ABG interpretation

ABG INTERPRETATION by StudyEM

STEP 1

Check ABG is for correct patient

 

STEP 2

Assess oxygenation

• pO₂ <10.5 kPa = hypoxic

 

Was the patient on oxygen when the ABG was performed?

• In general the pO₂ should be approximately: FiO₂(%) -10
  • For example:
    • On 40% O₂: pO₂ should be 30 kPa

 

Is there a significant alveolar-arterial gradient?

A-a gradient = PAO₂ – PaO₂

• Alveolar (A)
• Arterial (a)
• Partial pressure of oxygen in the airways (PAO₂)
• Partial pressure of oxygen in the artery (PaO₂)
• PAO₂ = FiO2 (P_atm –P_H20) – PaCO₂/kQ
  • P_atm-P_H2O = 713 at sea level
  • kQ = 0.8 (constant)
• Therefore:
  • PAO₂ = (FiO₂ x 713) – (PaCO₂/0.8)
• A-a gradient helps identify where the source of the hypoxia is coming from.  A high A-a gradient suggests a V/Q mismatch or right-to-left shunt.
• Normal A-a gradient is dependent on age as for every decade of life the A-a gradient increases by 1 mmHg.
• Age-adjusted normal A-a gradient = (age/4) + 4
  • Therefore a 40yo should have an A-a gradient less than 14.

 

Identify type of respiratory failure

• Type 1 – normal/low pCO₂
• Type 2 – high pCO₂

 

STEP 3

Determine the pH status

<7.35 – acidosis

>7.45 – alkalosis

 

Find the primary source of the pH disturbance

• Acidosis:

• pCO₂ > 6.0 kPa – respiratory
• HCO₃^– <22 mmol/l – metabolic

 

• Alkalosis
  • pCO₂ < 4.7 kPa – respiratory
  • HCO₃^– >26 mmol/l – metabolic

 

Is there a mixed picture?

• Respiratory and metabolic acidosis

or

• Respiratory and metabolic alkalosis

 

STEP 4

Is there compensation?

• Compensation present if:

• Respiratory acidosis: High HCO₃^– (commonly seen in COPD patients)
• Respiratory alkalosis: Low HCO₃^– (rare)
• Metabolic acidosis: Low pCO₂ (limited to pCO₂3 kPa)
• Metabolic alkalosis: High pCO₂ (only able to hypoventilate slightly)
  • Metabolic compensation take 3 days
  • Respiratory compensation occurs quickly
  • Over compensation CANNOT occur

 

Is there full or partial compensation?

Partial: pH remains abnormal

Full: pH has normalized

 

Is compensation adequate?

Not required at undergraduate level. For more information see http://lifeinthefastlane.com/investigations/acid-base/

 

STEP 5

Is there an anion gap?

Anion gap = Na⁺ – (Cl^– + HCO₃^–)

Normal = 12 ± 4

• Potassium is generally excluded as it is relatively stable
• Should be corrected in patients with albumin <40 g/L (add 2.5 per 10 g/L decrease)
• Sodium should be corrected in hyperglycaemia (cNa⁺ = Na⁺ + (glucose -5) ÷ 3)
• Anion gap is calulcating the difference in measured cations and anions. Since cations and anions will be balanced, a high anion gap is eluding to unmeasured substances ie. Ethanol
• Click here for causes of high anion gap metabolic acidosis (HAGMA) and non-anion gap metabolic acidosis (NAGMA) – see “ABG Mneumonics” on the data interpretation page.

 

Consider calculating other metabolic acid-base disorders eg

• Delta Gap = (anion gap – 12) ÷ (24 – HCO₃^–)
• Osmolality = (2 x Na⁺) + urea + glucose + ethanol
• Osmolar gap = measured osmolality – calculated osmolality

 

STEP 6

Check the other values especially:

• Hb
• Glucose
• Lactate

 

Remember to treat the patient and not the numbers. ABGs are a guide to management, but should be used in the wider clinical context (and don’t forget the possibility that the ABG machine may have given an incorrect result!)