Blood Gas of the Week #17
The following blood gas was obtained from a 6 year old, male castrated Golden Retriever. Interpret the blood gas and provide differentials:
Step 1: Evaluate the pH
This patient’s pH is 7.56 – quite high, which is alkalemia.
Step 2: Determine the primary process
Alkalemia can be caused by a high bicarbonate or a low carbon dioxide. In this case both the bicarbonate and carbon dioxide are high. Since the high bicarbonate is an alkalinizing process (and the high carbon dioxide is an acidifying process), the process represented by the bicarbonate is the cause of the high pH – this is a primary metabolic alkalosis.
Step 3: Is there compensation?
The expected compensation process with a metabolic alkalosis is a respiratory acidosis – a high pCO2. This patient’s pCO2 is 54 – over the high end.
The first-glance diagnosis is compensated metabolic alkalosis. 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 alkalosis, for every 1 point increase in the bicarbonate there should be a corresponding 0.7 point increase in the CO2.
Our patient’s bicarbonate is an impressive 34 points higher than normal:
54 – 20 = 34
This means there should be an approximately 26 point change in the carbon dioxide to compensate:
34 x 0.7 = 25.9 (we will round to 26)
The normal CO2 is about 40, so this means we expect the CO2 to be about 66 if our patient is compensating for his metabolic alkalosis
40 + 26 = 66
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:
66 – 4 = 62 (low end of range)
66 + 4 = 70 (high end of range)
Our patient’s CO2 is 54 – a little bit elevated but not as high as the formula says it should be if this was fully compensated. Since it is outside the calculated range it is technically classified as a mixed acid-base disturbance characterized by metabolic alkalosis and respiratory acidosis.
Next Steps: Provide a differential diagnosis
This is a metabolic alkalosis, so the next step is to look at the chloride. This patient has a particularly low Cl – 98mEq/L, so we will further classify this as a hypochloremic metabolic alkalosis. There are really two differentials for a veterinary patient with a hypochloremic metabolic alkalosis: administration of loop diuretics (inhibition of Na-K-2Cl pump), and vomiting – particularly with pyloric or duodenal obstruction (vomiting out hydrogen and chloride).
This patient had an intestinal mass causing near-complete obstruction of the duodenal lumen. Following resection and anastomosis the acid-base disturbance resolved quickly.
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.
