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

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 CONDITION - To 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. EXTERNAL INFLUENCES - 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 READINGS - Pulse 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.  ACTIVITY 1 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? ACTIVITY 2 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? ACTIVITY 3 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? AUTHORS _Mandy Howell, BSc (Hons), RN, OND, FETC, DPSN, DMS, Dip Asthma, Dip Resp Management_ _Senior Clinical Nurse, General Internal Medicine, City Hospital Sunderland

NHS Trust, Sunderland Royal Hospital, Sunderland_ CARROLL,_ P. (1997) Pulse Oximetry at your fingertips. RN 60: 2, 22-27._ CASEY, G. (2001) _Oxygen transport and the use of pulse oximetry.

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pitfalls. Journal of Emergency Nursing 17: 6, 437-439._ JENSEN, L.A., Onyskiw, J.E., Prasad, N.G.N. (1998) _Meta-analysis of arterial oxygen saturation monitoring by pulse oximetry in

adults. Heart and Lung 27: 6, 387-408._ JEVON, P. (2000) _Pulse oximetry: 1. Practical procedures for nurses. Nursing Times 96: 27, 43-44._ LOWTON, K. (1999) _Pulse oximeters for the

detection of hypoxaemia. Professional Nurse 14: 5, 343-350._ MEDICAL DEVICES AGENCY. (2001) _MDA SN2001(08): Tissue Necrosis Caused by Pulse Oximeter Probes. London: MDA._ PLACE, B. (2000) 

_Pulse oximetry: benefits and limitations. Nursing Times 96: 26, 42._ SCHNAPP, L.M., Cohen, N.H. (1990) _Pulse oximetry: uses and abuses. Chest 98: 1244-1250._ SIMS, J. (1996) _Making sense

of pulse oximetry and oxygen dissociation curve. Nursing Times 92: 1, 34-35._ STONEHAM, M.D., Saville, G.M., Wilson, I.H. (1994) _Knowledge about pulse oximetry among medical and nursing

staff. Lancet 344: 1339-1342._ WOODROW, P. (1999) _Pulse Oximetry. Nursing Standard 13: 42, 42-46._