Lab Cases: General Approach to Interpretation of Blood Gases

Here we provide a general systematic approach to interpretation of blood gases:

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          1pH 

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.

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