I love blood gas physiology. Internists and hospitalists should always attempt to define the hospitalized patient's acid base status to help rule out a stealth complex mixed acid base disorder. That process requires calculation of the anion gap. Performing the screening anion gap calculation is one of my 19 rules every internist flea should master. Being great at blood gas physiology is required if one is to practice competent and independent primary care. If you evaluate a patient in your office, whether you are a physician or nurse practitioner or physician assistant, you owe it to your patient to know how to screen for anion gap, part of the "A" in the AEIOU indications for acute dialysis. Failure to provide this basic level of competency is a failure to provide patients with the care they deserve.
Take for example this acid-base disorder example: A 52 year old homeless man who admits to "2 beers a day" was admitted with nausea, vomiting and weakness. No prior records were available. His last drink of alcohol was 48 hours prior, confirmed with an undetectable ethanol level. He adamantly denied any toxic alcohol ingestion. He had no diarrhea. His initial emergency room electrolyte panel was as follows:
Na 120K 2.7Cl 68HCO3 20BUN 32Cr 1.27
The hospitalist was called to the emergency room to admit him. The hospitalist noted him to have intravenous normal saline infusing at the time of his evaluation. The hospitalist calculated his bedside anion gap to be approximately 32. That's huge. The quick and dirty formula for calculating the anion gab is Na-(CL + HCO3). A normal anion gap is approximately 12. His delta anion gap was approximately 20 (32-12). That means he had "20 anions" floating around in his blood. That's a a big gap. To see this kind of delta anion gap usually requires critical illness. However, this guy didn't look all that sick. His lactate level was normal. The hospitalist attempted to order a serum ketone level to confirm the presence of starvation ketosis, however, this assay has continued to be unavailable for many months due to ongoing nationwide shortages of lots of stuff plaguing our hospitalized patients.
Prompted by this finding of a large anion gap, the hospitalist ordered a blood gas to help define the acid base status. Despite the large anion gap, the patient's blood gas returned with a significant alkalosis. Note, however, this lab was obtained almost 5 hours after the above lab was drawn and intravenous fluids had been administered:
pH 7.50pCO2 34HCO3 26
This patient has an impressive primary metabolic alkalosis that is being masked by his impressive primary anion gap metabolic acidosis, which is assumed to be from starvation ketosis. He probably also has a compensating respiratory alkalosis due to his attempts to "breath off" his metabolic anions. If one was to assume a normal measured bicarbonate of 24, then having an anion gap of 20 should force the measure bicarb to read 24-20, or 4. But his wasn't. It was originally 20. That means it was 16 higher than it should have been. That indicates he had a primary metabolic alkalosis as well, likely from his nausea and vomiting and lack of hydration. Interestingly enough his urine osmolality came back at over 600 with a urine sodium of nearly 60, which would appear at face value to be a problem of inappropriate antidiuretic hormone (SIADH). Normally these types of numbers would be treated with free water restriction. Though, I suspect, he was profoundly volume contracted to begin with.
The following day after normal saline hydration and potassium replacement, his new electrolyte panel confirmed the remarkable underlying metabolic alkalosis and resolution of his anion gap.
Na 128K 3.2Cl 89HCO3 31Bun 16Cr .8
My final answer on a quiz for this scenario would be primary metabolic alkalosis, primary anion gap metabolic acidosis, and probably a compensating respiratory alkalosis. Would I be right? Probably, but in the end, it probably doesn't matter, because the internist's best friend is often a cheap old bag of normal saline, no matter what the diagnosis.