ABG interpretation

 

See Eric Strong’s Tutorial: ABG Interpretation - YouTube

 

Normal values

·     pH: 7.4

·     PaO2: 100mmHg

·     PaCO2: 40mmHg

·     HCO3: 24mM

·     Base excess: 0mM

·     Anion gap: 16

1.Oxygenation

PaO2:

·     >60mmHg: safe

·     <60mmHg: not safe

A-a gradient:

·     Normal: 5-10mmHg (↑with age)

·     If room air: PAO2 = 150-PaCO2 / 0.8

·     If supplemented: PAO2 = FiO2 x 713 – PaCO2/R

P/F ratio: (valid if normal CO2)

·     Normal: 3.5-5

·     Mild: <3

·     Mod: <2

·     Sev: <1

2.Status

·     ↓pH = acidaemia

·     ↑pH = alkalaemia

3.Cause

If acidaemia:

·     ↑CO2: respiratory acidosis

·     ↓HCO3-: metabolic acidosis

If alkalaemia:

·     ↓CO2: respiratory alkalosis

·     ↑HCO3-: metabolic alkalosis

 

*If both metabolic and respiratory*

·     Eyeball: whichever looks worse

·     Frequency gamble: the primary disorder is usually metabolic

·     Calculation: ↑10mmHg CO2 = ↓0.03 pH if acute, ↓0.08 pH if chronic

4.Compensation

*If right direction = compensation*

*If wrong direction = secondary cause*

 

·     Resp acidosis:  ΔHCO3- = 0.1 x ΔCO2 if acute, 0.4 x ΔCO2 if chronic

·     Resp alkalosis:  ΔHCO3- = 0.2 x ΔCO2 if acute, 0.5 x ΔCO2 if chronic

·     Met acidosis: CO2 = 8 + 1.5 x HCO3-

·     Met alkalosis: ΔCO2 = 0.7 x ΔHCO3- (limited by ↓PaO2)

(For metabolic disorders, CO2 ≈ last two digits of pH)

5.Anion gap

Anion gap:

·     Purpose: a) characterize acidosis b) identify occult acidosis

·     Calculation: (Na + K) - (Cl- + HCO3)

·     Correction: ΔAG = 0.25 x Δalbumin

·     HAGMA: lactate, ketones, toxin, poison

·     NAGMA: normal saline, GIT losses, renal disease

Delta ratio:

·     Purpose: identify multiple metabolic disorders

·     Calculation: ∆anion gap / ∆HCO3

·     =1: HAGMA only

·     <1: HAGMA + NAGMA

·     >1: HAGMA + metabolic alkalosis

6.Don’t forget

·     Haemoglobin

·     Glucose

·     Electrolytes

·     Temperature

 

 

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