Tag: acid-base

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You cannot tell if a respiratory acid-base disorder from the ABG

This is a frequent cause of confusion. I know that I was confused by this when I was a young learner. And I believe the source of this confusion was garbled teaching from a resident that was still struggling with the concept.

Take this ABG:

pH: 7.26

PaCO2: 80

HCO3:
39

The Henderson-Hasselbalch variables are moving in discordant directions (pH down, pCO2 and HCO3 going up) so it is a respiratory disorder. The pH is decreased so this is a respiratory acidosis.

Now look at the compensation to see if there is a second primary disorder affecting compensation.

  • In acute respiratory acidosis the HCO3 rises 1 mEq/L for every 10 mmHg the CO2 rises.
  • In chronic respiratory acidosis the HCO3 rises 3 mEq/L for every 10 mmHg the CO2 rises.

This patient’s CO2 is 80, an increase of 40, so the HCO3 should rise 4 (4×1) if the respiratory acidosis is acute, yielding a bicarb of 28 (24+4). The actual bicarbonate is 39, too high, so there is an additional metabolic alkalosis if the respiratory acidosis is acute.

If the respiratory acidosis is chronic, to increase in CO2 of 40 should increase the bicarbonate by 12 (4×3), so a bicarb of 36 (24+12). The actual bicarbonate is 39, which is just outside of our ±2 so we’ll call it nearly appropriate with just a touch of metabolic acidosis.

The ABG can be “solved” with either acute or chronic respiratory acidosis. Patients cannot be diagnosed with Occam’s razor so the simpler explanation (chronic respiratory acidosis without the need for additional acid-base disorders) may not be the right one. In medicine we need to assume Hickam’s Dictum “A patient can have as many diseases as he damn well pleases.”

The ABG does not determine whether a patient has acute or chronic respiratory disorder, the physician must do that.

So what’s my beef with the two Bruces? Take a look at this question from chapter 8…

The simple acid-base disorders are:

  1. Metabolic acidosis
  2. Metabolic alkalosis
  3. Respiratory acidosis
  4. Respiratory alkalosis
  5. Respiratory acidosis
  6. Respiratory alkalosis

But this is answers the authors expect…

Since the question stipulates that these are simple acid-base disorders, one can extrapolate the acuity of the respiratory disorder by the degree the bicarb has adjusted, a large adjustment is chronic, and a smaller change is acute. But since patients don’t tell you if they have simple or complex acid-base disorders when the blood is drawn, this trains students to expect the ABG to provide information that it cannot provide.

Stupid book.

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I don’t come to laud the urinary anion gap, I come to bury it

While preparing for the electrolyte session for Epic Pathshala I added a new twist. For the first time I added the concept that Jaime Uribarri has been shouting for the last few years regarding the use of urinary anion gap. I first saw Dr. Uribarri at NKF Clinical at Boston in 2022. He gave a great lunch-time presentation on the utility (more specifically, the lack of urility) of urine anion gap. Perfect counter programming to Richard Sterns’ lecture on managing electrolyte disorders with the assistance of the BUMP (basic urine metabolic panel) (NDT).

Dr. Uribarri’s thoughts were crystalized in two recent articles:

  1. The Urine Anion Gap: Common Misconceptions
  2. Beyond the Urine Anion Gap: In Support of the Direct Measurement of Urinary Ammonium

The tweetorial can be found here:

Part One

Part Two

Here is my rough script that I use to write the tweets

This is going to be a long arc and it has a point but it is going to take a bit to get there.  #Tweetorial #MedThread #Electrolytes #AcidBase 1/11

Let’s start with acid base balance. Metabolizing proteins, specifically the sulfur containing amino acids, methionine and cysteine, generates hydrogen sulfide (H2S). This acid cannot be cleared by the lungs. #OnlyTheKidneys can clear this acid. 2/11

This acid load, generated by normal metabolism that must be excreted by the kidneys is the daily acid load. When patients with advanced CKD develop progressive metabolic acidosis it is because they are failing to clear the daily acid load. 3/11

On a “western diet” (I.e. carnivorous diet) it is about 50-100 mEq of acid (H+ ions) a day. 4/11

Boy it would be easy if we could excrete this as free hydrogen ions, alas that would require a urine pH of around 1 (50 mEq H+ in 2 liters of urine). At a minimal urine pH of 4.5, we would need to make 1200 liters of urine a day to clear the daily acid load. 5/11

So the kidney has to smuggle the hydrogen out as something other than free hydrogen. There are two solutions to this:

  1. Titratable Acid
  2. Ammonium 6/11

Titratable acid is just H2PO4–. Most of the daily acid load is excreted this way. The problem is that it is fixed by phosphate intake. We cannot manufacture new phosphate open the spot when we encounter a large acid load, so we cannot ramp up phosphate excretion to deal with an acid load.** 7/11

**Actually that is not entirely true. In addition to the serum bicarbonate, the bones are called upon to buffer an acid load. And as they are dissolved buffering acid (not an ideal state) they release phosphate which clears the acid from the body. 8/11

So when faced with a large acid load we call on system two: ammonium. The physiologists have two models for how this works. In one the production of NH4 from glycine produces two bicarbonate, the other urinary NH3 accepts a hydrogen ion to form NH4. 9/11

We’ll let the physiologists argue over these two models, for our purpose the only thing you need to know is that a healthy renal response to acidosis is an increase in urinary ammonium to excrete the excess daily acid load. 10/11

The problem comes from the fact that when you order a urine ammonium the labs tells you to pound sand. They won’t do it.They can use the same instrument they use to measure serum ammonia (though they would need to dilute the urine sample 40:1). Because the lab wouldn’t measure urine anion gap lead to 40 year distraction called the urinary anion gap… 11/11

Okay, on to part two. 

