2025 – PAGE 367 – ACID-BASE DISORDERS

ACID-BASE DISORDERS & PEARLS

ACIDOSIS

The only evidence of a primary metabolic acidosis may be tachypnea (occurring as a respiratory compensatory mechanism).

ANION GAP

An anion gap represents the unmeasured ions (phosphorus, organic acids, proteins, lactic acid, sulfate, etc.) outside of the Na+, Cl-, and CO3- that are in the routine chemistry panel. A normal gap is 12 or less. Up to 16 might be okay in a newborn.

ANION GAP METABOLIC ACIDOSIS

An anion gap metabolic acidosis can be noted in conditions including Methanol ingestion, Uremia, Diabetic ketoacidosis, Paraldehyde, IRON/INH ingestion, Lactic acidosis, and ingestion of Ethanol/Ethylene Glycol, Salicylates, and Theophylline. There should be a NORMAL chloride and LOW bicarb. Can also be found in acid ingestion, renal failure (because kidneys cannot excrete acids), and various inborn errors of metabolism (due to ketosis and lactic acidosis).

NOTE: “Anion gap metabolic acidosis” refers to an acidosis where the anion gap is increased beyond the normal maximum (usually 12 but can be lower if the albumin level is low).

MNEMONIC: Fill the ANION GAP with MUDPILES. The D = DKA, not DIARRHEA!

PEARL: The urea cycle defects have NO acidosis. They can, however, have a primary RESPIRATORY alkalosis.

NON-ANION GAP METABOLIC ACIDOSIS

Non-anion gap metabolic acidosis conditions include Ureterostomy, Small bowel fistula, Extra chloride, DIARRHEA (most common cause), Carbonic anhydrase inhibitors (acetazolamide), Adrenal insufficiency, Renal Tubular Acidosis, and Parenteral nutrition/Pancreatic fistula/PosthypOcapnea. Look for HYPERCHLOREMIA and a LOW BICARB level. Here are a few mechanisms to keep in mind:

• Extra chloride: Chloride levels go up to replace lost bicarbonate and maintain electrical neutrality.
• Diarrhea: Acidosis occurs from bicarbonate loss.
• RTA: Mechanism varies.
• Carbonic Anhydrase Inhibitor: Promotes renal bicarbonate loss.

PEARL: –CAPNEA refers to how much CO2 is in the blood and is NOT referring to how fast someone is breathing. So, hypOcapnea means there is low CO2 in the blood, and this occurs due to HypERventilation. Therefore, you get a RESPIRATORY ALKALOSIS and a compensatory non-gap metabolic ACIDOSIS. HypERcapnea means there is high CO2 in the blood, and this occurs due to hypOventilation. Therefore, you get a RESPIRATORY ACIDOSIS and a compensatory metabolic ALKALOSIS. Make sure you look carefully at the terminology in the question stem or vignette.

MNEMONIC: “Acid-azolamide”

MNEMONIC: NON-GAP CRAP! Several stool-related issues cause a non-gap acidosis (diarrhea, small bowel fistula, ostomies).

MNEMONIC: USED CARP (Ureterostomy, Small bowel fistula, Extra chloride, DIARRHEA, Acetazolamide/Adrenal insufficiency, Renal Tubular Acidosis, and Parenteral nutrition/Pancreatic fistula/PosthypOcapnea.

MNEMONIC: If you do not know your normal values for routine chemistries, a non-gap acidosis can be tricky. If you just calculate the gap and see that it’s normal, you could get in trouble. You should look closer at the chloride and bicarbonate levels. So I’m saying that the chemistry looks good on the outside (i.e., the gap looks good), but it’s actually falling apart on the inside (low bicarb and high chloride), LIKE A USED CARp!

MNEMONIC: Building on the one above mnemonic, a USED CARp can only travel on smooth roads WITHOUT GAPS/potholes!