A 4 month old male, intact golden retriever puppy is presented for ongoing seizure activity. The puppy got into the owner’s purse and chewed up a lot of things including an entire box of sugar free breath mints. The owners found him seizuring about 20 minutes ago and have come directly in to the emergency room. They report he intermittently seizures for 30-60 seconds at a time, and has not regained consciousness between seizures.
On presentation the puppy is having a generalized tonic-clonic seizure that terminated as the owners walked through the door. The dog is hot to the touch, breathing very hard, and remains comatose. An intraosseous catheter is placed, blood is collected from the catheter, and resuscitation commences. As IV fluids are being started and dextrose administered, the following vitals and STAT lab results become available. Interpret the blood gas:
T 104.8F P 88 R 60 mm brick red CRT 2-3s BCS 5/9 Weight 12kg BP – -/- – (- -mmHg)
Step 1: Evaluate the pH
This patient’s pH is quite low at 7.18 – this is acidemia.
Step 2: Determine the primary process
Acidemia can be caused by a low bicarbonate or high carbon dioxide. In this case the CO2 is low, so that is not the cause. The bicarbonate is low at 14 – this is a metabolic acidosis. So, the low bicarbonate is the acidifying process causing the change in the pH, so this is a primary metabolic acidosis.
Step 3: Is there compensation?
The expected compensation process with a metabolic acidosis is a respiratory alkalosis, which we see in this patient. This looks like we probably have respiratory compensation.
The diagnosis is metabolic acidosis, with respiratory compensation. 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 6 points lower than normal:
20 – 14 = 6
This means there should be an approximately 4 point change in the carbon dioxide to compensate (rounded to the nearest whole number):
6 x 0.7 = 4
The normal CO2 is about 40, so this means we expect the CO2 to be about 36 if our patient is compensating for his metabolic acidosis:
40 – 4 = 36
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 31-39:
36 – 4 = 32 (low end of range)
36 + 4 = 40 (high end of range)
Our patient’s CO2 is 30 – this doesn’t fall into the expected range. This means our patient has a mixed acid-base disorder that is characterized by a metabolic acidosis and concurrent respiratory acidosis, with a very low pH.
Mixed disorders can be a little bit confusing at first, but as you are first learning to interpret them, don’t over-think the changes. Just describe the blood gas abnormalities that are present – in this case the patient has acidemia with concurrent metabolic acidosis and respiratory alkalosis. Watch the end of the video above to hear a discussion about what might be causing the changes seen in this puppy’s blood gas.
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.
