Practical applications of pulse oximetry

Dr E Hill,
Dr MD Stoneham,
Nuffield Department of Anaesthetics, Oxford Radcliffe NHS Hospitals Headington, Oxford OX3 9DU

http://www.nda.ox.ac.uk/wfsa/html/u11/u1104_01.htm

Introduction

Pulse oximetry is a useful method of monitoring patients in many circumstances,and in the face of limited resources, the pulse oximeter may represent a wise choice of monitor, as with training it allows for the assessment of several different patient parameters.

Pulse oximeters are now a standard part of perioperative monitoring which give the operator a non-invasive indication of the patient's cardio-respiratory status. Having been successfully used in intensive care, the recovery room and during anaesthesia, they have been introduced in other areas of medicine such as general wards apparently without staff undergoing adequate training in their use⁽¹⁾. The technique of pulse oximetry does have pitfalls and limitations and it is possible that patient safety may be compromised with untrained staff. This article is therefore intended for the 'occasional' user of pulse oximetry.

Pulse oximeters measure the arterial oxygen saturation of haemoglobin. The technology involved⁽²⁾ is complicated but there are two basic physical principles. First, the absorption of light at two different wavelengths by haemoglobin differs depending on the degree of oxygenation of haemoglobin. Second, the light signal following transmission through the tissues has a pulsatile component, resulting from the changing volume of arterial blood with each pulse beat. This can be distinguished by the microprocessor from the non-pulsatile component resulting from venous, capillary and tissue light absorption.

The function of a pulse oximeter is affected by many variables, including: ambient light; shivering; abnormal haemoglobins; pulse rate and rhythm; vasoconstriction and cardiac function. A pulse oximeter gives no indication of a patient's ventilation, only of their oxygenation, and thus can give a false sense of security if supplemental oxygen is being given. In addition, there may be a delay between the occurrence of a potentially hypoxic event such as respiratory obstruction and a pulse oximeter detecting low oxygen saturation. However, oximetry is a useful non-invasive monitor of a patient's cardio-respiratory system, which has undoubtedly improved patient safety in many circumstances.

What does a pulse oximeter measure?

1. The oxygen saturation of haemoglobin in arterial blood - which is a measure of the average amount of oxygen bound to each haemoglobin molecule. The percentage saturation is given as a digital readout together with an audible signal varying in pitch depending on the oxygen saturation.

2. The pulse rate - in beats per minute, averaged over 5 to 20 seconds.

A pulse oximeter gives no information on any of these other variables:

• The oxygen content of the blood

• The amount of oxygen dissolved in the blood

• The respiratory rate or tidal volume i.e. ventilation

• The cardiac output or blood pressure

Systolic blood pressure can be estimated by noting the pressure at which the plethysmograph trace reappears during deflation of a proximal non-invasive blood pressure cuff.

Principles of modern pulse oximetry

Oxygen is carried in the bloodstream mainly bound to haemoglobin. One molecule of haemoglobin can carry up to four molecules of oxygen, which is then 100% saturated with oxygen. The average percentage saturation of a population of haemoglobin molecules in a blood sample is the oxygen saturation of the blood. In addition, a very small quantity of oxygen is carried dissolved in the blood, which can become important if the haemoglobin levels are extremely low. The latter, however, is not measured by pulse oximetry.

The relationship between the arterial partial pressure of oxygen (PaO2) and the oxygen saturation is described by the haemoglobin-oxygen dissociation curve. The sigmoid shape of this curve facilitates unloading of oxygen in the peripheral tissues where the PaO2 is low and oxygen is required for respiration. The curve may be shifted to the left or right by various patient characteristics e.g. recent blood transfusion, pyrexia.

A pulse oximeter consists of a peripheral probe, together with a microprocessor unit, displaying a waveform, the oxygen saturation and the pulse rate. Most oximeters also have an audible pulse tone, the pitch of which is proportional to the oxygen saturation - useful when one cannot see the oximeter display. The probe is placed on a peripheral part of the body such as a digit, ear lobe or the nose. Within the probe are two light emitting diodes (LED's), one in the visible red spectrum (660nm) and the other in the infrared spectrum (940nm). The beams of light pass through the tissues to a photodetector. During passage through the tissues, some light is absorbed by blood and soft tissues depending on the concentration of haemoglobin. The amount of light absorption at each light frequency depends on the degree of oxygenation of haemoglobin within the tissues.

