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Module title = Tutorial: Blood Gases
Lesson title = Degree of Compensation
This is lesson 5 of 9 in this module
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
(pCO
2
or the HCO
3
-
). The second value is how much the compensatory changed (again, pCO
2
or the HCO
3
-
)? These changes are called
deltas
.
For example, if the pCO
2
was 50, this would be a delta pCO
2
of 10 (because normal is 40).
If HCO
3
-
is 19, the delta HCO
3
-
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 / pO
2
/ 14. I will leave the pO
2
out so you don't have an extra distracting number. The delta pCO
2
is 40-30 = 10. The delta HCO
3
-
is 24-14 = 10. The pH is acidosis. The pCO
2
is low, which causes alkalosis. The HCO
3
-
is also low, and causes acidosis. Therefore, with a pH of acidosis,
we know that the low HCO
3
-
is the cause of the primary abnormality
, since a low pCO
2
causes alkalosis. Therefore, this is metabolic acidosis and respiratory compensation.
We compare the
ratio
of these 2 delta values for pCO
2
and HCO
3
-
. In other words, how much did the compensation value change compared to the primary abnormality.
Continuing with the example, we have the
primary
delta HCO
3
-
= 10 and the
compensatory
delta pCO
2
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 pCO
2
and the delta HCO
3
-
.
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 CO
2
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 CO
2
(1.0) is greater than raising CO
2
(0.7)
a similar observation is made for metabolic compensation too: bicarb is lowered more (0.5) than bicarb is raised (0.3)
Lesson 5 of 9
That was the last lesson!