Here we provide a general systematic approach to interpretation of blood gases:
Read stem carefully
1. pH
7.36-7.44 normal, compensated or mixed disorder
<7.36 acidaemia
>7.44 alkalaemia
2. Primary disorder
a. HCO3 (normal 24 +/- 2)
low – metabolic acidosis
high – metabolic alkalosis
b. BE -2 to +2
c. pCO2 35-45 mmHg
low – respiratory alkalosis
high – respiratory acidosis
3. Compensation Rules
a. metabolic acidosis – Winter’s formula
expected CO2 = 1.5 X HCO3 + 8 (+/- 2)
CO2 cannot usually fall to < 10
b. metabolic alkalosis
expected CO2 = 0.7 X HCO3 + 20
CO2 elevation limited by respiratory drive
c. respiratory acidosis
acute – every increase in CO2 of 10 causes an increase in HCO3 of 1
chronic – every increase in CO2 of 10 causes an increase in HCO3 of 4
d. respiratory alkalosis
acute – every decrease in CO2 of 10 causes a decrease in HCO3 of 2
chronic – every decrease in CO2 of 10 causes a decrease in HCO3 of 5
4. Oxygen
pO2 80 – 100 mmHg on room air
O2 saturation usually >95
5. Further Interpretation
a. Anion gap = Na-Cl-HCO3 normal 12 +/- 4
b. Delta ratio = AG – 12 / 24-HCO3 normal 0.8 – 2
assumes that in a pure AG metabolic acidosis the HCO3 decreases proportionately to the increase in anion gap
In mixed disorders, this is not always the case
Range:
0.4 – 0.8 mixed AG and Normal AG metabolic acidosis
< 0.4 pure non AG metabolic acidosis
> 2 metabolic alkalosis or compensated respiratory acidosis
c. Osmolar Gap = measured – calculated normal = 10
calculated osmolarity = 2Na + Urea + Glucose
measured is obtained by the lab
d. A-a gradient (alveolar – arterial O2 gradient)
estimated normal = Age/4 + 4 (rough guide)
Alveolar gas equation = (760-47)XFiO2 – (CO2/0.8) at sea level
FiO2 at Room air 21% or 0.21
6. Other Clues
a. Hyperglycaemia
correct for Na: corrected Na = (Glucose – 10)/3 + Na
and check for ketones ie. DKA, Alcoholic KA, starvation
b. Potassium
acidosis causes shift of K out of cells, this is then excreted in the urine, for every 0.1 decrease in pH the actual whole body Potassium decreases by 0.5
conversely hypoK (and hypochloraemia) suggests metabolic alkalosis.
c. Hyperchloraemia is common in non anion gap metabolic acidosis
d. Urea and Creat – if elevated consider pre renal / renal
Excess ketoacids may causes a high Creat due to Jaffe reaction in lab measurement (usually normal Urea)
7. Finally, look at the scenario and make necessary interpretation, decide on differential diagnosis, further investigations and treatment priorities.