A 4 year old female spayed terrier mix is presented with a 3 day history of vomiting, lethargy, anorexia, and weakness. The owners tell you that she is a master toy destroyer, and occasionally likes to eat pieces of the toys. At presentation she is weak but ambulatory, has depressed mentation, and is bradycardic at 50 beats per minute. Her femoral pulses are there when you feel for them, but you wouldn’t call them strong. Her blood pressure is a little soft at 92/53, and her ECG shows the tall tented T-waves that are characteristic of hyperkalemia.
The excellent veterinary technician lands an 18 gauge IV catheter in the cephalic vein on the first attempt, collects some blood for analysis, and your resuscitation efforts begin. A couple of minutes later your STAT lab results are back. Interpret the blood gas:
Step 1: Evaluate the pH
The patient’s pH is 7.40 – the exact midpoint of the normal range. There are two possibilities when we see a normal pH
- The patient’s acid-base status is normal OR
- The patient has a mixed acid-base status
The biggest mistake that people make is looking only at the pH, and stopping when they see it is normal. When you do this, you will miss many mixed disturbances that could give you clues about what is going on with the patient.
Step 2: Determine the primary process
When we have a mixed acid-base disturbance we have more than one primary process happening (as opposed to a simple disturbance where there is a primary process and a compensatory process). So we need to look at the respiratory component and the metabolic component to define the different problems.
Respiratory component: In this patient the pCO2 is 40, exactly in the middle of the normal range suggesting a normal respiratory status
Metabolic component: In this patient the bicarbonate is 23.1 – a little high indicating a metabolic alkalosis
Step 3: Name the process (we can’t calculate compensation here because this is a mixed disturbance)
To hear more discussion about this case and what caused the mixed-disturbance 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.
