Acid Base Balance & Interpreting ABG’s

A bit more on the oxygen dissociation curve…

This is a graph that plots the proportion of oxyhaemoglobin (haemoglobin in its oxygen-carrying form) on the vertical (y) axis against the partial pressure of oxygen on the horizontal (x) axis. It is a valuable tool in understanding how the blood carries and releases oxygen.

The oxygen dissociation curve has a sigmoid shape because of the the way oxygen binds to haemoglobin, a process called co-operative binding. Haemoglobin is formed by 4 polypeptide chains, each of which can bind to one oxygen molecule. Co-operative binding means that haemoglobin has a greater affinity to bind oxygen after a subunit has already done so. Also, a haemoglobin molecule that has 1 polypeptide chain bound to oxygen will have less affinity to bind another oxygen molecule than a haemoglobin molecule that has 3 of its polypeptide chains already bound to oxygen. 

The “top” part of the dissociation curve represents what happens at the pulmonary capillaries. Here, at high partial pressures of oxygen, haemoglobin binds to oxygen to form oxyhaemoglobin.  At low partial pressures of oxygen, the “steep” portion of the curve, oxyhaemoglobin releases the oxygen.

The oxygen dissociation curve can be shifted right or left by several factors. A right shift indicates decreased oxygen affinity of haemoglobin allowing more oxygen to be available to the tissues. A left shift indicates increased oxygen affinity of haemoglobin allowing less oxygen to be available to the tissues.

There are multiple online resources to help you improve your understanding of acid-base balance and gain more practice in ABG interpretation. Here are a couple:

  • https://litfl.com/acid-base-disorders/
  • https://geekymedics.com/abg-interpretation 

Now for the answers!

  1. Scenario 1: Hypoxia with respiratory acidosis
  2. Scenario 2: Hypoxia and metabolic acidosis with partial respiratory compensation
  3. Scenario 3: Respiratory alkalosis
  4. Scenario 4: Hypoxia & respiratory acidosis
  5. Scenario 5: Hypoxia & respiratory acidosis with complete metabolic compensation
  6. Scenario 6: Hypoxia & respiratory acidosis with partial metabolic compensation
  7. Scenario 7: Hypoxia and metabolic acidosis with partial respiratory compensation
  8. Scenario 8: Hypoxia and metabolic acidosis with partial respiratory compensation
  9. Scenario 9: Hypoxia and mixed metabolic and respiratory acidosis
  10. Scenario 10: Hypoxia and mixed metabolic and respiratory acidosis
  11. Scenario 11: Hypoxia and metabolic acidosis
  12. Scenario 12: Hypoxia and metabolic acidosis with partial respiratory compensation

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