The microprocessor can select out the absorbance of the pulsatile fraction of blood, i.e. that due to arterial blood, from constant absorbance due to non-pulsatile venous or capillary blood and other tissue pigments. Several recent advances in microprocessor technology have reduced the effects of interference on pulse oximeter function. Time division multiplexing, whereby the LED's are cycled: red on, then infrared on, then both off, many times per second, helps to eliminate background 'noise'. Quadrature division multiplexing is a further advance in which the red and infrared signals are separated in phase rather than time and then recombined in phase later. In this way, an artefact due to motion or electromagnetic interference may be eliminated since it will not be in the same phase of the two LED signals once they are recombined.

Saturation values are averaged out over 5 to 20 seconds. The pulse rate is also calculated from the number of LED cycles between successive pulsatile signals and averaged out over a similar variable period of time, depending on the particular monitor.

From the proportions of light absorbed at each light frequency, the microprocessor calculates the ratio of the two. Within the oximeter memory is a series of oxygen saturation values obtained from experiments performed in which human volunteers were given increasingly hypoxic mixtures of gases to breath. The microprocessor compares the ratio of absorption at the two light wavelengths measured with these stored values, and then displays the oxygen saturation digitally as a percentage and audibly as a tone of varying pitch. As it is unethical to desaturate human volunteers below 70%, it is vital to appreciate that oxygen saturation values below 70% obtained by pulse oximetry are unreliable.

Reflection pulse oximetry uses reflected rather than transmitted light on a single-sided monitor. It can therefore be used more proximally anatomically e.g. forehead, bowel, although it may be difficult to secure. Other than using specific reflection spectra, the principles are the same as for transmission oximetry.

Practical tips to the successful use of pulse oximetry:

• Plug the pulse oximeter in to an electrical socket, if available, to recharge the batteries.

• Turn the pulse oximeter on and wait for it to go through its calibration and check tests.

• Select the probe you require with particular attention to correct sizing and where it is going to go. The digit should be clean (remove nail varnish).

• Position the probe on the chosen digit, avoiding excess force.

• Allow several seconds for the pulse oximeter to detect the pulse and calculate the oxygen saturation.

• Look for a displayed waveform. Without this, any reading is meaningless.

• Read off the displayed oxygen saturation and pulse rate.
  * Be cautious interpreting figures where there has been an instantaneous change in saturation - for example 99% falling suddenly to 85%. This is physiologically not possible.

• If in doubt, rely on your clinical judgement, rather than the value the machine gives.

Alarms

• If the Low Oxygen Saturation alarm sounds, check that the patient is conscious if that is appropriate. Check the airway and make sure the patient is breathing adequately. Lift the chin or apply other airway manoeuvres as appropriate. Give oxygen if necessary. Call for help.

• If the Pulse Not Detected alarm sounds, look for the displayed waveform on the pulse oximeter. Feel for a central pulse. If there is no pulse, call for help, start the procedures for Basic and Advanced Life Support. If there is a pulse, try repositioning the probe, or put the probe on a different digit.

• On most pulse oximeters, the alarm limits for oxygen saturation and pulse rate can be altered according to your needs. However, do not alter an alarm just to stop it sounding - it could be telling you something important!

References

1.Stoneham MD,Saville GM,Wilson IH.Knowledge about pulse oximetry among medical and nursing staff.Lancet 1994:334:1339-1342.

2.Moyle JTB.Pulse oximetry.Principles and Practice Series.Editors:Hahn CEW and Adams AP.BMJ Publishing,London,1994.

3.Davidson JAH,Hosie HE.Limitations of pulse oximetry:respiratory insufficiency -a failure of detection.BMJ 1993;307:372-373.

4.Hutton P,Clutton-Brock T.The benefits and pitfalls of pulseoximetry.BMJ 1993;307:457-458