At what oxygen saturation (SpO2) level do symptoms like pre-syncope typically occur?

From the Guidelines

Symptoms of pre-syncope typically occur when oxygen saturation (SpO2) levels fall below 80-85%. At this level, the brain begins to experience insufficient oxygen supply, leading to lightheadedness, dizziness, confusion, and a feeling of impending loss of consciousness. The severity of symptoms generally correlates with how rapidly and severely the oxygen levels drop. Healthy individuals normally maintain SpO2 levels between 95-100%, and mild hypoxemia (90-94%) may not cause noticeable symptoms in many people. However, individuals with underlying cardiopulmonary conditions may experience pre-syncope at higher SpO2 levels than healthy people.

Key Points

• The body initially compensates for falling oxygen levels by increasing heart rate and respiratory rate, but when these compensatory mechanisms fail to maintain adequate oxygen delivery to vital organs, particularly the brain, pre-syncope symptoms emerge.
• If oxygen levels continue to drop below 75%, full syncope (fainting) may occur as a protective mechanism to position the body horizontally, which helps maximize blood flow to the brain.
• According to the BTS guideline for oxygen use in adults in healthcare and emergency settings 1, sudden exposure to low SaO2 levels (below about 80%) can cause impaired mental functioning even in healthy participants.
• The 2024 American Heart Association and American Red Cross Guidelines for First Aid 1 also discuss the importance of recognizing presyncope symptoms to prevent syncope.
• The Surviving Sepsis Campaign: Guidelines on the Management of Critically Ill Adults with Coronavirus Disease 2019 (COVID-19) 1 recommends starting supplemental oxygen if the peripheral oxygen saturation (SpO) is < 92%, and maintaining SpO2 no higher than 96% in adults with COVID-19 and acute hypoxemic respiratory failure on oxygen.

Recommendations

• Monitor oxygen saturation levels closely in individuals with underlying cardiopulmonary conditions, as they may experience pre-syncope at higher SpO2 levels than healthy people.
• Provide supplemental oxygen if SpO2 levels fall below 92% to prevent pre-syncope and syncope.
• Maintain SpO2 levels between 94-98% in most hypoxaemic patients to ensure adequate oxygen delivery to vital organs.

From the Research

Symptoms of Pre-Syncope and Oxygen Saturation Levels

• Symptoms like pre-syncope typically occur when the body, or a part of the body, is not receiving enough oxygen 2.
• The relationship between oxygen saturation (SpO2) levels and the onset of pre-syncope symptoms is not directly stated in the provided studies, but it is known that hypoxia can lead to symptoms such as dizziness and fainting 3, 4.

Oxygen Saturation Levels and Hypoxia

• A SpO2 level of less than 90% is considered a clinical emergency 3.
• If the SpO2 level is below 94%, the patient is considered hypoxic until proven otherwise, and may require supplemental oxygen administration 3.
• The Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines recommend evaluation with arterial blood gas (ABG) analysis if SpO2 is ≤92% 4.

Specific SpO2 Levels and Symptoms

• There is no specific SpO2 level mentioned in the studies as the threshold for feeling symptomatic like pre-syncope.
• However, it is suggested that a SpO2 level of ≤94% may be a cutoff for detecting severe resting hypoxemia in patients with COPD 4.
• In patients with COPD, a SpO2 level of 90% or lower may be associated with symptoms such as shortness of breath and fatigue 5, 6.