A 12 year old spayed female Border Collie is presented for coughing and difficulty breathing. The owners have noted an intermittent hacking cough for the last 3-5 days, and this morning they were awakened when the dig was breathing hard and fast, coughing, and bringing up white foamy phlegm. Four months ago she was diagnosed with multicentric lymphoma. Her last dose of chemotherapy was mitoxantrone 15 days ago, and at that time she was in remission.
On presentation the dog is standing with the neck outstretched, open-mouth breathing with increased inspiratory effort, and tachypneic. She has generally harsh breath sounds with quieter sounds in the right hemithorax compared to left. Pulses are fast and thready, and the peripheral lymph nodes are all normal in size. The respiratory rate and effort improve mildly with flow-by oxygen supplementation.
T 103.8P 150R 52mm paleCRT 3s BCS 2/9Weight 14kg BP 86/44 (58mmHg)
Alkalemia can be caused by a low pCO2, or by a high HCO3. In this case the carbon dioxide is low, and the bicarbonate is normal. The low pCO2 is respiratory alkalosis – alkalosis matches the pH, so this is a primary respiratory alkalosis.
The expected compensation process with a respiratory alkalosis is a metabolic acidosis (a low bicarbonate). This patient has a normal bicarbonate, so compensation doesn’t seem to be occurring. But since the changes we see in the bicarbonate when there is a primary respiratory problem are sometimes small, we can check the math to be sure:
With an acute respiratory alkalosis, for every 1 point decrease in the pCO2 there should be a corresponding 0.25 point decrease in the bicarbonate.
Our patient’s pCO2 is 14 points lower than normal:
40 – 26 = 14
This means there should be an approximately 3.5 point change in the bicarbonate to compensate:
14 x 0.25 = 3.5
The normal bicarbonate is about 20, so this means we expect the bicarbonate to be about 16.5 if our patient is compensating for her respiratory alkalosis:
20 – 3.5 = 16.5
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 for respiratory changes +/- 2. So for this patient the range for the CO2 would be about 14.5 to 18.5:
16.5 – 2 = 14.5 (low end of range)
16.5 + 2 = 18.5 (high end of range)
Our patient’s bicarbonate is 21, which falls outside our estimated range. This means our patient has an uncompensated acute respiratory alkalosis or a mixed acid-base disturbance. It is sometimes hard to tell which in the acute respiratory disorders, because it takes time for the kidneys to compensate for the respiratory changes. If we are seeing this patient very early in the disease process, this might be a pure respiratory alkalosis that the kidneys haven’t had time to compensate yet.
What else can we do to try to evaluate what is happening in this patient with the information we have?
We can evaluate thing like the anion gap and the strong ion difference to get a better idea of the metabolic components that might be affecting the patient.
This patient’s anion gap is 20.4 – normal for a dog (8-21 according to the inside cover of Silverstein & Hopper’s Small Animal Critical Care Medicine Textbook):
AG = (Na + K) – (HCO3 + Cl)
AG = (156 + 3.4) – (21 + 118)
AG = 159.4 – 139
AG = 20.4
We will get into the strong ion difference later on BGOTW, but for now, if you are interested check out chapter 55 in the second edition of Silverstein & Hopper’s Small Animal Critical Care Medicine Textbook or another reference on acid-base analysis using non-traditional methods.
Finally, we have discussed the delta gap and delta ratio to assess for mixed disorders previously (here). These two methods are typically used when the AG is high, so they are not really indicated in this case.
