 disease recommend that pulse oximetry is performed routinely as part of the assessment, management and monitoring of patients. In many parts of the UK, patients were provided with a pulse oximeter to help them monitor their condition at home, as part of a ‘virtual ward at home’ programme (Greenhlagh et al, 2021). An understanding of the technical and physiological principles involved in pulse oximetry can help clinicians to avoid errors in diagnosis arising from the incorrect use of equipment, or from the misinterpretation of SpO2 readings (Silverston, 2016).
In the respiratory form of COVID-19 disease, patients may become hypoxic due to the damage caused by the virus to the lungs, and pulse oximetry is used to detect and quantify the degree of hypoxia present. Patients with a SpO2 reading ≥95% are categorised as having mild disease, 93–94% as having moderate disease and ≤92% as having severe disease. Thus, the management of a patient varies with very small differences in the SpO2, which highlights the importance of ensuring that an SpO2 reading is both reliable and accurate. It is important, therefore, to appreciate the factors that can result in unreliable and inaccurate readings in clinical practice. Furthermore, it is also important to appreciate that there are a wide range of respiratory and non-respiratory medical conditions that can cause a low SpO2 reading in a patient with COVID-19 disease, so it is important to know how to interpret these readings in the context of COVID-19 disease, specifically. This article will describe the technical and physiological principles of pulse oximetry, along with the factors that can affect the reliability and accuracy of SpO2 readings, as well as the medical conditions that can result in a fall in the SpO2 reading in patients with COVID-19 disease.
Pulse oximeters
The fundamental principle in pulse oximetry is that when light is shone through a substance, some of that light is absorbed as it passes through the substance and the light that is not absorbed can be measured. A pulse oximeter probe emits light of two different wavelengths, one of which is absorbed by oxygenated haemoglobin (Hb-O2) and the other by deoxygenated haemoglobin (Hb), so the relative proportions of the two molecules can then be determined. So, pulse oximetry involves measuring the saturation of Hb with O2, hence the term ‘oxygen saturation’, or ‘Sats’, for short. However, in order to only sample Hb-O2 that is travelling in the arterial circulation, pulse oximeter software is designed to only measure Hb-O2 that is moving in a pulsatile fashion. This is important for understanding the factors that can affect the reliability and accuracy of SpO2 readings. A finger probe or finger clip pulse oximeter is applied to the finger, so it is necessary to consider the variables that could affect the absorption of light as it passes from one side of the probe to the other, such as false nails, certain colours of nail varnish and the presence of dirt, grease and oil (Yamamoto et al, 2008). This means that it is important to ensure that the finger is appropriately prepared and the inner surfaces of the finger probe kept clean to avoid incorrect readings. External light contamination from either sunlight, or artificial light of specific wavelengths, can also result in errors, which is why many finger probes are housed in a circumferential cover.
It is also important to ensure that the two sides of the finger probe are at a right angle to another and not at a more acute angle, as this can increase the amount of ambient light entering an unshielded probe, as well as the amount of light being detected by the sensor. Placing a probe on a thumb, or big toe, should be avoided and age and size-appropriate probes should be used. More recently, there has been discussion regarding the potential for dark skin colours to affect pulse oximeter readings, due to a difference in light absorption. In a recent study, patients with COVID-19 disease had their SpO2 reading taken, along with an arterial blood gas (ABG) measurement, and the two readings were compared (Sjoding et al, 2020). In patients with SpO2 readings of 92–96%, 3 times as many patients (17%) with black skin had ABG measurements of ≤88% than patients with white skin (6%). In other words, pulse oximetry failed to detect hypoxia in a significant proportion of patients with black skin. This has significant implications given that pulse oximetry is being used to assess large numbers of patients with COVID-19 disease, and also because this group of patients is at increased risk of developing severe disease.
The second factor that can influence both the reliability and accuracy of SpO2 readings is the requirement for pulse oximeters to only sample Hb-O2 that is moving in a pulsatile fashion. Both a low amplitude pulse and movement artefact can interfere with the sampling process (Gehring et al, 2002). A pulse oximeter may not be able to detect a pulse in a patient with low blood pressure, poor peripheral perfusion, cold hands, or Raynaud's disease, or may misinterpret the pulse in a patient with a tremor, or who is shivering. Pulse oximeters do contain software that is designed to filter out signals from low flow states and movement artefact, but the quality of the software is variable. This is why it is best practice to check the temperature of the finger, along with the capillary refill time in that finger, before applying the finger probe. Placing a cold hand in warm water for a few minutes prior to applying the probe can improve blood flow to the finger, especially during winter months, when the rates of respiratory illness are at their highest. Pulse oximeters do provide information about both the quantity and the quality of the signal that they are receiving from the probe, either in the form of a bar graph, or a plethysmograph. Plethysmograph waveforms are particularly helpful because the change from a sawtooth-type pattern to any other pattern is an indicator that there may be a problem with the quality or quantity of the signal being received. Many pulse oximeters also measure the Perfusion Index (Pi), which provides an assessment of the level of perfusion in the finger.
