Pulse oximetry is a quick, simple, noninvasive and reliable method of assessing arterial oxygen saturation (SaO2).
The importance of measuring SaO2 in primary care has been emphasised in the NICE guidelines on COPD and in the guidelines from the British Thoracic Society and SIGN on asthma and community acquired pneumonia. 1-3 SaO2 measurement should be used as an additional tool in conjunction with clinical assessment of patients, rather than in isolation.
Use in primary care
Although widely used in secondary care, the data on the use of pulse oximeters in general practice is limited.
The main indication of pulse oximetry is in the assessment of breathless patients, as it provides valuable information about the severity of the illness.
This helps when a decision is needed about hospital admission, and ensures that hypoxic patients are treated appropriately with oxygen. While the clinical sign of hypoxia is central cyanosis, studies have shown that without oximetry clinicians have difficulty in reliably detecting hypoxaemia until the saturation is <80%.4
SaO2 should be measured by pulse oximetry in all patients with worsening breathlessness; in conditions such as asthma, COPD and heart failure; and in acute respiratory infections such as pneumonia and influenza in both adults and children.
GPs should consider carrying a pulse oximeter in their medical bag. Breathing difficulty is a common reason for paediatric visits for GP consultation, home visits, out-of-hours and A&E attendance. Pulse oximetry may have a role in the acute assessment of children, but its added value to existing clinical assessment is still unclear.5
Pulse oximetry must be available in all locations where emergency oxygen is used. Guidance suggests that in primary care:
- SaO2 <90% in a COPD exacerbation requires hospital admission1
- SaO2 <92% in community acquired pneumonia requires hospital admission3
- SaO2 <92% in asthma requires admission.2
Pulse oximetry should be used if considering oxygen administration in acute stroke and MI, as high oxygen flow has been shown to produce vasospasm and can worsen the outcome. Oxygen should only be given if the patient is hypoxic and the level should be maintained at 94-98%.6
Patients with severe COPD with SaO2 <92% should be considered for long-term oxygen therapy (LTOT).1
Breathlessness in terminally ill patients is seldom due to inadequate oxygenation. Supplementary oxygen will not be of benefit if saturation is above 92%.7
Patients who may need oxygen regardless of oxygen saturation measurement include:
- Hypotensive patients whose systolic BP is less than 80mmHg
- Patients in respiratory or cardiac arrest
- Neonatal patients in distress
- Those with suspected sickle cell crisis
- Those with carbon monoxide poisoning.8
Using the device
The principle behind pulse oximetry lies in the red and infrared light absorption characteristics of oxygenated and deoxygenated haemoglobin. Oxygenated blood absorbs infrared light more and allows red light to pass through whereas deoxygenated haemoglobin absorbs more red light and allows more infrared light to pass through.
A pulse oximeter has a transmitter that transmits red and infrared light through the body part (usually finger, toe or earlobe) and a photo detector that detects the percentage of oxygenated versus deoxygenated haemoglobin through which the light passes.
The device measures the changing absorbance at each of the wavelengths, allowing it to determine the absorbance due to the pulsating arterial blood alone, excluding the venous blood. The percentage of oxygen saturation calculated is referred to as the percentage SpO2.6
|How to use the device|
The device is applied to a relatively translucent site of the body that has a good blood flow. Typical sites are fingers, toes, pinna or lobe of ear. In infants, the foot or palm of hands or the big toe or thumb can be used.
Interpretation of results
SpO2 >95% is generally considered to be normal. SpO2 <92% suggests hypoxaemia. Breathless patients with a saturation of ≤92% at room air are likely to be in respiratory failure. The explanation for this is given by oxygen-dissociation curve in the figure below.
The arterial oxygen tension (pO2), which is the amount of oxygen diffusing across the lungs to be made available to haemoglobin, can fall rapidly with disease. However, despite this drop, the oxygen saturation (the amount of oxygen made available to the tissues) stays at a reasonable level when O2 levels are 92% or over. At pO2 below 8kPa (equating to respiratory failure) oxygen saturation falls rapidly.
Patients with COPD can have a chronically low SaO2 and may have values of 80-90% but not be in exacerbation. It is not uncommon for longstanding COPD patients to have SaO2 levels between 88 and 92%. This so-called ‘well’ value must be documented in notes so that any further decline is recognised. Ensure that patients who have chronic hypoxia are aware of their normal oxygen level.
