Pulse oximetry in general practice: How would a pulse oximeter influence patient management?

Abstract

The pulse oximeter is a vital piece of equipment in secondary care for the non-invasive monitoring of oxygen saturation. With the increasing affordability of the oximeter and recognition of its clinical applications, there is an increasing interest in its role in primary care. The decision was made that a systematic review was not feasible due to the lack of data concerning the influence of pulse oximetry on patient management and on the extent of oximetry use in the general practice setting. In this article, a selection of studies is presented looking into its clinical use and limitations. The role and potential of the oximeter as a screening tool and aid in the assessment of hypoxia in a variety of clinical situations in primary care is discussed.

• Pulse oximetry
• general practice
• oxygen saturation
• hypoxia

Introduction to the pulse oximeter

The pulse oximeter has become a well-used tool in modern practice of emergency medicine and is being increasingly used in general practice. It monitors the percentage of haemoglobin which is saturated with oxygen (SaO₂). Pulse oximeters work on the principle of spectral analysis, that is, the detection and quantification of components by their unique light absorption characteristics [1]. Oxygenated and reduced haemoglobin have different absorption spectra, with arterial blood appearing red and venous, blue. Together in solution, their relative ratios are determined from the ratio of the light absorbed at two different wavelengths [2]. The pulse oximeter possesses two light-emitting diodes which each emit light of a specific wavelength, one red and one infrared, through a cutaneous vascular bed, and a photodiode at the other side derives the oxygen saturation from the intensity of transmitted light at each wavelength [3]. Absorption is measured at one point of the pulse wave and compared with absorption at another, so that the difference between the values is due to arterial blood alone, hence eliminating the effect of absorption by other tissue and blood components [2]. The pulse oximeter consists of a probe which is attached to the patient's finger or ear lobe, and a computerized unit which displays the percentage oxygen saturation of haemoglobin [1].

Uses and advantages of pulse oximetry

A pulse oximeter is useful whenever hypoxaemia may be present. Indeed, there is an argument for its use as a routine vital sign, as hypoxaemia may well be a contributing factor in a clinical presentation. As such, it is routinely used in the emergency department. Its application is seen as essential in anaesthesiology and critical care, as it provides continuous monitoring of patients’ oxygenation; and it is increasingly used during endoscopy and other diagnostic and interventional procedures. It is also used in the detection of anaemia, pulmonary function testing, periodic checks on medical and surgical wards, and in delivery suites [2], [4].

The gold standard for the measurement of arterial oxygen saturation has been, and remains, arterial blood gas (ABG) analysis. However, it is invasive, painful, time consuming, costly, provides only intermittent information on patient status, and there is a delay between sampling and results [4]. It can also give an inaccurate result due to patients’ oxygenation varying with their pattern of breathing, this being affected by the discomfort of the procedure [2]. Pulse oximetry provides an alternative when the major concern is oxygenation (ABG would be necessary when arterial CO₂ or acid–base balance is required). It has the advantages of being non-invasive, simple, and providing continuous monitoring. It is less prone to user error and avoids the aforementioned change in breathing pattern [2].

Limitations of pulse oximetry

Oximetry does have limitations of which the clinician must be aware. The full picture is not provided, only SaO₂; it does not replace clinical judgement, and an ABG may be unavoidable. Arterial CO₂ can rise to dangerous levels with still acceptable oxygen saturations.

Technological limitations include the fact that the accuracy of SaO₂ is dependent on the empirical calibration programmed into the oximeter, and the devices are calibrated by observations on healthy volunteers [4]. They are therefore most accurate at saturations of 70–99%, although this is of limited significance, as saturations lower than this require urgent intervention [2].

A number of factors can affect the accuracy of oximetry (Table I) because of the way in which the oximeter works. Dyshaemoglobins such as methaemoglobin and carboxyhaemoglobin absorb light at similar wavelengths to haemoglobin, thus affecting the SaO₂ result [4]. Other sources of error lead to signal losses or under-/overestimations of oxygen saturation.

