Pulse oximetry in primary care

Abstract

Most acutely unwell patients entering the hospital through either the emergency department or through an emergency assessment unit will have pulse oximetry performed as part of their initial clinical assessment. Although many GPs now have access to a pulse oximeter, pulse oximetry tends to be used much less frequently in primary care, and mainly to assess patients with respiratory conditions or symptoms. This article shows how pulse oximetry can be used as a screening tool during the initial assessment of acutely unwell patients to detect unanticipated hypoxia, as well as to identify those patients with respiratory symptoms who are beginning to decompensate.

The GP curriculum and pulse oximetry

Core statement 1: Being a GP requires GPs to:

Selectively gather and interpret information from the patient's history, physical examination and investigations, and use this to develop an appropriate management plan in collaboration with the patient, by performing clinical examinations and investigations that are in line with the patient’s problem, identifying abnormal findings and incorporating relevant results

Clinical module 3.19: Respiratory health requires GPs to:

Know the diagnostic and treatment guidelines for common respiratory diseases (asthma, chronic obstructive pulmonary disease, lung cancer) in primary care

Know the boundaries of primary care management and the role of specialist services in supporting the patient

Pulse oximetry

Foundation years doctors and GP registrars will have observed pulse oximetry being used during the clinical assessment of acutely unwell patients within both the Emergency Department and adult and paediatric Emergency Assessment Units. Pulse oximetry is performed routinely by both ambulance personnel and hospital clinicians to measure and monitor the oxygen saturation (SpO₂) in acutely unwell patients, to the extent that the SpO₂ is considered to be the ‘fifth vital sign’, alongside the pulse and respiratory rates, blood pressure and temperature (Porter, 1990). It is also considered to be negligent not to measure the SpO₂ during the clinical assessment of acutely unwell patients, especially children (Department of Health, 2011).

In primary care, the routine physiological assessment of patients, even acutely unwell patients, does not always involve taking the vital signs, which was confirmed in a study of the frequency with which GPs measured the vital signs in feverish children, where pulse oximetry was used in only 20% of children to assess their respiratory status (Thompson et al., 2008). It is not within the scope of this article to discuss the relative risks of not performing a set of observations as part of a clinical assessment, both medically and medico-legally, other than to say that performing a set of observations in an acutely unwell patient is considered to be a fundamental requirement in both prehospital and hospital practice.

Pulse oximetry also features in many of the guidelines for the assessment of specific conditions, such as acute asthma, and in specific patient groups, such as feverish children (British Thoracic Society, 2014; National Institute for Health and Care Excellence (NICE), 2013). However, as with any other medical diagnostic device, it is important to understand not just the uses and limitations of pulse oximetry, but also how to interpret the readings that it provides within the context of the rest of the clinical assessment of the patient.

SpO₂

In order to understand what the reading represents, clinically, we need to consider how oxygen is transported from the air that surrounds us to the cells that exist within us. Oxygen needs to be presented to the haemoglobin molecules being transported within the circulatory system at the alveolar interface in the lungs, so that it can be uploaded onto the haemoglobin carrier molecules and then transported by the circulation to the cells within the body. Pulse oximetry measures the effectiveness of this uploading process by measuring the proportion of oxygen-saturated haemoglobin to deoxygenated haemoglobin in the arterial circulation. Thus, a fall in the SpO₂ reading indicates that there is some impairment of the oxygen-uploading process, without defining where the problem occurs and the nature of that problem. This is why pulse oximetry needs to be interpreted within the context of the overall clinical assessment of the patient. There are a number of medical conditions that can impair the oxygen-uploading process, either by reducing the amount of oxygen being delivered to the alveolar surface, where gas exchange takes place, or by preventing the haemoglobin molecules from reaching the alveolar circulation.

The causes of a low SpO₂ are listed in Box 1, and it is important to consider all of these possibilities when faced with an acutely unwell patient with a low SpO₂, as the management of the patient will differ with the cause. Conditions that may reduce the amount of oxygen reaching the alveoli include a reduction in the amount of oxygen present in the atmosphere, such as high altitude, or in carbon monoxide poisoning. (It is also important to note that pulse oximetry cannot distinguish between oxygenated haemoglobin and haemoglobin that is saturated with carbon monoxide, so the patient may be hypoxic with a SpO₂ reading that appears normal). Obstruction of the upper airway from an infection, foreign body, or with anaphylaxis, will reduce the amount of air being delivered to the lungs, bronchospasm and mucous plugging within the middle and lower airways will also produce the same result. Anything that reduces the respiratory rate, such as respiratory depression due to a head injury, or drugs, will also reduce the amount of air being inspired, as will any condition that causes a reduction in the respiratory depth, such as pain from rib fractures, or the inability to create negative intrathoracic pressure, such as a flail chest, or tension pneumothorax. Finally, any condition that causes a physical barrier to gas exchange within the alveoli, either on the respiratory side (secretions, pus, pulmonary oedema), or the circulatory side (a pulmonary embolus), will prevent oxygen uploading.