The final factor that may affect the reliability and accuracy of a pulse oximeter is the quality of the software inside the pulse oximeter itself. There are now a large number of different pulse oximeters on the market, the majority of which are of the finger clip type, rather than having a separate finger probe and oximeter unit. It is important to distinguish between pulse oximeters that are for use by the general public and those that have been approved for use in medical settings. A study into the accuracy of inexpensive pulse oximeters not cleared by the American Food and Drug Administration concluded that ‘Many low-cost pulse oximeters demonstrate highly inaccurate readings’ (Lipnick et al, 2016). This should be of concern given the very small differences in readings that categorise patients as having mild, moderate or severe illness in both COVID-19 disease and other respiratory conditions. It is important, therefore, that health professionals only use pulse oximeters that have been certified by an appropriate body as producing accurate readings and that the accuracy of the device is re-certified annually. The technology contained in pulse oximeters is advancing rapidly, with some units now offering features such as a Plethysmograph Variability index (PVi), respiratory rate (RR) and a beat-to-beat assessment of the reliability of the signal. Not only are pulse oximeters now being fitted into wearable devices, such as smart watches and rings, but bluetooth-enabled probes are now available as skin patches and in contact lens form (Lee et al, 2018).
As with all physiological measurements, it is important to analyse these in the context of the patient and their disease, if errors in diagnosis and clinical decision-making are to be avoided. An assessment of the reliability and accuracy of the SpO2 reading is required and the reading has to be interpreted in the context of the patient's physiology and their illness. There are physiological reasons why a patient may have a low SpO2 reading. For example, the amount of air entering the lungs will be reduced if expansion of the diaphragm or chest wall is restricted. Chest wall and diaphragmatic excursion may be reduced in patients who are lying flat, or seated, especially if they are obese. Both musculoskeletal and pleuritic chest pain can lead to ‘splinting’ of the chest wall, resulting in shallow breathing, diminished air entry and a low SpO2. At night, nocturnal desaturation is not uncommon and up to 5 brief episodes per night is considered normal (Greenhalgh et al, 2021). Patients with underlying respiratory disease may have a chronically low SpO2 and it is important to interpret their pulse oximetry readings in the context of their pre-existing illness, as well as the new illness. A decrease in the SpO2 reading from their ‘normal’ baseline should be considered significant. Above all else, it is important to perform a comprehensive, holistic clinical assessment of the patient and not to rely on a single physiological parameter to assess the condition of the patient. This is why it is recommended that patients with COVID-19 disease being monitored at home with pulse oximetry should maintain regular contact with a health professional and not rely on symptom-reporting and pulse oximetry values alone (Greenhalgh et al, 2021). COVID-19 is a multi-system disease and pulse oximetry may miss the development of non-respiratory complications. Similarly, other respiratory infections and conditions may also cause a decrease in the SpO2 and be mistaken for COVID-19 disease. This is why it is not only important for safe practice to perform pulse oximetry correctly but also to interpret the readings correctly.
Pulse oximetry in COVID-19 disease
The guideline for the assessment and management of patients with COVID-19 disease in primary care advocates the use of pulse oximetry to help determine whether the patient is hypoxic and to what degree (NHS England and Wales, 2020). However, it is important that clinicians are aware of the limitations of pulse oximetry in assessing whether or not a patient is hypoxic and whether or not any hypoxia present is due to COVID-19 disease. This requires an understanding of what a SpO2 reading represents, both physiologically and clinically. An SpO2 reading is a measurement of the amount of O2 being carried by Hb in the bloodstream as Hb-O2, which is a function not only of how much O2 crosses into the circulation but also the quantity and quality of the Hb molecules being used to transport O2 to the cells. The first step in this process is inhaling O2 into the lungs, a complex process that involves the neurological, musculoskeletal and respiratory systems. A diverse range of medical conditions can prevent, reduce, or obstruct O2 from being inhaled into the lungs, or crossing from the respiratory system to the circulatory system in the alveoli (see Box 1). At the alveolar level, pulmonary oedema or pus within the alveoli can prevent O2 from entering the pulmonary circulation, while on the circulatory side of the alveolus, a reduction in blood flow in the pulmonary circulation can prevent Hb from reaching the alveolus, as is the case with a pulmonary embolus (PE), sepsis, or heart failure. Thus, a low SpO2 reading only tells you that there is a problem in the O2 uploading process, not where that problem is, nor what is causing it. This emphasises the importance of performing a comprehensive and holistic clinical assessment and considering a wide range of alternative diagnoses in patients with a low SpO2 level (Silverston, 2016).
