A 7 month old male intact domestic shorthair cat is presented in severe respiratory distress. About an hour before presentation the owners heard him cry out from another room and found he had been chewing on an electrical cord. He initially seemed fine however a short time later he seemed to be breathing a little faster than usual, and he has rapidly progressed to severe respiratory difficulty during the 15 minute drive to the emergency hospital.
At presentation the cat prefers to sit in sternal recumbency with the head and neck outstretched, has coarse crackles in all lung fields, and becomes frantic with any restraint. He is rapidly becoming exhausted in front of you. After a few minutes of oxygen supplementation in a quiet area you are able to minimally restrain him and obtain vitals and initial labs:
T 97.1F P 240 R 80 mm cyanotic CRT >3s BCS 4/9 Weight 3.2kg BP – -/- – (- -mmHg)
Step 1: Evaluate the pH
This patient has a low pH at 7.31 – this is an acidemia.
Step 2: Determine the primary process
Acidemia can be caused by a high carbon dioxide or low bicarbonate. In this case the bicarbonate and the carbon dioxide are both high. High bicarbonate is an alkalosis – that doesn’t match the pH (acidemia). The high CO2 is an acidosis which does match with the pH change, so the altered CO2 is causing the change in the pH – this is a primary respiratory acidosis.
Step 3: Is there compensation?
The expected compensation process with a respiratory acidosis is a metabolic alkalosis, which is occurring in this patient. This is metabolic compensation.
The diagnosis is compensated respiratory acidosis. If you want to check mathematically to be sure the changes in the bicarbonate are all due to compensation and being minimally altered by another problem, we can do that as well:
Step 4: calculate the expected compensation
With a respiratory acidosis, for every 1 point increase in pCO2 we expect a corresponding increase of 0.15 in the bicarbonate.
Our patient’s pCO2 is 12 points higher than normal:
52 – 40 = 12
This means there should be an approximately 2 point increase in the patient’s bicarbonate to compensate:
12 x 0.15 = 2 (rounding to the nearest whole number)
The normal bicarbonate is about 20, so this means we expect the patient’s bicarbonate to be about 22 if he is compensating for the acute respiratory acidosis:
20 + 2 = 22
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 +/- 2 for metabolic compensation. So for this patient the range for the bicarbonate would be about 20-24:
22 – 2 = 20 (low end of the range)
22 + 2 = 24 (high end of the range)
Our patient’s bicarbonate is 23, which falls into our estimated range. This means our patient has a
