Pulse oximetry is a non-invasive way to monitor a person's blood oxygen saturation. Peripheral oxygen saturation (SPÒ 2) readings are typically accurate within 2% of arterial blood gas analysis, which is more ideal for reading arterial oxygen saturation. But the correlation between the two is good enough, so a safe, convenient, non-invasive, and cheap pulse oximetry measurement method is valuable for oxygen saturation measurement in clinical use.
The most common method is transmission pulse oximetry. In this method, the sensor device is placed on a thin part of the patient's body, usually the fingertips or earlobes, or a baby's feet. Fingertips and earlobes have higher blood flow than other tissues, which aids in heat transfer. The device directs two wavelengths of light through the body part to a photodetector. It measures the change in absorbance at each wavelength, thereby determining the absorbance caused solely by pulsating arterial blood.
Reflection pulse oximetry is a less common alternative to transmission pulse oximetry. This method does not require thin sections of the human body, making it ideal for general applications such as the feet, forehead, and chest, but it does have some limitations. Due to impaired venous return to the heart, vasodilation and venous blood pooling in the head can cause arterial and venous pulsations in the forehead area to combine and lead to false SpO2 results. This situation occurs while undergoing anesthesia with endotracheal intubation and mechanical ventilation or while the patient is in the head-down position.
medical use
A pulse oximeter is a medical device that indirectly monitors the oxygen saturation of a patient's blood (as opposed to measuring oxygen saturation directly from a blood sample) and changes in skin blood volume, producing a photoplethysmogram that can be further processed into other measurements. .Pulse oximeters can be integrated into multi-parameter patient monitors. Most monitors also display pulse rate. Portable battery-operated pulse oximeters can also be used for transport or home blood oxygen monitoring.
advantage
Pulse oximeters are particularly convenient for non-invasive, continuous measurement of blood oxygen saturation. In contrast, blood gas levels must be measured in a laboratory from a blood sample drawn. Pulse oximetry can be used in any setting where patient oxygenation is unstable, including critical care, surgery, recovery, emergency and hospital ward settings, pilots on unpressurized aircraft, to assess oxygenation in any patient, and to determine supplementation Oxygen availability or need. Although a pulse oximeter is used to monitor oxygenation, it cannot determine oxygen metabolism, or the amount of oxygen the patient is using. To do this, carbon dioxide (CO2) levels also need to be measured. It may also be used to detect abnormalities in ventilation. However, the use of pulse oximetry to detect hypoventilation is compromised by the use of supplemental oxygen because respiratory dysfunction can only be reliably detected when the patient is breathing room air. Therefore, if the patient is able to maintain adequate oxygenation on room air, routine supplemental oxygen may not be necessary as this may result in undetectable hypoventilation.
Due to its simplicity of use and ability to provide continuous and instantaneous oxygen saturation values, pulse oximeters are vital in emergency medicine and are useful for respiratory or heart problems especially COPD or for diagnosing some sleep disorders such as apnea and breathing insufficient. For people with obstructive sleep apnea, pulse oximeter readings will be in the 70-90% range most of the time they are trying to fall asleep.
Portable battery-operated pulse oximeters are useful for pilots operating in non-pressurized aircraft requiring supplemental oxygen above 10,000 feet (3,000 meters) or 12,500 feet (3,800 meters) in the United States. Portable pulse oximeters are also useful for climbers and athletes whose oxygen levels may decrease at high altitudes or during exercise. Some portable pulse oximeters use software to chart a patient's blood oxygen and pulse as a reminder to check blood oxygen levels.
Advances in connectivity allow patients to continuously monitor their blood oxygen saturation without cables connected to hospital monitors, without sacrificing the flow of patient data back to bedside monitors and centralized patient monitoring systems.
In COVID-19 patients, pulse oximetry can help detect silent hypoxia early, where the patient still looks and feels comfortable, but their SpO2 is very low. This happens to patients in the hospital or at home. Low SpO2 may indicate severe pneumonia related to COVID-19, requiring a ventilator.
limit
Pulse oximetry only measures hemoglobin saturation, not ventilation, and is not a complete measurement of respiratory function. It is not a substitute for blood gases tested in the laboratory because it does not show base deficiency, carbon dioxide levels, blood pH, or bicarbonate (HCO 3 - ) concentration. Oxygen metabolism can be easily measured by monitoring exhaled CO2 , but saturation data provides no information about blood oxygen content. Most of the oxygen in the blood is carried by hemoglobin; in severe anemia, there is less hemoglobin in the blood, and although the hemoglobin is saturated, it still cannot carry as much oxygen.
Because pulse oximeter devices are calibrated in healthy subjects, accuracy is poor in critically ill patients and premature infants.
