Is the partial pressure of arterial oxygen (PaO2) or arterial oxygen saturation (SaO2) more important?

From the Guidelines

SaO2 is generally more clinically useful in everyday practice than PaO2, as it directly indicates how well hemoglobin is saturated with oxygen, which is what ultimately matters for tissue oxygenation. Both PaO2 (partial pressure of oxygen in arterial blood) and SaO2 (arterial oxygen saturation) are important measurements, but SaO2 can be continuously monitored non-invasively using pulse oximetry, making it practical for ongoing patient assessment 1. PaO2, measured through arterial blood gas analysis, provides valuable information about alveolar gas exchange and is more sensitive to early changes in respiratory function, particularly at higher oxygen levels where the oxyhemoglobin dissociation curve flattens (meaning SaO2 may remain normal despite falling PaO2) 1. However, PaO2 requires invasive blood sampling and cannot be continuously monitored.

In clinical practice, SpO2 monitoring is used for routine assessment, while PaO2 is obtained when more detailed information about gas exchange is needed, such as in critical care settings or when evaluating respiratory failure 1. The relationship between these values follows the sigmoid-shaped oxyhemoglobin dissociation curve, which explains why SaO2 can remain above 90% even when PaO2 drops significantly below normal 1. Most experts emphasize the importance of keeping the SaO2 above 90% for most acutely ill patients, and the guideline recommends a target SaO2 range of 94-98% for most hypoxaemic patients 1.

Some key points to consider when evaluating PaO2 and SaO2 include:

• The normal range for SaO2 is approximately 95-98% in healthy adults 1
• Hypoxaemia can be defined as SaO2 <94%, <92%, or <90%, or PaO2 <60 mm Hg or 8 kPa 1
• The brain is the most sensitive organ to the adverse effects of hypoxia, and sudden exposure to low SaO2 levels (below about 80%) can cause impaired mental functioning even in healthy participants 1
• The guideline recommends aiming for a normal or near-normal SaO2 range of 94-98% for acutely ill patients, except those at risk of hypercapnic respiratory failure when a target saturation range of 88-92% should be used 1

From the Research

Importance of PaO2 and SaO2

• PaO2 (arterial oxygen tension) and SaO2 (arterial oxygen saturation) are both important parameters in assessing oxygenation status in patients 2, 3, 4, 5, 6.
• PaO2 measures the amount of oxygen dissolved in the blood, while SaO2 measures the percentage of hemoglobin that is saturated with oxygen 5.

Comparison of PaO2 and SaO2

• A study found that PaO2 levels needed to avoid tissue hypoxia were higher in patients with pulmonary hypertension and decreased cardiac index than in those with preserved cardiac index 2.
• Another study suggested that precise control of arterial oxygenation, involving the specific targeting of arterial partial pressure of oxygen or arterial hemoglobin oxygen saturation, can help avoid the harms associated with inadvertent hyperoxia or hypoxia 3.
• SaO2 has been proposed as the fifth vital sign, with its measurement providing a holistic overview of respiratory functions and multidimensional conditions associated with hypoxemia 5.

Clinical Utility of PaO2 and SaO2

• PaO2 and SaO2 are both useful in detecting hypoxemic events, with PaO2 providing a more direct measure of oxygen levels in the blood and SaO2 providing a measure of oxygen saturation 2, 3, 5, 6.
• Supplemental oxygen can help blunt the drop in SaO2 caused by inhaled bronchodilator therapy, highlighting the importance of monitoring oxygen saturation in clinical practice 6.
• The choice between PaO2 and SaO2 may depend on the specific clinical context and the information needed to guide patient care 2, 3, 4, 5, 6.