Sail Through Acid-Base Disorders with Your ARMADA.

A third year friend shared this approach to acid-base disorders with me so I thought I'd share it since they can be problematic at times.  It centers around the mnemonic ARMADA which stands for:

A = Acidosis or Alkalosis?

R = Respiratory acidosis/alkalosis?

M = Metabolic acidosis/alkalosis?

A = Anion gap

D = Delta gap

A = Assess for appropriate compensation

Okay so let's work through an example to show you how this can work.  Here are some numbers:

PCO2 = 35 mmHg
[HCO3-] = 18 mmol/L

Now you are often given the pH on labs but in the event you do need to calculate it the Henderson equation (Henderson with out the "hassel") is quite useful:

[H+] will be given in nmol/L so you can either take the -log[H+] to find pH or you can try to memorize a few correlations on this table to get a good estimate of the pH.

Recall that physiologic pH is 7.4 and that, interestingly enough, correlates with a [H+] of 40 nmol/L.  For every 1 nmol/L increase in [H+] from 7.4, pH decreases by roughly 0.01 and vice versa. 
Anyway, our example has pH 7.33.

Other important labs to pay attention to are Na+ and Cl-.  For this example:
Na+ = 136 mmol/L
Cl-= 98 mmol/L

Okay on to ARMADA. 
A - Is the patient in acidosis or alkalosis?
With a pH of 7.33 this patient is acidotic.

R - Is this patient in respiratory acidosis/alkalosis?
PCO2 is slightly decreased from normal (40 mmHg) which would cause an increase in pH (more basic).  So this patient is not in respiratory acidosis.

M - Is this patient in metabolic acidosis/alkalosis?
HCO3- is below normal (24 mmol/L) which would cause a decrease in pH (more acidic).  So the primary disorder here is a metabolic acidosis.

A - Anion gap.
This only really applies to situations of metabolic acidosis.  To calculate the anion gap we are using the formula:

AG = Na - (Cl + HCO3).

A normal anion gap is no larger than 12 mmol/L.  Our AG = 136 - (98 + 18) = 20 mmol/L.  So we have an anion gap metabolic acidosis.  This means we can narrow down our causes of the acidosis to a handful of situations where an unmeasured anion has a signifcant presence in the body.  The classic mnemonic for this is MUDPILES.

What if the anion gap had been normal?  Well then that would typically mean a loss in one of the measured ions such as bicarbonate loss in diarrhea or renal tubule acidosis and often chloride anions increase to counterbalance the loss of HCO3 (hyperchloremic acidodis).

D - Delta gap. 

The delta gap will tell you if there is more than one process is going on alongside a anion gap metabolic acidosis.  It is calculated as follows:

or simplified will read

Essentially, we are seeing how much does the loss in bicarbonate match to the increase in acid. 

If the delta gap is between 1 and 2 there is probably no other process going on. 

If the delta gap is less than 1 then there is an additional normal anion gap metabolic acidosis (more bicarbonate lost than one would expect)If the delta gap is greater than 2 then there is an additional metabolic alkalosis going on. (less bicarbonate lost than expected).

If calculate the delta gap for our example patient we get 1.3.  There is probably no other issue going on.

A - Assess for appropriate compensation.

Now to check for compensation. You can memorize much easier things than formulas and it makes sense conceptually.

·         Normal P_CO2 = 40 mmHg

·         Normal serum [HCO₃^-] = 24 mmol/L

·         The ratio of change between the two parameters, which I summarize and simplify in my own tables below

*Note I am purposely ignoring 2 things for now: 1) +/- direction of the ratio because I think it is more important to think of the direction conceptually (later on).  2) the margins of error because I think we should normally allow for slight variation anyway. 

Respiratory acid-base changes and ratios of compensation (i.e. “for ___ increase/decrease in P_CO2 there is a ____ increase/decrease in [HCO₃^-]”)

	Δ[HCO3]/ΔPCO2
Acute Respiratory Acidosis	1/10
Acute Respiratory Alkalosis	2/10 or 1/5
Chronic Respiratory Acidosis	3/10
Chronic Respiratory Alkalosis	4/10

Metabolic acid-base changes and ratios of compensations (i.e. “for ____ increase/decrease in [HCO₃] there is a ____ increase/decrease in P_CO2)

	ΔPCO2/ Δ[HCO3]
Metabolic acidosis	1.2
Metabolic alkalosis	3/4

Ok so how are we going to use this?  Here’s an example with acute respiratory acidosis.  Respiratory acidosis is caused by an increase in P_CO2 so let’s say that someone has a P_CO2 is 60 mmHg so that means ΔP_CO2 = 60 mmHg – 40 mmHg (normal) = 20 mmHg. 

Now we use our ratio.  For acute respiratory acidosis Δ[HCO₃]/ΔP_CO2 = 1/10 so that means we can find the change in Δ[HCO₃] by taking (1/10) ΔP_CO2 = 2 mmol/L.  

So we should expect a change of 2 mmol/L from the normal [HCO₃^-].  Is it higher or lower?  Well in respiratory acidosis we would want to increase HCO₃^- to compensate for the increase in P_CO2.   So that means we add 2 mmol/L to the normal [HCO₃^-] (24 mmol/L) which means we would expect a [HCO₃^-] of 26 mmol/L.

Now let’s do one with metabolic acidosis which is what our example case actually has.  When we use the numbers for our case we get Δ[HCO₃] = 24-18 = 6 mmHg. 

Use the ratio and we find that we would expect a P_CO2 change of 1.2 x 6 = 7.2 mmHg. 

In metabolic acidosis we would expect a drop in P_CO2 to compensate so 40 – 7.2 = 32.8 mmHg is our expected P_CO2.  Assume a fudge factor of +/- 2 and we are pretty close to 35 mmHg so our patient is appropriately compensating.

Now, that being said, First Aid does go out of their way to provide the same formula we were provided in class for metabolic acidosis compensation.  This is Winter’s Formula and I confirmed with an third year student that this is the one to know for sure:

P_CO2 = 1.5[HCO₃^-] + 8 + 2

If we use Winter’s formula for the example I just gave we get P_CO2 = 1.5[18] + 8 + 2 = 35 + 2 mmHg.  Pretty damn close.  Two tools for the same job (like the MacIntosh or Miller blades for intubation), decide which you like better.

What if the numbers did not match up?  Well that would mean that the patient is not appriately compensating.  In real life this could mean a number of things depending on your case situation.  I worked up an example one time where the patient was not appropriately compensating her metabolic acidosis because she was going into respiratory failure and could not blow off enough CO2.  Likewise, I could see a situation if a patient was losing too much bicarbonate when it needs to be retained for compensation.

Hopefully, this was useful to my fellow medical students.  Good luck studying everyone!   

                                              

Pharmacy Review with Trollface - U Jelly?

For some reason, I have had trouble remembering the mechanism of action of metronidazole.  When I looked it up for the millionth time it finally dawned on me: metronidazole basically trolls bacterial proteins making them inactive.  A friend at le med school has been showing me a bunch of Troll Physics cartoons lately so I whipped up some "Troll Medicine" in about fifteen minutes:

And now I am never going to forget how metronidazole works or what it is used for.  U jelly?