Is pulse oximetry (PO) accurate in patients with shock?

Pulse Oximetry Accuracy in Shock States

Pulse oximetry is often unreliable in patients with shock, with forehead reflectance sensors being more accurate than traditional finger transmission sensors in severe shock states. 1, 2

Limitations of Pulse Oximetry in Shock

Shock states present several challenges for accurate pulse oximetry readings:

• Peripheral vasoconstriction: Reduced peripheral perfusion in shock significantly affects finger pulse oximetry accuracy 2
• Overestimation bias: Pulse oximetry typically overestimates arterial oxygen saturation by approximately 2.75% in severe sepsis and septic shock 3
• Worsened accuracy in hypoxemia: The overestimation is more pronounced (4.9% vs 1.89%) when patients are hypoxemic (SaO₂ <90%) 3
• Failure to obtain readings: Finger sensors may completely fail to obtain readings in severe shock requiring high-dose vasopressors 2

Alternative Monitoring Sites

When managing patients in shock:

• Forehead reflectance sensors are significantly more accurate than finger transmission sensors in patients requiring high-dose vasopressors (≥0.1 μg/kg/min) 2

  • Forehead sensors showed fewer clinically unacceptable outliers (15% vs 32%) compared to finger sensors 2
  • Limits of agreement were narrower for forehead sensors (-4.0% to 6.0%) than finger sensors (-8.0% to 10.9%) 2

• Pharyngeal pulse oximetry using an oropharyngeal airway with a pediatric pulse oximeter probe has been successfully used in severe shock when finger oximetry fails 4

  • Pharyngeal readings were only 0-2% lower than arterial samples in cases where finger readings were unobtainable 4

Clinical Implications and Recommendations

1. Verify with arterial blood gases: When accurate oxygen saturation measurement is critical in shock states, arterial blood gases should be obtained 3, 1

2. Be cautious with intermediate readings: When SpO₂ is in the 90-93% range in shock patients, 50% may actually have an SaO₂ <90% 3

3. Consider alternative monitoring sites:

   • Use forehead reflectance sensors in severe shock requiring vasopressors 2
   • Consider pharyngeal pulse oximetry in intubated patients with severe shock 4

4. Recognize specific effects of different shock types:

   • In septic shock with low systemic vascular resistance, pulse oximetry may underread by approximately 1.4% 5
   • This is hypothesized to be due to sensing of pulsatile venous flow from arteriovenous shunting in the skin 5

5. Monitor trends rather than absolute values: Pulse oximeters are better for monitoring trending phenomena than determining absolute magnitude of change 6

Factors Affecting Accuracy Beyond Shock

Additional factors that may compound inaccuracy in shock states:

• Dark skin pigmentation 6
• Motion artifacts 6, 7
• Nail polish or artificial nails 1
• Carboxyhemoglobinemia or methemoglobinemia 6, 7
• Severe anemia 7

Best Practices for Oxygen Management in Shock

• Aim for SpO₂ of 94-98% in most patients, or 88-92% in those at risk of hypercapnic respiratory failure 6, 1
• Document both SpO₂ reading and the oxygen delivery system being used 1
• Use arterial blood gas analysis for initial assessment and when SpO₂ readings are questionable 6
• Consider pulse oximetry as the "fifth vital sign" alongside other physiological parameters 1

Remember that while pulse oximetry is an invaluable tool in shock management, its limitations must be recognized, and clinical decisions should incorporate multiple parameters rather than relying solely on SpO₂ readings.