A 3 year old, spayed female pit bull is presented with a 2 day history of vomiting. The owners have noted pieces of toy stuffing in the vomitus on multiple occasions, and she has ingested toys and toy parts on multiple occasions.
On examination she is quiet but responsive and has moderate abdominal discomfort in the left caudal abdomen. Pulses are strong and thoracic auscultation is normal.
T 100.2F P 110 R 24 mm pink CRT 2s BCS4/9 Weight 24kg BP 127/83 (98mmHg)
Interpret the blood gas:
Step 1: Evaluate the pH
This patient has a high pH at 7.49 – this is called alkalemia
Step 2: Determine the primary process
Alkalemia can be caused by a low pCO2, or by a high HCO3. In this case the carbon dioxide and bicarbonate are both high. High pCO2 is a respiratory acidosis – that doesn’t match the pH (alkalemia). The high bicarbonate is an alkalemia which does match the pH change, so the altered bicarbonate is causing the change in the pH – this is a primary metabolic alkalosis
Step 3: Is compensation occurring?
The expected compensation process with a metabolic alkalosis is a respiratory acidosis, which is occurring in this patient. This is respiratory compensation.
This looks like a 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 another 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 9 points higher than normal:
29 – 20 = 9
This means there should be an approximately 6 point change in the carbon dioxide to compensate:
9 x 0.7 = 6
The normal CO2 is about 40, so this means we expect the CO2 to be about 46 if our patient is compensating for her metabolic acidosis
40 + 6 = 46
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 42-50:
46 – 4 = 42 (low end of range)
46 + 4 = 50 (high end of range)
Our patient’s CO2 is 53, which falls outside our estimated range. This means our patient has a
If you want to hear more discussion about this diagnosis, check out the video above.
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.