Purchasing a pulse oximeter
When purchasing a pulse oximeter for the surgery or yourself, it is sensible to make sure it is portable, accurate, simple-to-use, cost-effective and reliable. You may wish to contact neighbouring practices and see what they are using. You could also contact your CCG and see if they are able to provide them for the practice. A respiratory nurse specialist, community matron or a primary care COPD service delivery clinician can also advise you. The cost ranges from £30 to around £800. Ensure that the machine is calibrated regularly as per the instructions.
Pulse oximetry only measures haemoglobin saturation and not ventilation and therefore is not a complete measure of respiratory sufficiency. Arterial blood gas analysis is required for these purposes and this is not available in primary care.
Erroneous readings can be caused by:
- Poor perfusion due to hypotension, hypovolaemic shock, cold peripheries or cardiac failure, for example
- Anaemia, sickle cell disease
- Darker skin
- Carbon monoxide poisoning (haemoglobin has a higher affinity to carbon monoxide than oxygen and a high reading may occur despite the patient actually being hypoxaemic)
- Nail polish, dirt, artificial nails
- Bright artificial lights
- Motion such as shivering, seizures
- Hypotension, low cardiac output, vasoconstriction, vasoactive drugs (for example dopamine, dobutamine) that reduce tissue blood flow9
- Certain antiretroviral medications that affect oxygen’s affinity for haemoglobin10
Fitness to fly
Patients with respiratory problems planning to travel by air with a resting SaO2 of less than 92 % in room air should receive in-flight oxygen. If any of your COPD patients with SaO2 of less than 92% are planning air travel, it would be helpful to give them a letter containing this advice.
Those who are already receiving oxygen should have their flow rate increased, and those who have saturations of 92-95% should be assessed before a decision about in-flight oxygen is made.11 Patients should discuss their respiratory condition with their airline prior to travel.
- Oximetry aids the decision-making process with regard to hospital admission, so that hypoxic patients are diagnosed early and treated appropriately with oxygen.
- Breathless patients with a saturation of ≤92% are likely to be in respiratory failure
- Dr Anita Sharma is a GP in Oldham, Greater Manchester and GP member of NICE Quality Standards Advisory Committee.
- Dr Shalini Ghadiyar is a GP in Rochdale, Greater Manchester
- NICE. CG101. Guidance on Chronic obstructive pulmonary disease in over 16s: diagnosis and management. London, NICE. June 2010. Available from: https://www.nice.org.uk/guidance/cg101/chapter/1-guidance
- BTS/SIGN British Guideline on the Management of asthma. October 2014. (Update due July 2016). Available from: https://www.brit-thoracic.org.uk/guidelines-and-quality-standards/asthma-guideline
- BTS Guidelines for the Management of Community Acquired Pneumonia in Adults: 2009 Update. Available from:https://www.brit-thoracic.org.uk/guidelines-and-quality-standards/community-acquired-pneumonia-in-adults-guideline
- Hanning C, Alexander-Williams J. Pulse oximetry: a practical review. BMJ 1995; 311: 367-370.
- Plüddemann A, Thompson M, Heneghan C, Price C. Pulse oximetry in primary care: primary care diagnostic technology update. Br J Gen Pract 2011; 61: 358–359.
- Holmes S, Peffers SJ. Pulse oximetry in primary care. Opinion Sheet no 28. Primary Care Respiratory Society. Sept 2013.
- Guidance from the NHS Greater Glasgow and Clyde Respiratory Managed Clinical Network. Adult pulse oximetry in primary care. May 2014. Available from: http://www.nhsggc.org.uk/about-us/professional-support-sites/respiratory-managed-clinical-network
- DeMeulenaere S. Pulse Oximetry: Uses and Limitations. JNP 2007; 3: 312-17.
- Ibáñez J, Velasco J, Raurich J. The accuracy of the Biox 3700 pulse oximeter in patients receiving vasoactive therapy. Intensive Care Med 1991; 17: 484-6.
- Jubran A. Pulse oximetry. Intensive Care Med 2004; 30: 2017-20.
- Edvardsen A, Akerø A, Christensen C, Ryg M, Skjønsberg O. Air travel and chronic obstructive pulmonary disease: a new algorithm for pre-flight evaluation. Thorax 2012; 67: 964-9.