Why would arterial oxygen saturation (SaO₂) be higher than arterial oxygen partial pressure (PaO₂)?

Why SaO₂ Readings Are Higher Than PaO₂

SaO₂ is always numerically higher than PaO₂ because they measure fundamentally different things using different units: SaO₂ is a percentage (0-100%) representing hemoglobin saturation, while PaO₂ is a pressure measurement in mmHg or kPa, and these values cannot be directly compared numerically. 1

Understanding the Fundamental Difference

The confusion arises from comparing two completely different measurements:

• PaO₂ measures the partial pressure of dissolved oxygen in arterial blood, expressed in mmHg (or kPa) 1
• SaO₂ measures the percentage of hemoglobin binding sites occupied by oxygen, expressed as a percentage (%) 1, 2
• These are related through the oxygen-hemoglobin dissociation curve but use entirely different scales and units 1, 3

The Oxygen-Hemoglobin Dissociation Curve Relationship

The relationship between these values follows a curvilinear, non-linear pattern where you cannot accurately predict one from the other 3:

• At a PaO₂ of 60 mmHg, SaO₂ is approximately 90.7% 1, 3
• At a PaO₂ of 90 mmHg, SaO₂ is approximately 97.0% 1, 3
• At a PaO₂ of ≥127.5 mmHg, SaO₂ reaches only ≥99.0% (cannot exceed 100%) 1, 3

The upper portion of the curve is flat, meaning large increases in PaO₂ (from 90 to 127+ mmHg) produce minimal changes in SaO₂ (from 97% to 99%) because hemoglobin is already nearly fully saturated 1, 3

Clinical Implications of This Relationship

At High Oxygen Levels (Flat Portion of Curve)

• When PaO₂ is normal or elevated (>80 mmHg), SaO₂ remains in the 95-99% range regardless of how high PaO₂ climbs 1, 3
• Giving supplemental oxygen to a healthy person increases SaO₂ only slightly from 97% to 99-100%, despite potentially large increases in PaO₂ 1
• This protective mechanism ensures nearly complete oxygen saturation even with moderate drops in PaO₂ 1

At Low Oxygen Levels (Steep Portion of Curve)

• The steep lower portion means small changes in PaO₂ cause large changes in SaO₂ 1, 3
• At a PaO₂ of 45 mmHg, SaO₂ drops to 80.7% 1, 3
• At a PaO₂ of 30 mmHg, SaO₂ plummets to 57.4% 1, 3

Common Pitfalls to Avoid

Measurement Confusion

• Never directly compare the numerical values of PaO₂ (mmHg) and SaO₂ (%) as if they should match 1, 3
• SpO₂ (pulse oximetry) estimates SaO₂ but has accuracy limits of ±4-5% 2, 4
• Standard pulse oximeters cannot detect carboxyhemoglobin or methemoglobin, potentially giving falsely elevated SpO₂ readings in carbon monoxide poisoning 3, 2

Special Circumstances Where SaO₂ and PaO₂ Mismatch

• Hemoglobinopathies (e.g., Bassett hemoglobin) can cause decreased SaO₂ with normal PaO₂ due to altered oxygen-binding characteristics 5
• Factors that shift the dissociation curve rightward (increased temperature, increased PaCO₂, decreased pH, increased 2,3-DPG) cause lower SaO₂ at any given PaO₂ 1, 3
• Poor peripheral perfusion, movement artifacts, and dark skin color can interfere with pulse oximetry accuracy 2

Age-Related Variations

• Mean SaO₂ decreases with age: 96.9% in adults aged 18-24 years versus 95.5% in adults >64 years 1, 2
• The standard deviation is wider in older populations, indicating greater variability 1

When to Obtain ABG Instead of Relying on Pulse Oximetry

• When SpO₂ falls below 90%, pulse oximetry may not be accurate enough to reliably assess oxygenation 6
• In suspected alveolar hypoventilation, complement pulse oximetry with ABG analysis 6
• When carbon monoxide poisoning is suspected, laboratory-based SaO₂ measurements should confirm pulse oximetry readings 2
• In cases of unexplained desaturation with normal respiratory and cardiac investigations, consider hemoglobinopathy 5