Box 1.

Causes of a low SpO_2.

• Reduced atmospheric oxygen (i.e. high altitude)

• Respiratory depression (i.e. head injury, drug overdose)

• Upper airway obstruction (i.e. epiglottitis, croup, inhaled foreign body)

• Lower airway obstruction (i.e. bronchospasm, lung collapse/consolidation)

• Restriction to chest/diaphragmatic expansion (i.e. pain from rib fractures, muscular and neurological conditions)

• Restriction to lung expansion (i.e. haemothorax, large pleural effusion)

• Inability to create negative intrathoracic pressure (i.e. flail chest, tension pneumothorax)

• Obstruction to oxygen transfer across the alveolar wall (i.e. pulmonary oedema, airway secretions, pus, pulmonary embolism)

However, it is also important to recognise the limitations of pulse oximetry in clinical practice. First, pulse oximetry does not measure how well a patient is ventilating, as this requires a measurement of the carbon dioxide level and it does not provide an assessment of the oxygen-downloading process at the cellular level, which may be affected by factors such as the temperature, pH and carbon dioxide level within the tissue. A blood gas will be required to assess the situation. The World Health Organisation (WHO) has produced two excellent short tutorials on the principles and use of pulse oximetry in patient assessment, which can be viewed online (WHO, 2011a, 2011b).

Pulse oximetry in primary care

Pulse oximetry has been used to assess and monitor acutely unwell patients in both prehospital and primary care for over 30 years, and many GPs now have access to a pulse oximeter in the surgery and out-of-hours setting (Silverston, 1989a, 1989b, 1991). A number of papers have been published describing its use in the assessment and management of patients with either known respiratory disease, or with the symptoms of an acute respiratory illness (Ingram & Munro, 2005). A number of disease-specific guidelines provide information on the thresholds that can be used to determine whether a patient needs oxygen therapy, or referral to hospital (Potter, 2007). In primary care, measuring the SpO₂ can be of value in a number of different clinical situations. In patients with respiratory conditions, such as asthma, or with respiratory symptoms such as breathlessness, pulse oximetry can be used to determine whether or not the patient is hypoxic and, in conjunction with the respiratory rate and heart rate, it can be used to assess how well the patient is compensating for their illness.

In paediatric assessment, both the NICE guideline for the assessment and management of the feverish child and the ‘Spotting the Sick Child’ educational programme emphasise the importance of performing pulse oximetry as part of the clinical assessment of a child and referring the child to hospital if the SpO₂ is below 92% (Department of Health, 2011; NICE, 2013). There are a number of reasons why pulse oximetry can be helpful in assessing the sick child in primary care. A grizzly or distressed-looking child may, in fact, have normal physiological parameters, whereas a quiet, well-looking child may have deranged physiology, indicating that their protective, physiological responses are being challenged, and they may be struggling to compensate. In addition, clinical examination of the chest in a child is less helpful than it is in adults, due to the transmission of upper airway sounds throughout the lung fields, the small volume of airway being moved and, often, because the child is crying. This means that information gathered from performing a set of observations has a much larger role in the clinical assessment process than in adults.

However, babies and children often develop tachycardia and tachypnoea in response to fever, which can make it difficult to determine whether the raised heart and respiratory rates are physiological, or pathological, especially when the infection involves the respiratory system, as in bronchiolitis, or in pneumonia. A low SpO₂ in these patients indicates the need for a very careful assessment of the chest, especially if the child is unwell and has other signs of increased respiratory effort. Illness is a dynamic process and being able to detect the desaturated, but well-looking, child may enable the GP to identify those patients who are most at risk of sudden deterioration later, particularly if the SpO₂ does not improve with appropriate treatment in the surgery.

Pulse oximetry in acute illness

Pulse oximetry can be of great value in the assessment and management of acutely unwell patients in primary care. First, many of the patients who present with acute illness in primary care do so with symptoms and signs that are non-specific, such as fever, or breathlessness. It is generally recognised that measuring the patient’s temperature can help to establish whether the underlying cause is infective, or non-infective in nature, hence the significance of the phrase ‘If you don’t take a temperature, you won’t find a fever’. The same applies to measuring the SpO₂, as a low SpO₂ can be helpful in identifying the patient with a condition that is causing the oxygen-uploading process to be compromised, such as pneumonia, or a pulmonary embolus (Gupta & Woodhead, 2010). This is why its use as the fifth vital sign can make it so helpful in the assessment of acutely unwell patients with non-specific symptoms. For example, in a baby or young child with pneumonia, who often present without a cough to guide us to the presence of a chest infection, a low SpO₂ may be the only indicator of this serious condition (Department of Health, 2011).

