I posted this tweetorial to Twitter last month and I want to bookmark it here.
Internal Medicine Board Review: Fluids, Electrolytes and Acid Base
The chief resident at Providence asked me to do a board review class for the third-years. They have small sessions weekly (?) in the physician lounge. This afternoon we went over fluids, electrolytes and acid-base. We did the first 14 questions. Seems like it went pretty well.
iPhone Medical Applications
I have four medical applications on my iPhone, of which I use two. Here is a quick review.
To show how the iPhone equipped physician approaches clinical problems I will use the DB’s Medical Rants most recent acid-base problem. He presents a case with the following information:
49-year-old man, previously in good health, presents after a few weeks of progressive weakness and dizziness. He admits to polyuria. Your job is to extensively discuss his lab tests.
The first step in my mind is to fully interpret the ABG. To do this we will use the application ABG.
ABG
This simply named program is an ABG calculator that runs through the standard algorithms for detecting multiple primary acid-base abnormalities. Can’t remember Winter’s Formula. As long as you don’t have boards coming up you can just plug’n chug and turn DB’s ABG into the following:
This does two of the calculations that DB describes at length:
- Winter’s formula (16 * 1.5 + 8 ±2) shows that the predicted pCO2 is 30-34. The patient’s CO2 is 33 so the patient has isolated and appropriately compensated pCO2 of 33. ABG displays this information in the second line when it describes the acid-base disorder as “Compensated metabolic acidosis.” It does not describe a second primary condition such as respiratory acidosis or alkalosis.
- Gap-Gap or delat-delta. The patient has a dramatically elevated anion gap at 27 (15 over the upper limit of normal of 12) but his bicarb of 16 is only 8 below normal. The difference between the delta gap and the delta anion gap is 7 (15-8) when this is added to the normal bicarbonate you get 31; so the patient had a pre-existing metabolic alkalosis with a bicarbonate of 31. ABG displays this information as the corrected bicarbonate.
The next step is adjusting his sodium for the hyperglycemia. To do this we will use Mediquations though Medical Calc works just as well.
Mediquations
DB, in his discussion, states that he has unpublished data proving that no formula is effective at adjusting the serum sodium for the hyperglycemia. For those of us without his unpublished data should adjust the sodium using Katz’s traditional conversion (pdf of a letter to JAMA discussing adjusting sodium for hyperglycemia in DKA. Katz’s original conversion was discussed in a letter to the NEJM) of a drop in Na of 1.6 for every 100 the glucose is over 100 mg/dL. Nephrology fellows should additionally be aware of Hillier’s data showing the sodium falling 2.4 for every 100 of glucose. Both Mediquations and Medical calculator adjust the sodium using Katz’s conversion.
Of coarse you wouldn’t know it was Katz’s conversion because even if you tap on “More Info,” Mediquation does not provide the reference. Likewise you will not get the reference with Medical Calc.
I feel that using ABG and Mediquations will make you a more effective physician without forcing you to memorize equations used only periodically.
Of coarse you wouldn’t know it was Katz’s conversion because even if you tap on “More Info,” Mediquation does not provide the reference. Likewise you will not get the reference with Medical Calc.
Though DB did not explore free water defecits in his discussion of the case this is a clinically relevent point. You can use Mediquation to calculate the water deficit.
I feel that using ABG and Mediquations will make you a more effective physician without forcing you to memorize equations used only periodically.