Blood Gases Level 1 Tutorial: Blood Gases
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Tutorial: Blood Gases
Learn an organized approach to arterial blood gas analysis, incorporating serum and urine electrolyte values into your more advanced levels of analysis.
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Tutorial: Blood Gases Degree of Compensation
Lessons
42
Times Practiced
1284
Cases Completed
1h 24m
Total Time spent
1m 24s
Average Time
Progress
Accuracy
Efficiency
Accuracy
Efficiency
1 Acidosis or Alkalosis Acidosis or Alkalosis
2 Shifting chemistry Shifting chemistry
3 Metabolic or Respiratory Metabolic or Respiratory
4 Compensatory Processes Compensatory Processes
Current lesson 5 Degree of Compensation Degree of Compensation
6 Abnormal Compensation Abnormal Compensation
7 Anion Gap Mechanism Anion Gap Mechanism
8 The Delta Delta and Acid Base Ddx The Delta Delta and Acid Base Ddx
9 Summary of Approach to ABGs Summary of Approach to ABGs
Degree of Compensation
Please see lesson #4 for "Compensatory Processes" for the podcast on this lesson.

This lesson might be more challenging than the previous ones.

Why determine the degree of compensation?
It is because medicine wants you to do more math. Ok, obviously that is not why. It is important to determine the degree of compensation because either too much or too little compensation can provide you a big clue to a second diagnosis that is possibly less obvious.

Before we can begin, we need to define what we mean by appropriate compensation.

As a general statement, the body will do something to try to correct the pH to CLOSE to normal, but not exactly normal. For example, if you have a primary acidosis with a pH of 7.10, the compensatory mechanism will be trying to get the pH back to somewhere around 7.35. It will not compensate up to, nor greater than 7.40. Close is good enough. If you have an alkalosis of 7.55, the body will try to get back down to around 7.45.


The Concept of Deltas:
When we think about compensation, we need to look at 2 values. The first value is how much did the primary disturbance changed (pCO2 or the HCO3-). The second value is how much the compensatory changed (again, pCO2 or the HCO3-)? These changes are called deltas.

For example, if the pCO2 was 50, this would be a delta pCO2 of 10 (because normal is 40).

If HCO3- is 19, the delta HCO3- would be 5 because the normal is 24. The delta value is always positive (regardless if the change is increased or decreased).

Let's look at a quick example. A patient has a blood gas of 7.29 / 30 / pO2 / 14. I will leave the pO2 out so you don't have an extra distracting number. The delta pCO2 is 40-30 = 10. The delta HCO3- is 24-14 = 10. The pH is acidosis. The pCO2 is low, which causes alkalosis. The HCO3- is also low, and causes acidosis. Therefore, with a pH of acidosis, we know that the low HCO3- is the cause of the primary abnormality, since a low pCO2 causes alkalosis. Therefore, this is metabolic acidosis and respiratory compensation.

We compare the ratio of these 2 delta values for pCO2 and HCO3-.  In other words, how much did the compensation value change compared to the primary abnormality.

Continuing with the example, we have the primary delta HCO3- = 10 and the compensatory delta pCO2 is 10. The ratio between these 2 values is 10:10, or in other words, 1:1, or 1.0

If the compensatory delta was 6 and the primary delta was 12, then the delta ratios would be:  6:12 = 0.5

The Expected Delta Ratios
Ok, here is the super boring part. You need to memorize 4 numbers (sorry). These numbers compare the delta pCO2 and the delta HCO3-.

There are 4 different values because there are 4 different primary abnormalities (each with a different compensatory ratio):
  • metabolic acidosis will be compensated by respiratory alkalosis with a delta ratio of 1.0
  • metabolic alkalosis will be compensated by a respiratory acidosis with a delta ratio of 0.7
  • respiratory alkalosis will be compensated by a metabolic acidosis with a delta ratio of 0.5
  • respiratory acidosis will be compensated by a metabolic alkalosis with a delta ratio of 0.3

Do you think you will have difficulty remembering these compensatory delta ratios? I did. There are a couple little memory tip patterns that might help you remember:
  • it is easier to change your CO2 levels than it is to change your bicarb levels
    • therefore, the respiratory compensation ratios (1.0 and 0.7) are larger than the metabolic ratios (0.5 and 0.3).
  • it is easier to lower a value than it is to raise a value
    • therefore, within the respiratory compensation mechanisms, the ratio for lowering CO2 (1.0) is greater than raising CO2 (0.7)
    • a similar observation is made for metabolic compensation too: bicarb is lowered more (0.5) than bicarb is raised (0.3)