Let’s review, the kidneys excrete 50-100 mEq of H+ ions every day as what we call, “The daily acid load”

Most of the daily acid load is excreted as H2PO4–, AKA Titratable acid

An insignificant < 1% is excreted as free hydrogen and that can be detected as urine pH.

And most importantly, for our purposes, in the face of an acid load, that excess acid is excreted as NH4+ 1/

Clinical labs generally refuse to measure urinary ammonium so doctors have been forced to scramble to find ways to “estimate” urinary ammonium. 2/

In 1986 electrolyte legend, Mitch Halperin published this paper which discovered an amazingly tight correlation between urinary anion gap and urinary ammonium.  3/

This makes sense. The typically measured urine electrolytes are sodium, potassium, and chloride. If there are a lot of positively charged ammoniums in the urine, the urine chloride better increase to balance those cations out. Can’t have people peeing sparks. 4/

Discoveries are made in JCI, but standard of care changes with NEJM. And sure enough, 2 years later, NEJM published this “proof” of the urinary anion gap. What an amazing time that the NEJM would publish 60 person physiology studies 😍 5/

So for forty years this schema ruled the nephrology wards.

Patients with non-anion gap metabolic acidosis would have urine electrolytes checked in order to see if their kidneys were responding appropriately. 

A negative urine anion gap indicated a rich supply of urinary ammonium indicating a healthy renal response and would direct physicians to look to the gut for the cause of metabolic acidosis.

A positive urinary anion gap indicated a kidney that was unable to excrete excess ammonium and suggests a diagnosis of distal or hyperkalemic (type 1 or 4) RTA. 

But a few years ago Dr. Uribe began making noises that this whole urinary anion gap went against fundamental laws of nature. Na,Ley that urinary na, k, and cl are not there to merely balance charges but that their urinary excretion is dependent on dietary intake.

Uribe pointed out that patients would have volume depletion(due to extra-renal sodium losses) and as a result would lower urine Na, resulting in the negative gap. Similarly in the healthy controls ion Batille’s study, they had been loaded with oral NH3Cl. The excess chloride he argues would make the urinary anion gap negative. 

He then points to studies of DKA, respiratory acidosis, and systemic acidification that all result in increases in urine ammonium but do not make the urine anion gap more negative.

And here is the Keynote presentation I used to generate the images

Keynote

PowerPoint

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OUWB M2 questions and Answers

The Prince William Question

Lets do this number by numbers. This is an algorithm that will allow you to map out any acid-base question.

1. Primary disorder: pH is up (<7.4), pCO12 is down (< 40) and HCO3 is up (>24)The H-H variables are moving in discordant direction so this is an respiratory disorder, the pH is elevated so this is a respiratory alkalosis.

If you are really on the ball you will note that this breaks one of the fundamental guidelines of acid-base in that compensation is not in the same direction as the primary disorder (pCO2 and HCO3 almost always move in the same direction). This only happens when there are two primary disorders.

2. Is there a second primary acid-base disorder affecting compensation?Yes. In Respiratory alkalosis, for every 10 the pCO2 falls the bicarb falls 2 acutely and 4 chronically, so the target HCO3 is 22 for acute respiratory alkalosis and 20 for chronic respiratory alkalosis, well the bicarb did not drop at all, in fact it went up, so there is an additional primary metabolic alkalosis.

3. What is the anion Gap?148-(98+28)=22.So we did not talk about this, but the presence of an anion gap means there is an metabolic acidosis buried deep in the ABG.

4. Calculate the bicarb beforeBicarb before = HCO3 + (Anion gap -12)Bicarb before = 28 (22-12) = 38So without the anion gap the bicarb would be 38, revealing a pretty severe metabolic alkalosis, that is mostly hidden or covered up by the anion gap metabolic acidosis. The severity of the metabolic alkalosis by looking at the electrolytes without the anion gap. 

Put it all together and you have: A respiratory alkalosis, likely from the respiratory stimulant effect of feverA metabolic alkalosis from vomiting and/or some antacids he may have taken to soothe his stomach. An anion gap metabolic acidosis from the sepsis.

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How I made that short video about interpreting ABGs

Here are the tweets (I’m using WordPress’ ability to post a tweetstream, pretty cool)

We need to know if it is alkalosis or acidosis, so we ordered an ABG.

Thought I’d try my hand at a @HannahRAbrams style explanatory animation for the above ABG. Need to get it down under 2:20 to fit in a tweet.

And the last bit

Originally tweeted by Joel M. Topf, MD FACP (@kidney_boy) on October 7, 2020.

I made the video with Keynote, it is a single slide with a lot of animation. Here is the slide (all 750 kilobytes):

Creating the animation just takes patience. This slide has 44 steps to the animation. It is a mixture of build ins, actions, and build outs.

Once I had the animation perfect I used “Record Slideshow…” to record the animations and my narration, then exported the movie using “Export To Movie…”

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Curbsiders #104: Renal Tubular Acidosis

This is the back half of my Acid-Base talk, a detailed dive into non-anion gap metabolic acidosis with an examination of renal tubular acidosis. This one turned out pretty good.

Here is a link to the Curbsiders page for this episode.

This is the sequel to #88 Acid base, boy bands, and grandfather clocks with Joel Topf MD

Before that I did episode #67 and #69 on chronic kidney disease

Before that was #48 Hyponatremia Deconstructed

And I started my Curbsiders career with #31 Diuretics, leg cramps and resistant hypertension.

So non-anion gap metabolic acidosis is my fifth or sixth appearance on the Curbsiders. Thanks guys.