BGOTW #10
A 16 year female, spayed domestic shorthair cat is presented with a 3 day history of progressive vomiting, anorexia, and lethargy. At presentation she is very thin, has a gallop arrhythmia, painful right kidney, is very dehydrated, and obtunded. Her pulses are terrible and her blood pressure will not read.
T 96.2F P 150 R 40 mm pale pink CRT 3s BCS 2/9 Weight 2.9kg BP – -/- – (- -mmHg)
An IV catheter is placed, blood is collected, and her resuscitation begins. Interpret the blood gas:
Step 1: Evaluate the pH
This patient’s pH is 7.28, 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 the bicarbonate is low (the pCO2 is in the normal range). The low bicarbonate is an acidosis which matches 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 9 points lower than normal:
20 – 11 = 9
This means there should be an approximately 6 point change in the carbon dioxide to compensate:
9 x 0.7 = 6.3 (we will round to 6)
The normal CO2 is about 40, so this means we expect the CO2 to be about 34 if our patient is compensating for his metabolic acidosis
40 – 6 = 34
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 30-38:
34 – 4 = 30 (low end of range)
34 + 4 = 38 (high end of range)
Our patient’s CO2 is 37, which falls into our estimated range. So our patient probably has a compensated metabolic acidosis.
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.
