The Correlation Between PaO2 and SaO2
The relationship between partial pressure of arterial oxygen (PaO2) and arterial oxygen saturation (SaO2) follows a curvilinear pattern known as the oxygen-hemoglobin dissociation curve, where SaO2 cannot accurately predict PaO2 and vice versa due to the non-linear nature of this relationship. 1
The Oxygen-Hemoglobin Dissociation Curve
The oxygen-hemoglobin dissociation curve has two key features that protect against tissue hypoxia:
- The upper portion of the curve is flat, meaning a significant fall in PaO2 can still maintain nearly complete oxygen saturation 1
- The steep lower portion of the curve means that despite rapidly falling oxygen saturation, the PO2 remains relatively well preserved, facilitating continued oxygen delivery to tissues 1
Approximate Relationship Between PaO2 and SaO2
The following table shows the approximate relationship between PaO2 and SaO2:
| PaO2 (kPa) | PaO2 (mmHg) | SaO2 (%) |
|---|---|---|
| 4 | 30 | 57.4 |
| 5 | 37.5 | 71.4 |
| 6 | 45 | 80.7 |
| 7 | 52.5 | 86.8 |
| 8 | 60 | 90.7 |
| 9 | 67.5 | 93.2 |
| 10 | 75 | 94.9 |
| 11 | 82.5 | 96.2 |
| 12 | 90 | 97.0 |
| ≥17 | ≥127.5 | ≥99.0 |
Clinical Implications
- At lower PaO2 values (steep portion of the curve), small changes in PaO2 result in large changes in SaO2 1
- At higher PaO2 values (flat portion of the curve), large changes in PaO2 result in minimal changes in SaO2 1
- PaO2 of 50 mmHg can correspond to widely varying SaO2 values (80-92%) in different patients, demonstrating that individual variation exists 2
- At the upper limit of arterial oxygenation, PaO2 is a more sensitive indicator compared to SaO2 due to the flattening of the curve 2
Factors That Shift the Oxygen-Hemoglobin Dissociation Curve
The Bohr effect describes how the curve shifts in response to metabolic factors:
Right shift (decreased oxygen affinity, easier release to tissues) occurs with:
Chronic hypoxemia increases 2,3-DPG in erythrocytes, shifting the curve rightward and improving oxygen delivery to tissues 1
Clinical Measurement Considerations
- Pulse oximetry (SpO2) has moderate correlation with arterial oxygen saturation (SaO2) with correlation coefficients around 0.66-0.69 3, 4
- Pulse oximetry is generally reliable at SaO2 values above 65%, but may overestimate arterial SaO2 at lower values 5
- The mean difference between SaO2 and SpO2 is approximately 0.2% with limits of agreement of ±6% 4
- For patients with abnormal oxygenation near clinical decision thresholds, arterial blood gas measurement is more accurate than calculated values 3
Age-Related Variations
- PaO2 and SaO2 values tend to decrease with age 1
- In adults aged >64 years, mean SaO2 is approximately 95.5% compared to 96.9% in adults aged 18-24 years 1
- The standard deviation is wider in older age groups, indicating greater variability 1
Pitfalls and Caveats
- Carboxyhemoglobin (COHb) can confound oxygen saturation measurements 1
- Standard pulse oximeters using two wavelengths cannot differentiate carboxyhemoglobin from oxyhemoglobin, potentially leading to falsely elevated SpO2 readings 1
- In high-altitude settings, the correlation between PaO2/FiO2 and SpO2/FiO2 may be affected, requiring special consideration 6
- Perfusion index (PFI) has limited value in improving the accuracy of SpO2 for estimating arterial oxygenation 4
Understanding this non-linear relationship is crucial for proper interpretation of blood gas results and oxygen saturation measurements in clinical practice, especially when making decisions about oxygen therapy and ventilatory support.