What does a pulse oximeter measure and how does it work?

A pulse oximeter consists of the monitor containing the batteries and display, and the probe that senses the pulse.

The monitor contains the microprocessor and display. The display shows the oxygen saturation, the pulse rate and the waveform detected by the sensor. The monitor is connected to the patient via the probe.

What does a pulse oximeter measure?

What are the 2 readings on a pulse oximeter?

There are two numerical values obtained from the pulse oximeter monitor:

• The oxygen saturation of hemoglobin in arterial blood. The value of the oxygen saturation is given together with an audible signal that varies in pitch depending on the oxygen saturation.
  A falling pitch indicates falling oxygen saturation.
  Since the oximeter detects the saturation peripherally on a finger, toe, or ear, the result is recorded as the peripheral oxygen saturation, described as SpO2.
• The pulse rate in beats per minute averaged over 5 to 20 seconds.
  Some oximeters display a pulse waveform or indicator that illustrates the strength of the pulse is detected.
  This display indicates how well the tissues are perfused. The signal strength falls if the circulation becomes inadequate.

How does it work pulse oximeter?

During use, the monitor updates its calculations regularly to give an immediate reading of oxygen saturation and pulse rate.

The pulse indicator is continuously displayed to give information about the circulation.

The audible beep changes pitch with the value of oxygen saturation and is an important safety feature. The pitch drops as the saturation falls and rises as it recovers.

This allows you to hear changes in the oxygen saturation immediately, without having to look at the monitor all the time.

The monitor is delicate. It is sensitive to rough handling and excessive heat and can be damaged by spilling fluids on it.

The monitor can be cleaned by gently wiping with a damp cloth. When not in use, it should be connected to an electrical supply to ensure that the battery is fully charged.

Pulse oximeter probe types

The oximeter probe consists of two parts, the light emitting diodes (LEDs) and a light detector (called a photo-detector).

Beams of light are shone through the tissues from one side of the probe to the other. The blood and tissues absorb some of the light emitted by the probe.

The light absorbed by the blood varies with the oxygen saturation of haemoglobin. The photo-detector detects the light transmitted as the blood pulses through the tissues and the microprocessor calculates a value for the oxygen saturation (SpO2).

In order for the pulse oximeter to function, the probe must be placed where a pulse can be detected.

The LEDs must face the light detector in order to detect the light as it passes through the tissues.

The probe emits a red light when the machine is switched on; check that you can see this light to make sure the probe is working properly.

Probes are designed for use on the finger, toe or earlobe. They are of different types. Hinged probes are the most popular, but are easily damaged. Rubber probes are the most robust. The wraparound design may constrict the blood flow through the finger if put on too tightly.

Types of pulse oximeter

Ear probes are lightweight and are useful in children or if the patient is very vasoconstricted. Small probes have been designed for children but an adult hinged probe may be used on the thumb or big toe of a child. For finger or toe probes, the manufacturer marks the correct orientation of the nail bed on the probe.

The oximeter probe is the most delicate part of a pulse oximeter and is easily damaged. Handle the probe carefully and never leave it in a place where it could be dropped on the floor.

The probe connects to the oximeter using a connector with a series of very fine pins that can be easily damaged-see diagram. Always align the connector correctly before attempting to insert it into the monitor.

Never pull the probe from the machine by pulling on the cable; always grasp the connector firmly between finger and thumb.

When not in use, the oximeter probe cable may be loosely coiled for storage or carrying, but should not be coiled too tightly as this will damage the wires inside the cable. The lens and detector should be kept clean. Use soapy water or an alcohol soaked swab to gently clean dust, dirt or blood from the probe.

How to use pulse oximeter correctly?

• Turn the pulse oximeter on it will go through internal calibration and checks.
• Select the appropriate probe with particular attention to correct sizing and where it will go (usually a finger, toe, or ear). If used on a finger or toe, make sure the area is clean. Remove any nail varnish.
• Connect the probe to the pulse oximeter.
• Position the probe carefully; make sure it fits easily without being too loose or too tight.
• If possible, avoid the arm being used for blood pressure monitoring as cuff inflation will interrupt the pulse oximeter signal.
• Allow several seconds for the pulse oximeter to detect the pulse and calculate the oxygen saturation.
• Look for the displayed pulse indicator that shows that the machine has detected a pulse. Without a pulse signal, any readings are meaningless.
• Once the unit has detected a good pulse, the oxygen saturation and pulse rate will be displayed.
• Like all machines, oximeters may occasionally give a false reading-if in doubt, rely on your clinical judgement, rather than the machine.
• The function of the oximeter probe can be checked by placing it on your own finger.
• Adjust the volume of the audible pulse beep to a comfortable level for your theatre-never use on silent.

A medical use pulse oximeter

• Always make sure the alarms are on.

If no signal is obtained on the oximeter after the probe has been placed on a finger, check the following:

• Is the probe working and correctly positioned? Try another location.
• Does the patient has poor perfusion?
  1. o Check for low cardiac output especially due to hypovolemia, cardiac problems or septic shock. If hypotension is present, the resuscitation of the patient is required immediately. The signal will improve when the clinical condition of the patient improves.
  2. o Check the temperature of the patient. If the patient or the limb is cold, gentle rubbing of the digit or earlobe may restore a signal.

Tip: If you are uncertain that the probe is working properly, check it by testing it on your own finger.

What do the alarms on a pulse oximeter mean?

What do the alarms on a pulse oximeter tell you?

• Alarms alert the anaesthetist to clinical problems. The alarms are as follows:
  • Low saturation emergency (hypoxia) i.e. SpO2 <90
  • No pulse detected
  • Low pulse rate
  • High pulse rate

Low saturation alarm. The oxygen saturation in healthy patients of any age should be 95% or above. During anaesthesia, the SpO2 should be 95% or above. If SpO2 is 94% or below, the patient must be assessed quickly to identify and treat the cause.

Pulse rate alarms

Pulse rate alarms are useful to let the anaesthetist know that the heart is beating too fast or too slow.

However, alert anaesthetists will have already noticed the abnormal heart rate before the alarms sound.

Children normally have higher heart rates than adults, but the same oxygen saturation-see table below.

Age	Normal Heart Rate	Normal oxygen saturation (SpO2)
Newborn – 2 years	100 – 180	SpO2 of 95% or above
2-10 years	60 – 140	SpO2 of 95% or above
10 years -adult	50 – 100	SpO2 of 95% or above

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