An 11 year old, male castrated Golden Retriever is presented following an episode of collapse on a walk. The owners report a several day history of waxing and waning appetite and lethargy. This morning on his walk he seemed tired and suddenly groaned, collapsed, and seemed to be unconscious for about a minute. On the way to the hospital he is conscious but weak and breathing heavily.
On arrival he is non-ambulatory, laterally recumbent, and panting heavily. Mucus membranes are pale with prolonged capillary refill, the heart is difficult to hear, the pulses are weak with pulse deficits, and he has an abdominal fluid wave. IV access is immediately obtained and blood collected for analysis. He is connected to monitoring equipment.
T 99.7 P 180 R 80 mm pale CRT prolonged BCS 6/9 Weight 32kg BP 70/30 (43mmHg)
ECG: electrical alternans; FAST ultrasound: pericardial & abdominal effusion
Step 1: Evaluate the pH
This patient has a high pH at 7.48 – this is alkalemia.
Step 2: Determine the primary process
Alkalemia can be caused by high bicarbonate or low CO2. In this case the CO2 and the bicarbonate are both low. A low bicarbonate causes metabolic acidosis – since the pH is showing alkalemia, this isn’t the primary process. Low CO2 is a respiratory alkalosis – since this matches the pH change, the primary process is a respiratory alkalosis.
Step 3: Is there compensation?
The expected compensation process with a respiratory alkalosis is metabolic acidosis – we said in step 2 that this is occurring. This is metabolic compensation.
The diagnosis is 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.
Step 4: Calculating the expected compensation:
Our patient has an acute respiratory alkalosis. So, according to the chart, for every 1 point decrease in the pCO2, the bicarbonate should have a corresponding 0.25 decrease.
Our patient’s pCO2 is 14 points lower than normal:
40 – 26 = 14
This means there should be an approximately 3.5 point decrease in the bicarbonate from normal:
14 x 0.25 = 3.5
The normal bicarbonate is about 21, so this means we should expect the bicarbonate to be about 17.5 if our patient is compensating for his respiratory alkalosis:
21 – 3.5 = 17.5
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 15.5-19.5:
17.5 – 2 = 15.5 (low end of range)
17.5 + 2 = 19.5 (high end of range)
Our patient’s bicarbonate is 17, which falls into our estimated range. This means our patient has a