The following two cases highlight the benefits of taking this approach.

Case study 1.

Mr Jones is a 54-year-old man who has come to surgery with a ‘flu-like’ illness, present for 2 days, where his only symptoms are of ‘feeling hot and aching all over’. Symptom-based questioning provides no clues as to the focus of the infection. The only point of interest is that he reports that yesterday he returned from a cruise around the Mediterranean. He is normally fit and well and has no previous medical history of note. His observations show a pulse of 110 beats per minute, regular; blood pressure of 150/90 mmHg; respiratory rate of 24 breaths per minute; and a temperature of 38.6℃. His SpO₂ is 92%. Clinical examination of his chest is unremarkable. He denies having any respiratory symptoms, in terms of a cough, chest pain or dyspnoea. He is referred to the emergency assessment unit for blood tests, a chest X-ray, and urine and blood testing for Legionella (associated with cruise ship travel). Legionella is confirmed, along with grossly deranged liver function tests. Within hours of arriving in hospital, the patient becomes increasingly unwell from multi-organ failure, requiring transfer to the high dependency unit. With intravenous antibiotics and supportive care, the patient made a full recovery.

Case 1 demonstrates the role that pulse oximetry can play when assessing a patient with a raised temperature of unknown origin. If pulse oximetry had not been performed in this patient, the diagnosis of an atypical pneumonia would have been missed, and the deranged liver function tests might have led to an alternative diagnosis being made. Within the clinical reasoning process, the presence of a low SpO₂, in conjunction with the patient’s fever and history of cruise ship travel provided the clues as to the diagnosis of Legionnaire’s disease.

Case study 2.

Jenny is a 32-year-old woman with a 2-week history of increasing shortness of breath. Today, she was forced to return home from walking her dog a short distance, due to breathlessness. She denies having any other symptoms. On examination, Jenny is sitting comfortably and talking easily, without interruption to draw breath. At rest, her observations reveal a pulse rate of 100 beats per minute, regular; blood pressure of 130/80 mmHg; respiratory rate of 26 breaths per minute; and a normal temperature. Her SpO₂ is 94%. As her symptom is of shortness of breath on exertion, she is asked to walk around the room a few times and is then re-assessed. Her heart rate increases to 136 beats per minute, regular and with a good volume, while her respiratory rate increases to 38 breaths per minute. The SpO₂ falls to 86% and Jenny becomes quite breathless and uncomfortable, although her symptoms ease rapidly with rest and 100% oxygen. However, her SpO₂ remains stubbornly low at 95%, despite oxygen therapy. Jenny’s presentation is suggestive of a pulmonary embolus, with persistent unexplained breathlessness and a low SpO₂, exacerbated by exertion and not returning to normal in spite of 100% oxygen. Clinical examination of the chest and lower limbs was unremarkable and Jenny’s two-level Wells Score revealed no risk factors for a venous thromboembolism. Jenny was referred to the emergency assessment unit for a D-dimer and compuuterised tomography-pulmonary angiography, which established the diagnosis of multiple pulmonary emboli in both lung fields, although further investigation failed to establish an underlying cause for this problem.

Diagnosing a pulmonary embolism in primary care can often be difficult, due its varied and often non-specific presentations (Rossdale & Harvey, 2003). A low SpO₂ may be the only clue that the patient has suffered a pulmonary embolus (PE), which is present in around 60% of patients with this diagnosis. If the patient is complaining of breathlessness only on exertion, or if breathlessness is exacerbated by exertion, it is important to assess the patient both at rest and after exertion to uncover the ventilation/perfusion defect. The observations may be normal, or mildly deranged at rest, but will become more deranged with exertion and also tend not return to normal levels with the administration of 100% oxygen. An European study into deaths attributable to pulmonary embolism concluded that, overall, over 40% of patients who died from a PE were undiagnosed ante-mortem and that only 7% of the patients who died early were correctly diagnosed as having had a PE before death (European Society of Cardiologists, 2014). This study demonstrated that only 50% of patients with a PE presented with dyspnoea; 39% with pleuritic chest pain; 15% with substernal chest pain; and only 24% had clinical signs of a lower limb venous thromboembolism (Pollack et al., 2011). Patients with a large, or centrally-located, PE often present with non-specific symptoms and signs, such as unexplained syncope or hypotension, sweating, vomiting, central chest pain, persistent dyspnoea, or unexplained hypoxia (Raja et al., 2015). In addition, they may have no clinical evidence and few risk factors for a venous thromboembolism, so the presence of a low SpO₂ may be the only finding during a clinical assessment to indicate that this important diagnosis needs to be considered and excluded.

