Lab

Oxygen Saturation

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Oxygen Saturation, O2 Sat, Pulse Oximeter, Pulse Oximetry, Oxyhemoglobin Saturation, Arterial Blood Oxygen Content, CaO2

  • Mechanism
  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
  • 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
  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
  • Interpretation
  • Factors reducing O2 Sat 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
  • 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
  • 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)
  • 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%
  • Physiology
  • Shifted Oxygen Saturation Curve (Oxyhemoglobin Dissociation Curve)
  1. Physiologic stress shifts curve right
    1. Lower oxygen-Hemoglobin Affinity
    2. Greater oxygen release to tissue
  2. Left Shift (Same O2 Sat implies lower PaO2)
    1. Increased pH
    2. Decreased Temperature
    3. Decreased PaCO2
    4. Decreased 2,3-dpg
  3. Right Shift from physiologic stress (Same O2 Sat implies higher PaO2, lower O2 affinity, greater O2 tissue delivery)
    1. Increased Temperature
    2. Increased PaCO2
    3. Increased 2,3-dpg
    4. Decreased pH
  • References
  1. Killu and Sarani (2016) Fundamental Critical Care Support, p. 93-114