What is the difference between SpO2 (peripheral capillary oxygen saturation) and PaO2 (partial pressure of oxygen in arterial blood) in patients with respiratory or cardiovascular diseases?

Difference Between SpO2 and PaO2

SpO2 (peripheral capillary oxygen saturation) measures the percentage of hemoglobin saturated with oxygen using a pulse oximeter, while PaO2 (partial pressure of oxygen in arterial blood) measures the actual pressure of dissolved oxygen in arterial blood via blood gas analysis. 1

Key Distinctions

What Each Measurement Represents

• SpO2 is measured non-invasively by pulse oximetry and reflects the percentage of hemoglobin binding sites occupied by oxygen molecules 1
• PaO2 is measured invasively from arterial blood samples and represents the partial pressure of oxygen dissolved in plasma, expressed in kilopascals (kPa) or millimeters of mercury (mmHg), with normal range 90–110 mm Hg (11.98–14.82 kPa) for young adults 1
• SaO2 is the arterial oxygen saturation measured directly from arterial blood gas analysis, which is the gold standard for oxygen saturation measurement 1

Normal Values by Age

For SpO2 (pulse oximetry): 1

• Ages 18-24: Mean 98.0% (range 97-99%)
• Ages 25-34: Mean 97.6% (range 97-99%)
• Ages 45-54: Mean 96.8% (range 96-98%)
• Ages ≥65: Mean 95.8% (range 95-98%)

For PaO2 (arterial blood gas): 1

• Ages 18-24: Mean 13.4 kPa (100.5 mmHg)
• Ages 45-54: Mean 13.0 kPa (97.5 mmHg)
• Ages ≥65: Mean 11.89 kPa (89.2 mmHg)

The Oxygen-Hemoglobin Dissociation Curve Relationship

The relationship between SpO2 and PaO2 is non-linear and follows the oxygen-hemoglobin dissociation curve. 2

• When SpO2 is below 90%, small decreases in PaO2 cause large decreases in SpO2, making this a critical threshold 2
• A PaO2 of 63 mmHg corresponds to SpO2 of approximately 90-92%, placing it at the steep portion of the curve 2
• Above SpO2 of 90%, the curve flattens, meaning large changes in PaO2 produce minimal changes in SpO2 2

Factors That Shift the Curve

Right shift (decreased hemoglobin-oxygen affinity): 2

• Fever
• Acidosis
• Increased 2,3-DPG
• Results in lower SpO2 for the same PaO2

Left shift (increased hemoglobin-oxygen affinity): 3, 2

• Hypothermia
• Alkalosis
• Decreased 2,3-DPG
• Methemoglobinemia
• Results in higher SpO2 for the same PaO2 but impaired tissue oxygen delivery

Clinical Accuracy and Limitations

SpO2 Overestimation of Actual Oxygenation

• SpO2 commonly overestimates SaO2, with a bias of 3.8% at rest (limits of agreement 0.3% to 7.9%) 4
• At peak exercise, the bias is 2.6% (limits of agreement -2.9% to 8.1%) 4
• SpO2 may read 98% when PaO2 is as low as 70 mmHg, providing no advance warning of falling arterial oxygenation 5
• Pulse oximeters have an accuracy of ±2% under normal circumstances, but poorly placed probes and motion artifact lead to inaccurate measurements 2

Factors Associated with Greater SpO2-SaO2 Discrepancy

Patients with SpO2 ≥5% higher than SaO2 are: 4

• Older
• Current smokers
• Have lower forced expiratory volume in first second
• Have higher partial pressure of carbon dioxide
• Have higher carboxyhemoglobin levels (independently associated in multivariate analysis)

When SpO2 is Unreliable

• When SpO2 falls below 90%, pulse oximetry may not be accurate enough to reliably assess oxygenation 6
• In hypoxemic, hemodynamically compromised, and critically ill patients, SpO2 accuracy is lower 6
• Normal SpO2 readings may mask tissue hypoxia in patients with left-shifted oxygen dissociation curves 3

Clinical Application and Target Values

Oxygen Therapy Thresholds

Oxygen therapy is recommended when: 1

• SpO2 <90% or PaO2 <60 mmHg (8.0 kPa)

Target SpO2 ranges: 1

• Most patients: SpO2 should be above 90% and no higher than 96%
• Patients with strong respiratory drive (low/normal PaCO2): Target SpO2 ≥94%
• Patients with type 2 respiratory failure: Target SpO2 88-92%
• Pregnant patients: Target SpO2 92-95%

When to Obtain Arterial Blood Gas

ABG analysis should be considered when: 1

• SpO2 <90% and alveolar hypoventilation is suspected 6
• Patient has respiratory distress with SpO2 <90% and respiratory rate >25 breaths/min 1
• Measurement of blood pH and carbon dioxide tension is needed, especially in patients with acute pulmonary edema or COPD history 1
• In cardiogenic shock, arterial blood is preferable to venous blood 1

Age-Specific Considerations

• For adults ≥65 years, use an alveolar-arterial oxygen gradient (P[A-a]O2) cut-off of ≥20 mmHg instead of ≥15 mmHg, as mean oxygen saturation is approximately 2% lower than in young adults 2
• The 2SD range for SaO2 in adults >64 years is 92.7–98.3%, wider than younger adults 1

Common Pitfalls to Avoid

• Do not rely solely on SpO2 in critically ill patients – complement with ABG analysis when SpO2 <90% or clinical deterioration occurs 6
• Do not assume normal tissue oxygenation based on normal SpO2 alone – patients with left-shifted curves can have significant tissue hypoxia despite normal saturation 3
• Do not ignore carboxyhemoglobin levels in smokers – this independently affects the SpO2-SaO2 difference 4
• Do not use excessive oxygen supplementation in COPD patients – target SpO2 88-92% in those with type 2 respiratory failure to avoid suppressing hypoxic ventilatory drive 1
• Do not forget that stored blood has depleted 2,3-DPG – use fresh blood when possible for transfusions, as 2,3-DPG normalizes within 48 hours 3