BGOTW #12
A 4 year old, spayed female dachshund is presented for a 24h history of vomiting. She is still interested in food but vomits every time she eats. Initially the vomitus contained toy pieces and stuffing, and now is just food and yellow liquid. A venous blood gas was obtained as part of the initial diagnostic assessment. Interpret the blood gas:
Step 1: Evaluate the pH
This patient’s pH is 7.48, which is high – this is alkalemia.
Step 2: Determine the primary process
Alkalemia can be caused by a low carbon dioxide or a high bicarbonate. In this case both the pCO2 and the bicarbonate are high. The high pCO2 is acidosis – this does not match with the pH. The high bicarbonate is an alkalosis which does match with the pH change, so the altered bicarbonate is causing the change in the 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 48, which is higher than the ‘normal’ value of 40.
The 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 11 points higher than normal:
31 – 20 = 11
This means there should be an approximately 8 point change in the carbon dioxide to compensate:
11 x 0.7 = 7.7 (we will round to 8)
The normal CO2 is about 40, so this means we expect the CO2 to be about 67 if our patient is compensating for his metabolic alkalosis
40 + 8 = 48
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:
48 – 4 = 44 (low end of range)
48 + 4 = 52 (high end of range)
Our patient’s CO2 is 48, which falls into our estimated range (actually exactly what we calculated it should be!). This means our patient has a
