Blood Oxygen Saturation (SpO2) : Definitive Guide

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Blood Oxygen Saturation (SpO2) Definitive Guide

Sketchy Description

Arterial oxygen saturation (SaO2) refers to the degree to which oxygen in arterial blood binds to hemoglobin (Hb), which is the percentage of oxygen per unit Hb, i.e. SaO2=HbO2/total Hb * 100%=blood oxygen content/blood oxygen binding rate * 100%. Arterial oxygen saturation indirectly reflects whether the body is hypoxic and the degree of hypoxia, but its sensitivity to hypoxia is insufficient, and it has the potential to mask hypoxia. In clinical practice, SaO2 is detected through arterial blood gas analysis. Arterial oxygen saturation helps doctors diagnose whether a patient has hypoxia.

  • Normal level: 95% ~ 100%
  • Examination purpose: It helps to understand whether the patient has a hypoxic state
  • High risk value: less than 80%, which may cause multiple organ hypoxia and damage
  • Adjustment suggestion: After confirming the presence of hypoxemia, the cause will be identified and corresponding treatment measures will be taken

What Is The Normal Value of Arterial Oxygen Saturation?

What is the normal value of arterial oxygen saturation

The normal range of arterial oxygen saturation is 95% to 100%.

However, due to the S-shaped hemoglobin dissociation curve (ODC), when the arterial oxygen partial pressure (PaO2) is below 57mmHg, the ODC curve becomes steep and the oxygen partial pressure slightly decreases, resulting in a significant decrease in SaO2. However, ODC is influenced by factors such as pH, PaCO2, temperature, and the content of 2,3-diphosphoglycerate (2,3-DPG) in red blood cells, which in turn affects the speed of Hb binding to oxygen and the accuracy of arterial oxygen saturation in determining whether the body is hypoxic.

What Results Indicate Abnormal Arterial Oxygen Saturation?

What results indicate abnormal arterial oxygen saturation

Below 95% indicates that the body may be in a state of hypoxia. Arterial oxygen saturation is an indicator of hypoxemia, with arterial oxygen saturation reaching 100%. Due to increased intake of oxygen by the body, it is commonly seen in hyperbaric oxygen therapy.

For athletes and people living in high-altitude areas for a long time, it is normal for SaO2 to exceed 90% when the heart rate does not exceed 100. But if the patient is anemic or has acute massive bleeding, even if the arterial oxygen saturation value is normal, the body still has hypoxia (referred to here as hypoxia refers to tissue, organ, and cell hypoxia).

What Factors May Cause Inaccurate Arterial Oxygen Saturation Results?

Arterial oxygen saturation is detected by extracting arterial blood on a blood gas analyzer. Currently, arterial oxygen saturation on the blood gas analyzer is calculated based on arterial oxygen partial pressure and pH. So when conducting arterial blood gas analysis, attention should be paid to the collection and processing of arterial blood samples. Mixing venous blood during blood collection can lead to a decrease in arterial oxygen saturation compared to the actual value. After blood collection, if the blood remains stationary for too long and is not placed in ice water or low temperature, it can also affect the authenticity of the values. The syringe used for blood collection needs to undergo special anticoagulation pretreatment. The arterial blood samples collected should not be mixed with air, nor should they be exposed to the air.

How to Interpret The Results of Arterial Oxygen Saturation Examination?

How to interpret the results of arterial oxygen saturation examination

Arterial oxygen saturation is an indicator that reflects whether the body is hypoxic. Together with pulse oximetry (SpO2) and arterial partial oxygen pressure (PaO2), it can be used to determine whether a patient is in a state of hypoxia.

What Indicators Are Similar to Arterial Oxygen Saturation And What Are The Differences?

