Arterial Blood Gas Interpretation

Question 1

What is the expected respiratory compensation for metabolic acidosis?

Answer 1

PaCO2 = 40 - 1.2 x change in bicarbonate

approximately 1:1 decrease in paCO2 for drop in bicarb

Question 2

What is the expected respiratory compensation for metabolic alkalosis?

Answer 2

PaCO2 = 40 + 0.6 x change in bicarbonate

Question 3

What is the expected metabolic compensation for acute respiratory acidosis?

Answer 3

HCO3 = 24 + Change in PaCO2/10

Question 4

What is the expected metabolic compensation for chronic respiratory acidosis?

Answer 4

HCO3 = 24 + 4 x (change in PaCO2/10)

Question 5

What is the expected metabolic compensation for acute respiratory alkalosis?

Answer 5

HCO3 = 24 - 2 x (change in PaCO2/10)

Question 6

What is the expected metabolic compensation for chronic respiratory alkalosis?

Answer 6

HCO3 = 24 - 5 x (change in PaCO2/10)

Question 7

What is the effect of hypoalbuminemia on the interpretation of acid/base status?

Answer 7

In a metabolic acidosis, expected anion gap is decreased for hypoalbuminemia.
Normal AG = 12-14
AG in metabolic acidosis = 12- 0.25 x (change in albumin)

Question 8

What is the 1-2-3-4-5 Rule?

Answer 8

Rule to predict changes in HCO3 based on PaCO2:

For every 10 change in PaCO2:

Increase - acute = 1:10 +
Decrease - acute = 2:10 -
Increase - chronic = 4:10 +
Decrease - chronic = 5:10 -

i.e. For chronic resp acidosis with a PaCO2 of 60, HCO3 = 24 + 8 = 32

Question 9

What is the equation for the A-gradient?

Answer 9

A-a gradient = FiO2(Patm - PH2O) - PaCO2/RQ - PaO2

Where…
Patm = 760
PH2O = 47
RQ = 0.8

Question 10

What is the normal A-a gradient adjusted for age?

Answer 10

A-a gradient = 4 + (age/4)

Question 11

What is the differential diagnosis for hypoxemia with a normal A-a gradient?

Answer 11

1. Hypoventilation

2. Low PiO2

Question 12

What is the differential diagnosis for hypoxemia with an elevated A-a gradient?

Answer 12

1. Diffusion deficit
2. V/Q mismatch
3. R to L shunt
4. Increased O2 usage, i.e. low SvO2

Question 13

What is the importance of low albumin in interpretation of metabolic acidosis?

Answer 13

Hypoalbuminemia results in a decrease in expected anion gap.

i.e. normal gap = 12 and for every 10 decrease in albumin, expected anion gap decreases by 2.5