Table I.  Sources of error in pulse oximetry.

Methaemoglobin

Carboxyhaemoglobin

Intravenous dyes

Ambient light

Reduced perfusion

Motion

Venous pulsation

Hypoxaemia

Anaemia

Skin pigmentation

Nail vanish

Uses of pulse oximetry in general practice

Impact on patient management

Data are scare concerning the influence of pulse oximetry on patient management in the general practice setting. A study into the use of oximeters in an emergency department triage found that physicians were significantly more likely to change the medical treatment of patients with arterial oxygen saturations of <95% compared with patients with saturations of ≥95%, after being made aware of triage oximetry measurements. This study found that those patients with pneumonia, asthma, COPD, chronic bronchitis, emphysema, congestive heart failure, and pulmonary oedema were the most likely to have changes in their management following the oximetry results, and also suggested that the oximetry results altered management because the physicians failed to recognize underlying cardiopulmonary difficulties or because they did not realize the severity of the patient's illness [5]. The fact that pulse oximetry results led to changes in medical treatment of these patients suggests a need for research into whether the use of pulse oximetry in general practice could do the same.

In the following sections, the possible roles of pulse oximetry in the management of a number of conditions in general practice are discussed.

Recognition of hypoxia

There is not much available data on the use of pulse oximeters in general practice and their role in this setting for detecting hypoxia. However, its role in the emergency department has been widely studied. In one study looking into its role, this instrument measured the arterial oxygen saturation in 50 patients in A&E; the oximeter identified 21 patients (42%) with clinically unsuspected hypoxia [6]. The usefulness of the oximeter in this setting may also be applicable to general practice in detecting hypoxia. In clinical examination, the traditional sign of hypoxia is central cyanosis. However, the ability to detect this depends on factors such as the eyesight of the doctor, his or her experience, lighting, etc., and studies have shown that observers have difficulty detecting hypoxaemia until saturation is <80% [2]. This was supported by data from another study, which suggested that, in some instances, clinicians have problems in detecting those patients with mild to moderate degrees of hypoxia [7]. Pulse oximeters in general practice may thus have a role in recognizing hypoxia which otherwise may go undetected, leading therefore to a difference in management.

Also, data from a study indicated that respiratory rate, one of the standard vital signs used routinely to screen patients for cardiopulmonary disease and gas exchange abnormalities, does not screen reliably for oxygen desaturation and is not closely correlated with pulse oximetry measurements. This study suggested that patients with a low oxygen saturation (SaO₂) do not usually show an increased respiratory rate and increased respiratory rate is unlikely to reflect desaturations [8]. As respiratory rate alone is not a reliable screen for cardiopulmonary disease, this suggests a role for pulse oximetry in screening for these problems.

A study into the use of oximetry to screen for hypoxaemia concluded that arterial blood gas testing to detect hypoxaemia is unnecessary if oximetry values are sufficiently high [9]. Oximetry confers a number of advantages over arterial puncture where assessment of oxygenation is the aim, as discussed earlier.

Recognition of COPD

It is necessary to improve the early recognition of COPD in order to start treatment [10]. Arterial hypoxaemia in COPD occurs relatively early in the disease, and therefore measurement of arterial oxygen saturation may be a useful aid in identification. The use of spirometry has been recommended for early identification but has the problems of requiring experienced operators. One study sought to evaluate whether the measurement of arterial oxygen saturation by pulse oximetry can select patients who are suitable for screening spirometry. It concluded that although arterial oxygen saturation levels correlated with FEV₁, pulse oximetry was not a useful test for the selection of patients for screening spirometry and should be based on other findings [11].

As regards severe COPD, these patients are frequently undiagnosed, and a possible use of pulse oximetry may be in their detection. However, Garcia-Pachon [11] found that 23% of the patients in this study with significant COPD (FEV₁ <50% predicted) went undetected when using an arterial oxygen saturation of 98%, and that when pulse oximetry was used for the detection of patients with FEV₁ <80% predicted, results were also poor. These findings suggests that pulse oximetry alone in general practice is not sufficient for the diagnosis of COPD, although it may well still be a useful tool to provide evidence of failing lungs.

