Oxygen Saturation

Aka: Oxygen Saturation, O2 Sat, Pulse Oximeter, Pulse Oximetry, Oxyhemoglobin Saturation, Arterial Blood Oxygen Content, CaO2, Oxygen Saturation Curve, Oxyhemoglobin Dissociation Curve, Perfusion Index

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II. Physiology

  1. Most oxygen (97%) is transported in circulation bound to Hemoglobin
    1. Oxygen is otherwise poorly soluble in plasma
  2. Oxygen dissociation curve
    1. Facilitates oxygen binding in the alveoli even at low inspired oxygen concentrations
      1. Hemoglobin remains well saturated (90%) even when alveolar oxygen drops to 60 mmHg
      2. However, at very low levels of alveolar oxygen (<40 mmHg), Hemoglobin remains poorly saturated
    2. Facilitates oxygen release for tissue delivery at adequate levels, but not excessive
      1. Hemoglobin releases oxygen to tissues where oxygen concentration is typically 20-40 mmHg
      2. Peripheral tissue acidosis (lower pH) shifts the curve right, with greater release of oxygen to tissue
    3. Oxygen dissociation curve shifts with environmental conditions
      1. See Shifted Oxygen Saturation Curve below
      2. Curve shifts right with physiologic stress (fever, increased PaCO2, decreased pH)
        1. Hemoglobin releases more oxygen for a given Oxygen Saturation
    4. Images
      1. oxyhemoglobinDissociationCurve.png

III. Mechanism: Oxygen Saturation

  1. Oximeter probe applied to a finger, toe or ear lobe
  2. Light transmission circuit
    1. Light emitted by 2 LEDs (one red and one infrared)
    2. Light transmits through blood and soft tissue and is partially absorbed
      1. Oxygenated Hemoglobin Absorbs light at a different rate than deoxygenated Hemoglobin
    3. Light received by a photodiode opposite the LED
  3. Oxygen Saturation calculated based on the Oxyhemoglobin Saturation

IV. Mechanism: Perfusion Index

  1. Perfusion Index may also be interpreted from Pulse Oximeter waveform
    1. Reflects the strength of the pulse arriving at the detector
    2. Perfusion Index is the ratio of pulsatile to nonpulsatile blood
    3. Unlike oximetry which detects Oxygen Saturation, Perfusion Index reflects Blood Flow
      1. Waveform will not improve with oxygenation
      2. Waveform improves with increased Cardiac Output or vasodilation
  2. Normal Perfusion Index 0.3 to 20
    1. Oxygen Saturation waveform is lost at ratio <0.5
  3. Decreased Perfusion Index causes
    1. Decreased Cardiac Output (e.g. Cardiogenic Shock)
    2. Vasoconstriction (e.g. Hemorrhagic Shock)
  4. References
    1. Weingart and Swaminathan in Herbert (2021) EM:Rap 21(9): 6-7

V. Precautions

  1. Oxygen Saturation under ideal conditions is +/- 2%
  2. 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
    3. Normal PaO2 on FIO2 100% should be >500 mmHg
    4. Normal PaO2 on FIO2 50% should be >250 mmHg (linear relationship)
  3. Oxygen Saturation drop is delayed in apnea
    1. Supplemental Oxygen maintains oxygenation without desaturation for minutes despite apnea (see Apneic Oxygenation)
    2. Carbon dioxide however increases with apnea and Respiratory Acidosis develops
    3. End-Tidal CO2 is a better monitor of respiratory status (reflects apnea minutes before Oxygen Saturation drops)
  4. 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 Delivery is dependent on both Oxygen Saturation and Cardiac Output
      1. At normal Cardiac Output, a PaO2 50 mmHg might be sustained for months
      2. With decreased cardiac ouput and other comorbidity, PaO2 50 mmHg may trigger rapid decompensation
    4. Further drops in PaO2 become non-sustainable even with normal Cardiac Output
      1. PaO2 40 mmHg (75% O2 Sat) may be tolerated for days if otherwise stable
      2. PaO2 30 mmHg (50% O2 Sat) may be tolerated hours if otherwise stable
      3. PaO2 20 mmHg (little better than total anoxia) may be tolerated for minutes
  5. 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 if patient has ventilation-perfusion mismatch but has 100% O2 Sat on room air
    3. References
      1. Shipsey in Majoewsky (2012) EM:RAP 12(4): 3-4