Pulse oximetry in practice

Pulse oximetry is not a perfect tool and it has both clinical and practical limitations to its use, as it relies upon there being good tissue perfusion at the point where the sensor is located. Measuring the SpO₂ in patients with peripheral shutdown due to shock, or with cold hands or feet may not be possible or the readings may not be accurate, as is also the case with excess patient movement, or with ambient light entering the sensor. There continues to be a debate as to whether nail varnish may affect the readings (Yamamoto, Yamamoto, Yamamoto, Yamamoto, & Yamamoto, 2008). Most pulse oximeters now have software to counter this, or to alert the user as to the poor quality of the signal being received. Nevertheless, the reading should always be considered within the context of the other physiological parameters, especially the heart and respiratory rates and the patient’s higher brain functions, behaviour and level of responsiveness (Pluddemann, Thompson, Heneghan, & Price, 2011).

The value of pulse oximetry as the fifth vital sign in the clinical assessment of both acutely unwell adults and children is that it is often the only means of detecting hypoxia until a significant degree of oxygen desaturation has taken place. As many of our patients present early in their illness with symptoms and signs that are non-specific, the ability to detect hypoxia can both guide us as to the underlying cause of their illness and also as to its severity. The presence of a low SpO₂ in early acute illness is a marker of the need to perform a comprehensive clinical assessment of the patient, to establish the cause of the oxygen-uploading deficit. A SpO₂ of below 92% in a person with previously normal oxygen uploading indicates the need to consider oxygen therapy, along with disease-specific treatment and admission to hospital. Pulse oximetry is non-invasive, simple to perform and pulse oximeters are now relatively inexpensive to purchase, although there is a need to have sensors that can be used in patients of all ages. Finger clip-type pulse oximeters are now available in both adult and paediatric sizes, and more sophisticated units have sensors that include finger probes of different sizes, wrap sensors and ear lobe sensors, along with reflectance probes that can be applied to flat tissue surfaces, such as the forehead.

As the fifth vital sign, pulse oximetry should be performed not only in patients with known respiratory conditions or respiratory symptoms, but also as a screening tool in all acutely unwell patients to identify those patients with an oxygen-uploading deficit and those patients who are struggling to compensate physiologically for their illness. In primary care, pulse oximetry can be used as both a diagnostic tool and as a screening tool to help us reduce the risk of misdiagnosis in a patient presenting with non-specific symptoms, as well as to assess the severity of an illness and the need to initiate treatment and immediate referral of the patient to hospital.

Key points

Pulse oximetry measures the proportion of oxygenated haemoglobin in the peripheral arterial circulation

A low SpO₂ indicates that either insufficient oxygen is available in the alveoli to upload onto the haemoglobin molecules in the pulmonary circulation, or that there is something preventing its uploading at the alveolar respiratory/circulatory interface

Pulse oximetry has its uses and limitations and it is important to consider the SpO₂ reading within the context of the patient’s overall clinical assessment

Pulse oximetry can be helpful in determining the severity of respiratory compromise in patients with known respiratory conditions and symptoms, however, it can also be used to detect conditions that prevent the uploading of oxygen onto haemoglobin in acutely unwell patients with atypical presentations of serious illnesses, such as an atypical pneumonia, or a silent pulmonary embolus

Pulse oximetry can be used to aid the clinical reasoning process during the assessment of seriously ill patients and to assess the need for immediate medical care, including the need for oxygen therapy, assisted ventilation and prompt transfer to hospital

References

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Department of Health. (2011). Spotting the sick child. Retrieved from www.spottingthesickchild.com.

European Society of Cardiologists. (2014) ESC guidelines on the diagnosis and management of acute pulmonary embolism. European Heart Journal. doi: 10.1093/eurheartj/ehu283 3033-3073.

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Woodhead

(2010) Oxygen saturation and pneumonia: A complement to current practice or another burden for the GP? Primary Care Respiratory Journal 19(4): 301–303. doi: 10.4104/pcrj.2010.00055.

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Pulse oximetry in primary care

Abstract

The GP curriculum and pulse oximetry

Pulse oximetry

SpO2

Pulse oximetry in primary care

Pulse oximetry in acute illness

Case study 1.

Case study 2.

Pulse oximetry in practice

Key points

References

SpO₂