Assessment for long-term oxygen therapy in patients with COPD

Epidemiological data have suggested that long-term oxygen therapy (LTOT) in patients with severe COPD is under prescribed by GPs [12]. LTOT at home for more than 15 hours a day has been shown to increase life expectancy in COPD patients with severe hypoxia [13]. There exists strict prescribing criteria for LTOT, and recommendation for it is usually by a hospital physician; however, this is dependent upon primary care referral. A study aimed to evaluate pulse oximetry in general practice and to screen for undetected hypoxaemia fulfilling the LTOT prescription criteria in COPD patients. Its results indicated that COPD patients with unsuspected severe hypoxaemia may be prevalent in primary care and that screening by pulse oximetry is one method of detecting patients that may benefit from LTOT. Of 114 patients screened in two practices, three received new prescriptions for oxygen concentrators. This was achieved by referral to hospital for formal arterial blood gas analysis of patients with saturations ≤92%. It concluded that oximetry provides a readily usable, non-invasive method of screening and, when applied to all COPD patients seen in general practice, it can reveal those who fulfil the criteria for LTOT [12]. In this way, the use of pulse oximeters in general practice could have a valuable role in detecting COPD patients who should be referred with a view to starting oxygen therapy.

A second study found a sensitivity of 100% and a specificity of 69% for oximetry in the detection of PaO₂ <7.3 kPa, and a 100% sensitivity and 86% specificity for detecting a PaO₂ <8kPa. This study concluded that oximetry used alone is unsuitable for prescription of LTOT because of its poor specificity in the crucial PaO₂ range. However, as suggested above, it may have a valuable role in the selection of patients who need definitive arterial blood gas analysis [14], thus influencing GP management of the patient by referring him or her to hospital.

It is also worth noting that in this study all principles found the oximeter easy to operate and acceptable with respect to time constraints when the equipment is in the consulting room. Patients also found the measurement acceptable [12].

Acute exacerbations of COPD

An exacerbation is defined as a “sustained worsening of the patient's symptoms from their usual stable state that is beyond normal day-to-day variations and is acute in onset” [15]. One study sought to determine the correlation and agreement between arterial oxygen saturation and oxygen saturation measured by pulse oximetry in order to establish where oximetry could be used instead of arterial values in clinical management of patients with exacerbations of COPD. The authors concluded that there was not sufficient agreement and therefore that oximetry would not be suitable to replace arterial blood gas analysis in oxygenation evaluation of COPD patients presenting to the emergency department. They did, however, find that it may be an effective screening tool for systemic hypoxia in this group of patients. Their results suggested a best screening cut-off of 92% (100% sensitivity and 86% specificity), so that patients with SaO₂ below this would have arterial blood gases performed, whilst those above this level could avoid it [16]. In general practice, there may therefore be a role of oximetry in assessing COPD patients with exacerbations to measure their SaO₂ to see if it is above or below this 92% value.

The NICE guideline has emphasized the need to attempt to reduce the severity and frequency of acute exacerbations in COPD, and the use of pulse oximetry has been encouraged in primary care to help in the assessment of exacerbation severity. If values from the pulse oximetry aid the assessment that the patient is experiencing an exacerbation, primary care management can then be started: increase in bronchodilators, antibiotics if purulent sputum, and prednisolone [15].

Community-acquired pneumonia

Another study, in the United States, looked into arterial blood gas and pulse oximetry in initial management of patients with community-acquired pneumonia (CAP). It was found that outpatients who had either test performed were more likely to be admitted to hospital than patients who had neither performed. Even in the absence of risk factors, 10% of the patients tested where found to be hypoxic. This study suggested that the routine use of pulse oximetry in patients suspected of having CAP would detect clinically unrecognized hypoxaemia, particularly in the outpatient setting, and that for most CAP patients, pulse oximetry is the most suitable test, being less invasive, less costly, and the most readily available [17].

