Oxygenation

ABG's can tell us some important things about the body's oxygenation. Every living cell in the body needs oxygen. Hemoglobin binds and then releases O2 in response to physiologic conditions.

• At sea level, pH of 7.4, 37 degrees C, and room air, only 3% of the body's oxygen is dissolved in the plasma.
  • PaO2 is a measurement of the partial pressure of oxygen dissolved in the plasma only. It is measured in mm Hg (millimeters of mercury).
  • The PaO2 does not tell us about the body's total oxygen content, but it does indicate how much oxygen was available in the alveoli to dissolve in the blood.

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• The remainder of the body's oxygen is carried attached to hemoglobin molecules.
  • SaO2, or oxygen saturation, measures the degree to which oxygen is bound to hemoglobin.
  • Sa02 is expressed as a percentage, which is calculated by dividing the maximum oxygen capacity into the actual oxygen content and multiplying by 100.
  • Oxygen's affinity for hemoglobin changes depending on pH and temperature (see Oxyhemoglobin Dissociation Curve.)
  • Each hemoglobin molecule has four oxygen binding sites. When those sites are occupied, the hemoglobin molecule cannot hold any more. This is the oxyhemoglobin molecule.
  • Hemoglobin binding sites can hold molecules other than oxygen.
    • For example, smokers and people exposed to smoke, automobile exhaust, or other chemicals can have hemoglobin saturated with carbon monoxide (CO).
      • Carboxyhemoglobin (HbCO) is a hemoglobin molecule that has carbon monoxide where the oxygen should attach. The blood will have a cherry red color.
    • Methemoglobin (MetHb) is produced when certain poisons or a genetic condition affect the iron portion of the hemoglobin subunit. It changes blood to a brownish color.
  • The presence of either carbon monoxide or methemoglobin changes the affinity of oxygen for hemoglobin. Oxygen is much less likely to be carried on the hemoglobin molecule.
    • The hemoglobin molecule in these conditions is unusable. If enough hemoglobin is inactivated like this, it can cause tissue hypoxia.
    • Hypoxia, or reduced oxygen supply to the tissues, can occur even in the presence of 100% oxygen. This can be a life-threatening condition.

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• Hypoxia can result from ventilation or circulatory problems.

• The term shunt is used to describe a situation where there is ventilation without oxygenation. Some textbooks call a mild shunt a "mismatch."
  • Calculations can be done to determine the extent of the problem.
  • It is normal for a small percentage of air in the lungs not to reach blood. This is "dead space". It's an anatomical necessity. Air in the nasopharynx, trachea and bronchi does not reach the alveoli before exhalation.
    • More than this amount of "dead space" can lead to hypoxia.
  • Circulatory problems that result in a shunt are called "V-Q shunt " or "V-Q mismatch", where:
    • V represents "ventilation" and
    • Q represents "perfusion"("P" usually stands for "pulmonary", so apparently the next letter in the alphabet was used for "perfusion.")
    • V and Q can be compared directly using a nuclear medicine procedure called a "V-Q scan" or a "lung scan."
  • When dealing with shunts and mismatches, you may also come upon a value known as "A-a gradient."
    • This value has traditionally been used to compare oxygenation of the alveoli and that of the arteries.
    • Recent studies suggest that using "A/a ratio" can give a more accurate prediction of V-Q mismatch.
    • Newer studies have shown a much less complex formula, FiO2-PaO2, may be just as accurate.

Points to remember:

1. PaO2 is oxygen dissolved in plasma, not total O2.
2. SaO2 is saturated hemoglobin minus HbCO and MetHb.
3. Ventilation does not equal oxygenation.
4. A shunt is normal alveolar O2, but low blood O2.

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