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An 11 year old male castrated cat is presented with a primary complaint of being non-responsive to the owner. He has a 2-3 week history of progressive anorexia, vomiting, regurgitation, and inappetence. The owner reports significant weight loss and recently noticed the abdomen appears bloated. The cat was on antibiotics for an upper respiratory infection but the owner stopped them 1 week ago due to inappetence and inability to administer the medication. At triage the cat is in sternal recumbency with the head ventroflexed and is completely unresponsive to normal stimuli.
T 97.2 P 150 R 46 mm pink CRT 3s BCS 2/9 Weight 3.1kg (previously 5.3kg) MAP 92mmHg
Physical exam findings: Sitting in sternal recumbency, head ventroflexed, only responsive to painful stimuli. Harsh breath sounds in all lung fields. The abdomen is painful on gentle palpation and tympanic.
An IV catheter was placed and resuscitation initiated. The patient vomited a large amount of green fluid during IV catheter placement. The initial blood work results are as follows. Interpret the blood gas. Once you are have an answer, watch the video or read on below.
Step 1: Evaluate the pH
This patient has a very high pH at 7.55 – this is alkalemia.
Step 2: Determine the primary process
Alkalemia can be caused by a low carbon dioxide or high bicarbonate. In this case the bicarbonate and the carbon dioxide are both high. High CO2 is an acidosis – that doesn’t match the pH (alkalemia). The high bicarbonate is an alkalosis which does match with the pH change, so the altered bicarbonate is causing the change in the pH – this is a primary metabolic alkalosis
Step 3: Is there compensation?
The expected compensation process with a metabolic alkalosis is a respiratory acidosis, which is definitely occurring in this patient. This is respiratory compensation.
The 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 38.9 points higher than normal:
58.9 – 20 = 38.9
This means there should be an approximately 27 point change in the carbon dioxide to compensate:
38.9 x 0.7 = 27
The normal CO2 is about 40, so this means we expect the CO2 to be about 67 if our patient is compensating for his metabolic alkalosis
40 + 27 = 67
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 63-71:
67 – 4 = 63 (low end of range)
67 + 4 = 71 (high end of range)
Our patient’s CO2 is 71, which falls into our estimated range. This means our patient has a