Respiratory illness in children

A study sought to determine if pulse oximetry alone or in conjunction with clinical examination is predictive of pneumonia in infants, thereby removing the need for a chest X-ray (CXR). They concluded that pulse oximetry in the emergency department setting was not a useful method of excluding pneumonia in infants and should not influence the decision as to whether to obtain a CXR or not. Pulse oximetry was not found to be a statistically significant predictive variable for radiographic pneumonia [18]. In general practice, therefore, in children less than 2 years of age presenting with respiratory problems, the use of oximetry would seem not to be useful in ruling out pneumonia, and the measurement would not have a role influencing management of the patient.

However, also in the paediatric emergency department, pulse oximetry was evaluated and the findings were that SaO₂ measurements changed the previously assessed degree of illness in 53% of the patients in the study; more specifically, 13% were deemed more ill and 37% less ill than at the initial assessment. With respect to management of these patients, 17% had their management plan changed on account of their SaO₂ readings; 8% were treated more aggressively and 11% less aggressively [19]. This study would seem to suggest that pulse oximetry provides useful information which does have an important impact on both assessment and management of patients.

Another study looked at the use of pulse oximetry as a routine fifth vital sign in acute paediatric assessment. They found that the use of oximetry did result in important changes in the treatment of a small proportion of paediatric patients. Those patients with pulmonary disease such as pneumonia, asthma, viral respiratory tract infections, and bronchitis were the most likely to have abnormal pulse oximetry readings and to have their medical treatment altered after values became known and the proportion of treatment changes showed a peak at 89% saturation [7]. Although highlighting the usefulness of oximetry in acute settings, the findings demonstrate how oximetry can alter management and its potential for use in general practice playing a supporting role alongside history and examination.

Future uses of pulse oximetry in primary care

Altered vascular responses to thermal stimuli correlate with changes of autonomic neuropathy in diabetic patients. It has been suggested that pulse oximetry could be useful in its assessment, and a study designed to investigate this found abnormal vascular responses demonstrated by abnormal change in oxygen saturation as measured by oximetry, on exposure and removal of cold stimuli in diabetic patients with autonomic neuropathy [20]. Also, in patients with diabetes mellitus (DM), lower-extremity arterial disease (LEAD) is common and under diagnosed. Pulse oximetry of the toes was found to be as accurate as the ankle-brachial index to screen for LEAD in patients with DM, and a combination of the two tests was found to increase sensitivity [21].

Conclusion

The pulse oximeter was introduced in the early 1980s and is a non-invasive method of monitoring the SaO₂ of patients’ blood, and is able to detect hypoxia before the patient becomes clinically cyanosed [1]. It is used in a variety of situations in hospital settings and, as the cost of oximeters has fallen in recent years, there has been increasing interest in its role in primary care.

Oximetry in primary care has the potential to help in the assessment of hypoxia and, in some instances, identify unsuspected hypoxia. The values obtained from pulse oximetry in studies conducted in emergency departments did influence patient management [5], [7], [19]. The question is then raised as to whether it could also do the same in less acute presentations seen in general practice. This report suggests that although there are clinical situations in which oximetry may not be of use, it does have numerous indications in this setting and could provide valuable information for the practitioner (Table II).

Table II.  Clinical situations for oximetry use in general practice.

Situations when useful

Assessment for LTOT in patients with COPD

Exacerbations of COPD

Suspected CAP

Acute severe asthma in adults, or any other acute respiratory condition

Respiratory illness in children

Situations when not useful

Selection of patients for spirometry for identification of COPD

Detection of patients with severe COPD

Detection of hypoxaemia in patients younger than 2 years

It is clear that more studies are needed into the use of pulse oximetry in general practice and what impact they could have on patient management. Although it is important for the clinician to bear in mind its limitations, the pulse oximeter can provide useful additional information whilst being easy to use, acceptable within the time constraints of a busy practice, and acceptable to patients.

Acknowledgements

The author would like to thank Dr Paul Thornton.