Pulse oximetry is one of the most commonly used modalities to assess and monitor the respiratory status of infants and children. It is non-invasive and may be used continuously or intermittently.

Pulse oximetry is a simple, non-invasive monitoring modality. It is used to measure the percentage of oxygen saturation (SpO₂) of haemoglobin in peripheral capillary blood. Pulse oximetry is used in the clinical setting in the hospital, the community and the home. It is one part of a patient assessment and should be used in conjunction with a complete respiratory assessment. It can be used for spot readings or for continual monitoring.

Pulse oximetry is based on two physical principles. First, the presence of a pulsatile signal generated by arterial blood which is reasonably independent of non-pulsatile arterial blood and, second, oxygenated and deoxygenated blood have different absorption spectra. Two light-emitting diodes emit red and infrared wavelengths through the tissues to a photo detector which work together. The detector measures the colour difference between the oxygenated and the deoxygenated haemoglobin during each cardiac cycle so the probe requires a constant supply of arterial blood. This information is then analysed in the calibration algorithm of the microprocessor of the pulse oximeter and the estimated arterial saturation level is displayed. This is displayed as a percentage and a waveform. A normal signal shows a sharp waveform with a clear dicrotic notch. Movement artefact and decreased perfusion will distort the waveform.

A measurement of 95–99% in room air denotes that the haemoglobin is adequately saturated with oxygen. However, pulse oximetry cannot detect anaemia so the nurse needs to be aware of the patient’s haemoglobin level, otherwise a false high reading will occur. Oximetry measures the percentage of haemoglobin that is saturated by oxygen, so if there is less haemoglobin available, then the saturated blood will have reduced oxygen-carrying capacity that is not reflected in the oximetry readings, putting the child at an increased risk of hypoxia.

Pulse oximetry should be used to monitor infants and children and as a screening tool when the following conditions are present:

• the potential for respiratory failure
  
• respiratory illness
  
• haemodynamic instability
  
• requiring sedation or anaesthesia
  
• receiving oxygen therapy
  
• have undergone complex surgical procedures of longer than 6 hours
  
• under 1 year of age and are post-surgery
  
• during the administration of continuous respiratory depressant medication, e.g. patient-controlled analgesia
  
• during the transportation of infants and children intra departmental or intra hospital, who are at risk of respiratory compromise or who are already receiving oxygen therapy.

Pulse oximetry has a number of limitations that the user needs to be aware of as these may lead to inaccurate readings. These include the following:

• When the child has low cardiac output, hypothermia or vasoconstriction, peripheral perfusion may be impaired, and as oximetry relies on detecting a pulse, it may be difficult for the sensor to detect a true signal.
  
• When the SpO₂ is < 70%, pulse oximetry is unreliable, due to the presence of carboxyhaemoglobin which the two wavelengths of light cannot distinguish.
  
• Elevated methehaemoglobin caused by either structural changes of iron in the haemoglobin or drug-induced as with local anaesthesia may lead to tissue hypoxia as the oxygen binding to the haemoglobin is inhibited.
  
• Smoke inhalation and carbon monoxide poisoning. The oximeter cannot distinguish between haemoglobin saturated with oxygen and that saturated with carbon monoxide.
  
• Motion artefact accounts for a significant number of errors and false alarms, thus shivering can cause problems with detecting saturation levels and give a false high pulse.
  
• The use of intravenous dyes such as methylene blue can give false low readings so nurses need to know which dye has been used and what the half-life of this is.
  
• The presence of oedema will lead to inaccurate measurement of the saturation level.
  
• Inaccurate reading will also occur in the presence of nail varnish and acrylic nails. Dried blood and dirt will also affect the accuracy of the readings and need to be removed.
  
• Inaccurate readings have been reported in people with dark skin and in pigmented patients. This has not been reported in jaundiced patients.
  
• Bright overhead lighting and external light may cause overestimation of the saturation level.

Various studies on the use of pulse oximetry as a monitoring tool for patients with sickle cell anaemia, who have acute vaso-occlusive disease, have reached different conclusions about the accuracy of the readings with up to 8% bias. Therefore, the nurse should state the child’s diagnosis when reporting saturation levels.