From the Guidelines
Arterial oxygen saturation (SaO2) is more important than arterial partial pressure of oxygen (PaO2) for immediate patient assessment, as it directly reflects the blood's oxygen-carrying capacity and is easily monitored continuously via pulse oximetry. According to the British Thoracic Society guideline for oxygen use in adults in healthcare and emergency settings 1, SaO2 is a critical parameter in assessing oxygenation, with a desirable target saturation range of 94–98%. This guideline emphasizes the importance of maintaining SaO2 above 90% for most acutely ill patients to prevent hypoxic tissue injury. In contrast, PaO2, while valuable for assessing alveolar gas exchange, is more sensitive to early changes in respiratory function and becomes particularly important in specific contexts, such as respiratory disorders or high altitudes. The relationship between SaO2 and PaO2, as described by the oxyhemoglobin dissociation curve, explains why SaO2 remains relatively stable until PaO2 drops below about 60 mmHg, making SaO2 a more practical choice for routine monitoring.
Some key points to consider when evaluating the importance of SaO2 and PaO2 include:
- The ease of continuous monitoring of SaO2 via pulse oximetry, making it a practical choice for real-time patient assessment 1
- The sigmoid shape of the oxyhemoglobin dissociation curve, which explains why SaO2 remains relatively stable until PaO2 drops below about 60 mmHg 1
- The importance of maintaining SaO2 above 90% for most acutely ill patients to prevent hypoxic tissue injury, as emphasized by the British Thoracic Society guideline 1
- The value of PaO2 in assessing alveolar gas exchange and its particular importance in specific contexts, such as respiratory disorders or high altitudes 1
In the context of post-cardiac arrest care, the American Heart Association guidelines recommend maintaining arterial oxyhemoglobin saturation at 94% or higher, with the goal of avoiding hyperoxia while ensuring adequate oxygen delivery 1. This further supports the importance of SaO2 in immediate patient assessment and management.
From the Research
Importance of Arterial Oxygen Saturation and PaO2
- Arterial oxygen saturation (SaO2 or SpO2) and partial arterial oxygen pressure (PaO2) are both crucial parameters in assessing a patient's oxygenation status 2, 3, 4, 5, 6.
- SaO2 measures the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen, while PaO2 measures the pressure of oxygen dissolved in the blood 2, 5.
Comparison of SaO2 and PaO2
- Studies have shown a good correlation between SaO2 and PaO2, with SaO2 being a reliable indicator of PaO2 in many cases 2, 5.
- However, SaO2 values cannot be used as an exact substitute for PaO2, as the relationship between the two is not always linear 5.
- The target SpO2 level to maintain PaO2 within a physiological range is around 95% 5.
Clinical Utility of SaO2 and PaO2 Monitoring
- Continuous monitoring of SaO2 using pulse oximetry has become a common practice in critically ill patients, allowing for early detection of hypoxemic events and timely intervention 2, 6.
- PaO2 monitoring is also essential in guiding oxygen therapy and avoiding hyperoxia, which can have adverse effects on patients 3, 4.
- The measurement of trends in SaO2, combined with respiratory rate, provides clinicians with a comprehensive overview of respiratory function and conditions associated with hypoxemia 6.