VI. Interpretation: Factors reducing Oxygen Saturation reliability and accuracy

  1. Conditions with falsely depressed Oxygen Saturation
    1. Interference
      1. Nail polish or false nails
      2. Excessive sensor motion or poorly adherent detector
      3. Bright or intense Ambient light
        1. Amar (1989) J Clin Monit 5(2):135-6 [PubMed]
    2. Severe Anemia (Hematocrit <15%)
    3. 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
    5. Lipid suspensions (e.g. Propofol) or Hyperlipidemia
    6. Darker skin (e.g. black)
      1. Target Oxygen Saturation >94% in dark skin, and Oxygen Saturation >92% in light skin

VII. Interpretation: Newborns

  1. Oxygen Saturation does not normally increase to >85% until after 10 minutes of life in newborns
  2. Normal Oxygen Saturation by minutes of life
    1. Oxygen Saturation at 1 minute: 60-65%
    2. Oxygen Saturation at 2 minutes: 65-70%
    3. Oxygen Saturation at 3 minutes: 70-75%
    4. Oxygen Saturation at 5 minutes: 80-85%
    5. Oxygen Saturation at 10 minutes: 85-90%
  3. References
    1. Claudius, Behar, Nichols in Herbert (2015) EM:Rap 15(1): 3-4

VIII. Physiology: Arterial Blood Oxygen Content (CaO2)

  1. CaO2 = Hgb * 1.34 * SaO2 + (0.003 * PaO2)
    1. Where CaO2 is Arterial Blood Oxygen Content in ml O2/dl
    2. Where Hgb is Hemoglobin in g/dl
    3. Where SaO2 is Oxygen Saturation in % (O2Sat, fraction e.g. 0.95 = 95%)
    4. Where PaO2 is Partial Pressure of oxygen in mmHg
  2. Normal CaO2 = 18-20 ml/dl
    1. Given normal Hemoglobin And Oxygen Saturation
  3. Hemoglobin (as key oxygen transporter) is the most important contributor to oxygen availability to tissues
    1. Each gram Hemoglobin transports 1.34 ml oxygen
  4. Dissolved oyxgen contributes minimally to Oxygen Delivery at tissue level
    1. Reflected in the equation by (0.003 * PaO2)

IX. Physiology: Oxygen Saturation to PaO2 Relationship

  1. Non-Shifted Oxygen Saturation (unreliable above 97%)
    1. 97%: 110 mmHg PaO2
    2. 96%: 90 mmHg PaO2
    3. 95%: 80 mmHg PaO2
    4. 94%: 73 mmHg PaO2
    5. 93%: 68 mmHg PaO2
    6. 92%: 64 mmHg PaO2
    7. 91%: 60 mmHg PaO2
    8. 90%: 58 mmHg PaO2
    9. 89%: 56 mmHg PaO2
    10. 88%: 54 mmHg PaO2
    11. 87%: 52 mmHg PaO2
    12. 86%: 51 mmHg PaO2
    13. 60%: 30 mmHg PaO2
    14. 50%: 27 mmHg PaO2
  2. Mnemonic:
    1. 30-60%
    2. 60-90%
    3. 40-75%

X. Physiology: Shifted Oxygen Saturation Curve (Oxyhemoglobin Dissociation Curve)

  1. Left Shift (pathologic causes)
    1. Background
      1. Same O2 Sat implies lower PaO2, higher O2 affinity, less O2 tissue delivery
      2. Hemoglobin releases less oxygen for a given Oxygen Saturation
    2. Increased pH
    3. Decreased Temperature (Hypothermia)
    4. Decreased PaCO2
    5. Decreased 2,3-DPG (2,3-Diphosphoglycerate)
    6. Carbon Monoxide Poisoning
    7. Methemoglobinemia
  2. Right Shift (physiologic stress)
    1. Background
      1. Same O2 Sat implies higher PaO2, lower O2 affinity, greater O2 tissue delivery
      2. Hemoglobin releases more oxygen for a given Oxygen Saturation
    2. Increased Temperature (hyperthermia)
    3. Increased PaCO2
    4. Increased 2,3-DPG (2,3-Diphosphoglycerate)
      1. Chronic Hypoxemia increases 2,3-DPG, and allows for maximal Oxygen Saturation of Hemoglobin
    5. Decreased pH (acidosis)
  3. Images
    1. oxyhemoglobinDissociationCurve.png

XI. References

  1. Killu and Sarani (2016) Fundamental Critical Care Support, p. 93-114

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