Pulse oximetry | Oxygen Status Measurement


The pulse oximeter, which is used to assess the oxygen status of patients in various clinical settings, has become increasingly common monitoring equipment.

It provides continuous, non-invasive monitoring of hemoglobin oxygen saturation in arterial blood. Its results are updated with each pulse wave.

Pulse oximeters do not offer information about hemoglobin concentration, cardiac output, tissue oxygen delivery efficiency, oxygen consumption, oxygen sufficiency or adequacy ventilation. However, they allow deviations from a patient’s oxygen baseline to be immediately noticed, as an early warning signal for clinicians to help prevent the consequences of desaturation and detect hypoxemia before it does not produce cyanosis.

It has been suggested that the increase in the use of pulse oximeters in general wards may see it become as commonplace as the thermometer. However, personnel would have limited training in the operation of the device and limited knowledge of its operation and the factors that may affect readings (Stoneham et al, 1994; Casey, 2001).

This article aims to promote greater awareness of the importance of having a proper 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 hemoglobin versus that of reduced hemoglobin. Oxygenated arterial blood is red due to the quality of oxyhemoglobin it contains, which allows 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 an appropriate part of the body, usually a fingertip or earlobe, and the LEDs transmit light wavelengths through the pulsing arterial blood to a photodetector on the other side of the probe. Infrared light is absorbed by oxyhemoglobin; red light by reduced hemoglobin. Pulsating arterial blood during systole causes an influx of oxyhemoglobin into the tissues, absorbing more infrared light and allowing less light to reach the photodetector. Blood oxygen saturation determines the degree of light absorption. The result is processed in a digital display of oxygen saturation on the oximeter screen, which is symbolized by SpO2 (Jevon, 2000).

There are different brands and models of pulse oximeters available (Lowton, 1999). Most offer a visual digital waveform display, an audible display of arterial pulse and heart rate, and a variety of sensors to accommodate individuals regardless of age, height, or weight. The selection depends on the framework in which it is used. All personnel using the pulse oximeter should be familiar with its functions and proper use.

Arterial blood gas analysis is more accurate; however, pulse oximetry is considered sufficiently accurate for most clinical purposes, having recognized that there are limitations.

Factors affecting the 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). For pulsatile flow to be detected, there must be sufficient perfusion in the monitored area. If the patient has a weak or absent peripheral pulse, the pulse oximeter readings will not be accurate. Patients most at risk for low perfusion states are those with hypotension, hypovolemia, and hypothermia and those in cardiac arrest. Patients who are cold but not hypothermic may have vasoconstriction of the fingers and toes which can also compromise arterial flow (Carroll, 1997).

Non-arterial pulses can be detected if the probe is attached too firmly, creating venous pulsations in the finger. Venous pulsations are also caused by right heart failure, tricuspid regurgitation (Schnapp and Cohen, 1990) and the tourniquet effect of a blood pressure cuff above the lead.

Cardiac arrhythmias can cause very inaccurate measurements, especially if there are large apex/radial deficits (Woodrow, 1999).

Intravenous dyes used in diagnostic and hemodynamic testing can lead to inaccurate, usually lower, estimates of oxygen saturations (Jenson et al, 1998). The effects of deeply pigmented skin, jaundice, or low bilirubin levels should also be considered.

Using pulse oximetry correctly involves more than just reading the digital display, because 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 hemoglobin in the body is filled with oxygen molecules. Therefore, interpretation of oxygen saturations must be made in the context of the patient’s total hemoglobin level (Carroll, 1997). Another factor that affects oximeter readings is the degree of hemoglobin and oxygen binding, which can change with various physiological conditions.

External influences – Since the pulse oximeter measures the amount of light transmitted through arterial blood, bright light shining directly on the sensor, whether artificial or natural, can affect the readings. Dirty sensors (Sims, 1996), dark colored fingernail polish (Carroll, 1997) and dried blood (Woodrow, 1999) can affect the accuracy of readings by interfering with or altering the light absorption of sensor probes. contact.

Optical shunt affects accuracy and occurs when the sensor is incorrectly positioned so that light passes directly from the LED to the photodetector without crossing the vascular bed.

Sensor displacement and dislodgement, which can be caused by rhythmic movement such as parkinsonism tremors, seizures, or even chills, can cause inaccurate readings. Exercise and vibration can also prevent the pulse oximeter from determining which tissue is pulsating.

False high readings – Pulse oximeters can give a falsely high reading in the presence of carbon monoxide. Carbon monoxide binds to hemoglobin about 250 times more strongly than oxygen and, once in place, prevents oxygen binding. It also turns hemoglobin bright red. The pulse oximeter is unable to distinguish between hemoglobin molecules saturated with 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, inhaling car exhaust, and prolonged exposure to heavy traffic environments.

There is also evidence that anemia leads to false high readings (Jensen et al, 1998).

Dangers associated with the use of a digital probe

Continued use of the probe may cause blisters on the fingertips or pressure sores on the skin or nail bed. Burns are also a hazard with continued use of the catheter, which must be repositioned every two to four hours (MDA, 2001; Place, 2000).

Woodrow (1999) suggests that if a probe is placed on a paralyzed limb, the patient may not be able to alert staff to any discomfort and potential burns.

Pulse oximetry is, like any other form of monitoring, an adjunct to care. Care must remain focused on the person, 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 only benefits patients if the staff using it are able to use the equipment correctly. and interpret the results with full knowledge of the facts.


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?


Think about the last time you used pulse oximetry monitoring in your clinical area:

  • What aspects of the patient’s condition did you need to consider before determining the accuracy of the readings?
  • What external or technical factors, if any, did you consider to determine the accuracy of the readings?


After reflecting 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?


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

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