An overview of the
Blood oxygen probes are widely used to assess oxygen saturation (SpO 2) to guide patient care and monitor response to treatment. However, improper placement of the oximetry probe can affect the oximetry of healthy and normal oxygen content outpatient patients. One study evaluated how treatment decisions were influenced by an SpO 2 value, obtained by placing a finger-clip-on blood oxygen probe on the auricle of 46 patients undergoing the trial, compared with an arterial blood gas (ABG) (SaO 2) analysis measured from the finger. There was no statistical difference between finger probe saturation and SaO 2, with a mean difference of -0.66% (P>0.05). There was a significant difference in ear saturation (-4.29%; P < 0.001). Analysis of hypoxic patients (SaO 2 <90%) showed significant differences between ABG SaO 2 and finger and ear SpO 2. The study provides evidence that placing finger clip blood oxygen probes on the ear is unsafe clinical practice and may lead to poor patient management.
Blood oxygen probe is an indirect, non-invasive, accurate and safe method for measuring oxygen saturation (SpO 2). It is widely used to record clinical observations in a range of outpatient and inpatient Settings. Many treatment regiments are guided by oxygen saturation measurements, such as during the evaluation and response to interventions such as oxygen therapy and non-invasive ventilation (NIV). More recently, blood oxygen probes have been used to monitor the condition of COVID-19 patients while in hospital.
The oximetry probe is designed to record SpO 2 by measuring the absorption of oxygenated hemoglobin (HbO 2) and deoxygenated hemoglobin (Hb) to specific wavelengths of light. The oximeter probe contains light-emitting diodes (leds) that project two wavelengths of light -- red (660 nm) and infrared (940 nm) -- from one side of the probe to a photodetector on the other. The pulsating arterial blood during heart contraction delivers oxygenated hemoglobin (HbO 2) to the tissues, which results in the absorption of more infrared light, so less light reaches the photodetector. Therefore, the level of oxygen saturation in the blood determines the degree of light absorption. The results are processed and a digital reading of the oximetry results is displayed on the oximeter screen as SpO2.
The accuracy of the oximeter depends on the difference between the SpO 2 measured by the oximeter and the oxygen saturation measured by the concurrent arterial blood gas (ABG) sampling (SaO 2). The manufacturer claims an accuracy of 2% with a standard deviation (SD) of 2-3% for the difference, but there is evidence that the measurement error is usually closer to 3-4%. It is also highly dependent on the continued reliability of the wavelength of the emitted light, but functional changes (such as damage caused by general wear or cleaning of leds) can alter the rate of light absorption, which will affect the accuracy estimates of SpO 2. Thus, the average difference between SaO 2 and SpO 2 becomes larger when SpO 2 drops below 80% due to inaccuracies in calibration and blood oxygen saturation measurements.
The accuracy of an oxygen saturation reading is affected by arterial pulse strength, body movement, color interference, venous pulsation, and a variety of physical factors. Measured saturation also fluctuates significantly with changes in ventilation associated with coughing, speaking, breath-holding, and physical activity. Although easy to perform, the oximetry probe requires clinical skills training to ensure accurate readings are obtained. There is evidence that clinicians are not always aware of the limitations of blood oxygen probes and that motion artifacts and poor signal quality can lead to inaccurate readings. Because of these factors, the saturation measurement must be observed for several minutes to determine the most frequently measured value, rather than relying on the first value provided.
Because the blood-oxygen probe measures the amount of light transmitted through tissue, any bright light directly shone on the sensor has the potential to produce inaccurate readings. If the sensor is improperly applied or applied to a tissue site not specified by the manufacturer, such as the use of a finger clip oximetry probe on the ear, optical shunt may occur where light reaches the detector and does not infuse the tissue through the blood. The effect on the measured SpO 2 will depend on which wavelengths of light are subject to optical shunt, as well as external light sources, which may therefore increase or decrease the recorded true value. For this reason, oximetry probes must be used only where they are designed.
advice
Measurements of SpO 2 should always be checked for accuracy by visually evaluating the photoelectric plethysmography waveform on the oximetry equipment using a probe in the appropriate location. Reduced peripheral circulation, as determined by poor waveform strength, should be considered a measurement limitation, which will result in inaccurate SpO 2 readings. In these cases, a suitable alternative oximetry probe should be determined, such as an ear clip/forehead clamp oximetry probe or a blood gas measurement.