Figure 20.1 Pulse oximetry
20
Pulse oximetry
Pulse oximetry is used in both the acute care environment and the community. It is routinely used as part of the assessment of the respiratory status of infants and children by measuring the oxygen saturation of the arterial blood flow through their extremities. The oxygen saturation level is expressed in SpO2. Advantages of using pulse oximetry are that it is non-invasive, can be used for continuous monitoring, intermittent monitoring and is accurate if the SaO2 is >70%.
The pulse oximeter
Pulse oximetry consists of a pulse oximeter monitor which is connected to the patient by a probe. The pulse oximeter display shows the oxygen saturation of the patient, a plethysmographic waveform that indicates the pulsatile nature of the blood flow through the patient’s extremities and the heart rate. The heart rate displayed is an average recorded over 5–20 seconds. There is also an audible signal that varies in pitch depending on the saturation level and heart rate level. The pulse oximeter displays a motion indicator that indicates the signal quality, thus identifying the accuracy of the saturation level and heart rate.
As 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, secondly, oxygenated and deoxygenated blood have different absorption spectra. The 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 deoxygenated haemoglobin during each cardiac cycle so the probe requires a constant supply of arterial blood. The absorption of light by the haemoglobin is dependent on the level of oxygenation. 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 plethysmographic waveform. A normal signal shows a sharp waveform with a clear dicrotic notch. Movement artefact and decreased perfusion will distort the waveform.
The pulse oximeter probe
The probe is available in two types: hinged clip-on type for the older child or adhesive single use only for the neonate, infant and young child. Some probes are weight specific.
The probe consists of two parts: light-emitting diodes and a photo detector. This needs to be place where a pulse can be detected. In the infant, this will be on the big toe or the lateral aspect of the foot. For the child, the adhesive probe may be sited on the finger or big toe over the nail-bed area or across the hand. With the older child, a finger clip probe may be used on the thumb or big toe. The orientation of the nail-bed is identified on the probe by the manufacturer. The probe position should be changed on a regular basis and only secured as indicated by the manufacturer’s instructions.
Indications for use and clinical application
Pulse oximetry is used for monitoring and as a screening tool in infants and children when the following are present:
- Potential for respiratory failure
- Respiratory illness
- Oxygen therapy is being received
- Haemodynamic instability
- Sedation or anaesthesia is required
- Complex surgical procedures have been undertaken
- In infants who are post surgery
- During the administration of continuous respiratory depressant medication (e.g. patient-controlled analgesia)
- During transportation of infants and children between departments or hospitals, who are at risk of respiratory compromise or who are already receiving oxygen therapy.
Limitations of pulse oximetry
Pulse oximetry has a number of limitations that the user needs to be aware of as these may lead to inaccurate readings:
- Inadequate positioning of the probe: excessive light entering straight through the photo detector may give a false high reading
- 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%. The presence of carboxyhaemoglobin which the two wavelengths of light cannot distinguish make pulse oximetery unreliable
- 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 oxygen binding to haemoglobin is inhibited
- Smoke inhalation and carbon monoxide poisoning. The pulse 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 level and give a false high pulse
- Use of intravenous dyes such as methylene blue can give false low readings so nurses need to know which dye has been used and its half-life.
- Presence of oedema will lead to inaccurate measurement of saturation level
- High bilirubin levels will affect the accuracy of readings
- Inaccurate readings will also occur in the presence of nail varnish and acrylic nails
- Dried blood and dirt on the skin can also effect accuracy of readings and need to be removed
- Bright overhead lighting and external light may cause overestimation of saturation level.
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