A 10 year old, castrate male mix-breed dog is presented with a 2 week history of progressive tachypnea and labored breathing. As part of his diagnostic evaluation a blood gas is performed. Interpret the blood gas:
Acidemia can be caused by low bicarbonate or high CO2. In this case the CO2 and the bicarbonate are both high. A high bicarbonate causes metabolic alkalosis – since the pH is showing acidemia, this isn’t the primary process. High CO2 is a respiratory acidosis – since this matches the pH change, the primary process is a respiratory acidosis.
The expected compensation process with a respiratory acidosis is metabolic alkalosis – we said in step 2 that this is occurring. This is metabolic compensation.
The diagnosis appears to be a compensated respiratory alkalosis. 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.
Our patient has an chronic respiratory acidosis (his breathing has been abnormal for 2 weeks). So, according to the chart, for every 1 point increase in the pCO2, the bicarbonate should have a corresponding 0.35 increase.
Our patient’s pCO2 is 15 points higher than normal:
55 – 40 = 15
This means there should be an approximately 5 point increase in the bicarbonate from normal:
15 x 0.35 = 5.25 (we will round to 5)
The normal bicarbonate is about 20, so this means we should expect the bicarbonate to be about 25 if our patient is compensating for his respiratory acidosis:
20 + 5 = 25
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 compensations. So for this patient the range for the bicarbonate would be about 23-27:
25 – 2 = 23 (low end of range)
25 + 2 = 27 (high end of range)
Our patient’s bicarbonate is 26, which falls into our estimated range. This means our patient has a
