Oxygen Saturation

1. See Also
   1. Arterial Blood Gas
2. Precautions
   1. Oxygen Saturation can miss a large A-a Gradient
      1. Oxygen Saturation can be 100% when PaO2 is 90 mmHg or 300 mmHg
      2. At a high FIO2 of Supplemental Oxygen, Oxygen Saturation cannot distinguish a PaO2 above 90 mmHg
   2. Oxygen Saturation has a very steep drop off below 90-93%
      1. See Oxygen Saturation to PaO2 Relationship below
      2. PaO2 falls off rapidly, dropping to 60 mmHg at 90% Oygen Saturation
   3. Oxygen Saturation may be falsely increased by dehydration
      1. Expect an imperfect Oxygen Saturation (93-95%) on room air in patients with underlying cardiolpulmonary disease
      2. Suspect dehydration in a patient who should have ventilation-perfusion mismatch but instead has 100% O2 Sat on room air
      3. References
         1. Shipsey in Majoewsky (2012) EM:RAP 12(4): 3-4
3. Interpretation: Factors reducing O2 Sat reliability and accuracy
   1. Conditions with falsely depressed Oxygen Saturation
      1. Severe Anemia
      2. Reduced blood flow (consider using central location for detector such as forehead)
         1. Vasoconstriction
         2. Hypotension
         3. Blood Pressure cuff on arm with sensor
         4. Hypothermia
         5. Raynaud's Phenomenon
   2. Conditions with falsely elevated Oxygen Saturation
      1. Severe dehydration
      2. Carboxyhemoglobin (Carbon Monoxide Poisoning)
      3. Methemoglobinemia
      4. Tachypnea
   3. Miscellaneous factors
      1. Intense Ambient lighting
4. Physiology: Oxygen Saturation to PaO2 Relationship
   1. Non-Shifted Oxygen Saturation
      1. 100%: 90 mmHg PaO2
      2. 90%: 60 mmHg PaO2
      3. 60%: 30 mmHg PaO2
      4. 50%: 27 mmHg PaO2
   2. Mnemonic:
      1. 30-60%
      2. 60-90%
      3. 40-75%
5. Physiology: Shifted Oxygen Saturation Curve
   1. Left Shift (Same O2 Sat implies lower PaO2)
      1. Increased pH
      2. Decreased Temperature
      3. Decreased PaCO2
      4. Decreased 2,3-dpg
   2. Right Shift (Same O2 Sat implies higher PaO2)
      1. Increased Temperature
      2. Increased PaCO2
      3. Increased 2,3-dpg
      4. Decreased pH