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Emergency physicians assess the patient's hemoglobin oxygen saturation multiple times per shift and use this information as a vital sign. We basically make important clinical decisions based on the real-time readings obtained by the pulse oximeter. However, the complexity and mechanism of the pulse oximeter process are unknown to most people who use them.
The pulse oximeter probe only needs to be placed on the patient, and the hemoglobin oxygen saturation will be displayed on the oximeter screen. Clinical decisions and interventions are often made within a few seconds. Due to COVID-19 (a disease that mainly affects the lungs), even home oxygen saturation measurements are becoming more and more common.
The harmful effects of excessive oxygen are well known to EP. A common conventional reflex is to use high concentrations of inhaled oxygen to produce 100% hemoglobin saturation. Doctors often seem to want to produce excessive oxygenation, but this result has been questioned. Oxygen is obviously a toxin, and its use should be titrated to avoid hyperoxia.
Hyperoxia is harmful to newborns and adults with myocardial infarction and can easily occur without monitoring. Aims to obtain 100% hemoglobin oxygen saturation for all patients was once a common goal, but this may be a mistake. Titration of oxygen therapy for all patients can avoid hyperoxia. These goals and tasks are best achieved using a pulse oximeter.
U.S. Food and Drug Administration
https://bit.ly/3CUL4do
This communication has been sent to the doctor to update the use of pulse oximetry, including the limitations and possible inaccuracies of the procedure. Due to COVID-19, the use of pulse oximeters has increased significantly, and this alert includes some common-sense instructions for patients using pulse oximeters at home.
When the hemoglobin oxygen saturation is less than 80%, the pulse oximeter is the least accurate. Pulse oximeter readings should only be used as an estimate of arterial oxygen saturation. A 90% reading actually represents an arterial oxygen saturation of 86%-94%. Clinical decisions are based on the trend of oximeter readings over time rather than a single reading.
A pulse oximeter is usually placed on your fingertips. It uses a light beam to estimate the oxygen saturation of the patient's hemoglobin, reflecting the amount of oxygen carried in the blood. The pulse oximeter also reports the pulse rate. The true analysis of hemoglobin oxygen saturation is performed using blood samples. Most healthy people have an oxygen saturation of 95%-100%, but people with lung problems (such as COPD) and people living in high altitude areas have lower oxygen saturation.
Prescription pulse oximeters are reviewed and approved by the FDA, and tested to confirm accuracy before being sold. Over-the-counter oximeters are sold directly to consumers, but these devices have not been reviewed by the FDA and should not be used for medical purposes.
It is important to understand the limitations and risks of inaccurate results reported on the oximeter screen. Unrecognized hypoxic saturation may be clinically important. FDA-approved prescription pulse oximeters are evaluated through desaturation studies conducted on healthy individuals. The test compares the pulse oximetry reading with the arterial blood gas saturation reading, which is between 70% and 100%. The oximeter reading must be within 2%-6% of the arterial blood gas value to be certified. But the oximeter reading is only an estimate of the oxygen saturation, so if the FDA-approved pulse oximeter reading is 90%, the true oxygen saturation in the blood is usually between 86% and 94%.
The accuracy of the pulse oximeter is highest when the saturation is 90%-100%, medium when the saturation is 80%-90%, and lowest when the saturation is lower than 80%. The accuracy varies between light and dark skin pigmentation. The frequency of occult hypoxemia detected by blood gas analysis in black patients is almost three times that of white patients, while pulse oximetry does not . (New Engl J Med. 2020;383[25]:2477; https://bit.ly/3CTdAMs.) The FDA recently required oximeter analysis to include the effect of skin pigment on accuracy. Doctors must realize that older pulse oximeters may not be able to recognize hypoxic saturation in black patients.
Comment: Pulse oximetry is a non-invasive measurement of the percentage of hemoglobin bound to oxygen. The device provides real-time estimation of arterial blood oxygen saturation in the range of 80%-100%. It provides early warning of decreased capillary perfusion without the risk and time required to perform arterial puncture and use hospital laboratories. It relies on the concept that the concentration of an unknown substance (oxyhemoglobin) dissolved in a solvent (blood) can be determined by light absorption.