Falsely low readings may be caused by hypoperfusion of the limb being monitored (usually due to cold limb, or vasoconstriction due to use of vasopressors); incorrect sensor application; highly calloused skin; or movement ( such as shivering), especially during periods of low perfusion. To ensure accuracy, the sensor should return stable pulses and/or pulse waveforms. Pulse oximetry technology varies in its ability to provide accurate data during exercise and low perfusion conditions.
Obesity, hypotension (low blood pressure), and some hemoglobin variations can reduce the accuracy of the results. Some home pulse oximeters have very low sampling rates, which can significantly underestimate drops in blood oxygen levels. ] The accuracy of pulse oximetry decreases significantly when readings fall below 80%.
Pulse oximetry is also not a complete measure of circulating oxygen sufficiency. If there is insufficient blood flow or hemoglobin in the blood (anemia), the tissues will be starved of oxygen despite high arterial oxygen saturation.
Because pulse oximeters only measure the percentage of bound hemoglobin, false high or false low readings can occur when hemoglobin is bound to something other than oxygen:
- Hemoglobin has a higher affinity for carbon monoxide than for oxygen, and high readings may occur even though the patient is actually hypoxemic. In the case of carbon monoxide poisoning, this inaccuracy may delay recognition of hypoxia (low cellular oxygen levels).
- Readings are higher in cyanide poisoning because it reduces the extraction of oxygen from arterial blood. In this case, there is no error in the reading because arterial blood oxygenation is indeed high in early cyanide poisoning.
- In the mid-1980s, methemoglobinemia was characterized as causing pulse oximetry readings.
- COPD [especially chronic bronchitis] may cause false readings.
One non-invasive method that allows continuous measurement of hemoglobin abnormalities is the pulse oximeter, manufactured in 2005 by Masimo. By using additional wavelengths, it provides clinicians with a way to measure hemoglobin abnormalities, carboxyhemoglobin and methemoglobin, as well as total hemoglobin.
Common pulse oximeter devices may have higher error rates in adults with darker skin tones, research suggests, raising concerns that inaccuracies in pulse oximeter measurements could be exacerbated in countries with racially diverse populations such as the United States. of systemic racism. Pulse oximetry is used to screen for sleep apnea and other types of sleep breathing disorders, which are more common among minorities in the United States.
equipment
In addition to pulse oximeters for professional use, there are many inexpensive "consumer" models. Opinions about the reliability of consumer oximeters vary; a typical comment is "Study data on home monitors are mixed, but they tend to be accurate within a few percentage points." Some smartwatches with activity tracking capabilities include an oximeter feature.
mobile application
Mobile app pulse oximeters use a flashlight and phone camera instead of the infrared light used by traditional pulse oximeters. However, the app cannot produce accurate readings because the camera cannot measure light reflection at both wavelengths, so oxygen saturation readings obtained through the app on a smartphone are inconsistent with clinical use. In fact, one study shows these are unreliable. So even though pulse oximeters aren't perfect, they're still much more accurate than smartphone app pulse oximeters.
mechanism
A blood oxygen monitor shows the percentage of oxygen in the blood. More specifically, it measures the percentage of hemoglobin (the protein in the blood that carries oxygen) that is loaded. For patients without lung disease, the acceptable normal Sa O range is 95% to 99%. For people breathing room air at or near sea level, arterial pO 2 can be estimated from blood oxygen monitor "peripheral oxygen saturation" (SpO 2 ) readings.
Operation method
A typical pulse oximeter uses an electronic processor and a pair of small light-emitting diodes (LEDs) that face a photodiode through a translucent part of the patient's body (usually a fingertip or earlobe). One LED is red with a wavelength of 660 nm and the other is infrared with a wavelength of 940 nm. The absorption of light at these wavelengths differs significantly between oxygenated and deoxygenated blood. Oxygenated hemoglobin absorbs more infrared light and allows more red light to pass through. Deoxygenated hemoglobin allows more infrared light to pass through and absorbs more red light. The LEDs cycle sequentially, one on, then the other, and then off approximately 30 times per second, which allows the photodiodes to respond to red and infrared light separately and adjust to the ambient light baseline.
The amount of light transmitted (in other words, the amount of light that is not absorbed) is measured and a separate normalized signal is produced for each wavelength. These signals fluctuate over time because the amount of arterial blood present increases with each heartbeat. By subtracting the minimum transmitted light from the transmitted light at each wavelength, the effects of other tissues are corrected to generate a continuous signal for pulsatile arterial blood. The ratio of the red light measurement to the infrared light measurement (representing the ratio of oxyhemoglobin to deoxygenated hemoglobin) is then calculated by the processor and this ratio is then converted to SpO2 by the processor. Signal separation has other uses as well: typically the plethysmograph waveform representing the pulsatile signal ("volume wave") is displayed to visually display pulse and signal quality, and a numerical ratio between the pulsatile signal and baseline absorbance ("perfusion index") is available To assess perfusion.
Where HbO2 is oxyhemoglobin (oxyhemoglobin) and Hb is deoxygenated hemoglobin.
