Blood Gas of the Week #16
A 16y, spayed female cat has been hospitalized for the past 7 days for management of severe pancreatitis with ongoing profuse watery diarrhea. The following blood gas was obtained on day 2 of hospitalization (day 6 of illness). Interpret the blood gas:
Step 1: Evaluate the pH
This patient’s pH is 7.082 – severely low.
Step 2: Determine the primary process
Acidemia can be caused by a low bicarbonate or a high carbon dioxide. In this case both the bicarbonate and carbon dioxide are very low. Since the low bicarbonate is an acidifying process (and the low carbon dioxide is an alkalinizing process), the process represented by the bicarbonate is the cause of the low pH – this is a primary metabolic acidosis.
Step 3: Is there compensation?
The expected compensation process with a metabolic acidosis is a respiratory alkalosis – a low pCO2. This patient’s pCO2 is 8.65 – incredibly low (and putting the patient at risk of poor cerebral perfusion and seizures, among other things).
The first-glance diagnosis is compensated metabolic acidosis. If you want to check mathematically to be sure the changes in the CO2 are all due to compensation and being minimally altered by a true respiratory problem, we can do that as well.
Step 4: Calculating the expected compensation
With a metabolic acidosis, for every 1 point decrease in the bicarbonate there should be a corresponding 0.7 point decrease in the CO2.
Our patient’s bicarbonate is a whopping 18 points lower than normal:
20 – 2.5 = 17.5 (we will round to 18)
This means there should be an approximately 13 point change in the carbon dioxide to compensate:
18 x 0.7 = 12.6 (we will round to 13)
The normal CO2 is about 40, so this means we expect the CO2 to be about 27 if our patient is compensating for his metabolic alkalosis
40 – 13 = 27
BUT there is a range for normal (both bicarbonate and CO2) that we need to account for, so generally we say that the range is the calculated value +/- 4. So for this patient the range for the CO2 would be about 63-71:
27 – 4 = 23 (low end of range)
27 + 4 = 31 (high end of range)
Our patient’s CO2 is 8.65 – quite impressively low, which falls outside our estimated range. Since it is outside the calculated range it is technically classified as a mixed acid-base disturbance characterized by severe metabolic acidosis and respiratory alkalosis. We don’t know what this patient’s concurrent respiratory rate was, but based on the incredibly low pCO2 it is likely very fast. With this severe of a metabolic acidosis and hyperventilation (low pCO2) a significant tachypnea is probably present , my opinion is to consider at least part of the pCO2 to be a compensatory response to the profound metabolic acidosis. In fact, an extreme ventilatory rate is probably keeping this cat alive for the moment.
The next steps in the blood gas analysis: defining the type of metabolic acidosis, and calculating the anion gap
Last week on BGOTW we introduced the concept of the anion gap. If you haven’t looked at that discussion yet, go read this, then come back to this discussion.
When using the traditional acid-base analysis approach (the approach presented above) metabolic acidosis is subdivided into two classes:
- High Anion Gap Metabolic Acidosis (HAGMA)
- Normal Anion Gap Metabolic Acidosis (NAGMA)
This patient has a normal anion gap. This is useful because there are a relatively limited number of things that can cause a metabolic acidosis without increasing the anion gap, limiting our differential diagnosis.
Several mnemonics exist for the medically relevant ‘things’ that cause a normal anion gap metabolic acidosis. My preference is the ABCD mnemonic:
- Addison’s
- Bicarbonate loss (GI & Renal)
- Chloride
- Drugs (acetazolamide, acids)
So the ABCD list above is where our initial list of differential diagnoses for the NAGMA come from. The patient presented here is having profuse watery diarrhea which is probably resulting in significant bicarbonate loss and thus the impressive metabolic acidosis. Additionally, she has a mild azotemia based on the limited blood work available here, so renal loss of bicarbonate might also be contributing. Other things to consider would be the type of fluid therapy she might be getting (0.9% NaCl solutions for fluid resuscitation, replacement, and drug delivery contain 154mEq/L of Cl contributing to iatrogenic hyperchloremic metabolic acidosis – a type of NAGMA), if she is receiving (not commonly used in veterinary medicine, but may be present in the owner’s household – very unlikely in this case), and if she might have adrenal insufficiency (if there is a reasonable index of suspicion we could consider measuring a baseline cortisol test to rule out, but not rule in, Addison’s disease).
Want to learn about reading blood gases? You’ve come to the right place. Every week a new blood gas case will be posted. The scenario will post on Monday and the solution will post on Wednesday.
New to blood gasses? Check out the quick guide to acid-base interpretation here.
