All-time favorites: Arterial blood gases (ABGs) demystified

All-time favorites: I originally wrote this post in April 2007, and it has gotten a lot of traffic over the years. I hope readers enjoy the post again...or for the first time!

I get a lot of Google hits by folks looking for ABG tricks, hints, and tips, so I'm going to post the system I was taught. It's the best (meaning "most likely I will remember when I'm handed ABG values and asked, 'What is wrong with this patient?'") I've seen in the numerous treatises I've read on the subject. 1. Look at pH: if high, alkalosis. If low, acidosis. 2. Look at PaCO2 (normal range, 35-45); note if high or low. 3. Look at HCO3 (normal range, 22-26); note if high or low.

First, check for the respiratory aspect, which involves the PaCO2 value. Look for a "seesaw": does pH go up while PaCO2 goes down or vice versa? If so, you've got yourself respiratory alkalosis or acidosis, respectively.

Second, check for the metabolic aspect, which involves the HCO3 value. Look for an "elevator" (if pH takes the same elevator, meaning both values go up or both go down), you've got yourself metabolic alkalosis or acidosis, respectively.

The problem is compensated if the pH lies within the normal range, and it is uncompensated if the pH lies outside the normal range. And if you have both a seesaw and an elevator, you have a respiratory AND metabolic problem ("alkalosis" or "acidosis" according to the pH).

Examples:

pH 7.05 (low) PaCO2 12 (low) HCO3 5 (low) Uncompensated metabolic acidosis (elevator, low pH)

pH 7.38 (WNL *on low side*) PaCO2 76 (high) HCO3 42 (high) Compensated respiratory acidosis (seesaw appears if you count the nearly compensated pH as low; bicarb is high to make up for increased PaCO2)

pH 7.10 (low) PaCO2 50 (high) HCO3 15 (low) Respiratory and metabolic acidosis (both elevator and seesaw present)

Hope this helps!