Chapter 19: ACID-BASE DISORDERS

A GUIDE TO CALCULATIONS AND SHORTCUTS FOR ACID BASE DISORDERS

NOTE: In order to do acid-base questions, you have to know some of the fundamentals. The upside is that if you learn the rules below, you’ll get some very quick points on the exam because these questions tend to be short and simple. Also, there ARE some shortcuts that can help you answer certain questions within seconds!

THE ULTIMATE ABG CALCULATOR BIBLE!

ABG FUNDAMENTALS AND TERMINOLOGY

• pH: LOW pH = ACIDOSIS!
• pCO2: HIGH pCO2 = RESPIRATORY ACIDOSIS!
• TERMINOLOGY: In a mixed acid-base situation, you can have MULTIPLE “primary” acid-base disturbances. For our purposes, let’s refer to the acid-base disorder that is winning as the “overarching” acid-base disorder. This is the disorder you diagnose by simply looking at the pH and pCO2. All other acid-base disturbances we find will be considered additional primary acid-base disorders.
• PEARL: Do NOT try to calculate for compensation in any other acid-based disturbance aside from the overarching This is EXTREMELY IMPORTANT. If you try to calculate for compensation for additional primary disorders, you will go in circles. For example, if the overarching diagnosis is a respiratory acidosis, ALWAYS check for appropriate metabolic compensation with the information provided in this section. NEVER check for respiratory compensation in such a patient if you note that the patient also has an additional primary metabolic acidosis (gap or non-gap).
• “SOAC” = THE BASIC STEPS FOR DIAGNOSING SINGLE AND MIXED ACID-BASE DISORDERS
  • S – Check the ABG and chemistry for any Shortcuts to quickly diagnosis additional acid-base disorders.
  • O – Diagnose the Overarching acid-base disturbance using the pH and pCO2.
  • A – Check for an Anion gap to see if a gap acidosis is present.
  • C – Check for Compensation but do so ONLY for the overarching acid-base disorder. If compensation is NOT appropriate, you can diagnose an additional primary acid-base disorder.

ABG & CHEMISTRY NUMBERS – THE BASICS

• • When evaluating an ABG and chemistry, start by looking at the pH and pCO2 on the ABG to identify the primary disorder. This refers to the single, overarching disorder, often easily diagnosed simply by looking at the pH and pCO2. Look at the HCO3 on the CHEMISTRY to possibly support your diagnosis. Use 7.4 as your normal pH, 40 as your normal pCO2 and 24 as your normal HCO3.
  • Next, look at the chemistry and ALWAYS calculate the anion gap. A normal anion gap, ([Na⁺] − ([Cl^–] + [HCO₃⁻]), can be up to 12. Know that the upper limit of normal for the anion gap decreases by 2.5 for every 1 g/dL drop in the patient’s albumin, relative to a normal albumin assumed to be 4 g/dl. So if a patient’s albumin is 2.0 g/dl, the difference between a normal albumin and the measured albumin is 2. Multiply 2 by 2.5 to get a decrease of 5 from 12. So the upper limit of normal for this patient’s anion gap is now 12 – 5 = 7, which means that any measured anion gap greater than 7 should be considered a positive anion gap. So if a patient has an albumin level of 2 and a gap of 10, then there is a positive anion gap.