The pulse oximeter, which is used for evaluating the oxygen status of patients in a variety of clinical settings, has become an increasingly common piece of monitoring equipment. It provides continuous, non-invasive monitoring of oxygen saturation of haemoglobin in arterial blood. Its results are updated with each pulse wave.
Pulse oximeters do not offer information about haemoglobin concentration, cardiac output, efficiency of oxygen delivery to the tissues, oxygen consumption, sufficiency of oxygenation, or adequacy of ventilation. They do, however, provide an opportunity for deviations from a patient’s oxygen baseline to be noticed immediately, as an early warning signal to clinicians to help prevent the consequences of desaturation and detect hypoxaemia before it produces cyanosis.
It has been suggested that the increase in the use of pulse oximeters in general wards could see it becoming as commonplace as the thermometer. However, staff are reported to have limited education in the operation of the device and limited knowledge of how it works and what factors may affect the readings (Stoneham et al, 1994; Casey, 2001).
This paper aims to promote a greater awareness of the importance of having an appropriate knowledge base before using pulse oximetry and to provide a source of education and reference for teaching purposes. See Activity 1.
How does the pulse oximeter work?
Pulse oximeters measure the absorption of specific wavelengths of light in oxygenated haemoglobin as compared with that of reduced haemoglobin. Arterial oxygenated blood is red due to the quality of oxyhaemoglobin it contains, causing it to absorb light of certain wavelengths. The oximeter probe has two light-emitting diodes (LEDs), one red and one infrared, located on one side of the probe. The probe is placed on a suitable part of the body, usually a fingertip or ear lobe, and the LEDs transmit light wavelengths through pulsating arterial blood to a photodetector on the other side of the probe. Infrared light is absorbed by the oxyhaemoglobin; red light by the reduced haemoglobin. Pulsatile arterial blood during systole causes an influx of oxyhaemoglobin to the tissue, absorbing more infrared light, and allowing less light to reach the photodetector. The oxygen saturation of the blood determines the degree of light absorption. The result is processed into a digital display of oxygen saturation on the oximeter screen, which is symbolised as SpO2 (Jevon, 2000).
There are various makes and models of pulse oximeters available (Lowton, 1999). Most provide a visual digital waveform display, an audible display of arterial pulsations and heart rate, and a variety of sensors to accommodate individuals regardless of age, size or weight. Selection depends on the setting in which it is used. All staff using the pulse oximeter must be aware of its functions and correct usage.
Arterial blood gas analysis is more accurate; however, pulse oximetry is considered sufficiently accurate for most clinical purposes, having recognised that there are limitations.
Factors which affect accuracy of readings
Patient conditionTo calculate the difference between full and empty capillaries, oximetry measures light absorption over a number of pulses, usually five (Harrahill, 1991). In order for a pulsatile flow to be detected, there must be sufficient perfusion in the monitored area. If the patient has a weak or absent peripheral pulse, pulse oximeter readings will not be precise. Patients most at risk of low perfusional states are those with hypotension, hypovolaemia and hypothermia and those in cardiac arrest. Patients who are cold but not hypothermic may have vasoconstriction in their fingers and toes that may also compromise arterial flow (Carroll, 1997).
Non-arterial pulses may be detected if the probe is secured too tightly, creating venous pulsations in the finger. Venous pulsations are also caused by right-sided heart failure, tricuspid regurgitation (Schnapp and Cohen, 1990), and the tourniquet effect of a blood pressure cuff above the probe.
Cardiac arrhythmias may cause very inaccurate measurements, especially if there are significant apex/radial deficits (Woodrow, 1999).
Intravenous dyes used in diagnostic and haemodynamic testing may cause inaccurate, usually lower, estimates of oxygen saturations (Jenson et al, 1998). The effects of deeply pigmented skin, jaundice or bilirubin levels should also be considered.
Using pulse oximetry correctly involves more than just reading the number display, since not all patients with the same SpO2 have the same amount of oxygen in their blood. A saturation of 97% means that 97% of the total amount of haemoglobin in the body is filled with oxygen molecules. Therefore the interpretation of oxygen saturations must be in the context of the patient’s total haemoglobin level (Carroll, 1997). Another factor that affects the oximeter readings is how tightly the haemoglobin and oxygen are bound together, which may change with various physiological conditions.
Because the pulse oximeter measures the amount of light transmitted through arterial blood, bright light that shines directly on the sensor, whether artificial or natural, may affect readings. Dirty sensors (Sims, 1996), dark-coloured nail polishes (Carroll, 1997) and dried blood (Woodrow, 1999) may affect the accuracy of the readings by hindering or altering the light absorption of the contact probes.
Optical shunting affects accuracy and occurs when the sensor is improperly positioned so that light goes directly from the LED to the photodetector without passing through the vascular bed.
Moving and dislodging of the sensor, which may be caused by a rhythmic movement such as the tremors of Parkinsonism, seizures or even shivering, may leads to inaccurate readings. Exercise and vibrations can also make it difficult for the pulse oximeter to determine which tissue is pulsatile.
False high readingsPulse oximeters can give a falsely high reading in the presence of carbon monoxide. Carbon monoxide binds to haemoglobin about 250 times more strongly than oxygen and, once in place, prevents the binding of oxygen. It also turns haemoglobin bright red. The pulse oximeter is unable to distinguish between haemoglobin molecules saturated in oxygen and those carrying carbon monoxide (Casey, 2001). False high readings are also always obtained from smokers - readings are affected for up to four hours after smoking a cigarette (Dobson, 1993). Other sources of carbon monoxide include fires, car-exhaust inhalation and prolonged exposure to heavy-traffic environments.
There is also some evidence that anaemia leads to false high readings (Jensen et al, 1998).
Hazards of using a finger probe
Continuous use of the probe may cause blisters on the finger pad or pressure damage to the skin or nail bed. Burns are also a hazard of continuous use of the probe, which should be repositioned every two to four hours (MDA, 2001; Place, 2000).
Woodrow (1999) suggests that if a probe is placed on a paralysed limb, the patient may not be able to warn staff of any discomfort and potential burns.
Pulse oximetry is, like any other form of monitoring, an adjunct to care. Care should remain focused on the person and not the machine. The accuracy of routine pulse oximetry should not be taken for granted and nursing and medical staff should be aware that the technology benefits patients only if the staff using it are able to use the equipment correctly and interpret the results knowledgeably.
Before reading further, consider your current knowledge and skills regarding the use of pulse oximetry:
- How does a pulse oximeter work?
- What does it measure?
- What factors affect accurate readings?
Reflect on the last time you used pulse oximetry monitoring in your clinical area:
- Which aspects of the patient’s condition did you have to consider before determining the accuracy of the readings?
- What external or technical factors, if any, did you consider to determine accuracy of the readings?
Having reflected on the previous activity, are there any factors that you now consider may have affected the accuracy of the readings the last time you used pulse oximetry?
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