Box 1.Causes of a low SpO2
- Reduced atmospheric oxygen
- Respiratory depression
- Upper airway obstruction
- Middle airway obstruction
- Lower airway obstruction
- Obstruction to oxygen transfer across the alveolar wall
- Reduced chest/diaphragmatic expansion
- Reduced lung expansion
- Inability to create negative intrathoracic pressure
The significance of this in patients with COVID-19 is that a low SpO2 may or may not be the result of COVID-19 respiratory illness. Where a diagnosis of COVID-19 has been made on the basis of symptomatology alone, it is important to appreciate that respiratory infections other than COVID-19 can cause a low SpO2 reading, such as a community-acquired pneumonia, or influenza (Gupta and Woodhead, 2010). In a patient with confirmed COVID-19 disease, it is also important to consider the possibility that a low SpO2 reading may not be due directly to COVID-19 but rather to a COVID-induced exacerbation of a pre-existing cardiac or respiratory condition, or a cardiac or respiratory complication of COVID-19 disease, such as a PE, or a secondary bacterial pneumonia (Rossdale et al, 2003). This is why it is necessary to interpret a low SpO2 within the context of the findings from the rest of the patient's clinical assessment and to always consider alternative, less obvious, causes for the patient being hypoxic. The difficulty of establishing the correct diagnosis in patients with acute respiratory symptoms and chronic respiratory disease in primary care is well-recognised (Couturaud et al, 2021). The risk in patients with COVID-19 disease is that a patient with increasing respiratory symptoms and a falling SpO2 may have a non-COVID cause for this, which would lead to an incorrect diagnosis and clinical decision being made. For example, if a patient's SpO2 reading was 95%, the management plan would differ for that patient depending on the cause of the low SpO2 reading. In a patient with COVID-19 disease that reading would represent mild illness, but that reading is also consistent with a diagnosis of a PE, a community-acquired pneumonia, or heart failure, in which case the clinical decision would be to send the patient to hospital, rather than allow them to remain at home.
It is inferred from a SpO2 reading that the amount of Hb-O2 in the bloodstream equates to the amount of O2 that is being delivered to the cells, but this is not always the case. There are clinical situations where the SpO2 and an ABG reading do not correlate and a patient may have cellular hypoxia in the presence of a normal SpO2. This occurs because the amount of O2 being delivered to the cells is a function not just of the amount of O2 crossing from the respiratory system into the circulatory system but also other factors, such as the quantity and quality of the circulating Hb molecules. This may render the SpO2 unreliable at detecting cellular hypoxia in patients with anaemia, sickle cell disease and other haemoglobinopathies (Caboot and Allen, 2014).
It should also be noted that many pulse oximeters are unable to distinguish between Hb-O2 and Hb-CO (carboxyhaemoglobin). A person who smokes heavily might have up to 8% of their oxygen displaced by CO, so their true SpO2 may be 8% lower than their pulse oximetry reading is showing (Moyle, 2021).
In addition, the cellular environment has an effect on the downloading of O2 from Hb-O2 and the uptake and utilisation of O2 by the cells, such as the pH, temperature and CO2 level, which is why an ABG provides a more comprehensive, holistic and accurate assessment of what is happening at a cellular level, including whether the cell is hypoxic, than an SpO2 reading. In a patient with severe COVID-19 disease, the presence of viral and bacterial sepsis and respiratory failure, with resulting respiratory and metabolic acidosis may mean that cellular hypoxia is worse than the SpO2 reading would indicate.
When using a pulse oximeter in clinical practice, there are a number of factors that can affect the reliability and accuracy of SpO2 readings, which those performing pulse oximetry in the surgery, or in the patient's home, should be aware of (Luks and Swenson, 2020). Differences in the software in pulse oximeters can lead to variations in the time that a pulse oximeter takes to obtain its first reliable reading, which can vary from 5–30 seconds. Patient factors, such as a cold or under-perfused finger, can also contribute to this delay. A pre-warmed, non-elevated hand that is comfortably resting on a pillow to prevent movement can facilitate this. The position of the patient is also important, as a patient who is laid flat, or slouched in a chair, will not be moving air normally, especially if they are obese, and this cause a positional fall in the SpO2 reading. Asking the patient to take deep breaths, sit-up straight, or walk around should result in an increase in the SpO2 reading and there should be concern if the SpO2 reading does not return to normal with these measures, or if these measures cause the SpO2 reading to fall further.