  • Pulse blood oxygen saturation (SpO2): refers to the percentage of hemoglobin (HbO2) bound to oxygen in the blood of terminal or other peripheral blood vessels, which is the concentration of oxygenated hemoglobin. After excluding interfering factors, SpO2 is non-invasive compared to SaO2, but when there is abnormal hemoglobin in the blood, weakened peripheral pulse, and the presence of dyes in the vein, SpO2 may experience reading errors.
  • Arterial oxygen partial pressure (PaO2): refers to the pressure generated by physically dissolved oxygen molecules in the blood. The oxygen partial pressure of healthy adults is influenced by age, and as age increases, arterial blood oxygen partial pressure decreases. The expected age formula is PaO2=100mm Hg – (age * 0.33)+-5mm Hg. Its reference value is 95-100mmHg (12.6-13.3kPa). Like SaO2, it is an indicator to determine whether the body has hypoxia and the degree of hypoxia. According to the relationship between oxygen partial pressure and arterial oxygen saturation, the dissociation curve of oxygenated hemoglobin shows an S-shape. When PaO is greater than 60mmHg, the curve is flatter, with blood oxygen saturation above 90%. PaO2 changes 40mmHg, but arterial oxygen saturation changes less. Therefore, arterial oxygen partial pressure is more sensitive than arterial oxygen saturation. When PaO2 is less than 60mmHg, the curve is steep, and oxygen partial pressure slightly changes, resulting in a sharp change in blood oxygen saturation, Therefore, arterial oxygen partial pressure less than 60mmHg is a diagnostic indicator for respiratory failure.
  • Pulmonary alveolar arterial oxygen partial pressure difference (PAO2): refers to the difference between the oxygen partial pressure in the alveoli and the arterial oxygen partial pressure. It is an indicator of the body’s lung ventilation function and can reflect the oxygen uptake in lung tissue earlier. More sensitive than PaO2. However, in clinical practice, the alveolar arterial oxygen partial pressure difference cannot be directly measured, and is calculated by simplifying the alveolar gas equation. The difference in alveolar arterial oxygen partial pressure in normal young people is 15-20mmHg (2-2.7kPa), which is also influenced by age. The older the age, the greater the difference in alveolar arterial oxygen partial pressure.
  • Arterial blood oxygen content (CaO2): refers to the total amount of oxygen contained in arterial blood per unit volume or the milliliters of oxygen per 100ml of arterial blood. The normal range is 8.55~9.45mmol/L (19~21ml/dl). CaO2, like SaO2, is an indicator that reflects the oxygen content carried in arterial blood.

Common Questions about Blood Oxygen Saturation

Common questions about blood oxygen saturation

Does The Body Suffer from Hypoxia Due to 90% Arterial Oxygen Saturation?

Not necessarily, arterial oxygen saturation is affected by the ODC curve. Although PaO2 has significantly decreased during mild hypoxia, SaO2 may not show significant changes. Specific analysis should be based on the results of blood gas analysis. When clear symptoms of hypoxemia appear, such as cyanosis, immediate medical attention should be sought, and oxygen therapy for hypoxemia should be sought, while identifying the cause and providing targeted treatment.

Is The Blood Oxygen Saturation Monitored by Daily Fingertip Monitoring Arterial Blood Oxygen Saturation?

No, the common blood oxygen saturation of fingertips in daily life is pulse blood oxygen saturation (SpO2), which reflects the concentration of oxygenated hemoglobin in the blood vessels at the end of the finger and is more affected by external factors. But the advantage of nail oximeter is that it is portable, intuitive, and can be continuously monitored, which can be applied to continuous monitoring and home monitoring of blood oxygen content. Especially in patients with obstructive sleep apnea syndrome, hypoxemia during nighttime sleep often occurs. Pulse oximetry can effectively assess the severity of the condition and the improvement after treatment.

But when the patient’s condition is severe, with severe hypoxemia or other abnormalities, arterial oxygen saturation monitoring is necessary to more clearly reflect the body’s hypoxia status.

Does Occasional Low Arterial Oxygen Saturation Require Treatment?

Arterial oxygen saturation occurs when there is not enough oxygen in the body to meet the body’s needs.

Hypoxia can cause damage to multiple organs in the body, such as the central nervous system, liver, kidneys, and circulatory system. The severity of hypoxia has different damage effects on the body. Severe hypoxemia is the most common cause of death during surgical general anesthesia, mostly caused by cardiac arrest caused by hypoxia or severe damage to brain cells in the central nervous system. Therefore, it is necessary to actively investigate the cause of decreased arterial oxygen saturation. If there are no other abnormalities after examination, it is necessary to monitor finger pulse oxygen, and no other special treatment is required. If there is hypoxemia, targeted oxygen therapy is required. In addition to targeted treatment, it is also necessary to identify the cause and treat it accordingly. If pneumonia exists, anti infection treatment is required. Chronic obstructive pulmonary disease requires targeted treatment such as low flow oxygen therapy and airway dilation.

Arterial oxygen saturation is affected by exercise, and vigorous exercise should be avoided.

Is Arterial Oxygen Saturation Normal, So There’s No Need to Worry?

No, it’s not.

Monitoring arterial oxygen saturation is an assessment of lung oxygenation and hemoglobin carrying capacity, but arterial oxygen saturation is influenced by many factors. In the case of acute CO poisoning, arterial oxygen saturation cannot distinguish between hemoglobin bound to CO and hemoglobin bound to oxygen, and therefore cannot accurately reflect the oxygen content in the body’s arterial blood. At this time, arterial oxygen saturation is normal and the body is in a state of hypoxia. For patients with CO poisoning, high concentration oxygen therapy is necessary according to the severity of the disease to solve the problem of hypoxia in the body.

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