Blood Gas Analysis

Question 1

What is the anion gap, and how do you calculate it?

Answer 1

The anion gap is the difference between measured cations (positively charged ions, eg, Na+) and measured anions (negatively charged ions, eg, Cl- and HCO3-).

A normal anion gap ranges from 4 to 12 mmol/L.

The calculation for the anion gap is the following:

Anion Gap = [Na+] - ([Cl-] + [HCO3-])

Question 2

What is the alveolar gas equation?

Answer 2

A simplified version of the alveolar gas equation states the following:

PAO2 = PIO2 - (PACO2 / R)

Where PAO2 is the partial pressure of oxygen in the alveoli, PIO2 is the partial pressure of the inspired oxygen, PACO2 is the partial pressure of CO2 in the alveoli, and R is the respiratory ratio.

Note that R is equal to 0.8 , and PIO2 is equal to 150 mm Hg at ambient temperature and pressure