My thoughts on March 8 acid-base

First, thanks to the great discussion.  Readers will learn as much from the discussion as they will from me.  To repeat the presentation:

The patient is an 81 year old man found with altered mental status.  He has known diabetes mellitus, hypertension, COPD and CHF, but has not taken any medications for the past year.

Alb 3.1

ABG on 4 liters nasal oxygen

So please address these questions:

1. What is the acid-base disorder?

Great job here.  The patient has an increased anion gap – defining an increased anion gap metabolic acidosis.  Note that has expected gap is approximately 9 (quick rule of thumb – multiply the albumin by 3 to get the expected gap) with an observed gap of 25.  Thus his "delta gap" is 16.  Adding 16 to the measured bicarbonate of 21 we get 37.   Thus he starts with either a metabolic alkalosis or compensation for a respiratory acidosis.  Since he now hyperventilates I strongly favor metabolic alkalosis.  Finally, doing the Winter's equation his pCO2 is lower than expected.  Thus he does have a triple disorder – metabolic acidosis, metabolic alkalosis and respiratory alkalosis.

2.Provide a differential for the causes of the acid-base disorder?

Another great job.  We must exclude salicylates – any time you have an anion gap acidosis and respiratory alkalosis salicylates enter the differential.  He could have ketoacidosis or he could have lactic acidosis.  As I have written once before I prefer KILU to MUDPILES – An iatrogenic cause of increased anion gap acidosis.

Students find KILU easier to remember because it organizes anion gap acidosis into physiologic causes.

The metabolic alkalosis is usually secondary to volume contraction.  His BUN/creatinine ratio strongly supports that.

The respiratory alkalosis is puzzling.  We need more information.

3. What other information do you need?

1. Vital signs – he was relatively hypotensive – supporting severe volume contraction
2. Ketones, lactic acid and salicylate results – ketones negative, lactic acid high, salicylates negative

When presented this patient at morning report, I had this reasoning – diabetes untreated for a long time – leading to osmotic diuresis, severe volume contraction and the volume contraction causing both lactic acidosis and metabolic alkalosis. 

The team caring for him added that with volume expansion he revealed a septic picture, probably explaining his respiratory alkalosis.

If admitting this patient I would start with aggressive normal saline and low dose insulin.  He does have a free water deficit, but I must first restore his volume prior to addressing the free water deficit. 

Thanks for the comments.  You stimulate me to find more cases to present for your discussion.