The probe used in this process is a reusable clip or disposable patch, which contains two photodiodes, which can generate red light and infrared light, which are detected by a photodetector placed on the pulsating blood vessel bed. Disposable clips can now be used for infection control. The vascular bed to be analyzed is usually under the nail bed, not the fat pad, but blood from the earlobe, toe, or forehead can also be analyzed.
For oxyhemoglobin and reduced hemoglobin, the absorption characteristics of the two emission wavelengths are completely different. The substance to be analyzed is determined by the wavelength of the light source. Pulse oximetry is based on the principle that oxyhemoglobin and deoxyhemoglobin absorb red light and infrared light differently. The absorption properties of skin, connective tissue, bone and venous blood remain unchanged. Compared with deoxyhemoglobin, oxyhemoglobin absorbs more infrared light and less red light. Oxygenated blood with a high content of oxygenated hemoglobin is bright red on naked eyes because it scatters more red light than hypoxic blood that absorbs red light. In visual inspection, deoxygenated blood containing a large amount of hemoglobin that absorbs red light does not appear bright red.
The pulse oximeter uses the relative amounts of red and infrared light absorbed by the two hemoglobins to determine the ratio of hemoglobin bound to oxygen. The ability of a pulse oximeter to detect only the oxygen saturation of hemoglobin in arterial blood is based on the principle that the amount of absorbed red light and infrared light fluctuates with the cardiac cycle or pulse. The small increase in fingertip arterial blood volume produced by each heart contraction results in a change in the absorption of oxyhemoglobin that exceeds the baseline absorption and a decrease in hemoglobin because the arterial blood volume in the fingertip increases with each heartbeat.
The ratio of oxyhemoglobin to reduced hemoglobin can be calculated by comparing the ratio of the pulsating absorption and the baseline absorption of the two wavelengths. The pulse oximeter uses this ratio calculated on a series of pulses and a calibration curve that is generated by analyzing blood collected from volunteers with various hemoglobin oxygen saturations to determine the specific individual under study Hemoglobin oxygen saturation.
Chen ED, Chen MM, Chen MM
This is a well-written review article on the principles of pulse oximetry measurement, and the full text should be read to understand its information. It discusses the basic principles of pulse oximetry and reviews the various conditions that can lead to false readings and the mechanisms behind them. The basic concept that pulse oximeter only detects arterial blood oxygen saturation is based on the fact that the amount of red light and infrared light absorbed by blood fluctuates with the cardiac cycle. As blood occurs, red light and infrared light are absorbed A measurable change has occurred. The volume under the probe changes with each heartbeat. Therefore, the pulse oximeter excludes the influence of venous blood and other resting tissues in its calculations.
The pulse oximetry probe, whether it is a reusable clip or a disposable sticky probe, can detect areas with high blood vessel density such as fingers, nose, earlobes, or forehead. Ear and forehead probes are more reliable in patients with hypotension, vasoconstriction or hypothermia, because these areas are less likely to respond to endogenous and exogenous catecholamines than fingers.
In some cases, pulse oximeters will report false normal or elevated oxygen saturation. The real impact is usually minimal, and in most cases is reduced by newer machines. For example, carbon monoxide poisoning will produce carboxyhemoglobin, which has similar light absorption properties to oxyhemoglobin, so pulse oximeters cannot distinguish between the two. Even with a large amount of carboxyhemoglobin, a pulse oximeter using only two beams will report normal oxygen saturation.
Finger probes can also cause false readings, including probes that are too tightly bound to cause venous pulsation, severe tricuspid regurgitation, colored dyes (such as methylene blue), indigo carmine used to detect amniotic fluid leakage, and evaluation Liver function of indolecyanine and excessive patient exercise, such as seizures or patient tremors. Newer models of pulse oximeters will not report low concentrations that were previously incorrectly reported due to nail polish, but it is better to remove the polish from the fingers used by the probe. The effects of artificial nails vary, and it is best to remove artificial nails from the fingers under evaluation. Severe anemia, methemoglobinemia, thiohemoglobinemia, severe hyperbilirubinemia, and bright ambient light can cause false readings. Fetal hemoglobin has red and infrared light absorption similar to normal adult hemoglobin, so pulse oximeters are as reliable for newborns as adults.
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Dr. Robertsis is Professor of Emergency Medicine and Toxicology at Drexel University School of Medicine in Philadelphia. Read his past columns at http://bit.ly/EMN-InFocus.
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