An 11 year old male castrated German Shorthair Pointer is presented with a 6 hour history of acute onset vomiting and diarrhea. The last bout of diarrhea was quite bloody prompting presentation to your hospital.
On presentation the patient is ambulatory but very lethargic. Examination identifies non-localizing abdominal pain, bloody fecal staining on the perineum, bounding pulses, tachycardia, and significant dehydration.
T 100.4 P 160 R 36 mm bright red CRT <1s BCS 5/9 Weight 32 BP 95/50 (68mmHg)
Step 1: Evaluate the pH
This patient has a low pH at 7.21 – 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 and the carbon dioxide are both low. Low CO2 is an alkalosis – that doesn’t match the pH (acidemia). 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, which is occurring in this patient. This is respiratory compensation.
The diagnosis is compensated metabolic acidosis. If you want to check mathematically to be sure the changes in the CO2 are all due to compensation and being minimally altered by another 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 8.7 points lower than normal:
20 – 11.3 = 8.7
This means there should be an approximately 6 point change in the carbon dioxide to compensate:
8.7 x 0.7 = 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 31, which falls into our estimated range. This means our patient has a
