Blood Gas of the Week #24
A 17 year, spayed female domestic shorthair cat is hospitalized for management of anorexia, ileus, and vomiting secondary to GI lymphoma. Vomiting resolved on day 2. Diagnosis of lymphoma was confirmed on day 3 at which point the family elected steroid therapy, palliative care, and was discharged home. Today is day 5. The cat does not have a feeding tube and remains anorexic. This afternoon she is weak and obtunded prompting additional assessment.
T 99.4 P 156 R 18 mm pink CRT 1.5s BCS 7/9 Weight 7.7kg BP 100/75 (83mmHg)
Step 1: Evaluate the pH
This patient’s pH is 7.57 – quite high, which is alkalemia.
Step 2: Determine the primary process
Alkalemia can be caused by a high bicarbonate or a low carbon dioxide. In this case both the bicarbonate and carbon dioxide are high. Since the high bicarbonate is an alkalinizing process (and the high carbon dioxide is an acidifying process), the process represented by the bicarbonate is the cause of the high pH – this is a primary metabolic alkalosis.
Step 3: Is there compensation?
The expected compensation process with a metabolic alkalosis is a respiratory acidosis – a high pCO2. This patient’s pCO2 is 58 – definitely increased.
The first-glance diagnosis is compensated metabolic alkalosis. If you want to check mathematically to be sure the changes in the CO2 are all due to compensation and being minimally altered by a true respiratory problem, we can do that as well.
Step 4: Calculating the expected compensation
With a metabolic alkalosis, for every 1 point increase in the bicarbonate there should be a corresponding 0.7 point increase in the CO2.
Our patient’s bicarbonate is an impressive 29 points higher than normal:
49 – 20 = 29
This means there should be an approximately 20 point change in the carbon dioxide to compensate:
29 x 0.7 = 20.3 (we will round to 20)
The normal CO2 is about 40, so this means we expect the CO2 to be about 60 if our patient is compensating for his metabolic alkalosis
40 + 20 = 60
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 56-64:
60 – 4 = 56 (low end of range)
60 + 4 = 64 (high end of range)
Our patient’s CO2 is 58 – neatly inside the range for a compensated process. So, this patient has a compensated metabolic alkalosis.
Interested in the management plan? Come up with a plan and check back on Wednesday for the case discussion.
