Acid Base Approach...and Metabolic Acidosis

Acid-Base Problems...

Step 1: look at the pH. Whichever side of 7.4 the pH is on will be your Primary abnormality. Eg. if the pH is less than 7.4, you have a primary acidemic issue.

Step 2: find out if this is a metabolic or respiratory disorder (see flow chart below).

normal values:
pH 7.4
HC03 25
pCO2 40

Step 3: Look for “Compensation”
The body’s goal is to attain a pH of 7.4, this is a pH which our enzymes, muscles, nerves, etc. function at their best. The direction of compensation is almost always the same as the primary disorder to help the pH get back to normal.

Eg.1. you have a primary metabolic acidosis (low pH, low HCO3), you would expect your pCO2 to also fall, hence the direction of compensation of pCO2 is in the same direction as the primary disorder, they are both lowered.

Eg.2. if you have a primary respiratory alkalosis (high pH, low pC02), you would expect your HCO3 to fall as well.

Step 4: Look at the magnitude of compensation (discussed below).

What is the mechanism of compensation?
respiratory disorders: the kidneys increase their rate of HCO3 production in response to respiratory acidosis, and decrease their rate of HCO3 production in response to respiratory alkalosis
↑ pCO2 → HCO3↑
↓ pCO2 → HCO3 ↓

metabolic disorders: Patients hyperventilate in response to metabolic acidosis, and hypoventilate in response to metabolic alkalosis.
↓ HCO3 → pCO2 ↓
↑ HCO3 → pCO2 ↑


Metabolic Acidosis

Today we discussed a case of Diabetic Ketoacidosis. We talked about precipitants of DKA earlier and you can read about it here, we have also discussed the management of DKA here. There are some good links on both pages.

Ask yourself a few questions:

1. Firstly: Is this an anion gap, or non-anion gap metabolic acidosis? See below for details.

2. What is the etiology? you can use your “mudpiles,” but I find this a simpler way…you only have to remember 4 things...

1. Lactic acidosis: hypoperfusion of tissue vs Liver disease and cannot metabolize lactate.
2. Ketones: from Diabetes, alcohol, starvation
3. Uremia
4. Poisons/Toxins: ethanol, methanol, ethylene glycol, ASA, isopropyl alcohol, etc.

3. Is there compensation? for each 1 mmol/L fall in HCO3, you would expect a 1 mmHg drop in pCO2

4. Calculate the Anion Gap: AG = Na – [Cl + HCO3], a normal value is 12 or less

Remember to correct for Albumin: for every drop of albumin by 10, add 3 to your AG. So if you calculate your AG to be 12 (seemingly normal), but your albumin is 30, you must add 3 to your AG, making it 15… now start looking for excess anions (see above etiologies).

5. The Delta Delta: Delta what? Not to worry, it's pretty straight forward, and it adds some valuable information. Compare the change in anion gap to the change in HC03. What does this mean? Let’s say our patient has an AG of 20 and a HCO3 of 17. Well, compared to our normal values, the delta AG is 8 (20 – 12), and the delta HCO3 is 8 (25 – 17), so the delta AG is equal to the delta HCO3. We would expect the change in AG and the change in HC03 to be similar in a pure metabolic acidosis. If the delta AG is bigger than the delta HCO3, there is also a concurrent metabolic alkalosis present. If the delta AG is less than the delta HCO3, there is a concurrent AG metabolic acidosis, and non-AG metabolic acidosis.

6. Calculate the Osmolar Gap:

OG = [2x Na] + Urea + Glucose ....Remember "two salts and a sugar-bun"

Measured OG – Calculated OG should be less than 10.

Is this greater than 10? think about EtOH, methanol, ethylene glycol, isopropyl alcohol

Non-AG Metabolic Acidosis? think about diarrhea, and renal tubular acidosis. You can read more about RTA's here.