Oxygen saturation, commonly referred to as blood oxygen level and also known as SpO2, measures the amount of oxygen that is in your blood. More precisely, oxygen saturation is the measure of what percentage of hemoglobin is bound to oxygen molecules.
The pulse oximeter is the non-invasive instrument that allows to take such measure thanks to the fact that hemoglobin absorbs light differently when it is bound to oxygen: this is also the reason why oxygenated arterial blood is bright red in color, while deoxygenated venous blood is darker.
The saturation of oxygen in blood, taken with a fingertip pulse oximeter, is often considered one of the main vital signs.
What does SpO2 mean?
SpO2 stands for peripheral oxygen saturation and is the value detected by pulse oximeters.
Another common and more reliable measure of blood oxygen levels is the arterial blood gas analysis or SaO2. Although more precise, arterial blood gas analysis is an invasive test as it requires a healthcare professional to draw blood from an artery, usually from the wrist (radial artery) in an emergency setting, and it is not immediate as it needs a lab to analyze the sample.
Normal oxygen saturation (SpO2)
In a healthy person, a normal oxygen saturation is above 95%; it is considered abnormal in the 90-94% range and it requires medical attention below 90%.
Obesity, smoking, old age are factors that affect oxygen saturation making it lower.
Pulmonary diseases also may lower the SpO2: COPD patients for example have 88-92% as target saturation.
Are pulse-oximeter readings useful?
In general, symptoms and clinical signs should come before the numbers, with the latter being useful to better understand and to have an objective measure of the former.
Shortness of breath, chest tightness, rapid breathing, feeling tired and weak, wheezing, mental confusion, numbness or tingling of the extremities, cyanosis and the use of accessory respiratory muscles for breathing are some of the clinical signs and symptoms that you may have a respiratory problem.
In some cases though, you may feel ok and have no shortness of breath, but at the same time you may have low blood oxygen levels, a condition known as silent hypoxia which is also associated with the CoViD-19.
Pulse oximeters in COVID-19 patients: silent hypoxia
While in the general healthy population having a pulse oximeter at home may not be useful, for COVID-19 patients this instrument may actually help detect a complication of the disease: silent hypoxia. By measuring the SpO2 a couple of times per day you may be able to detect a lowering of your blood oxygen levels even when this is not reflected by your symptoms, allowing for a timely medical intervention.
Pulse oximeters in patients with chronic pulmonary disease
Patients with COPD may use pulse oximeters when a worsening of the usual symptoms of the disease happen, both to understand the seriousness of the exacerbation and to facilitate home treatment by adjusting the oxygen therapy to reach the target saturation of 88-92%.
Warning about pulse oximeter readings
Pulse oximeters are a very useful tool, but they are not perfect. More expensive oximeters may have more accurate sensors and the ability to provide more reliable readings, while cheaper versions may not even be intended for medical use and may not have passed any accuracy test.
Moreover, some factors may influence the readings such as:
- Nail polish. For a proper reading apply the sensor to a finger where nail polish was removed, or to a toe, or to the ear lobe. You may also try and place the pulse oximeter sideways on the finger.
- Cold fingers. Poor circulation due to cold fingers may influence the reading. Warm up the fingers by placing them in warm water and/or massage them to increase the perfusion to obtain a more accurate reading.
- Skin pigmentation and skin thickness may also influence the reading.
- Ambient light may reduce the accuracy: hospital pulse oximeters have rubber shields on the sides to reduce ambient light input.
- In case of carbon monoxide intoxication (CO), the oximeter may show a normal saturation: this is because the carboxyhemoglobin absorbs light roughly at the same wavelength as oxyhemoglobin.
- SpO2 below 83%. When oxygen levels are too low, the pulse oximeter may not provide an accurate reading. In these cases, an arterial blood gas analysis is usually performed in a hospital setting.
Don’t just rely on the self-measurement of oxygen levels and self-assessment of the symptoms: always consult your GP when you feel something is not right.
Warning about Oxygen Toxicity
Some people having access both to a pulse oximeter and to an oxygen gas cylinder, such as relatives of patients who have it prescribed or scuba divers, may think it is a good idea to get some oxygen when their saturation is on the low-end side.
Supplemental oxygen is a medical therapy and like any other prescription drug it should be used only when directed by a physician and as directed by a physician. Air-oxygen percentage mix (ranging 21-100%) and flow rate (ranging 0.5-15 L/min) should be decided by a doctor according to your specific needs.
Moreover, oxygen is actually toxic to the human organism: we would not survive in an atmosphere with 100% oxygen. Long term exposure to high levels of oxygen causes oxygen poisoning with effects on the central nervous system, on the lungs and in general to all organs and tissues. More oxygen means also more oxidative processes in the body which result in an increased production of reactive oxygen species (ROS) commonly known as free radicals. Oxygen also causes vasoconstriction of the blood vessels in the tissues. In COPD and myocardial infarction patients, high levels of supplemental oxygen are correlated to a worse outcome.
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