What conditions can cause a discrepancy between hypoxemia on finger pulse (pulse oximeter) and normal arterial oxygenation on arterial blood gas?

Conditions Causing Pulse Oximetry Hypoxemia with Normal Arterial Blood Gas

The primary conditions causing falsely low pulse oximetry readings despite normal arterial oxygenation include carbon monoxide poisoning, methemoglobinemia, severe anemia, and technical/physiological factors affecting pulse oximeter accuracy.

Dyshemoglobinemias (Most Important Clinical Causes)

Carbon Monoxide Poisoning

• Carboxyhemoglobin causes pulse oximeters to read falsely normal or high SpO2 despite severe tissue hypoxia because pulse oximeters cannot distinguish carboxyhemoglobin from oxyhemoglobin 1
• The device measures only two wavelengths of light and interprets carboxyhemoglobin as oxygenated hemoglobin, masking true hypoxemia 1
• Arterial blood gas with co-oximetry is required for diagnosis, as standard ABG PaO2 will be normal 2
• Smoking significantly affects oxygen-carrying capacity and SpO2 readings due to elevated carboxyhemoglobin levels 3

Methemoglobinemia

• Methemoglobin absorbs light at both wavelengths used by pulse oximeters, typically causing SpO2 readings to plateau around 85% regardless of actual arterial oxygen saturation 1
• PaO2 on arterial blood gas remains normal because dissolved oxygen is unaffected 1
• Co-oximetry is essential for diagnosis 2

Severe Anemia

• Pulse oximeters can provide readings even at extreme anemia (hemoglobin as low as 1.9 mmol/L or 3.0 g/dL), though accuracy varies 4
• When high SpO2 values are displayed in severe anemia, they may reflect true arterial oxygen saturation, but the oxygen content (CaO2) is critically low despite normal PaO2 4, 5
• Oxygen content is calculated as: CaO2 = (SaO2 × Hb × 1.35), making hemoglobin concentration crucial for tissue oxygen delivery 5
• This represents a critical distinction: hypoxemia (low PaO2) versus tissue hypoxia (inadequate oxygen delivery) 5

Technical and Physiological Factors Affecting Pulse Oximetry Accuracy

Hemodynamic Instability

• Pulse oximeters may give inaccurate readings with extremes of heart rate, cardiac index, or pulmonary arterial wedge pressure, potentially misrepresenting arterial saturation by up to 7% 6
• Poor peripheral perfusion, cold extremities, and vasoconstriction can prevent accurate readings 7

Other Interfering Factors

• Dark skin pigmentation can affect accuracy 7
• Jaundice may interfere with readings 7
• Nail polish, particularly dark colors, can cause spurious readings 1
• Motion artifact during measurement 1

Clinical Scenarios with Normal PaO2 but Low SpO2 Readings

Alkalemia

• Indicated SpO2 may substantially overestimate arterial PaO2 if the patient is alkalemic due to leftward shift of the oxygen-hemoglobin dissociation curve 7
• The pulse oximeter reads saturation accurately, but the corresponding PaO2 is lower than expected 7

Exercise-Related Discrepancies

• Pulse oximetry readings may be inaccurate during exercise, particularly in patients with cardiopulmonary disease 7
• Exercise can unmask latent hypoxemia by increasing oxygen demand 3

Important Clinical Pitfalls

Pulse oximetry cannot detect hypercapnia or acidosis, which are critical in evaluating respiratory failure 7. A patient may have:

• Normal SpO2 with severe hypercapnia (elevated PaCO2) 7
• Normal SpO2 while on supplemental oxygen, masking underlying severe gas exchange abnormalities 3

In pulmonary embolism, up to 20% of patients have normal PaO2 despite the diagnosis, and 15-20% have normal alveolar-arterial oxygen gradient 6. This represents true normal oxygenation rather than a pulse oximetry artifact.

Diagnostic Approach

When pulse oximetry shows hypoxemia but you suspect normal arterial oxygenation:

1. Obtain arterial blood gas with co-oximetry to measure:

   • PaO2 (dissolved oxygen) 2
   • Carboxyhemoglobin levels 2
   • Methemoglobin levels 2
   • Actual oxygen saturation by co-oximetry 1

2. Check hemoglobin concentration to calculate oxygen content, as severe anemia can cause tissue hypoxia despite normal PaO2 and SpO2 4, 5

3. Assess for technical factors: peripheral perfusion, skin pigmentation, nail polish, motion artifact 1, 7

4. Consider clinical context: recent smoke exposure, chemical exposure (methemoglobin-inducing agents), hemodynamic status 6, 7

For initial selection of patients requiring intervention, pulse oximetry should not substitute for arterial blood gas analysis 7. Use pulse oximetry for monitoring stable patients and titrating oxygen, but always confirm with ABG when clinical decisions depend on accurate oxygenation assessment 7.