Practical applications of Pulse Oximeter
Dr E Hill,
Dr MD Stoneham,
Nuffield Department of Anaesthetics, Oxford Radcliffe NHS Hospitals Headington, Oxford, UK
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 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?
- 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.
- 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
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.
- 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.
Uses of pulse oximetry
- Simple, portable all-in-one monitor of oxygenation, pulse rate and rhythm regularity, suitable for field use.
- As a safe, non-invasive monitor of the cardio-respiratory status of high-dependency patients - in the emergency department, during general and regional anaesthesia, postoperatively and in intensive care. This includes procedures such as endoscopy, where often frail patients are given sedative drugs such as midazolam. Pulse oximeters detect the presence of cyanosis more reliably than even the best doctors when using their clinical judgement.
- During the transport of patients - especially when this is noisy - for example in aircraft, helicopters or ambulances. The audible tone and alarms may not be heard, but if a waveform can be seen together with an acceptable oxygen saturation, this gives a global indication of a patient's cardio-respiratory status.
- To assess the viability of limbs after plastic and orthopaedic surgery and, for example, following vascular grafting, or where there is soft tissue swelling or aortic dissection. As a pulse oximeter requires a pulsatile signal under the sensor, it can detect whether a limb is getting a blood supply.
- As a means of reducing the frequency of blood gas analysis in intensive care patients- especially in paediatric practice where vascular (arterial) access may be more difficult.
- To limit oxygen toxicity in premature neonates supplemental oxygen can be tapered to maintain an oxygen saturation of 90% - thus avoiding the damage to the lungs and retinas of neonates. Although pulse oximeters are calibrated for adult haemoglobin, HbA, the absorption spectra of HbA and HbF are almost identical over the range used in pulse oximetry, so the technique remains reliable in neonates.
- During thoracic anaesthesia - when one lung is being collapsed down - to determine whether oxygenation via the remaining lung is adequate or whether increased concentrations of oxygen must be given.
- Fetal oximetry - a developing technique that uses reflectance oximetry, using LEDs of 735nm and 900nm. The probe is placed over the temple or cheek of the fetus, and needs to be sterile and sterilisable. They are difficult to secure and the readings are variable, for physiological and technical reasons. Hence the trend is more useful than the absolute value.
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