In a patient who is complaining of breathlessness on exertion rather than shortness of breath at rest, it is important to appreciate that the patient may only become hypoxic when they are symptomatic. An SpO2 reading taken when the patient is at rest and asymptomatic may result in a serious condition being missed. Post-exertional pulse oximetry can reveal patients with hypoxia that is ‘silent’ at rest, although care must be taken to limit the exertion to the point where the patient becomes symptomatic and to avoid doing so in patients who are at risk of ischaemic events (Greenhlagh et al, 2020). It is also advisable and appreciated by the patient if a supply of O2 is available to help alleviate the symptom. Finally, a low SpO2 should not be ignored in a patient with COVID-19 disease, even if the patient is asymptomatic, as it is recognised that some patients have ‘silent’ hypoxia and have few or minimal respiratory symptoms. Patients with a pre-existing medical condition, such as chronic obstructive lung disease, will often have a low SpO2 level due their underlying condition and may be aware of what their normal SpO2 level is. In COVID-19, a fall from this level of 1–2% represents mild disease, 3–4% moderate disease and >4% severe disease, although it is essential to perform a holistic assessment of the patient's overall condition and respiratory status, as pulse oximetry does not provide information on the patient's CO2 level (NHS England and Wales, 2020).
Discussion
One of the consequences of the COVID-19 pandemic is that there has been an increase in the use of pulse oximetry in both primary and community care to assess and manage patients with COVID-19 disease. Pulse oximetry is a non-invasive, simple to use and inexpensive method of detecting and quantifying the degree of hypoxia in patients with COVID-19 disease and identifying patients with the disease who have ‘silent’ hypoxia.
Pulse oximetry can also be used as a screening tool to help identify older patients with COVID-19 disease, as older patients do not always develop a high fever with COVID-19 infection and may not present with the classic symptoms of the disease (Van Son and Eti, 2021).
However, it is important that clinicians are aware of the factors that can affect the reliability and accuracy of SpO2 readings, given the impact on clinical decision-making and patient management that result from small differences in these readings. It is also essential for safe practice that clinicians understand the need to interpret SpO2 readings in the context of the physiological principles of pulse oximetry, the rest of the findings elicited from the patient's clinical assessment and COVID-19 disease. In patients with the symptoms of COVID-19 disease, a low SpO2 reading may indicate that the patient is hypoxic due to COVID-19 respiratory illness but other serious causes of hypoxia also need to be considered, especially in patients with underlying cardiac and respiratory conditions. It should also be recognised that there are a number of serious respiratory and non-respiratory complications of COVID-19 disease that can cause a low SpO2, which need to be considered when assessing the SpO2 in patients with COVID-19. There is increasing concern that this winter will see a surge in the number of patients with COVID-19 disease, as well as patients with seasonal influenza and respiratory syncytial virus. In primary care, this will increase the requirement to be able to assess and monitor patients with acute respiratory illnesses and to determine which patients need to be admitted to hospital. Pulse oximetry will form an important part of that assessment and monitoring, so it is essential for safe practice that all health professionals are aware of the uses and limitations of pulse oximetry in clinical practice.
KEY POINTS:
- Pulse oximetry measures the amount of oxygenated haemoglobin travelling in the peripheral arterial circulation
- A low SpO2 reading indicates that there is either insufficient oxygen reaching the lungs, or that oxygen is not being uploaded to haemoglobin in the pulmonary circulation
- The SpO2 reading should be interpreted in the context of other findings elicited during the patient's clinical assessment
- In patients with COVID-19 disease, pulse oximetry can be used to categorise patients as having mild, moderate or severe disease
- In COVID-19 disease, it is essential that other causes of a low SpO2 are considered, including an exacerbation of a pre-existing illness, a complication of COVID-19 disease, or an illness that presents with the same symptoms as COVID-19 disease
CPD reflective practice:
- How can you improve the reliability and accuracy of your SpO2 readings when you are performing pulse oximetry in your patients?
- In your patient population, what are the potential causes of a low SpO2 reading and how would you distinguish one cause from another in clinical practice?
- When performing a clinical assessment in a patient with suspected or confirmed COVID-19 disease with a SpO2 reading of 95%, which findings would support a diagnosis of pneumonia, heart failure, or a pulmonary embolus (PE), rather than mild COVID-19 disease?