Pulse oximetry

Pulse oximetry is a simple, relatively cheap and non-invasive technique to monitor oxygenation. It monitors the percentage of haemoglobin that is oxygen-saturated. Oxygen saturation should always be above 95%, although in those with long-standing respiratory disease or cyanotic congenital heart disease, it may be lower, corresponding to disease severity. The oxyhaemoglobin dissociation curve becomes sharply steep below about 90%, reflecting the more rapid desaturation that occurs with diminishing oxygen partial pressure (PaO₂)¹. On most machines the default low oxygen saturation alarm setting is 90%.

Pulse oximetry does not provide information on the oxygen content of the blood or on ventilation. Thus care is needed in the presence of anaemia and in patients developing respiratory failure due to carbon dioxide retention, for example.

Principles of pulse oximetry

Oximeters work by the principles of spectrophotometry: the relative absorption of red (absorbed by deoxygenated blood) and infrared (absorbed by oxygenated blood) light of the systolic component of the absorption waveform correlates to arterial blood oxygen saturations. Measurements of relative light absorption are made multiple times every second and these are processed by the machine to give a new reading every 0.5-1 second that averages out the readings over the last three seconds.

Two light-emitting diodes, red and infrared, are positioned so that they are opposite their respective detectors through 5-10 mm of tissue. Probes are usually positioned on the fingertip, although earlobes and forehead are sometimes used as alternatives. One study has suggested that the ear lobe is not a reliable site to measure oxygen saturations². However, a more recent study advocated their use in patients admitted to intensive care units for coronary artery bypass surgery³. Probes tend to use 'wrap' or 'clip' style sensors.

Uses

Central cyanosis, the traditional clinical sign of hypoxaemia, is an insensitive marker occurring only at 75-80% saturation. Consequently, pulse oximetry has a wide range of applications including:

• Individual pulse oximetry readings - can be invaluable in clinical situations where hypoxaemia may be a factor - for example, in a confused elderly person.

• Continuous recording - can be used during anaesthesia or sedation, or to assess hypoxaemia during sleep studies to diagnose obstructive sleep apnoea. Peri-operative monitoring has not, however, been shown to improve surgical outcomes⁴.

• Pulse oximetry can replace blood gas analysis in many clinical situations unless PaCO₂ or acid-base state is needed. It is cheaper, easier to perform, less painful and can be more accurate where the patient is conscious (hyperventilation at the prospect of pain raises PaO₂).

• Pulse oximetry allows accurate use of O₂ and avoids wastage. For example, in patients with respiratory failure, rather than limit the use of O₂ to maintain hypoxic ventilatory drive, it can be adjusted to a saturation of ~90% which is clinically acceptable.

• Neonatal care - the safety limits for oxygen saturations are higher and narrower (95-97%) compared to those for adults⁵. Pulse oximetry is not yet a standard of care in the screening of neonates for asymptomatic congenital heart disease but may become so. A UK pilot concluded that many babies found to have a low oximetry reading were either normal or had a non-cardiac cause for their low oxygen level. Further evaluation is required⁶.

• Intrapartum fetal monitoring - the use of fetal pulse oximetry in combination with routine cardiotocography (CTG) monitoring has been studied and found not to reduce the operative delivery rate⁷.

Pulse oximeters are now used routinely in critical care, anaesthesiology, and A&E departments, and are often found in ambulances. They are an increasingly common part of a GP's kit. Pulse oximetry's role in primary care may include:

• Diagnosing and managing a severe exacerbation of chronic obstructive pulmonary disease (COPD) in the community.

• Grading the severity of an asthma attack. Where oxygen saturations are less than 92% in air, consider the attack potentially life-threatening⁸.

• Assessing severity and oxygen requirements for patients with community-acquired pneumonia⁹.

• Assessing severity and determining management in infants with bronchiolitis.

General pointers to the management of hypoxaemia

Using an oximeter

• Resting readings should be taken for at least five minutes.

• Poor perfusion (due to cold or hypotension) is the main cause of an inadequate pulse wave. A sharp waveform with a dicrotic notch indicates good perfusion whilst a sine wave-like waveform suggests poor perfusion.

• If a finger probe is used, the hand should be rested on the chest at the level of the heart rather than the affixed digit held in the air (as patients commonly do) in order to minimise motion artefact.

• Checking that the displayed heart rate correlates to a manually checked heart rate (within 5 beats per minute) generally rules out significant motion artefact.

• Emitters and detectors must oppose one another and light should not reach the detector except through the tissue. Ensure the digit is inserted fully into the probe and that flexible probes are attached correctly. Appropriately sized probes should be used for children and infants.

• Oximeter accuracy should be checked by obtaining at least one simultaneous blood gas, although this rarely happens. Oximeters may correct average oximeter bias based on pooled data but this does not eliminate the possibility of larger individual biases.

Sources of error

• Pulse oximetry cannot differentiate between different forms of haemoglobin. Carboxyhaemoglobin is registered as 90% oxygenated haemoglobin and 10% desaturated haemoglobin, thereby causing an overestimation of true saturation levels.

• Significant venous pulsation such as occurs in tricuspid incompetence and venous congestion.

• Environmental interference: vibration at 0.5-3.5 Hz and excessive movement. Ambient light - including infrared heat lamps - makes a difference of less than 5%¹⁰.

• Cold hands - warm extremity if local poor perfusion.

• Nail polish should be removed, as it may cause false readings¹⁰.

• Intravascular dyes, such as methylthioninium chloride, may also temporarily falsely reduce saturation readings.

Improving an oximeter signal¹¹