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.