October 24, 2022
Pulse oximeters are used to assess patient oxygen status in a variety of clinical Settings and have become an increasingly common monitoring device.
It provides continuous, non-invasive monitoring of hemoglobin oxygen saturation in arterial blood. Its results are updated with each pulse.
Pulse oximeters do not provide information about hemoglobin concentration, cardiac output, efficiency of delivering oxygen to tissues, oxygen consumption, oxygen recharge, or degree of ventilation. They do, however, provide an opportunity to immediately notice deviations from a patient's oxygen baseline as an early warning sign to clinicians to help prevent the consequences of desaturation and detect cyanosis from hypoxemia before it occurs.
It has been suggested that increasing the use of pulse oximeters in general wards may make them as common as thermometers. However, staff reportedly had limited operational knowledge of the device, and little was known about how it worked and the factors that might affect readings (Stoneham et al. 1994; Casey, 2001).
How does pulse oximeter work?
In contrast to reduced hemoglobin, pulse oximeters measure the absorption of light at specific wavelengths in oxidized hemoglobin. Arterial oxygenated blood has a red color due to the mass of oxygenated hemoglobin it contains, which allows it to absorb certain wavelengths of light. The blood oxygen probe has two light-emitting diodes (leds) on one side of the probe, one red and one infrared emitting tube. The probe is placed in a suitable part of the body, usually a fingertip or earlobe, and the LED transmits light wavelengths through pulsating arterial blood to a photodetector on the other side of the probe. Oxygenated hemoglobin absorbs infrared light; Reduced hemoglobin glows red. Pulsatile arterial blood during systole causes oxygenated hemoglobin to flow into 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 results were processed on the oximeter screen into a digital display of oxygen saturation, denoted by SpO2 (Jevon, 2000).
Pulse oximeters are available in a variety of manufacturers and models (Lowton, 1999). Most display with visual digital waveforms, audible arterial beats and heart rate displays, and a variety of sensors to suit the age, size or weight of the individual. The choice depends on the Settings in which it is used. All personnel using pulse oximeters must be aware of their function and proper usage.
Arterial blood gas analysis is more accurate; However, having recognized its limitations, pulse OXImetry is considered accurate enough for most clinical purposes.
Factors affecting the accuracy of readings
Patient Status - To calculate the difference between capillaries and empty capillaries, blood oxygen saturation is measured by light absorption through multiple pulses (usually five) (Harrahill, 1991). To detect pulsatile blood flow, adequate perfusion must be performed in the monitored area. If the patient's peripheral pulse is weak or absent, the pulse oximeter reading will be inaccurate. Patients at high risk for hypoperfusion are those with hypotension, hypovolemia and hypothermia and those in cardiac arrest. Patients with a cold but not hypothermia may have vasoconstriction in the fingers and toes and may also impair arterial blood flow (Carroll, 1997).
If the blood oxygen probe is fixed too tightly, nonarterial beats may be detected, creating venous beats in the finger. Venous pulsations are also caused by right-side heart failure, tricuspid regurgitation (Schnapp and Cohen, 1990), and tourniquet of the blood pressure cuff above the probe.
Cardiac arrhythmias can lead to very inaccurate measurements, especially in the presence of significant cusp/radius defects (Woodrow, 1999).
Intravenous dyes used in diagnostic and hemodynamic tests may result in inaccurate and often low oxygen saturation estimates (Jenson et al., 1998). The effects of skin pigmentation, jaundice or elevated bilirubin levels should also be considered.
Proper use of pulse oximetry involves more than just reading the digital 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 hemoglobin in the body is filled with oxygen molecules. Therefore, the interpretation of oxygen saturation must be done in the context of the patient's total hemoglobin level (Carroll, 1997). Another factor that affects oximeter readings is how tightly hemoglobin binds to oxygen, which may vary with various physiological conditions.
External Influences - Because pulse oximeters measure the amount of light transmitted through arterial blood, bright light directly shining on the oximeter (whether artificial or natural) may affect the reading. Dirty sensors (Sims, 1996), dark nail polish (Carroll, 1997), and dry blood (Woodrow, 1999) may affect the accuracy of readings by obstructing or altering the light absorption of contact probes.
Optical shunting affects accuracy and can occur when the sensor is incorrectly placed in order to allow light to reach the photodetector directly from the LED without crossing the vascular bed.
The sensor may shift and shift due to rhythmic movement (e.g., Parkinson's tremor, seizures, or even shivering), which may cause inaccurate readings. Movement and vibration can also make it difficult for pulse oximeters to determine which tissue is pulsing.
False high readings - Pulse oximeters give false high readings in the presence of carbon monoxide. Carbon monoxide binds hemoglobin 250 times more strongly than oxygen, and once fixed prevents oxygen from binding. It also turns hemoglobin bright red. Pulse oximeters cannot distinguish between hemoglobin molecules saturated with oxygen and those carrying carbon monoxide (Casey, 2001). Smokers also consistently get falsely high readings - readings up to four hours after smoking are affected (Dobson, 1993). Other sources of carbon monoxide include fire, vehicle exhaust inhalation, and prolonged exposure to high flow environments.
There is also evidence that anemia can lead to falsely high readings (Jensen et al., 1998).
The dangers of using finger probes
Continuous use of blood oxygen probes may cause blisters on the finger pads and pressure damage to the skin or nail bed. Continuous use of the probe also poses a risk of burns, and the probe should be repositioned every two to four hours (MDA, 2001; Place, 2000).
Woodrow(1999) suggested that patients may not be able to alert staff to any discomfort and potential burns if the probe is placed on a paralyzed limb.
Like any other form of monitoring, pulse oximetry is an adjunct to care. Care should always focus 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 this technology will only benefit patients if those using it are able to use the device properly and understand the results proficiently.