BGOTW #11
A 6 year, castrated male bull mastiff is presented in status epilepticus. The owners report he was seizuring for at least 45 minutes while they got him into the vehicle and transported him to the hospital. At presentation he is actively seizuring, with generalized tonic-clonic movement. The seizures were immediately controlled with IM midazolam, an IV catheter placed, blood collected for analysis. Volume resuscitation and active cooling were initiated.
Vitals obtained after seizure activity was controlled are as follows:
T 104.8 P 154 R 72 mm brick red CRT <1s BCS 6/9 Weight 124kg BP – -/- – (- -mmHg)
Interpret the blood gas:
Step 1: Evaluate the pH
This patient’s pH is 7.35, which is low – this is acidemia.
Step 2: Determine the primary process
Acidemia can be caused by a high carbon dioxide or low bicarbonate. In this case both the pCO2 and the bicarbonate are low. The low pCO2 is alkalosis – this does not match with the pH. The low bicarbonate is an acidosis which does match with the pH change, so the altered bicarbonate is causing the change in the 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 37, which is lower than the ‘normal’ value of 40, but still in the range of normal for a population of patients. This makes it a little confusing when we evaluate the blood gas. To try to clear things up some, we can calculate the expected compensation…
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:
6 x 0.7 = 4.2 (we will round to 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 32-40:
36 – 4 = 32 (low end of range)
36 + 4 = 40 (high end of range)
Our patient’s CO2 is 14, which falls into our estimated range. So our patient has a mixed acid-base disturbance characterized by a metabolic acidosis and respiratory alkalosis.
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.
