Blood Gas of the Week #22
A 3-year old spayed female beagle, 2 year old male castrated beagle, and 5 year old male castrated beagle are presented with a primary complaint of vomiting and diarrhea. Yesterday they caught, killed, dismembered a squirrel. managing to consume most of it before the owner could take it away from them. Today the dogs all seemed fine at lunch, however in the evening the owner came home from work to find an impressive amount of vomit and diarrhea around the house. All three dogs have been seen to have diarrhea, and at least two of them are vomiting. It has been about 7 hours from the time the owner returned to work after lunch and time of presentation to the emergency department.
On exam the dogs are all bright, happy, and checking out the garbage can to see if they can find anything else to eat. All three dogs show mild signs of dehydration and have normal vital signs. The males both have normal physical exam findings. The female has moderate abdominal pain and vomits what appears to be a piece of squirrel forelimb when her abdomen is palpated. A blood gas is performed as part of a diagnostic panel on the female dog. Interpret the blood gas:
Step 1: Evaluate the pH
This dog’s pH is 7.44 – a normal (just barely) pH. There are two possibilities when we see a normal pH
- The patient’s acid-base status is normal OR
- The patient has a mixed acid-base status
The biggest mistake that people make is looking only at the pH, and stopping when they see it is normal. When you do this, you will miss many mixed disturbances that could give you clues about what is going on with the patient.
Step 2: Determine the primary process
Compensation should not take the pH back to completely normal. When we have a mixed acid-base disturbance we should generally interpret this as the patient having more than one primary process happening (as opposed to a simple disturbance where there is a primary process and a compensatory process). So we need to look at the respiratory component and the metabolic component to define the different problems.
Respiratory component: In this patient the pCO2 is 42, which is normal
Metabolic component: In this patient the bicarbonate is 26.6 – high, which is metabolic alkalosis
Step 3: Name the process (we can’t calculate compensation here because this is a mixed disturbance)
This is technically a mixed acid-base disorder characterized by a metabolic alkalosis with a normal pH and pCO2.
HOWEVER – we should apply this information in the context of the patient. This patient has had multiple bouts of vomiting and diarrhea just since the owner arrived home from work this evening, and presumably a few more prior to the owner arriving home. All of this has occurred over a relatively short period of time – seven hours. The pH is at the top end of the normal range – a trend toward alkalosis, which would fit with the high bicarbonate and the multiple bouts of vomiting. It’s likely that if the patient continued to vomit and was presented a few hours later this would have been a clear metabolic alkalosis.
Next week the ‘metabolic alkalosis’ series will continue with a discussion of the clinical implications of a metabolic alkalosis, followed by treatment.
Oh, for those interested in the case resolution, the beagles were treated symptomatically and all returned to their happy squirrel chasing selves without further incident. The living room rug, however, could not be saved. Cheers!
