OXYGEN SATURATION
Pulse oximetry is a vital tool in patient assessment. Nurses must use the correct procedure and be aware of situations where accuracy of reading may be compromised. Detecting low oxygen levels in patient’s is important but not always easy; central cyanosis – when a patient’s lips, tongue and mucous membranes acquire a blue tinge – can be missed, even by skilled observers, until significant hypoxemia is present. Pulse oximetry can be undertaken to measure a patient’s oxygen levels and help identify earlier when action must be taken.
Pulse oximetry: pulse oximetry is a simple, non-invasive method of measuring oxygen levels and can be useful in a variety of clinical settings to continuously or intermittently monitor oxygenation. An oximeter is a device that emits red and infrared light, shone through a capillary bed (usually in a fingertip or earlobe) onto a sensor. Multiple measurements are made every second and the ratio of red to infrared light is calculated to determine the peripheral oxygen saturation (SpO2). Deoxygenated hemoglobin absorbs more red light and oxygenated hemoglobin absorbs more infrared light
- Pulse oximetry is a non-invasive procedure that is used to assess patients oxygen levels
- It should be available in all clinical settings where hypoxemia may occur
- Pulse oximetry enables early identification of hypoxia
- It requires good pulsatile blood flow
- Nurses should record whether the patient is breathing room air or oxygen when the reading is taken, along with other factors that may affect accuracy.
Oxygen Saturation Measurement
Pulse oximetry should be available for use in all clinical settings where hypoxemia may occur and is used to:
- Assess breathless patients or those who are acutely ill, including those who have acute confusion
- Provide an objective indication of the severity of an acute respiratory episode and need for hospital admission – for example, exacerbation of chronic obstructive pulmonary disease, asthma
- Determine the need for emergency oxygen therapy in acute illness
- Provide a continuous oxygen saturation recording, for example, during anesthesia or sedation, or in the assessment of oxygenation during sleep studies
- Undertake routine monitoring in chronic respiratory disease to screen for suitability for assessment for domiciliary oxygen therapy
Pulse oximetry does not give a measure of arterial blood oxygen content or ventilation; oxygen delivery to the tissues is dependent on adequate ventilation and circulation. However, oximetry can add to the clinical picture to aid diagnostic and treatment decisions
Procedure
- Ensure oximeter is in good condition and probe sensor is cleaned according to local infection control policy/manufacturer’s guidance
- Explain procedure to the patient and gain consent where possible
- Select the most appropriate probe for the site chosen; in adult patients, the most common sites are the fingertip and earlobe. Using the incorrect probe will lead to inaccuracies in the readings obtained
- Consider choice of device:
Fingertip devices with integrated sensor and display may be appropriate for spot checks of SpO2
Handheld devices with detachable sensor allow the most appropriate probe to be selected
Wrist-worn devices have a sensor attached by a short cable and are useful for overnight oximetry and exercise testing
- Desktop/bedside devices may be more appropriate in the acute setting for continuous monitoring
- Ensure the chosen site is warm and well perfused
- Apply the probe to the site, ensuring the sensor is correctly positioned
- Ask the patient to rest the hand with the sensor on it down gently to reduce interference of motion
- Check the pulse strength signal and ensure the pulse-rate reading correlates with the manual pulse
- Allow the pulse oximetry to remain in situ for at least five minutes to ensure it equilibrates
- Document the reading, noting whether the patient is breathing room air or oxygen, then record the oxygen delivery device used and flow rate or percentage. Note any other factors that may influence accuracy, such as movement, cold hands, etc
- In acute illness, resting saturations are usually most useful, but in non-acute settings, oximetry may be used during exercise testing to determine exertional desaturation. Record whether the readings have been taken at rest or during/after activity in addition to inspired oxygen/air
Competencies
A range of competencies relate to the safe undertaking of pulse oximetry:
- Be aware of, and understand, local infection control policy/guidelines in relation to monitoring equipment
- Demonstrate a basic understanding of how oxygen saturations are derived
- Be able to discuss the indications for, and limitations of, pulse oximetry
- Demonstrate an ability to use a pulse oximeter safely and effectively, selecting the appropriate probe and device for the clinical situation
- Demonstrate accurate documentation of results
Limitations
Pulse oximetry requires a good pulsatile blood flow and no interference with measurement of light absorption and detection. Pulse strength can be checked by ensuring the recorded heart rate correlates with a manual pulse rate; some devices have a pulse amplitude indicator in addition to pulse detector. Where a good signal is obtained, pulse oximetry readings are accurate within saturation range of 70-100% but cannot be relied on outside of this range
Common Causes of Inaccuracy
- Poor peripheral circulation
- Cold peripheries
- Constriction, e.g. from blood pressure cuff, tight clothing or tight oximeter probe
- Poor perfusion due to hypovolemia, marked hypotension or cardiac arrhythmias, peripheral vascular disease
- Raynaud’s syndrome
- Motion artefact
- Gross movement may cause loss of signal
- Fine vibration may interfere with accuracy
- Carbon monoxide/smoke inhalation/intravenous dyes (e.g. methylene blue) used in diagnostic tests
- Carboxyhemoglobin (from carbon monoxide) is detected as oxyhemoglobin and will overestimate true oxygen saturation
- Ambient light interference
- Light emitters and detectors must be directly opposite each other and light should only reach the detector via tissues. Inappropriately sized probes or excessive ambient light may result in inaccuracies
- Interference with transmission/detection of light signals
- Dirty probe sensors
- Nail varnish/synthetic nails
- Anemia/skin discoloration (very dark skin/jaundice) may affect readings, but is rarely clinically significant
Documentation
- The patient’s oxygen saturation
- The site where you measured oxygen saturation
- Any signs or symptoms of abnormal oxygen saturation
- Type of oxygen therapy (nasal cannula, mask) and flow rate
- Oxygen saturation after a specific treatment (nebulizer therapy)
- Your nursing interventions
- The patient’s response to care
