48 - understanding ABGs part 5. Compensation.

KYJ 48 - Blood Gases series. Part 5.

In the last episode we looked at basic analysis of ABGs, normal values and some ABG examples.  Two sets of homework ABGs were offered :  did you have a go??
A).
pH 7.54
paCO2 =24
HCO3 = 24
This is Respiratory Alkalosis (The Justin Beiber effect)

And
B).
pH 7.61
PaCO2 = 42
HCO3 = 28
This is Metabolic Alkalosis commonly caused by Hyperemesis in morning sickness.

Plug: in all my two day Respiratory nursing seminars, I do a two hour face to face session in ABGs with 20 examples.

Today we start to tackle the term "compensation".

Compensation is the process of bringing an abnormal pH back into the normal range.  Now we talked about the range being 7.35-7.45, but if you had to pin one number to absolute normal, then 7.4 is absolute.  So less than 7.4 (7.35-7.4) would be the acidic side of normal, and higher than 7.4 (7.4-7.45) is the alkaline side of normal.

In the last episode we mentioned that abnormal pH (acidosis and alkalosis) is either caused by an abnormal CO2 or an abnormal HCO3.

Remember if the problem is abnormal CO2 then it is called "respiratory".  If the HCO3 is abnormal it is called "metabolic".

Compensation is where the respiratory system fails to excrete CO2, the kidneys (metabolic) adjust the amount of HCO3 to balance pH. And visa versa,
If the HCO3 is the cause of abnormality, the lungs will adjust CO2 levels to balance pH close to 7.4.

So let's start with a simple set of Respiratory Acidosis values.
The patient has had a heavy dose of morphine post op, and not breathing up well.  He is not blowing off adequate CO2 which accumulates and drops his pH.

pH 7.31
paCO2  50
HCO3  25

Now over time (60-90 mins) his kidneys will start retaining alkali (HCO3) to buffer his acidic pH.  This early compensation is seen in the next set of gases.

pH  7.34
PaCO2  50
HCO3  29. **

Note the increase of bicarb which has dragged his pH up close to normal.  At this stage, because pH is still technically acidotic, the diagnosis of the gases is called:
Respiratory Acidosis with partial compensation.

10 mins later the gases are :
pH  7.36
PACO2  50
HCO3  31**

Note HCO3 has risen further bringing pH into normal range.
- this is called
Fully Compensated Respiratory Acidosis.
...

Let's look
At another example:
pH 7.22
PaCO2 36
HCO3 18 very low

Metabolic Acidosis ( this girl is a diabetic with DKA.  Note how fast and deep she is breathing (Kussmaul respirations) (video)

5 minutes lapse
pH 7.34
PaCO2 28* (blowing off her CO2)
HCO3 18

You can see her lungs are compensating - but this is still a Metabolic Acidosis with partial compensation.

5 more mins
pH  7.36
paCO2  24 (blown more off)
HCO3  18

Fully compensated metabolic acidosis.  The pH has corrected by excreting all the acidic CO2 by rapid deep breathing.

Remember the difference between partially compensated and fully compensated is directly related to the pH value.

If it's a HCO3 problem the lungs try to compensate
If it's a CO2 problem the Kidneys try to rescue.

In the final episode, we will look at the often unused value called Base Excess (BE).  What we have covered to date has served me well for my whole career, and every exam, and I have never needed to look at BE for ABG analysis, however many of you would have covered it at some point in your education or practice, so I will address this forgotten ABG value. Then I think we are done.